How to treat acidic soil?


pH acidity: what it does to your plants

In this article we will look at pH and acidity and what this means for a plant and the growing environment. First of all, we’ll look at what acidity and pH really are.

What is acidity?

Acidity is essential for life on earth. Acidity often determines the characteristics, quality, absorbability and solubility of many substances. This is how enzymes, which are responsible for almost all biological processes in organisms, work, but only with the correct acidity. A small fluctuation in the blood’s acidity is deadly.

What is pH?

The pH (pondus Hydrogenii) indicates a solution’s acidity or alkalinity. The pH value usually varies between 0 and 14. A solution with a pH value between 0 to 7 is acid and one between 7 to 14 is alkaline. Vinegar and cola have a pH value of less than 3. Soda and soap have a pH value higher than 8. A pH value of 7 is considered neutral. Pure water at room temperature has a pH of 7. The pH of tap water is generally a little higher due to the presence of calcium.

Many natural environments such as our skin, plant substrates and nutrient mediums are mildly acidic and have a pH value of between 5 and 6.5. If we look at the things that people like we see that they are generally mildly acidic or neutral substances such as water. Plants also prefer mildly acidic substances. A pH value of around 5.5 occurs so often in nature that some plant experts regard this value as ‘neutral’.

Why is acidity important?

Acidity has a substantial influence on the absorbability and solubility of a number of food elements (see figure 1).

In addition acidity has considerable influence on the structure, breakdown of organic substances, and the micro life in the ground. The pH also influences the way in which food elements, heavy metals, and pesticides are flushed out of the ground.

A pH value that is too low or too high can be detrimental to your plants, so it is important to get it right. But how do you know when the pH is wrong? By experience! So to help you, we’ve set out some of the symptoms you might observe:

Symptoms of a pH that is too low (substrate is too acid):

  • Most nutrients can be dissolved easily, which can cause an excess of manganese, aluminium and iron;
  • Phosphorus, potassium, magnesium and molybdenum deficiencies can be caused by excessive rinsing;
  • Magnesium deficiency, especially in cold substrates;
  • The soil is generally poor;
  • Soil life is inhibited.

Symptoms of a pH that is too high (substrate is too alkaline):

  • Most nutrients dissolve less easily, causing calcium, iron and phosphate compounds to precipitate;
  • Reduced absorption of manganese, phosphate, and iron in particular but also copper, zinc and boron. This will cause deficiencies, particularly in wet, cold growing mediums In sandy soils the breakdown of organic substances increases considerably if the pH is high.

What determines the pH?

One of the most important factors determining the pH value in a solution or in the substrate is the buffering capacity. The buffering capacity in this instance means that there is a sort of balance present that continually restores itself. For example, if one puts a drop of acid into 1 litre of tap water that has a pH of 7 it will have little influence on the acidity. However, if one puts one drop of acid in 1 litre of demineralised water (battery water), the pH will immediately fall dramatically. This is because tap water contains bicarbonate while demineralised water doesn’t. Bicarbonate is the most important buffering substance for pH values between 5.5 and 7.5 in water.

Bicarbonate binds itself to acid in the solution which releases carbon dioxide into the atmosphere. This is how the acid is neutralised and the changes in the acidity will only be minor so long as there is still bicarbonate present.

With a pH value of 5.3 all the bicarbonate has been used up and the solution has no more buffer. The pH is now unstable and it will change immediately if acid is added (see figure 2). The amount of acid that is needed to get a feeding solution to the correct acidity can therefore be calculated based on the bicarbonate content. The bicarbonate content of tap water is generally given by the water company in milligrams per litre.

The buffering capacity and the substrate’s acidity depend on its composition and freshness. The presence of organic material, calcium and bicarbonate generally determine the pH. Clay always contains calcium carbonate and has a relatively high pH value which is difficult to change, while peat and sandy soils are acid.

The plant itself also has great influence on the acidity. The roots will secrete either acid or alkaline substances depending on the crop’s stage of development, the food available, the differences in root temperature and light intensity. So you see why the pH of the root environment can constantly fluctuate. A sophisticated feeding balance during the different phases of development will keep the pH in the root environment within acceptable limits.

Micro life, CO2 levels, and algae growth can also have an effect on the acidity of the root environment and the nutrient tank.

Measuring the pH value

It is quite easy to measure the pH – you need some pH indicators such as litmus paper or a pH testing set. These are relatively cheap but are not always accurate and can sometimes deviate by 1 to 2 pH units. pH meters are generally more expensive and the accuracy depends on the type of meter and regular calibration with calibration fluid.

Taking samples

The pH of the water used to water the plants is important but the acidity around the roots is essential. So when you measure the pH it is very important to take the sample in the correct way to get good results. The sample has to represent the average acidity in the root environment.

It is easy to take samples and measure the pH in a recirculation system, simply measure the recirculated feeding solution.

In substrate systems without recirculation, feeding solution is drawn out of the substrate (rock wool, agrofoam) at a number of locations. Experts have been discussing the question of where to take the samples from for a year and a day. We recommend, just like a number of reputable laboratories do, taking samples from the places where the roots are which is under and around the drippers. Take small samples from as many places as possible. Always take all samples at the same time and preferably after the second drip-feeding during the light – daytime – cycle.

In soil, coco and peat substrates just take a small amount of substrate from several locations.

You can best measure the acidity of your sample using the ‘1:1.5 volume extract’ method. You can easily do this yourself by making the growing medium so wet that the water runs through your fingers when it is kneaded and squeezed quite hard. Use a 250 ml measuring beaker for example. Fill the measuring beaker to 150 ml with demineralised water. Add growing medium until the volume is 250 ml (photo 3). Shake it well and let it stand for a few hours. Then filter it and measure the pH.

The correct pH values for every medium

When cultivating in substrate pH values of between 5.0 and 6.4 are fine for the root environment. There will not be any adverse effects if the values are a little higher or lower. Immediate adverse effects will only be seen with values lower than 4 and higher than 8. pH values lower than 4 often cause immediate damage to the roots. In addition, heavy metals, including manganese and iron are absorbed so well that they can poison the plant (necrosis). Values between 7 and 8 are not immediately harmful for the plant. Nutrients such as iron, phosphate, and manganese are less available then which will lead to deficiencies (chlorosis and development problems) in the long run.

Correcting the pH value

If the acidity in the root environment is between 5 and 6.4 then the pH of your growing environment is OK and you do not have to take any corrective measures. Try to avoid correcting the pH unless it is really necessary. It’s more likely to do harm than good; the plant likes its peace and quiet. It is more important to monitor how the acidity changes over a longer period. If the value falls below pH 5 or rises above pH 6.4 then it is advisable to gradually start making adjustments (see figure 2).

Correcting the acidity is most easily done by lowering the acidity of the feeding solution with nitric acid during the growing phase and phosphoric acid during the flowering phase or, as the case may be, to raise it with caustic potash, potassium bicarbonate or soda and CANNA RHIZOTONIC. Ensure that the pH in the solution that is used does not fall too far below 5.0. When growing in rock wool the fibres will be harmed causing a lot of alkaline material to be released at very low pH values. In addition, the pH is more difficult to control due to the absence of bicarbonate.

A high pH in the root environment can also be caused by bicarbonate that has built up. To remedy this maintain 20% drainage or rinse through with a more acid solution.

It is useful to note the pH measurements from both the solution added and the feeding solution in the substrate. You will get a good idea of the progression of the pH and the effect of the measures taken.

  1. Protein splitting enzymes need an acidic environment (gastric juices) and carbohydrate splitting enzymes need an alkaline environment (saliva).
  2. The acidity of a solution is determined by the ratio of hydrogen ions (= acid) and hydroxide ions (= alkaline).
  3. Shortages can occur because the plant has to secrete protons to be able to absorb these molecules. A growing medium with a low pH already has a very large quantity of protons. These elements also get rinsed out because the protons repel the molecules from the medium in the substrate.
  4. Bicarbonate is the substance that, when combined with calcium, is responsible for scale. In combination with sodium, bicarbonate is used in medicaments to counter excess gastric acid (Alka-seltzer).
  5. Some laboratories also work with the bicarbonate hardness. In order to translate this to mg/l bicarbonate you must multiply the bicarbonate hardness by 21.8.
    For example: the bicarbonate hardness is 11, then 1 litre of water contains (11 x 21.8=) 240 mg/litre bicarbonate.
  6. Sandy ground: Grass land pH 4.6 … 5.2 Building land pH 5.0 … 5.6
    Clay: Sea Clay pH 6.0 … 7.2 River Clay pH 6.2 … 6.4
    Peat: Unprocessed pH 4.0
  7. If there is significant algae growth then the pH will increase because carbon dioxide will be removed from the solution. Bacteria can transform certain forms of nitrogen so that they have an acidifying effect. Large amounts of CO2 in the air generate more carbon dioxide in the feeding solution and vice versa.
  8. Only use soda in small quantities, because it contains sodium, and plants only need a very small quantity of sodium. Remember, high concentrations of sodium will damage the plant.

Figure 3: illustration of the pH values of tap water from various areas with differing levels of bicarbonate. We added 33 ml of nitric acid (38%) to each 100 litre sample of each water type. The pH curve drops faster after pH 5.3 because for this type of water the acid neutralises all the bicarbonate. Below pH 5.3 the acidity level will accelerate fast.

Soil pH

Soil pH is a measure of the acidity or alkalinity (basicity) of a soil, and is reported as a value between 0 and 14. A soil test for pH measures the concentration of hydrogen ions in the soil solution.

A pH of 7.0 is considered neutral. A pH value below 7.0 indicates that the soil is acidic, with lower values representing increasing acidity. A pH value above 7.0 indicates that the soil is alkaline (basic), with higher values representing increasing alkalinity.

The pH scale is logarithmic, so a change in 1 pH unit reflects a 10 fold change in acidity or alkalinity.

Alkaline Soils

Soils may be alkaline due to over-liming acidic soils. Also, alkaline irrigation waters may cause soil alkalinity and this is treatable, but alkaline soils are primarily caused by a calcium carbonate-rich parent material weathering (developing) in an arid or dry environment.

These types of soils are common in many areas of the western United States. The average pH of these carbonate-containing arid soils is 8.0. Most landscape and garden plants do best at pH values between 6.0 and 7.2.

Popular climbing clematis vines, grow well in high pH soils. Photo credit: Qsimple Flickr CC BY-NC-SA 2.0

Problems Caused by Alkaline Soils

The availability of many plant nutrients in soils, including iron, zinc, copper, and manganese, is reduced at high pH values. Iron chlorosis in plants, caused by inadequate iron, is a common problem in alkaline soils.

Iron chlorosis, or iron deficiency in plants, causes yellowing of leaves while the veins remain green. Photo source: Charlotte’s Place Landscape and Interior Decor Consultant

Phosphate, a macronutrient, may also be limited in these high pH soils due to its precipitation in the soil solution.

The pH of a soil can be readily and inexpensively tested by a soil laboratory. County Extension Agents can give advice on how to sample soil and where to have the samples analyzed. Soil pH test kits may also be purchased and will give an estimate of the soil pH.

Treatment of High pH Soil

Fertilizers and chelates can be added to soil to increase concentrations of plant nutrients. It is important to note that addition of phosphate fertilizer alone will further reduce the availability of other nutrients.

Lowering the pH of alkaline soils, or acidifying the soil, is an option. Elemental sulfur can be added to soil as it forms sulfuric acid when it reacts with water and oxygen in the presence of sulfur-oxidizing bacteria. Iron and aluminum compounds can be added to soil, as they cause the release of hydrogen when they react with water. Sulfuric acid may also be added directly.

Additions of appreciable amounts of organic matter will help to acidify the soil as microbes decompose the material, releasing CO2 which then forms carbonic acid. Organic acids are also released during humus decomposition. Peat and peat moss are highly acidic forms of organic matter but can be costly.

Application of acidifying fertilizers, such as ammonium sulfate, can help lower soil pH. Ammonium is nitrified by soil bacteria into nitrate and hydrogen ions.

Soils naturally containing carbonates, or lime, are very difficult to acidify, and it may take years before a significant change in soil pH is seen. Even then, the carbonatic parent material will continue to weather, producing more soluble carbonate and buffering the soil solution pH.

Many plants can tolerate pH values between 7 and 8, and some actually thrive at these higher pH values. Choosing plants that grow well in mildly alkaline soils can be selected. This is the most reasonable “treatment” option for soils that have developed from carbonatic parent material.

Vegetable garden plants such as asparagus, beets, cabbage, cauliflower, celery, carrots, lettuce, parsley and spinach grow well in soils whose pH is between 7 and 8.

Alkaline-tolerant landscape plants include boxelder, Japanese barberry, hackberry, Russian olive, sargent crabapple, mockorange, locust, bridalwreath, and arrowwood.

pH of Soil Above 8.5

A soil pH value above 8.5 indicates the presence of sodium. High-sodium soils may reach pH values up to 10. Such high-sodium soils are termed “sodic” soils, and they may also be saline. Sodic soils contain so much sodium that the soils become dispersed and almost impervious to water. To remediate sodic soils, gypsum or sulfuric acid is added, and the soil is leached.

Sphagnum peat moss comes from formerly living sphagnum growing in bogs. Photo credit: lastonein Flickr CC BY-NC-ND 2.0


Vermont – pH for the Garden

Pennsylvania – Understanding Soil pH


Many growers&face a problem of a low pH of their soil. Some soils are acidic by nature and, in other cases, low pH is the result of prolonged and intensive fertilization and irrigation.

Soil pH below 5.5 might result in reduced yields and damages to the crop. Under these pH conditions, the availability of micronutrients such as manganese, aluminium and iron increases and toxicity problem of micronutrients might occur.
On the other hand, at low soil pH, the availability of other essential nutrients, such as K, Ca and Mg decreases and this might result in deficiencies.

In growing media, pH changes are much more rapid than in soils. Although various growing media are available with different baseline/starting pH levels, the effect of fertilization and irrigation on their pH levels can be enormous.


The most commonly used technique to raise the soil pH is applying agricultural lime. The Solubility of lime is relatively low, so if it is applied only to the soil surface, it usually affects only the top layer of the soil, not more than a few centimeters deep.

But which liming material should you use? What should be the lime application rate? .

In soilless media, lime should be incorporated into the media prior planting and the process is usually logistically difficult. Waiting until after planting only makes it more complicated, because the lime should then be individually applied to each growing container or each plant. Again, due to its very low solubility, it’s impossible to apply it through irrigation.


Unlike lime, potassium carbonate is highly soluble and therefore can be applied by drip irrigation. Due to its high solubility, potassium carbonate can be easily distributed throughout the root zone together with irrigation water and reach deeper soil profile.

In both soils and growing media, potassium carbonate can rapidly affect chemical reactions in the root zone, thus elevate root zone pH.

Irrigation with water that have a low buffering capacity (low bicarbonate content) might drastically decrease pH levels in growing media. In this case, and especially when using inert media, pH drop can present a constant problem.
Applying potassium carbonate periodically, or even regularly, as part of the fertilization program, can prevent the pH drop.


Potassium carbonate also contributes potassium to the nutrient content of the irrigation water.
Therefore, potassium carbonate can be regarded as a fertilizer and its K contribution should be considered.

When applying potassium carbonate through the irrigation water, it is important to keep the pH below 7.0 in order to avoid emitter clogging.

Sometimes growers need to increase the buffer capacity of the irrigation water, while keeping pH levels low enough. In this case, it is possible to add potassium carbonate to water, and at the same time to acidify the water. The acid will neutralize some of the carbonate ions, while the pH level will still be low enough to prevent emitter clogging.


Prior to applying materials that increase pH, make sure that the low pH is not caused by an inappropriate fertilization regime. Often, an adjustment of such a regime may solve the acidity problem.

This is especially true for growing media (soilless media): ammonium/nitrate ratio is a major factor that can determine the media pH, and it can be controlled by proper a fertilizers application.

In soils, intensive fertilization with ammonium-based fertilizers or ammonium-forming fertilizers (urea) may lower soil pH.

Other factors affecting soil pH include:

Parent material – type of rocks from which the soil developed.
Rainfall – soils under high rainfall conditions are more acid than soils formed under dry conditions.

Soil organic matter – soil organisms are continuously decomposing organic matter. The net effect of their activity is that hydrogen ions are released and the soil becomes more acidic.
Native vegetation – the type of the native vegetation under which the soil was formed affects the pH of the soil. Soils formed under forest vegetation tend to be more acidic.

SMART Fertilizer Management – Your Digital Plant Nutrition Expert

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  • Recommends the ideal fertilizer mixture/ blends
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  • Interprets soil, water and tissue test results


How To: Test Soil pH

Photo: .com

Soil matters. To the plants you wish to grow in your garden, nothing is as important as what’s in or missing from the soil on your property. After all, the soil composition makes or breaks your plants’ chances of thriving. While there are many details to learn about your soil’s makeup, the pH level is the best place to start, because some plants prefer soil that is acid (for example, azaleas) or alkaline (carnations) or neutral (grass). To plan and maintain a healthy garden, therefore, you’ve got to know where you stand pH-wise. Fortunately for backyard gardeners, you don’t need a chemistry degree to test soil pH. In fact, it’s simple.

Photo: .com

Here’s perhaps the easiest of all ways to test soil pH: Head to the nearest home center or nursery, and pick up a testing kit. (Alternatively, buy one from an online retailer like Amazon). When used correctly, such kits are fairly reliable. Not every kit involves the same order of operations, but generally the process begins with digging a small hole, two to four inches deep.

Move any twigs or stones to the side, then fill the hole with distilled water—that is, water that is neither acidic nor alkaline. (If you don’t have any on hand, you can buy a bottle from almost any grocery store or pharmacy.) As the hole you created in the soil turns into a muddy pool, insert the test probe. Now wait.

After about a minute, you should get a reading. If the pH registers as being lower than 7, that means your soil is acidic. Higher than 7? Your soil is alkaline. (Exactly 7 means your soil is neutral.) Bear in mind that most plants do well in soil with a pH between 6.5 and 7. If yours falls within that range, consider yourself lucky. No small number of gardeners must amend their soil to make it hospitable.

Before calling it a day, take the time to test the soil pH in several different parts of your garden. Even in a small yard, pH variations—sometimes considerable ones—are common. A plant that wouldn’t adjust well to one corner of your property might live very happily in another location.

An alternative method of testing soil pH involves—believe or not—the red cabbage that’s been lurking at the rear of your refrigerator. What you do is chop the cabbage into small pieces before boiling it in a pot of distilled water (again, refrain from using tap water; the H20 used must have a neutral pH).

After about 10 minutes, the boiling water should turn violet. Remove the pot from the stove, strain out the cabbage, and pour a portion of the water into two separate containers. To one container, add a small amount of vinegar. Into the other container, sprinkle a couple of pinches of baking soda. The result, assuming no missteps, should be one container of pink (acidic) liquid, another of blue-green (alkaline).

Now pour the remaining violet water into yet another empty container, only this time add in a spoonful of soil. If the water turns pink, that means your soil is acidic; if blue-green, your soil is alkaline. The stronger the color change, the more acidic or alkaline the sample. If the liquid does not change color at all, then your soil is neutral. Science!

The Ultimate Guide to Testing Soil pH

Ever wonder what else you can do to help your plants thrive? Try testing the pH of your soil! Robust plants start from the ground up, and maintaining healthy soil is the first step towards a successful crop.

This all-encompassing guide will help anyone who is just starting out with soil pH testing. (And experienced growers will be able to learn something, too!) In the first part, we cover everything from “What is pH?” to elements and events that affect your soil’s pH, and how to fix it. After that we go over all the different soil testing methods and tools, so you will have all the information you need to choose the best soil testing plan for you and your crops.

First, let’s go over the basics. (Or use one of the links below to skip to a topic!)

  1. What is Soil?
  2. What is pH?
  3. Why Test Soil pH?
  4. Things That Affect Soil pH
  5. Optimal Soil pH Ranges
  6. How to Test Soil pH
  7. Soil pH Testing Options
  8. Care & Maintenance of Your Soil pH Electrode

What is Soil?

Water, air, and soil are the largest groups of natural resources that humans use. Soil is the loose material on the surface of the Earth that allows plants to grow. Think of the Earth as an onion with layers. Soil is only the very thin (about 200cm) top layer of the onion. All plants are grown in that thin layer.

No matter where you are on earth, soil has three main components: inorganic, organic, and microorganisms.

  • The inorganic part of soil has minerals from rocks that have broken down over time. Plants use these minerals to help them grow. When thinking about soil, we tend to think of only these mineral components. Yet, the inorganic part of soil also contains liquids (water) and gases (air).
  • The organic section of soil is the broken down remains of plants, animals, and other living things.
  • Microorganisms live in the soil and are usually too small to see with the naked human eye. They help to break down dead material into soil.

There are five main factors which affect how soil is formed: climate, organisms, an area’s geology, topography, and finally, time. With all these variables, it’s no surprise that soil varies greatly from location to location, even within relatively small areas.

pH is a measurement of how acidic or how basic (alkaline) a substance is. When you test pH, you’re measuring the amount of hydrogen atoms that carry a positive charge.

The higher the concentration of hydrogen ions, the more acidic the sample is. The lower the concentration of hydrogen ions, the more basic a sample is. Acidic substances fall between pH 0 and pH 7 on the pH scale. Basic substances fall between pH 7 and pH 14 on the pH scale. pH 7 is completely neutral; it is neither acidic nor basic.

Common acidic substances include orange juice, soda pop, and black coffee. Common basic items include gin, baking soda, and household cleaners. Pure water is completely neutral at pH 7.

Why Test Soil pH?

Correct soil pH is essential to ensure optimal plant growth and crop yield, because it allows nutrients to be freely available for plants to take in. Testing the pH of your soil helps to determine what plants are best suited for that area.

Sometimes soil needs supplements, like fertilizers and soil pH adjusters, for plants to be able to thrive. Measuring the pH can help you figure out what and how much you need.

Things That Affect Soil pH

Many things can affect the pH of your soil. The most common factors are climate and weather, other plants in the area, the pH of your irrigation water, soil type, the kind of fertilizer you use, and nutrient availability.

Climate and Weather

Temperature, precipitation, sunlight, and seasonal weather changes all influence the soil pH. High precipitation, for example, will wash essential nutrients out of the soil. Many nutrients, such as calcium carbonates, are basic so as these nutrients leave the soil, it becomes more acidic.

Water coming into contact with decaying material in the soil (like leaves, for example) can also cause the pH to drop, because decaying matter releases carbon dioxide. When carbon dioxide mixes with water, acids can form.

Drier climates or regions going through a drought will have a more alkaline soil pH. Because there isn’t as much water moving through the soil, minerals and salts become concentrated, increasing the pH.


Native plants and local ecology can determine the starting pH of your soil. Soil underneath grasses is usually less acidic, while soils formed under trees tend to be more acidic. This is due to there being more decaying matter (leaves) near trees. The very crop you are growing can even alter your soil’s pH.

Irrigation Water

The water that you use to irrigate your crops will also influence the pH of your soil. If the water used is more acidic or more basic than the soil it is irrigating, the pH of the soil will shift.

Soil Type

Did the soil in your region form from granite, limestone, or shale? These parent materials will determine if your soil is more acidic or more basic. Areas with a lot of shale tend to be more acidic, while areas rich in limestone are more basic.

The texture of your soil will also determine how easy or how difficult it is to adjust the pH; this is known as the soil’s buffering capacity. Sandy soils have a lower buffering capacity while soils with more clay will have a higher buffering capacity. It’s harder to change the pH of soils with higher buffering capacities.


Fertilizing soil is very important to get the best crop yields. Because pH will affect how easily available nutrients are to plants, it’s important to check soil pH before and after adding any type of fertilizer. By knowing your pH you can decide how much and what type of fertilizer you need.

Artificial nitrogen fertilizers tend to lower pH the most in soils. Organic fertilizer will acidify the soil once they come in contact with water, because of the soluble organic acids they contain.

Nutrient Availability

Plants cannot absorb nutrients if the soil pH is too low or too high. When soil pH is off, nutrients such as calcium and phosphorus will bind up with other things in the soil. When the nutrients become bound up, plants will not be able to take in what they need to grow.

Most nutrients are available when the soil is slightly acidic, but different plants thrive in different pH ranges depending on their specific nutrient requirements. If the pH is too low, aluminum toxicity can occur. When this happens, aluminum becomes unbound and the plants take it in at toxic levels.

If the pH is too high, nutrients like iron become bound. Without adequate iron uptake, plants will lose their chlorophyll and start to turn yellow, indicating the plants can no longer make food for itself. Molybdenum poisoning can also occur in soils with alkaline pH, resulting in stunted crops.

Optimal Soil pH Ranges

Plants that thrive in more acidic soil include apple trees (pH 5 – pH 6.5), potatoes (pH 4.5 – pH 6), and orchids (pH 4.5 – pH 5.5). Alkaline loving plants include acacia and walnut trees (they both like soil between pH 6 – pH 8).

To figure out the best pH for your needs, do a little bit of research on the type of plants that you want to grow. Natural soil is typically between pH 4 and pH 8. If your soil’s pH doesn’t match the plants optimal range, you’ll need to treat your soil.

Types of Soil Treatments

Soil too acidic? Popular options for treatment are lime, calcium carbonate, and ground up eggshells. If the soil is too basic then gypsum, iron sulfate, sulfuric acid, or calcium chloride can be added.

Irrigating the soil frequently can help lower the pH if it is too high as well. However, be careful not to over-water the plants if the treatment of the soil is in a planted area. This can cause diseases to set in, and nutrients can get diluted or washed away.

Hanna Tip: The cost of the materials and the size of the planting space will also be a factor in how you treat your soil. For example, it’s much more workable and affordable to treat a small home garden with ground eggshells than to do so in several acres of field.

How to Test Soil pH

The two primary ways to test soil pH from field samples are slurry testing and direct soil testing. It is important that the soil samples and tests take place in the same spots and the same way every time.

Slurry pH Testing

The slurry method allows you to get a representative sample and measurement of an entire area with just one test. Because soil pH can vary within a small area, be sure to take a representative sample. The soil should be taken from the same depth below the surface each time you test.

When using the slurry method, take soil from next to the plants, as well as some from further away. (Keep these two samples separate.) While this means a little extra work, you will get measurements that are more accurate since the amount of nutrients, types of soil, and moisture content can vary across a planted area.

All these things affect the pH of soil, so it’s important to track your pH at many points.

How to Test Soil Using the Slurry Method

  1. Gather some soil from the test area.
  2. Take the homogeneous sample and add equal parts of soil and distilled or deionized (DI) water in a 1:1 ratio. So, for 25 grams of soil you would add 25 mL of water.
  3. Stir the sample for 5 seconds.
  4. Let it sit for 15 minutes.
  5. Start stirring the sample again after 15 minutes, and take your measurement.

Video: How to Prepare a Soil Slurry (1 minute 40 seconds)

Direct Soil pH Testing

Direct soil pH testing gives you the benefit of not needing to take soil samples, because the pH is tested right in the ground.

How to Test Directly in Soil

  1. Using an auger or ruler, first put a hole down into the soil. The hole needs to be the same depth each time you test to avoid pH discrepancies.
  2. Add some distilled or deionized water to the hole; the soil should be damp but not saturated with water.
  3. Insert your testing instrument into the hole, and allow for the reading to develop or stabilize.


Soil pH Testing Options

Now that we have gone over soil and how important pH is, let’s talk about what you can use to test your soil pH. We’ve narrowed it down to four main groups: test strips, chemical test kits, digital pocket testers, and portable meters.

pH Test Strips

Pros: Easy to use, inexpensive
Cons: Hard to read, loss of precision, hidden costs

pH test strips (aka litmus paper) are paper strips that have been saturated with pH-sensitive dyes. When exposed to a damp substance, the strips will change color relative to that substance’s pH. This color change corresponds to a color chart provided with the test strips. This method for testing is quick, easy, and inexpensive, but it does have some disadvantages.

Soil is very dark in color, even when mixed with water. The muddy color could stain the test strips and make them hard to read. Even when a color change can be seen, it’s subjective since colors can look different depending on the lighting, as well as from person to person. This leads to inconsistent and poor results.

Tests strips will not give the most accurate test results either; they only have a resolution of 0.5 pH units. This means the closest your test results can get to your soil’s true pH would be 0.5 pH +/-. Being 0.5 pH units off means a greater cost to treat the soil. If the soil treatment is not accurate you can have low crop yield and dead plants.

Soil pH Chemical Test Kits

Pros: Easy to use, all-inclusive
Cons: Multiple kits needed, hard to read & dispose of, limited number of tests

pH chemical test kits are like test strips in that they are easy to use, but also have several drawbacks.Using a soil test kit involves adding your soil, distilled or DI water, and some chemicals (these will be included when you purchase a kit) to a tube. The chemicals, like test strips, react with the pH levels in your sample to create a color change. Also like test strips, the color change of the test kits will be subjective, and readings will vary between different people.pH test kits have lower resolution, generally between 1 or 0.5 pH points, and tend to test specific ranges of pH. That means you need to buy many kits to test your different types of soil, or when you are just beginning and don’t know your starting pH.

The number of tests you perform with a chemical test kit limited to the number of reagents included. Regular kits include enough chemicals for anywhere between 1 and 10 tests. Disposal of these chemicals poses another issue; chemical compositions differ from manufacturer to manufacturer, and many cannot just be poured down the drain or into the trash.

Digital Soil pH Pocket Testers

Pros: Pocket size, better accuracy, easy to keep clean
Cons: Need to know how to care for the device

Soil pH pocket testers are digital, portable testing instruments that utilize a pH electrode. The integration of a pH electrode in the durable casing of a tester allows for much greater accuracy than test kits or strips. The pH electrode takes a pH reading in your soil or soil slurry and displays it on an LCD screen.Testers have fewer things to interfere with taking a reading compared to test kits and strips. You no longer have to worry about the dark soil sample interfering with color change, or the subjectivity of color change tests in general. Many testers also have a much higher resolution and accuracy than chemical options, generally between 0.1 and 0.01 pH units.

Hanna Tip: Testing more than pH? Some testers can test other things as well, such as conductivity.

Soil pH testers allow for easy testing out in the field, with simple, single-handed, one- or two-button functions. Some testers have special features like waterproof capabilities or durable bodies which let you test in humid environments without affecting the readings.

Certain pocket testers also feature a cloth junction, which helps prevent clogging of the electrode. To unclog soil particles from the junction, the cloth can be gently tugged with tweezers to remove debris and uncover fresh cloth in the junction. This gives you more accurate readings and a longer electrode life.

As the temperature changes, the way the electrode behaves will also change; this can affect your pH readings. To help counter this, some pH soil testers have automatic temperature compensation, a feature that allows the device to correct the error.

These handy instruments do take a little bit more know-how to operate and properly care for. You’ll need solutions to keep the electrode calibrated, hydrated, and cleaned. More information on care for a pH electrode can be found under Care and Maintenance below.

Hanna Tip: On some soil pH testers, it can be hard to tell if the pH readings have stabilized. Look for a tester that features a stability indicator for an easier testing experience.

Portable Soil pH Meters

Pros: Portable laboratory accuracy, customizable, no more guesswork
Cons: Larger investment, more technical

Portable soil pH meters are the next step up from soil pH testers. They are a convenient way to have laboratory accuracy in field testing. A bit larger than the testers, portable soil pH meters offer many functions from multiparameter testing to data logging.

All choice portable soil pH meters have automatic temperature compensation; they will come with either an integrated temperature sensor, or a separate temperature probe. Portable soil pH meter measurements are nearly exact, with resolutions as low as 0.001 pH units. Both of these functions give you readings with much greater accuracy.If you need to report your pH values, a portable soil pH meter is a great choice. Some of these meters are able to provide Good Laboratory Practices (GLP) data, which includes things such as date, time, calibration data, and logged data. This gives traceability to your readings.

Hanna Tip: Many electrode choices make portable soil pH meters a versatile testing option. Some are even waterproof!

Certain soil pH meters come with an electrode diagnostics feature called CAL Check™. The meter will check the condition of the electrode and pH buffers during calibration. Some of these meters also come with a HELP button which will bring up tutorials right on the screen.

Looking for something smaller than a typical portable soil pH meter, but need the accuracy of one? Some of the portable soil pH meters do not even need a regular meter! There are electrodes on the market that can to a smartphone or tablet.

Hanna Tip: Like a tester, the electrodes will need some upkeep to give them a long working lifespan. You will need calibration, cleaning, and storage solutions. More information on care and maintenance is next!

Care & Maintenance of Your Soil pH Electrode

Proper care and maintenance of your pH electrode is essential. Appropriate care of electrodes will extend its useful life. Our maintenance motto will help you to remember the three main concepts in electrode maintenance: Clean Regularly, Calibrate Often, and Condition Always.

Clean Regularly

When testing the pH of soil, it’s important to properly clean the pH electrode since soil can clog the junction. If soil gets stuck on the electrode, do not wipe it! Instead, rinse the electrode with distilled water.

Is the soil really sticking to the electrode? Soak it in a cleaning solution specially formulated for soil or humus deposits . Both solutions help to remove residue left behind after rinsing the electrodes with distilled water. The cleaning solution for soil deposits is great for general agricultural samples; the humus solution is best highly organic soils (such as compost).Cleaning the electrode will give you maximum efficiency and accuracy when taking pH readings. After using a cleaning solution, the electrode should be placed in storage solution for at least one hour before using it again. The cleaning solutions are available in disposable one-use-only packets as well as bottles.

Electrode Cleaning Steps

    1. Fill a squeeze bottle or spray bottle with deionized (DI) or distilled water.
    2. Using the bottle, rinse down the electrode with the water.
    3. Gently shake down the electrode to remove residual water.
    4. The electrode is ready for use or storage!
    5. Further Cleaning:
    1. Rinse the electrode with DI or distilled water.
    2. Let the electrode soak in cleaning solution for at least 15 minutes. Use a cleaning solution formulated for soil deposits or humus deposits.
    3. Remove the electrode from the cleaning solution.
    4. Rinse the electrode with DI or distilled water.
    5. Place the electrode in storage solution for at least 1 hour before using it again.

Calibrate Often

Calibrating your electrode will give you the greatest accuracy when testing pH. Calibration will help to correct your electrode as its response changes over time, due to aging and other factors.

Electrode response changes due to several factors. It’s important to calibrate to at least two pH points which bracket your expected pH value. Bracketing simply means calibrating to one pH point below the expected range, and one pH point above the expected range. (For example, if your expected reading is pH 8.6, then pH 7 and pH 10 buffers should be used.)

Hanna Tip: Daily calibration is recommended, but if you can tolerate a little bit of error in your measurement, it is not completely necessary – though still highly recommended!

Electrode Calibration Steps

  1. Use a spray or squeeze bottle and rinse down the electrode with DI or distilled water.
  2. If using a disposable one-use-only packet of pH buffer, tear or cut open the packet.
    1. Enter calibration mode on your soil pH meter.
    2. Insert the electrode until the bulb and junction on the side of the electrode are covered by the buffer.
    3. Let the reading stabilize and accept the buffer.
    4. Remove the electrode from the buffer and rinse it again.
    5. Repeat these steps for the other pH buffers.
    6. When finished, exit calibration mode.
  3. If using a bottle of pH buffer, pour the buffer into a clean beaker and place a magnetic stir bar in the beaker.
    1. Place the beaker on a stir plate to keep the buffer stirring while taking a measurement.
    2. Enter calibration mode on your soil pH meter.
    3. Insert the electrode until the bulb and junction on the side of the electrode are covered by the buffer.
    4. Let the reading stabilize and accept the buffer.
    5. Remove the electrode from the buffer, and rinse it again.
    6. Repeat these steps for the other pH buffers.
    7. When finished, exit calibration mode.

Hanna Tip: Fresh pH buffers are recommended every time you calibrate an electrode. This will give you the best possible calibration.

Condition Always

The most important part of the pH electrode is the sensing bulb at the bottom. The bulb is made of glass that is sensitive (responsive) to hydrogen ions. It’s important to maintain equilibrium in the electrode to keep your readings stable by keeping the bulb hydrated.

Proper hydrating means always storing your electrode in storage solution . Storage in other liquids, like distilled or DI water, can damage the glass bulb and cause slow, inaccurate pH readings.

Hanna Tip: Damage from improper storage may be repaired in a refillable electrode by filling it with fresh electrolyte, then rehydrating it in storage solution. This should bring the electrode back to equilibrium, though it’s not a guarantee.

Electrode Conditioning Steps

  1. Rinse the electrode with distilled or deionized water.
  2. You can use either a disposable one-use-only packet of conditioning solution, or a beaker with some of the solution poured into it.
  3. Place the electrode in the conditioning solution. (It’s important to make sure the junction on the side of the electrode is covered by the conditioning solution. Both the pH glass bulb and the junction need to be hydrated for the probe to work!)
  4. Let the pH electrode sit in conditioning solution from an hour to overnight to completely rehydrate.

Looking for more pH testing tips? For more information on pH electrode care, check out our pH Best Practices Checklist, and our blog on the Top 10 Mistakes in pH Measurement.

Soil may be complex but…

…testing your soil’s pH doesn’t have to be! The options for testing pH are as diverse as the different types of soils. For help in choosing the best option for your testing needs please contact us using one of the channels below.

Written by Allison Hubbard

Allison graduated from Bryant University with a Master’s Degree in Global Environmental Studies. She is passionate about nature, and how science is connected to the world around us. At Hanna, she provides an array of content and support to customers through the Hanna Blog, SOPs, and Data Sets.

Allison may be reached at [email protected]

How To Change Your Soil’s pH

Have you ever tried to grow blueberries or azaleas only to have them turn yellow, then brown and eventually die? If you have, chances are you planted them in an alkaline soil.

“Acid loving” plants, like blueberries and azaleas, succeed only in acidic soils like those typically found in parts of Minnesota. In contrast, many plants that are native to Iowa are adapted to alkaline soils. However, on highly alkaline soils even some Iowa native plants grow poorly. These include pin oak, river birch, and white pine.

The standard measurement of alkalinity and acidity is known as pH. The pH scale ranges from 0 to 14. A pH of 7 is neutral, which is neither acid nor alkaline. Below 7 is acid and above 7 is alkaline. A pH of 5.5 is 10 times more acidic than a pH of 6.5. Conversely, a pH of 8.5 is 10 times more alkaline than a pH of 7.5. A soil test will determine pH.

The soil pH is important because it affects the availability of nutrients in the soil. Many plant nutrients are not readily available to plants in highly alkaline or acidic soils. These essential nutrients are most available to most plants at a pH between 6 to 7.5.

Consequently, most horticultural plants grow best in soils with a pH between 6 (slightly acid) and 7.5 (slightly alkaline). Most Iowa soils are in this range. If your soil is not, then you will need to make a choice. Either choose plants adapted to your soil’s pH or alter your soil’s pH to fit the plants.

But before attempting to raise or lower your soil’s pH, you should first conduct a soil test to determine your current soil pH. Contact your local county Extension office for advice on collecting and sending a soil sample to a laboratory for analysis.

Some soils in Iowa (especially those in western Iowa) are slightly alkaline to very alkaline, with pH’s that range from 7.2 to 9.5. This is due mainly to the limestone parent material from which the soils were formed. In addition, home builders may remove topsoil during construction and replace it with more alkaline subsoil. Alkaline building materials, such as limestone gravel and concrete, and high pH irrigation water may also contribute to a soil’s alkalinity.

If your soil is alkaline, you can lower your soil’s pH or make it more acidic by using several products. These include sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, and organic mulches.

An excellent way to lower the pH of small beds or garden areas is the addition of sphagnum peat. (The pH of Canadian sphagnum peat generally ranges from 3.0 to 4.5.) Sphagnum peat is also a good source of organic matter. On small garden plots, add a one to two inch layer of sphagnum peat and work it into the top 8 to 12 inches of soil before planting. The addition of sphagnum peat to large areas would be cost prohibitive.

Granular sulfur is the safest, least expensive but slowest acting product to use when attempting to lower your soil’s pH. The table below shows the pounds of elemental sulfur needed per 10 square feet to lower the pH of a loam or silt-loam soil to the desired pH indicated in the table. Reduce the rate by one-third for sandy soils and increase by one-half for clays.

To avoid plant injury, don’t exceed 2 pounds of sulfur per 100 square feet per application. Wait at least 3 months to make another application.

Aluminum sulfate and iron sulfate react more quickly with the soil than elemental sulfur. However, aluminum sulfate and iron sulfate must be applied at a 5 to 6 times greater rate. Do not apply more than 5 pounds per 100 square feet of aluminum or iron sulfate at any one time. Excessive amounts of these two sulfates can also injure plants.

Some types of fertilizers can help to acidify the soil and most of them are safe to apply. Acidifying fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea, and ammonium nitrate. Read the label on the fertilizer bag to determine if it is an acidifying fertilizer.

Research suggests that wood chips as a surface mulch may actually allow greater nutrient absorption by some trees. Spread a layer about three inches thick at least out to the dripline. Each spring add more mulch to keep the depth at about three inches.

If the pH of your soil is greater than 7.5, then the soil may contain a large amount of free calcium carbonate. This compound strongly resists changes in soil pH. Lowering the pH becomes difficult or impractical on soils that have a pH above 7.5.

The pH of highly acidic soils can be raised by incorporating limestone into the soil. Hydrated lime works quicker, but over liming is more likely. The table below shows pounds of ground limestone needed per 100 square feet to raise the pH to 6.5 in the top 6 inches of soil.

Soil pH Sandy loam Loam Clay loam
5.0 8 10 15
5.5 6 8 10
6.0 3 4 6

Wood ash will also raise the soil pH and make the soil more alkaline. Do not apply wood ash, limestone, hydrated lime, or other liming materials to alkaline soils.

Modifying a soil’s pH is usually a slow process and may require repeat treatments. It is often most effective to use a combination of treatments. However, don’t expect a quick fix or a miracle cure.

This article originally appeared in the April 6, 1994 issue, pp. 1994 issue, pp. 42-43.

Soil pH

Soil pH is a measure of the acidity or alkalinity (basicity) of a soil, and is reported as a value between 0 and 14. A soil test for pH measures the concentration of hydrogen ions in the soil solution.

A pH of 7.0 is considered neutral. A pH value below 7.0 indicates that the soil is acidic, with lower values representing increasing acidity. A pH value above 7.0 indicates that the soil is alkaline (basic), with higher values representing increasing alkalinity.

The pH scale is logarithmic, so a change in 1 pH unit reflects a 10 fold change in acidity or alkalinity. On average, garden and landscape plants grow best in soils with pH values between 6.0 and 7.2.

Causes of Soil Acidity

Soil acidity can be caused by a number of factors:

  • Soils in areas with large amounts of rainfall tend to be acidic because the water leaches basic cations (calcium, magnesium, sodium, and potassium) out of the soil profile, and these cations are then replaced by acidic cations (hydrogen and aluminum).
  • Carbonic acid formed from carbon dioxide and water acidifies soils in high-precipitation areas.
  • Acidic soils tend to be high in iron and aluminum oxides, as they are the slowest minerals to weather in soil. Aluminum in these increasingly acidic soils is solubilized and will combine with water to release additional hydrogen ions (acidity).
  • The soil parent material (or mineral types from which the soil developed) can be a source of acidity in soils.
  • Nitrification of ammonium fertilizers yields hydrogen ions.
  • Acid rain contains nitric and sulfuric acid.
  • Added elemental sulfur oxidizes to form sulfuric acid.
  • Plants take up, and thus remove, basic cations from the soil.
  • Plant roots excrete hydrogen ions in exchange for nutrients in the soil.

Detrimental Effects of Soil Acidity

  • Soil acidity can lead to elemental toxicities for plants by aluminum, iron, manganese, and zinc due to the increased solubility of these elements at low pH values.
  • Soil acidity can cause limited availability of some macronutrients and micronutrients such as phosphorus which binds to iron and aluminum oxides in acidic soils.
  • Other elements in their plant-available forms, such as molybdate, exibit decreased solubilities at low pH values.
  • Microbial activity drops off in acidic conditions which can lower nitrogen (the key plant nutrient) concentrations, reducing nitrogen fixation and nitrogen mineralization, two processes vital to creating plant-available forms of nitrogen.
  • Organic matter decomposition by soil organisms slows.
  • Calcium, magnesium, and potassium deficiencies develop.

Influence of soil pH on the relative availability of plant nutrients in a typical mineral soil. Image source: Chart of the Effects of Soil pH on Nutrient Availability

Treatments for Soil Acidity

Soil acidity can be ameliorated and the pH of the soil increased by the addition of lime/limestone (calcium carbonate) and similar compounds that have been ground fine for use. Types of lime-like amendments include:

  • Dolomitic limestone
  • Quicklime
  • Hydrated lime
  • Marl
  • Chalk
  • Oystershell
  • Wood ashes
  • Fluid lime

Each lime-like amendment has its benefits and drawbacks, such as effectiveness, price, and purity. Lime is most effective at neutralizing acidity when it is incorporated/tilled into the soil to the full depth of the plow layer or root zone.

Surface application of lime to a field. Plants should be irrigated after lime application to ensure the foliage is free of the amendment. Photo credit: Dwight Sipler Flickr CC BY 2.0

Benefits of Liming:

Lime treats acidity by combining with carbon dioxide gas, water, and hydrogen ions to form free calcium ions and carbonic acid (weak acid). The carbonic acid then dissociates to form carbon dioxide gas and water, ridding the soil of hydrogen ions. Liming is also effective at accomplishing the following:

  • The calcium addition by the lime displaces aluminum and hydrogen off the soil particle surfaces and replaces calcium in the soil (dolomitic lime furnishes magnesium as well).
  • As the pH of the soil increases, excess metals, such as aluminum, iron, manganese, and zinc, precipitate out of the soil solution and are no longer plant-available.
  • Phosphorus solubilizes and become plant-available.
  • Molybdenum solubility increases.
  • Microbial activity resumes.

Other lime-like amendments neutralize acidity, but may follow different reaction paths. For instance, calcium oxide combines with water and hydrogen ions to immediately form free calcium ions and water.

Applying Lime to Acidic Soil

The amount of lime needed to treat the soil acidity depends on the following:

  • Crop tolerance to acidity/alkalinity and the lime requirement of the selected plants
  • pH of the untreated soil and the desired pH of the treated soil
  • The amount of soluble and exchangeable acidity
  • Cost of the lime amendment will affect choice of product
  • Less product is needed when applying a lime-like amendment that has a large calcium carbonate equivalent
  • Fineness of grind of the lime amendment
  • Amount of organic matter in the soil
  • Type of clay present in the soil

The SMP, Adams-Evans, and Mehlich buffer methods are used to determine the lime requirement of soils. These different methods were developed for distinctly different soil types. Other methods, such as titratable acidity and reactive aluminum, may also be used to determine soil lime requirements. A soil sample can be submitted to an analytical laboratory to determine the lime requirement, which will be given in the results report.

Time of Application

Lime should be applied a few months before planting, approximately one time per year. Lime applied to turf should be irrigated after application to wash any lime off the leaves. Lime should not be applied if a soil test indicates that liming is not necessary; similarly, caution must be taken to avoid over-liming. A soil with too high of a pH poses a whole new set of problems.

Rhododendrons are very tolerant of acid soils and grow best in soil with a pH between 4.5 and 5.5. Photo credit: Uwe Fischer Flickr CC BY-NC-SA 2.0

Additional Resources:


Iowa – How to Change Your Soil’s pH


California – Changing pH in Soil

Effects of Organic and Inorganic Materials on Soil Acidity and Phosphorus Availability in a Soil Incubation Study


We tested the effects of two organic materials (OMs) of varying chemical characteristics that is, farmyard manure (FYM) and Tithonia diversifolia (tithonia), when applied alone or in combination with three inorganic P sources, that is, triple superphosphate (TSP), Minjingu phosphate rock (MPR), and Busumbu phosphate rock (BPR) on soil pH, exchangeable acidity, exchangeable Al, and P availability in an incubation study. FYM and tithonia increased the soil pH and reduced the exchangeable acidity and Al in the short term, but the inorganic P sources did not significantly affect these parameters. The effectiveness of the inorganic P sources in increasing P availability followed the order, TSP > MPR > BPR, while among the OMs, FYM was more effective than tithonia. There was no evidence of synergism in terms of increased available P when organic and inorganic P sources were combined. The combination of OMs with inorganic P fertilizers may, however, have other benefits associated with integrated soil fertility management.

1. Introduction

Soil acidity and phosphorus deficiencies limit crop production in many tropical soils . Lime and inorganic phosphate fertilizers are used in developed countries to remedy these problems. However, due to increasing costs and unavailability when needed, their use among smallholder farmers in developing countries is not widespread. This coupled with concerns for environmental protection and sustainability has renewed interest in the use of alternative cheaper locally available materials. The use of phosphate rocks (PR) and organic materials has in particular received increased attention in recent years in eastern Africa . In addition to provision of P, PRs have Ca and Mg which makes them assume a significant role as a potential tool for sustaining soil productivity by reducing soil acidity through its liming effect . Although most OMs are low in P, they can influence soil parameters such as soil pH, exchangeable Al, and Ca, which greatly influence crop growth .

There are a number of PR deposits of variable reactivity in eastern Africa which, however, differ greatly in their suitability as sources of P in P-deficient soils . The most promising of these PRs are Minjingu in northern Tanzania and Busumbu in eastern Uganda , but their low solubility makes them unsuitable for direct application . Techniques aiming to increase the solubility of BPR through blending with soluble phosphate fertilizers such as TSP or partial acidulation are likely not to have positive effects in terms of increasing P availability and uptake by plants . Enhancing the solubility of PRs by combining them with OMs has been tried in western Kenya, but there is no consensus as to whether or not these combinations enhance P availability . Interactions of OMs with inorganic P nutrient inputs and their effect on P availability and soil acidity therefore merit further study. The objective of this study was to investigate the effect of inorganic phosphorus sources (TSP, MPR, and BPR) when applied alone or in combination with OMs (tithonia or FYM) on soil pH, exchangeable acidity, exchangeable Al, and P availability acid P-deficient soils.

1.1. Materials and Methods

The study was conducted from April to July 2008 at Moi University, using soils collected at two sites in western Kenya which were selected on the basis of contrasting characteristics (Table 1).

Table 1 Initial surface (0–15 cm) soil properties.

Surface soil (0–15 cm) samples were randomly taken from each site and thoroughly mixed by hand to produce one homogenous sample per site. Two hundred gram samples of air-dried soil (<2 mm) from each site were weighed into plastic polythene bags which were kept in upright positions in a laboratory. Finely ground (<1 mm) tithonia, FYM (obtained from cattle), BPR, MPR, or TSP were added to the soils according to the treatments given in Table 2 and thoroughly mixed. The treatments were arranged in a completely randomized design with three replications. The procedure used by was used with slight modifications. This involved incubation of the samples for 16 weeks at room temperature. Moisture content in the soil samples was adjusted to field capacity and maintained at that level during the entire period of incubation. Soils were sampled twice from each treatment, that is, at 4 and 16 weeks after the start of the incubation (WAI), air-dried, and sieved before being analyzed.

Table 2 The experimental treatments.

1.2. Analyses of Soils and the Organic Materials

The soils and the OMs were analyzed using the following methods; organic C was determined by Walkley and Black sulphuric acid-dichromate digestion followed by back titration with ferrous ammonium sulphate . Total N and P in the soils were determined by digesting 0.3 g of the soil sample in a mixture of Se, LiSO4, H2O2, and concentrated H2SO4 . The N and P contents in the digests were determined colorimetrically. Total soluble polyphenols in tithonia and FYM were determined by the Folin-Ciocalteau method , while the lignin content was determined using the acid detergent fiber (ADF) method as described by . Soil pH was determined using a glass electrode pH meter at 1 : 2.5 soil : water ratio . The basic cations (Ca, Mg, and K) were extracted using ammonium acetate at pH 7 . Exchangeable Ca and Mg in the extract were determined using atomic absorption spectrophotometry, and exchangeable K by flame photometry. Exchangeable acidity and exchangeable Al were extracted using unbuffered 1 M KCl .

2. Results

2.1. Characteristics of the Organic Materials Used in the Study

Tithonia contained higher amounts of C, N, Ca, Mg, and K than FYM, but its total P content and pH were lower (Table 3). The C : N ratios of tithonia and FYM were 13.5 and 20, respectively, and a net mineralization of N would therefore be expected to occur from both OMs . The C : P ratios were 140 for tithonia and 90 for FYM. Tithonia had low (<15%) while FYM had high (>15%) lignin content. Both OMs had low polyphenol content (<4%). According to the criteria proposed by , tithonia would be a high-quality OM, while FYM would be a medium-quality OM.

Table 3 Average chemical composition of tithonia and farmyard manure used in the study over the three seasons.

2.2. Effect of Organic and Inorganic Amendments on Soil pH

Results for soil pH as affected by the treatments for the Bukura and Kakamega soils are presented in Tables 4 and 5, respectively. Averaged across all treatments, the soil pH at Bukura at 4 WAI (4.91) and 16 WAI (4.27) was lower than at Kakamega at similar times (5.31 and 4.65, resp.). The pH of the soils at 4 WAI was lowest for the control treatment and highest for tithonia applied in combination with MPR for both soil types. All the tithonia treatments (applied alone or in combination with the inorganic inputs), apart from Tithonia (20 kg P ha−1), showed a significant increase in pH above the control treatment at 4 WAI for the Bukura soil. All the other treatments had no significant effect on soil pH at this time for this soil. At Kakamega, all the tithonia treatments with the exception of Tithonia (20 kg P ha−1) + TSP (40 kg P ha−1) and Tithonia (20 kg P ha−1) significantly increased the soil pH above that of the control. FYM when applied alone or in combination with the inorganic P sources generally increased soil pH of both soil types, although statistical significance was not always attained. There was no significant treatment effect on soil pH at 16 WAI for soils from both sites.

Table 4 Effect of organic and inorganic materials on soil pH, exchangeable acidity and exchangeable Al for the Bukura soils in the incubation study. Table 5 Effect of organic and inorganic materials on soil pH, exchangeable acidity, and exchangeable Al for the Kakamega soils in the incubation study.

Averaged across the three inorganic P sources, the soil pH followed the trend Tithonia > FYM > no OM at both sites. Averaged across the OMs, MPR gave a significantly higher soil pH than TSP and BPR at both sites at 4 WAI. There was a decline in soil pH in all the treatments at 16 WAI compared to 4 WAI for both soil types. Averaged across all the treatments, the pH of the Bukura and Kakamega soils declined by 0.67 and 0.64 units, respectively. In general, the acidification over time was more pronounced with the tithonia treatments at both sites.

2.3. Exchangeable Acidity and Exchangeable Aluminum

At 4 WAI, tithonia when applied alone or in combination with the inorganic P sources significantly reduced the exchangeable acidity with respect to the control for the Bukura soil (Table 4). The largest reduction (65%) at this sampling time was obtained with tithonia applied at a rate of 60 kg P ha−1. FYM also significantly reduced exchangeable acidity at 4 WAI, but only when it was applied at rate of 60 kg P ha−1 (26%) or in combination with MPR (31%). There was generally an increase in exchangeable acidity in the soils sampled at 16 WAI compared to those at 4 WAI. At this time (16 WAI), all the tithonia treatments, other than tithonia (20 kg P ha−1), gave significant reduction in the exchangeable acidity with respect to the control at Bukura. The inorganic P sources did not significantly reduce the exchangeable acidity at both sampling times at Bukura although the MPR treatments had generally lower levels of exchangeable acidity than TSP or BPR.

There were no significant treatment effects on exchangeable acidity for the Kakamega soil at 4 WAI (Table 5). However, at 16 WAI, all the treatments with tithonia applied alone or in combination with inorganic P sources, except tithonia (20 kg P ha−1) + BPR (40 kg P ha−1), significantly reduced the exchangeable acidity at this site. FYM, when applied alone at 60 kg P ha−1 or in combination with MPR, also significantly reduced exchangeable acidity but not when applied at 20 kg P ha−1 or in combination with TSP or BPR. The inorganic P sources had no significant effect on exchangeable acidity when applied alone at 16 WAI at Kakamega (Table 5).

The exchangeable Al trends among the treatments were generally similar to those of exchangeable acidity for the Bukura soil, at both sampling times (Table 4). The Kakamega soil showed wide variations especially in the samples taken at 16 WAI in which exchangeable Al could not be detected in several treatments. When averaged across the three inorganic P sources, tithonia gave significantly lower exchangeable acidity and exchangeable Al levels compared to FYM and no OM. The effect of inorganic P sources on exchangeable acidity and exchangeable Al was not significant at Bukura, but at Kakamega, MPR had significantly lower amounts of exchangeable acidity than TSP and BPR at 16 WAI. Although FYM gave lower exchangeable acidity and exchangeable Al levels than when no OM was applied at both sampling times at Bukura, these differences were not statistically significant.

There was a strong significant negative correlation between the soil pH with both the exchangeable acidity ( 𝑟 2 = 0 . 7 4 ; 𝑃 < 0 . 0 0 1 ) and exchangeable aluminum ( 𝑟 2 = 0 . 7 3 ; 𝑃 < 0 . 0 0 1 ) at 4 WAI at Bukura. At 16 WAI, there was also a significant but weak correlation between the soil pH and exchangeable acidity ( 𝑟 2 = 0 . 3 4 ; 𝑃 < 0 . 0 5 ), but the correlation between soil pH and exchangeable Al was not significant at this time for the Bukura soil. At Kakamega, there was no significant correlation between the soil pH and exchangeable acidity or exchangeable Al at both sampling times.

2.4. Effect of Phosphorus Sources on the Olsen Phosphorus in Soils

All the applied inputs generally increased the Olsen P levels compared with the control for both soil types at 4 WAI (Table 6). The highest Olsen P values for both soil types, at both sampling periods, were obtained with TSP (60 kg P ha−1). When applied alone at the same P rate of 60 kg P ha−1, there were no significant differences in Olsen P between FYM and TSP, but the two P sources had significantly higher Olsen P levels than tithonia, MPR, and BPR for the Bukura soil at 4 WAI. A similar trend was also observed for the Kakamega soil. FYM gave slightly higher but non significant Olsen P levels compared to tithonia at a similar P application rate applied at 20 kg P ha−1. In general, at the same P rate, the effectiveness in increasing the available P among the inorganic sources followed the order, TSP > MPR > BPR, while among the OMs, FYM was more effective than tithonia.

Table 6 Effect of organic and inorganic P amendments on Olsen P (mg P kg−1) at Bukura and Kakamega in the laboratory incubation study.

The combined application of the OMs, that is, tithonia or FYM, with TSP or the PRs did not result in synergy, whereby the available P increased more than the sum of the increase from either of the P sources applied singly. This is illustrated in Figures 1, 2, 3, 4, 5 and 6 for the Bukura soil. In general, the expected increase in the available P due to the additive effects of applying the inorganic and organic P sources separately was always greater than the actual increase obtained by combining the inorganic and organic P sources, at the same total P application rate (Figures 1–6). Combined application of organic and inorganic P sources generally resulted in observed increases in Olsen P intermediate to those of sole applications of the organic or inorganic P sources (Figures 1–6).

Figure 1
Increase in Olsen P above the control treatment as affected by tithonia and TSP at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with TSP (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when TSP was applied alone at 40 kg P ha−1.
Figure 2
Increase in Olsen P above the control treatment as affected by tithonia and MPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with MPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when MPR was applied alone at 40 kg P ha−1.
Figure 3
Increase in Olsen P above the control treatment as affected by tithonia and BPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with BPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when BPR was applied alone at 40 kg P ha−1.
Figure 4
Increase in Olsen P above the control treatment as affected by FYM and MPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with MPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when MPR was applied alone at 40 kg P ha−1.
Figure 5
Increase in Olsen P above the control treatment as affected by FYM and TSP at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with TSP (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when TSP was applied alone at 40 kg P ha−1.
Figure 6
Increase in Olsen P above the control treatment as affected by FYM and BPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with BPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when BPR was applied alone at 40 kg P ha−1.

3. Discussion

The application of both FYM and tithonia generally increased the soil pH at 4 WAI with tithonia-treated soils having a higher pH than the FYM-treated soils at this time. The soil pH, however, declined by 16 WAI with tithonia-treated soils showing the highest pH reductions. The increase in soil pH due to application of OMs at 4 WAI in this study is consistent with results reported by several other workers (e.g., ). The principal mechanisms involved in increasing soil pH by various types of OMs differ considerably and according to , and a broad distinction can be made between the mechanisms of undecomposed plant materials such as tithonia and humified materials such as FYM and composts. The initial increase in the soil pH by FYM in the present study can primarily be attributed to the high pH of FYM (7.7) at the time of its application. It may also partly be explained by proton (H+) exchange between the soil and the added manure . During the initial decomposition of manures, prior to their collection, some formation of phenolic, humic-like material may have occurred . It is these organic anions that consume protons from the soil, thus tending to raise the equilibrium pH . Another mechanism that has been proposed to explain the increase in soil pH by such materials as FYM is the specific adsorption of humic material and/or organic acids (the products of decomposition of OMs) onto hydrous surfaces of Al and Fe oxides by ligand exchange with corresponding release of   O H −   as suggested by . On the other hand, attributed the soil pH changes observed with fresh materials, for example, tithonia, in an incubation study, mainly to nitrogen transformations and release of metal cations as tithonia decomposed. In this incubation study, soils were amended with the OMs in a closed system without growing plants. Therefore, the effects of plant uptake, root exudates, and leaching are not relevant and the processes responsible for the pH changes are limited to the decomposition and nutrients held in tithonia and N transformations . Under anaerobic conditions,   N H 4 +   produced by the ammonification process would accumulate due to inhibition of nitrification, and the pH would increase. However, under conditions favorable for microbial activity, such as those in the present study, the initial alkalization from plant residue amendment may be neutralized by subsequent nitrification, which is an acidifying process . This is likely why there was a decline in soil pH in all the treatments by 16 WAI. The higher acidification observed for the tithonia-treated soils at 16 WAI in the incubation study is ascribed to its high nitrifiable N content (3.3%) compared to the other treatments. Similar variations in soil pH with time, when different OMs were mixed with soil, were observed by . The failure of the PRs to increase the pH is attributed to their low reactivity and low rates used.

3.1. Exchangeable Acidity and Exchangeable Aluminum

Addition of tithonia, FYM, and MPR had the effect of reducing both the exchangeable acidity and exchangeable Al, but the magnitude of the reduction varied with each of these materials. Tithonia appeared to be more effective in reducing exchangeable Al, but not exchangeable acidity, compared to FYM. The reduction in exchangeable acidity can partially be attributed to an initial increase in soil pH that was observed with the OMs. Several other workers have measured an increase in soil pH with concomitant decrease in exchangeable Al during decomposition of organic residues in soils . An increase in soil pH results in precipitation of exchangeable and soluble Al as insoluble Al hydroxides , thus reducing concentration of Al in soil solution. However, there are other mechanisms involved in the reactions of Al with OMs which are intricate and according to probably involve complex formation with low-molecular-weight organic acids, such as citric, oxalic, and malic acids, and humic material produced during the decomposition of the OMs and adsorption of Al onto the decomposing organic residues. Complexation by soluble organic matter may partially explain why the tithonia treatments were able to significantly reduce exchangeable acidity and Al relative to the control treatment, despite the fact that they had at times low pH that was comparable to that of TSP or BPR. Both TSP and BPR, however, failed to significantly reduce exchangeable Al, likely due to their low content of CaO (19% and 35% CaO for TSP and BPR, resp.).

The Al complexing effect of tithonia is likely to have been stronger than that of FYM given that FYM gave higher soil pH (5.17) than tithonia but still ended up with a higher level of exchangeable Al (0.35 cmol kg−1). Tithonia was applied as a green manure and was thus likely to produce large quantities of organic acids, which would be involved in complexation reactions . On the other hand, FYM had been exposed to the weather elements for a long time (one year) before its collection for use in this study. It was well rotten and hence likely to be at an advanced stage of decomposition and is therefore unlikely to have had substantial amounts of organic acids .

3.2. Soil Olsen P Changes as Affected by Application of Organic and Inorganic Inputs

Addition of P from both organic and inorganic sources generally resulted in increase in the Olsen P relative to the control. The magnitude of the increase in the Olsen P depended on the soil type, time of soil sampling, P source, and rate of P application. On average, addition of P inputs generally resulted in larger increases in Olsen P for the Bukura soil than the Kakamega one. Similar site-specific differences in extractable soil P, in response to applied P fertilizers, were found by . The increase in the Olsen P with time of incubation contrasts with most studies which have reported a decline in the Olsen P with time, usually ascribed to P sorption by the soil (e.g., ). However, a few studies have obtained results similar to those of the present study. These authors explained that the increase in P availability with time is likely due to microbially mediated mineralization of soil organic P, to form inorganic P at a faster rate than that of P sorption by the soils of low to moderate P sorption capacity, such as those used in the current study. Also, due to the absence of plants in such incubation studies, the mineralized P is not taken up by plants and hence the observed increase in available P with time.

TSP gave the highest amount of Olsen P compared to the PRs, tithonia, or FYM, applied at the same total P rate at all times. This is ascribed to the higher solubility of TSP compared to the PRs whose dissolution is usually low and slow . The OMs generally gave higher Olsen   𝑃   values than the PRs at comparable total P rates. This reflects the large percentage of soluble P in both the tithonia tissues and the FYM. High levels of water soluble P in plant tissues (50–80%) have also been reported by . Immediate net P mineralization would in addition be expected to occur because both OMs had a higher P concentration (0.3% in tithonia and 0.4% in FYM) than the critical level of 0.25% required for net P mineralization .

The significant increase in Olsen P above the control by MPR indicates that the soil conditions at both sites were conducive to its dissolution. Some of the factors known to increase the dissolution and subsequent release of P in PRs include low soil pH, low exchangeable Ca, and low P . The soils at both sites generally met these criteria. The higher amounts of Olsen P as a result of MPR application compared to BPR application can be attributed to differences in their solubility arising from varying extents of carbonate substitution in the PR . Results of chemical analyses indicate that the BPR is a low-carbonate-substituted type of igneous origin. It has low reactivity in acid solvents with a neutral ammonium acetate (NAC) solubility of 2.3% compared to 5.6% of MPR .

The interaction between the OMs and inorganic P sources was significant only on a few occasions. In such instances, it was observed that combining the PRs with tithonia or FYM gave higher Olsen P values than when the PRs were combined with urea. However, when the TSP was combined with tithonia or FYM, it gave lower amounts of Olsen P than when it was combined with urea. This may suggest that tithonia and FYM were enhancing the dissolution of PRs, but retarding the availability of P from TSP. However, closer examination of the data reveals that tithonia and FYM were unlikely to have enhanced the dissolution of the PRs and that combining these two OMs with the PRs has no advantage in terms of increasing the Olsen P compared to their application with urea. There was therefore no synergistic effect in terms of increased Olsen P, when PRs were applied in combination with organic materials. In general, the combined application of organic and inorganic P sources generally resulted in observed increases in Olsen P intermediate to those of sole applications of the organic or inorganic P sources.

The likely reason why the PRs when combined with tithonia and FYM gave higher Olsen P levels compared to their combination with urea is because both tithonia and FYM were generally more effective in increasing the Olsen P compared to the PRs, and therefore, a portion of the insoluble PRs (20 kg P ha−1) was substituted for by the more available tithonia or FYM in the combinations. However, when combined with urea all the 60 kg P ha−1 was from the low soluble PRs and thus the lower Olsen P levels. On the other hand, TSP when combined with urea, gave higher Olsen P levels compared to its combination with tithonia or FYM. In this case, TSP was more effective in increasing the Olsen P compared to tithonia and FYM whose P is mostly in organic forms initially, and hence, substituting a portion of it (20 kg P ha−1) in the combination with tithonia or FYM yielded less Olsen P than when it (TSP) was applied at the full rate of 60 kg P ha−1 with urea.

The findings of the present study are in contrast to others (e.g., ) who reported synergism when OMs such as manures were combined with PRs. These authors combined PRs with OMs of diverse composition and concluded that due to acidifying effect organic acids produced during the decomposition of the OMs, the solubilization of PRs was enhanced thus leading to the higher extractable P values in treatments where PR was combined with OMs than from application of PR alone. The most probable reason, however, why the combined application of PR and OM gave higher extractable P values compared to sole application of PR in these studies was because the contribution of P by the OM in the OM/PR combination was not considered, thus leading to a higher total P rate in the OM/PR combination than the sole PR application, and hence the higher amounts of available P in the combination. The results reported herein are, however, in agreement with other recent works where total P among the treatments to be compared was the same . The common conclusion in these studies was that combination of PR with OMs does not enhance the dissolution of the PR mainly because OMs can increase the soil pH and Ca levels which are negatively correlated with PR dissolution. If the cost was not a limiting factor, then replenishing soil P using TSP would be a more appropriate strategy, as it results in more available P than when it is applied in combination with tithonia or FYM (at the same total P rate). Likewise, if availability and cost were not a constraint, then it would be better to apply tithonia or FYM alone at 60 kg P ha−1 than combining them with MPR or BPR because the combination results in a lesser amount of available soil P than if the OMs are applied alone.

4. Conclusion

Tithonia and farmyard manure were more effective in increasing the soil pH and reducing exchangeable acidity and Al than the inorganic P sources (MPR, BPR, and TSP) in the early stages of incubation suggesting that these OMs can substitute for lime. Addition of P from both organic and inorganic sources generally resulted in an increase in the Olsen P, relative to the control, whose magnitude depended on the soil type, time of soil sampling, P source, and rate of P application. The effectiveness of the inorganic P sources in increasing P availability followed the order, TSP > MPR > BPR, while among the OMs, FYM was more effective than tithonia. There was no synergistic effect, in terms of increased Olsen P, when inorganic P sources were applied in combination with OMs. In general, the combined application of organic and inorganic P sources resulted in observed increases in Olsen P intermediate to those of sole applications of the organic or inorganic P sources. The combination of OMs with inorganic P fertilizers may, however, have other benefits associated with integrated soil fertility management.


The authors thank Moi University for financial assistance and for providing laboratory facilities, Mary Emong’ole for conducting laboratory analyses, and Laban Mulunda of Bukura Agricultural College for assistance with collection and preparation of the soil samples.

Azaleas prefer acidic soil with a pH of 5 to 6.5.

Soil pH is a very important factor in plant health – if the soil is too acidic or too alkaline, plants will be unable to absorb nutrients properly, and your garden won’t grow. The degree of acidity and alkalinity is measured on a scale of 0-14, with a pH of 7 neutral, 0-7 acidic, and 7-14 alkaline.

The ideal soil pH for vegetables and lawn grasses is 6.5, just a little on the acidic side. A soil test is needed to determine the pH of your soil.

It’s important to identify the plants in your yard before attempting to adjust the pH level of your soil, since some flowers and shrubs thrive in a slightly higher or lower pH soil. If the results of a soil test indicate that the pH of your soil needs adjusting, here’s how to go about adjusting it.

Acidifying fertilizers can help lower soil pH over time to make soil more acidic.

Tips for Soil pH Correction

  • Read Label: No matter which product you choose, it’s important to follow the instructions on the package to the letter, even if you have to buy a special spreader or applicator to get it right. For example, one brand of sulfur may be more finely ground than another, and over application could damage your plants. While your soil test results will provide general guidelines about how much amendment is needed, follow the label on the particular product.
  • Proceed Slowly: Make one application of the product, wait at least three months, retest the pH of the soil, and reapply again if needed. It might take a year or more to get your soil on track, and overdosing can cause more harm than good.
  • Fall Application: For best results, incorporate pH-correcting amendments in the fall, to give them plenty of time to break down for spring planting. Many gardeners make soil testing and pH-fixing an annual fall ritual.
  • Plant Selection: You’ll have much better luck if you choose plants well suited to the soil you already have. It’s OK to tweak the pH a little to optimize the growing conditions, but your overall soil makeup pretty much is what it is.

Lime is the most popular additive for acidic soils to raise the pH.

How to Raise the pH in Acidic Soil

  • Lime: Limestone is the most common soil additive for raising pH of your soil to make it less acidic. You’ll generally see two types: calcitic limestone (which is mostly calcium carbonate), and dolomitic limestone (which also adds magnesium to the soil). Both work equally well at raising soil pH. Liming products come in granular, hydrated, pelletized, or pulverized forms. Pulverized lime is a fine powder that is faster-acting, but it tends to clog spreaders. The granular or pelletized types of limestone spread more easily and take longer to break down. Hydrated lime is the fastest-acting but is very easy to overdose. All lime products will work much better if they can be worked down into the soil, rather than left on top. This is why applying lime to lawns is often paired with core aeration and fall watering.
  • Wood Ash: For an organic way to make your soil less acidic, sprinkle about 1/2″ of wood ash over your soil and mix it into the soil about a foot deep. This method takes small applications over several years, but it can be very effective, as well as a great way to recycle fireplace ashes!

Sulfur is commonly applied to alkaline soils to make them more acidic.

How to Lower the pH in Alkaline Soil

  • Sulfur: Plain elemental sulfur (or sulphur) is probably the easiest and most common way to make soil more acidic, since it’s cheap, relatively safe, and can be spread on top of the soil. Since sulfur is pretty slow-acting, you shouldn’t apply more than 2 pounds per 100 square feet at a time.
  • Sphagnum Peat: This is a great organic solution, since sphagnum peat also adds organic matter to your soil and increases water retention. Simply work a 2” layer of sphagnum peat into your soil at least a foot deep. Larger areas will probably require a tiller.
  • Aluminum Sulfate and Iron Sulfate: These two products are very fast-acting, but they can also be the most damaging by adding salts and elements that can build up in the soil. Be sure not to apply more than about 5 pounds per 100 square feet.
  • Acidifying Fertilizer: Fertilizers that contain ammonia (such as ammonium nitrate), urea, or amino acids can, over time, have an acidifying effect on the soil in your yard.
  • Mulches and Compost: As organic matter breaks down, it tends to make soil more acidic. Regular use of organic compost and mulches will, over time, bring the soil pH closer to the desired neutral to slightly acidic level. The easiest way to lower your soil pH is just to keep heaping on the rotten stuff. Mother nature sure is smart!

Further Information

  • Changing the pH of your Soil (Clemson University)
  • Soil pH and Fertilizers (Mississippi State University Extension)
  • Changing pH in Soil (University of California Extension, PDF/151 Kb)
  • Soil Acidity and Liming (Clemson University)

How To Lower pH In Soil Fast: what to add and why

admin April 16, 2019 6985 Views 0 SaveSavedRemoved 4

In chemistry, pH is a scale used to specify how acidic or alkaline a substance is. Acidic solutions have a lower pH, while alkaline – or basic – solutions have a higher pH. If the pH level is exactly 7, such as that of pure water, it’s neutral.

Different types of plants need different levels of soil pH to survive and thrive. And although soil pH varies from region to region and one garden to the next, most plants need a pH between 6.5 and 7. But there are other plants, such as flowers like azalea, marigolds or heather and fruits and vegetables such as blueberries, potatoes, peppers – which need and thrive in slightly acidic conditions.

Once you’ve decided what to plant, or decided which plant may benefit from lower pH it’s best to verify the soil’s current pH level before making any adjustments. This will give you the base line to start making amendments from.

How to lower pH in soil

In order to correct alkaline soil, you will typically need to introduce a source of acid. You can add compost, manure or organic soil amendments like alfalfa meal to increase the nitrogen level of the soil which will also gradually decrease the pH.

Organic gardeners commonly use elemental sulfur to decrease the pH level of their soil; however, sulfur requires some time (6 months+) for the soil bacteria to convert it to sulfuric acid.

The speed of the conversion is dependent on the particle size of the sulfur, the temperature and the degree of moisture of the soil and the amount of bacteria present.

Sulfur will only work during the warmest months of summer when bacterial activity peaks. It can therefore take up to several months for this method to decrease the soil pH value.

Elemental sulfur is the best choice for lowering the pH of very dense soil, such as soils with a heavy clay component. Also by adding organic materials such as manure to heavy clay soils will help it become more workable, and as a bonus the manure will gradually lower the soil ph naturally.

As it takes so long to act, element sulfur is best introduced at the end of the last planting season.

How to lower pH in soil fast

What is the fastest way to lower pH in soil? – While there are methods that will make the soil more acidic very quickly, their results may vary, and in some cases, they may over-correct the soil pH and do more harm than good. The following methods will lower the soil pH quicker than elemental sulphur but as they are fast acting, you should add them to the soil in measured doses.

Using Coffee to lower soil pH

While it’s a well-known myth that coffee grounds are a quick fix for lowering soil pH, in fact most of the organic acids in coffee are water-soluble, and flush out into the brew. Used coffee grounds have a pH around 6.8, which is so close to neutral that they won’t lower pH much; however, they do add a little nitrogen, so they can help reduce pH over time, just like manure or compost.

Freshly ground or brewed coffee has an average pH of about 4.5, depending on the region in which it was grown. So if you need to drop soil pH more quickly, try watering your plants with leftover (cold) coffee that is diluted 50-50 with water. This method works especially well for smaller volumes of soil such as for houseplants or container vegetables.

How to lower pH in soil with vinegar

Vinegar is a kitchen staple, because it has a wide variety of uses; it can be used as a condiment, to add flavor to cooked dishes, and even to clean sinks and counters when the cooking is done. This potent liquid is also useful to gardeners, and it can be used to naturally adjust the pH level of soil without the need for harsh, commercially manufactured products.

Vinegar is a diluted, liquid form of acetic acid, and depending on what the vinegar is made from and how it’s processed, it may also contain other things, like traces of vitamins and minerals. The average pH of commercially manufactured white vinegar, like that sold in supermarkets, is 2.4, making it highly acidic. Organic gardeners can find an organically-made vinegar.

Vinegar can be sprayed onto the soil or introduced through an irrigation system. A cup of vinegar mixed into a gallon of water is ideal for plants like azaleas and rhododendrons.

Aluminum sulfate to lower soil pH

One of the quicker-acting acidic soil additives is aluminum sulfate; it produces acidity in the soil as soon as it dissolves, which is basically instantly as long as moisture is present. If you need to urgently lower the pH level of your soil, aluminum sulfate is a great choice.

Keep in mind that using too much additive can be harmful for your plants, so it’s best to verify the usage details based on the starting pH of your soil. Aluminum sulfate shouldn’t be used for large applications because it can lead to aluminum accumulation or even aluminum toxicity in the soil.

Mix around 5lbs of aluminium sulphate around the base of the plant you want the soil ph lowered to reduce the pH by around 1 unit. Always check the dosage on your label before application.

Sulfur-coated urea to lower soil pH

A common ingredient in many slow-release commercial fertilizers, sulfur-coated urea is a fairly quick-acting soil additive. It can lower the pH level of the soil considerably over time, yet will produce some effect within a week or two of being introduced.

If you were already planning to fertilize the soil as well as decreasing its pH, simply choose a fertilizer that contains urea; the sulfur-coated urea content does vary from one brand of fertilizer to another, so remember to consult the mixing instructions to determine the proper amount to use.

Iron sulfate to lower soil pH

A good choice for heavily compacted soil with a high clay content, iron sulfate and aluminium sulphate rely on a chemical reaction to create acidity in the planting beds, making it less dependent on temperature conditions than elemental sulfur which relies on a slower biological reaction to begin any changes in soil ph.

Both Iron sulfate and Aluminium sulphate act faster than elemental sulfur and can significantly reduce pH in as little as three or four weeks; therefore, it can be used during the same season you decide to plant acid-loving plants.

It may take more than 10 pounds of iron sulfate per 100 square feet of soil to reduce the pH level by one; if you do need to add more than that, it’s best to split the quantity into two applications that are spaced a month or two apart. This will give the soil enough time to absorb the iron sulfate between applications.

Iron sulfate can leave rusty stains on clothes, so it’s best to wash any clothes that have come into contact with them separately to avoid damaging other items; they can also stain cement surfaces such as patios or sidewalks. Have a look at what other users had to say about Iron Sulphate on


In summary i would say that the most important thing to do first is to measure the soil pH. After that you can begin to amend the pH by whichever way you choose. The coffee and vinegar methods would be ok in small potted areas but not really realistic for large areas.

For larger areas i would use elemental sulphur if i wasn’t in a rush- but usually you do want the benefit within that growing season. In which case I would choose Iron Sulphate as my first choice (helping to reduce aluminium toxicity in the soil) and if i couldn’t get that then my second choice would be Aluminium Sulphate. Thanks for reading and good luck- Richard.

Fixing Your Soil When Soil Is Too Acidic

Many gardens start out as great ideas only to find that things don’t grow quite as planned. This could very well be because the soil is too acidic to support the life of some plants. What causes acid soil? There are many things that can cause the soil to be too acidic.

Effect of Acid Soils on Plant Growth

Sometimes there could be too much aluminum in the soil, making it acidic. Sometimes there is too much manganese, which is toxic to plants. If the soil is too acidic, it can be because of a calcium and magnesium deficiency, which is just as bad for plants as it is for humans. Iron and aluminum in great amounts can tie up phosphorus, which also makes the soil too acidic for plants.

Another thing to consider if your soil is too acidic is poor bacterial growth. This is because with bacteria, the soil becomes more alkaline,

and if there isn’t enough of the good bacteria, your soil will not be fertile enough to support life.

So what causes acid soil? Many things can do it, from natural soil pH to the types of mulch you use. Acidic soil can have mineral deficiencies just like the human body, and unless these deficiencies are fixed, the plants won’t live. So if your soil is too acidic, you’ll need to correct it.

How to Lower Acid Amount in Soil

The most common way to raise the pH of soil is to add pulverized limestone to the soil. Limestone acts as a soil acid neutralizer and consists of either calcium and magnesium carbonate or calcium carbonate. These are called dolomitic limestone and calcitic limestone respectively.

The first thing that needs to be done is a soil test to see how acidic the soil actually is. You want your soil pH to be around 7.0, or neutral. Once you have run the soil test and have the results, you will know which kind of pulverized limestone to add as a soil acid neutralizer.

Once you know the kind of soil acid neutralizer to add to your soil, apply the lime according to the instructions given to you by the garden center. Never apply more than necessary.

Making sure you know what causes acid soil is important, but be careful not to add too much limestone in your efforts to correct it. If you end up with alkaline soil, you could have other problems like iron, manganese and zinc deficiencies, which also won’t support life. Further, you could end up with an overgrowth of bacteria in the soil, which can kill those things that spend a long time underground, like potatoes.

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