Super phosphate vs bone meal

Organic Fertilization

In order for crops to grow and develop, soil nutrients (chemical elements) need to be absorbed by roots and moved into the plant. These nutrients, coming from soil parent material or from added fertilizers, function in structural and metabolic systems to enable the plant to carry on processes necessary to living. To obtain the full yield potential and produce a high?quality product, the soil must contain enough nutrients to support vigorous plant growth throughout the life cycle. Since the weathering of soil parent material is a slow, long?term process, assuring proper nutrition for crop plants generally requires the addition of fertilizers to the soil.

When a grower decides to fertilize, he is faced with the choice of which fertilizer to use and the task of determining how much fertilizer to apply. No universal guidelines are available for answering these questions as soils differ in fertility, and crops differ in their nutritional requirements. The grower must apply what seems to be the correct amount of fertilizers based on soil tests, plant analyses, previous experience and advice from others. Modifications can be made during the season or in the following year based on how the crop is growing, and on the amount of time remaining in the current season.

Personal beliefs often enter into the selection of a fertilizer with many individuals choosing to use organic fertilizers rather than manufactured products. Both types of fertilizer can be used to supply soil nutrients to the plant, but with few exceptions, natural or organic fertilizers contain nutrients in low concentrations and relatively insoluble forms as compared with synthetic fertilizers. The concentration and solubility of the nutrients govern how much fertilizer to apply because the nutrients must be dissolved in soil water before they can be absorbed by plants.

Nutrients in organic fertilizers become soluble through a process termed mineralization or weathering where the complex molecular structure is broken into smaller water?soluble mineral ions that can be utilized by the plant. Some organic substances have a rapid rate of mineralization and release all of their nutrients during the first growing season with little to no residual value for crops in subsequent years. Other organic substances have a slow rate of mineralization and release portions of their nutrients over several years and may be considered soil?building materials. Materials slow to mineralize generally have limited value for growth of plants in the year they are first applied. Some natural rock materials that are very slow to weather may have little value in supplying nutrients to a crop or in enhancing the fertility of a soil.

In the fertilization of crops, growers must first be concerned with supplying the primary macronutrients—nitrogen, phosphorus and potassium. Since organic or natural fertilizers have variable chemical composition, a balanced or adequate supply of all of the primary macronutrients from one organic fertilizer is unlikely. Therefore, more than one kind of organic fertilizer is usually needed to provide sufficient crop nutrition in any system of organic gardening. This situation differs from that of synthetic fertilizers that are manufactured to contain from one to all three of the macronutrients and can be purchased in practically any formulation.

Nitrogen, one of the most likely nutrients to be deficient in most soils, is seldom lacking in organic fertilization programs because of the relative availability and slow release of this source of nutrient in organic materials. Some nitrogen accumulates in the soil from rainfall and from nitrogen fixation (the conversion of gaseous nitrogen in the atmosphere to organic nitrogen) by free?living microorganisms. This accumulation rarely exceeds 10 pounds per acre per year, however, and cannot support much crop production. A grower who relies on precipitation for sources of nitrogen is unlikely to have much production success.

Manure, one of the oldest known fertilizers, can serve as a source of organic nitrogen. The actual nitrogen content of manures varies with the type of animal and feed given to the animal. Poultry manure is considerably higher in nitrogen than the manures from larger farm animals, and the better any livestock is fed the richer the nutrient content of the manure. Bedding, such as straw, wood chips, or sawdust, that is added to manure greatly reduces the nitrogen value. The nutrients are diluted and, if the manure is incorporated into the soil, nitrogen is immobilized and unavailable as microorganisms “tie?up” this nutrient in the process of decomposing the bedding. Although nitrogen content is reduced by one-half, manures with a high proportion of bedding should be composted before they are mixed in the soil to prevent immobilizing soil nitrogen.

Fresh manures with low bedding content should be turned into the soil as soon as possible after spreading to prevent volatilization and the loss of nitrogen from the manure to the atmosphere in the form of ammonia. Manures that are left on top of the ground for 2-4 days have only half of the nitrogen value of those that are plowed or tilled in immediately after spreading. Manures incorporated into the soil release approximately 50 percent of their nitrogen content for plant growth the first year. The rest of the nitrogen is retained in the soil and builds the soil fertility for subsequent cropping years. The nitrogen in organic fertilizers such as dried blood, alfalfa meal and seed meals is released almost entirely in the first season they are applied to the soil, leaving little residual nitrogen available for the following season unless the application was in excess of the requirements for the crop.

The organic grower has only a limited number of phosphorus fertilizers from which to choose. Plant residues, farm manures, and composts are, practically, too low in phosphorus analysis to be considered for any purpose other than maintaining soil fertility following a build?up of this element in the soil through the use of more concentrated materials. Residues from the bodies of animals are excellent sources of phosphorus with bone meal being the most significant among available animal residues. Unfortunately, bone meal, the oldest phosphorus fertilizer, is expensive, so its use is generally limited to garden?sized plots.

Rock phosphate, mined from deposits, has a high phosphorus analysis, but the material is of such low solubility that special application techniques are required to achieve any benefits in the soil. Colloidal rock phosphate, taken from a lower grade ore than regular rock phosphate, is claimed to release phosphorus more readily than regular rock phosphate. Although colloidal rock phosphate costs the same as regular rock phosphate, the phosphorus content is only about 2/3 that of rock phosphate. The low pH of the soil helps release the phosphorus from the phosphate source. Super-phosphates are manufactured by treating rock phosphate with sulfuric or phosphoric acid, simulating the action of acid soil on the rock. Unless restricted by marketing or philosophy, organic growers may want to use the super-phosphates during an initial build?up of phosphorus reserves in the soil. Once a soil test indicates an adequate level of phosphorus, organic materials can be used to maintain phosphorus fertility.

Most plant residues and farm manures can serve as sources of fertilizers in building and maintaining the available potassium reserves of a soil. Except for rinds and peelings, the vegetative portions of plants are higher in potassium than are the fruits and seeds. Hay, straw, hulls (or shells) and any other plant residues contain from 2-9 percent potash with the exact amount depending on the plant product and on the fertility of the soil in which the plants were grown.

Dried manures are about 2 percent available potash, and fresh manures with bedding have about 10 pounds of available potash per ton. Actually, soils already contain thousands of pounds of essentially unavailable potassium in the form of primary minerals.

Fertilization programs to provide other plant nutrients such as calcium, magnesium, sulfur, and the minor elements (iron, zinc, copper, manganese, boron, molybdenum, and chlorine) are usually not necessary in an organic system. Other minor elements come as impurities in limestone.

The application of farm manures, composts, or plant residues to meet the nitrogen requirements of a crop will satisfy the calcium, magnesium, sulfur and most of the minor element requirements of the crop. Other than the reserves in the soil, this type of organic matter is the most important source of micronutrients and sulfur. Organic matter produced from soils deficient in minor elements, however, will be deficient in those nutrients. In these situations, organic matter should be brought in from outside sources to enrich and maintain the soil or minor elements must be added.


Chapter 11. Phosphorus


The primary function of phosphorus is the transfer of energy from plant leaves to its storage in sugars and starches. Its observable effect is to enhance root development, seed size and flower development.

Once in the soil, it is so active that it is almost completely immobile. A plant needs a good root structure to find it; on the other hand phosphorus is not subject to leaching losses,

Both the pH and biological activity affect the availability of phosphorus to plants.

The value of fertilizer depends on how it is spread and the availability of water.

Table 20. Comparison Of Phosphorus Fertilizers is a comparison of fertilizers.

Phosphorus In The Plant

Phosphorus is the Power Broker. It controls and distributes the energy trapped by photosynthesis preparatory to storing that energy in sugars and starches.

It is also an essential element in every metabolic process. It is a constituent of DNA and RNA and necessary in protein synthesis. Root nodules associated with the fixation of nitrogen require an ample supply of phosphorus.

But the role phosphorus plays in energy transfer is its most important activity and the one which is most affected by a deficiency.

Seeds contain a large amount of phosphorus. A phosphorus deficiency reduces the number and size of seeds. Larger seeds can germinate from deeper into the soil, and the sprouting plants have more resistance to drought.

Phosphorus is a stimulus to root development. Roots branch out and root hairs form profusely in the vicinity of a source of phosphorus. Owing to its effect on roots, phosphorus is a major factor in determining the early growth of a plant and its vigor throughout the season.

Nitrogen and phosphorus have complementary tendencies. Nitrogen enables the plant to trap energy from sunlight, and phosphorus facilitates the actual use of the energy. Nitrogen is a necessary component of proteins, but phosphorus manages the synthesis of proteins.

In field crops, nitrogen encourages grasses, while phosphorus encourages legumes.

However, nitrogen in the nitrate form (slightly acid to alkaline soils) competes with phosphorus for takeup by the plant roots. But it is much more mobile, and phosphorus can be overwhelmed by an excess of nitrogen even if it is adequate otherwise.

A deficiency of phosphorus also, like nitrogen, produces stunted growth. On some plants the underside of leaves may be purplish, owing to the accumulation of underutilized sugars. A phosphorus deficiency delays the growth of new shoots and the development of flowers.

Phosphorus In The Soil

Limitations On Phosphorus Mobility

Owing to its high reactivity with almost anything which it contacts, phosphorus has a lower mobility than any other nutrient. It can be bound up by soil organisms, by mineral elements (particularly aluminum, calcium and iron), and by clay minerals containing aluminum or iron. Consequently phosphorus does not remain in a free state for long, and any amount taken up by plants usually comes from an area within a fraction of an inch around the roots.

One of the few agricultural benefits of a temporarily anaerobic condition is that it causes iron phosphate to change from ferric phosphate to ferrous phosphate, which is more soluble.

Otherwise phosphorus is only slowly available to plants. Furthermore, in cool weather, particularly in the spring, biological activity is low, and phosphorus availability may be low even if a soil test indicates an adequate amount.

An advantage of this immobility is that it limits leaching losses to such low levels as to be measurable only over periods of 50-100 years1. Water pollution from phosphates is caused not by leaching of phosphorus through the soil but by runoff of phosphorus-containing fertilizers from the surface.

Factors Affecting Phosphorus Mobility

Soil pH

The pH affects the limitation on phosphorus availability in several ways:

  • a low pH reduces biological activity and diversity, which limits the effectiveness of soil organisms in promoting the release of phosphorus
  • in weathered soils – particularly in humid areas of the east, south and northwest – a low pH increases available aluminum, which ties up phosphorus.
  • a high pH limits mobility by precipitation with calcium

The net effect is to create a window – in most cases in the pH range from 6.5 to 6.8 – where these tendencies to immobilize phosphorus drop off.

On most acid soils the pH can be adjusted with lime, but on alkaline soils pH control is not easy. Where it is high because of arid conditions, constant irrigation to leach the salts may help, although excess irrigation washes away some of the important nutrient salts in the process. Gypsum may help by dissolving insoluble sodium carbonates. Where the soil is on top of a limestone bed, or in dry conditions, peat moss and finely ground mined sulfur are common natural materials for increasing acidity; but they may be expensive. Aluminum sulfate and sulfuric acid are synthetic alternatives.

Fortunately, biological activity reduces the damage from alkaline soils. The production of organic acids as a metabolic byproduct creates a separate environment with a reduced pH around plant roots, where activity is strongest.

Phosphorus And Water

The major mechanism for plant roots to absorb phosphorus – as well as other anion nutrients (nitrogen, sulfur, boron, molybdenum, and silicon) – is by solution in soil water. Although the solubility of phosphorus in water is low, it is adequate for plant growth if water flow is steady throughout the growing season.

The reverse is also true. Phosphorus is important in good root development, and good root development is necessary to enable the plant to find water. Consequently, an adequate supply of phosphorus is essential at the beginning of the season.

Moreover, the placement of phosphorus fertilizers affects its availability. Phosphorus fertilizer topdressed or banded results in high growth within a small volume of the soil. In dry weather, the lack of well-spaced roots limits the plant’s ability to take up water. Dry weather will also cause root development downward in search of moisture, away from the fertilized zone. More care than usual is necessary in order to assure a satisfactory supply of water.

Consequently, if an irrigation system is in place to assure a sufficient supply of water throughout the season, topdressing or banding is probably the most efficient way to utilize fertilizer. Otherwise a better procedure is to broadcast the fertilizer and thoroughly till it under.

Saturation of the Phosphorus Reservoir

One way to overcome the tendency of the soil to absorb phosphorus is to load it down with fertilizer to such an extent that all the mechanisms which can tie up phosphorus are overpowered. This is one rationale for banding phosphorus fertilizer.

The strategy often works, but it can be dangerous. Phosphorus may be present at such excessive levels as to have a harmful effect on crop growth. The major hazard of a phosphorus overload is the reduction of trace element availability, particularly of iron, manganese and zinc. Phosphorus must be unusually high to be so detrimental, but occasionally it is.

The role of organic matter And biological activity

Organic matter and the activity of soil organisms have a strong influence on the availability of phosphorus. Any which is released by decaying residues is readily available.

Phosphorus picked up by fungi is distributed throughout the innumerable extensions of their microscopic threads (mycelia). Upon death of the fungi, the released phosphorus is apportioned more evenly throughout the soil. A consequence is that phosphorus broadcast onto a pasture is soon well distributed. This reduces the need for irrigation, stated earlier, in soils topdressed with phosphorus fertilizer.

Organic matter can break up the aluminum-phosphate bond in an acid soil, because aluminum has a stronger affinity for organic matter than it does for phosphorus.

Soil organisms cause the production of organic acids as waste products of their metabolism. These acids are effective in dissolving inorganic phosphorus. The particularly high activity surrounding plant roots produces a high concentration of acids, which is especially favorable to phosphorus availability.

Some fungi invade the roots of plants for the purpose of extracting carbohydrates. The value of these fungi – mycorrhizae – is that they accumulate minerals, including phosphorus, which they pass on to the roots. Fungi assisting the plant in obtaining phosphorus is analagous to nodule-forming bacteria which provide nitrogen to legumes.

As is true with nitrogen fixation, however, this exchange and cooperation between plant and microorganism is an agent of last resort. If the plant can obtain phosphorus (or other minerals) by an easier route with less expenditure of carbohydrates, it will do so in order to divert its energy elsewhere. Mycorrhizae are useful only when available phosphorus is low. They are probably responsible for the success of trees in soils poor in phosphorus and may be most useful to perennials.

Organic matter and biological activity are often the predominant sources of phosphorus, especially in alkaline soils. Plowed sods produce a good crop the first year because of the phosphorus released by the decaying residues.

Root activity

By a straightforward but technical chemical process, the roots of plants facilitate the breakdown of insoluble calcium phosphates, releasing the phosphorus. This process may occur with any plant having a high calcium requirement. It has been demonstrated with squash and undoubtedly is a factor in the ability of many calcium-loving legumes to make direct use of rock phosphate.

Phosphorus Fertilizers


Table 20. Comparison Of Phosphorus Fertilizers lists only those organic materials which offer a generous supply of phosphorus. Others, such as cow manure, hay and seed meals are good for maintaining phosphorus, and possibly they might supply enough to growing plants even though soil phosphorus is low. But their value is questionable for building up soil phosphorus where the phosphorus/nitrogen balance is low.

Of the inorganic materials listed, two natural products are hard rock and colloidal rock phospage – also called soft rock phosphate. The alternative name for hard rock phosphate from Florida is pebble phosphate. The usual use for these is in the production of commercial phosphorus fertilizers, four of which are listed in table 20. Comparison Of Phosphorus Fertilizers .

Colloidal rock phosphate is the variety commonly available to organic agriculture. When rock phosphate from Florida is mined, a very finely-divided low-grade ore is removed by washing it away to a settling basin. After the water has evaporated, any sediment which has a phosphate content of 20% or more is sold as an animal feed supplement; the rest is marketed as colloidal phosphate for fertilizer use.

Of the four synthetic fertilizers listed in table 20. Comparison Of Phosphorus Fertilizers , superphosphate is the oldest. It was first manufactured in England in the middle of the nineteenth century by dissolving bone meal in sulfuric acid. The new product became so popular among farmers that bones soon became scarce. Englishmen scoured Europe looking for them and earned a reputation as “the Ghouls of Europe”. Eventually, the industry was rescued when rock phosphate was discovered in North Africa, and production of superphosphate increased steadily up to recent years.

Today, however, superphosphate is considered inefficient because of its relatively low phosphorus content and has been superseded by triple phosphate and by mono-ammonium phosphate and di-ammonium phosphate. Owing to its sulfuric acid parentage, superphosphate is a combination of calcium phosphate and calcium sulfate, or gypsum, while triple phosphate contains no sulfate. The two ammonium phosphate fertilizers are a mixture of ammonia and phosphoric acid and are now the most popular phosphorus fertilizers in the world.


The ideal fertilizer appears to be poultry manure; it is cheap and loaded with nitrogen and phosphorus. However, cage layer manure, the strongest, is difficult to deal with nonprofessionally and should be used carefully for several reasons:

  • it is messy, hard to clean up, and leaves an odor for days or weeks
  • it can inhibit germination and injure seedlings because of its high ammonia and salt content
  • it can pollute groundwater faster than other manures
  • it is comparatively low in potassium but usually has a high lime content; heavy or constant applications can drive the pH to an excessive level.

Applications of cage layer manure on most soils should not exceed 5 tons/acre, or 250 lbs/1000 sq ft. It is not a pleasant material, but no soil which regularly receives it is acid or low in phosphorus.

Bone meal is the oldest phosphorus fertilizer. Owing to its high cost, it is popular today principally among caretakers of small gardens. Its nutrient content is usually specified by available NPK content, typically 1-11-0, rather than the total content referred to in table 20. Comparison Of Phosphorus Fertilizers .

At one time, farmers manufactured their own bone meal by roasting the bones of slaughtered livestock or by soaking bones in urine or water and allowing them to ferment. Bones have also been composted by mixing them with wood ashes or quicklime and covering them with soil for several weeks.

In terms of the cost per pound of phosphorus, hard and colloidal rock phosphates is less expensive than bone meal; but the availability of the phosphorus is much lower. Rock phosphates are the skeletal remnants of marine animals, which have a similar composition to the bones of land animals, namely a combination of calcium phosphate and lime. Over long periods of time, however, while the deposits were still under water, the carbonates in the lime were slowly replaced by fluorides, resulting in a much more stable material. Colloidal phosphate has about 2% immediately available phosphate compared to 11% for bone meal; hard rock phosphate may have 3% immediately available phosphate.

Experimental results comparing hard and colloidal rock phosphate do not seem to exist, but in any event available phosphorus is low in both products. The choice of one or the other on the basis of a miniscule availability misses the point of using rock phosphate. Rock powders are applied either because they are cheap or because of the decision to use fertilizers whose nutrients are released by the biological activity of the soil.

Colloidal rock phosphate particles are so fine that they are hazardous to lungs and should be handled with the use of a respirator.

Rock phosphate does have a high availability in acid soils. It has been used with great success in the black soils of Illinois.

Despite its high cost, bone meal is often preferred to rock phosphate, particularly on small gardens, for three reasons:

  1. it is easier to obtain
  2. its higher availability is significant
  3. it is easier to spread.

Where immediate effect on plant takeup is essential, the limited availability of phosphorus in rock phosphate is the major impediment to its widespread use. Where the soil pH is above 6, usually optimum for other reasons, phosphorus availability in either rock phosphate or colloidal phosphate is low without the help of biological activity. Bone meal offers a higher initial availability and is more suitable in a near-neutral soil, but that portion which is not initially available is slow to dissolve. Rock and colloidal phosphate, and bone meal to a lesser extent, are useful mainly for their long-term benefits.

Rock phosphate, as well as other rock powders, are reputed to become more available when spread with animal manure. Although the evidence for this belief is weak, I have sometimes recommended the combination as a desperate measure. Some studies conclude that the mixture is effective, and others that it is not. The effectiveness may depend on the state of the manure. In theory, the organic acids of manure are said to dissolve the rock phosphate. But fresh manure tends to have a high pH, which may cancel the effectiveness of the acids. Rotted manure, however, is somewhat acid and may be more efficient.

An alternative to increasing the near-term usefulness of rock powders is to spread them before turning under a planting of green manures. The decay of the vegetation stimulates a high biological activity and the production of organic acids; this will hasten the availability of the rock powders.

Despite its limited availability, rock phosphate can be efficiently utilized by some plants. There is little universal agreement on what those plants are, but on everybody’s list are buckwheat, sweetclover and mustard; other recommendations are Indian corn and rape. Most legumes are better than average at picking up rock phosphate, and most grasses and small grains are worse than average.

For a philosophical comparison of rock phosphate and the synthetic, acidulated fertilizers, see chapter 1. Introduction . Where organic certification standards forbidding the use of synthetic phosphorus fertilizers are in force for philosophical reasons or marketing purposes, rock or colloidal phosphate has to be the preference. Otherwise, the synthetics are worth considering, especially if they are less costly and their use is restricted to an initial period of building up phosphorus reserves. Among the synthetics, triple phosphate or superphosphate should be the choices2.

Before any inorganic phosphorus fertilizer is used, one should determine that phosphorus is indeed deficient. Organic residues contain more phosphorus than most people realize and are often sufficient for maintaining the soil supply. Most of the phosphorus in residues is inorganic, but both the organic and inorganic forms have a high availability. One exception is starting a crop in a cold Spring; this may warrant supplemental phosphorus even if the soil reserve is adequate.

Spreading Rates

The phosphorus test differs from tests for other major nutrients in that the result does not state how much phosphorus the soil contains, but only whether or not adding fertilizer is warranted. Consequently a low test result does not by itself indicate how much fertilizer is likely to be necessary.

The amount that is necessary depends upon the fixing power of the soil, that is, the power of the soil to lock up fertilizer phosphorus. The fixing power depends upon the nature of the soil and upon the soil pH. Several states have developed tests to measure this fixing power, or they have successfully correlated the phosphorus test with the fixing power. The University of Vermont, for example, bases a fertilizer recommendation on phosphorus and aluminum tests, on the assumption that aluminum is responsible for locking up phosphorus. This method works very well for acid soils in Vermont and possibly in other states in New England but not for soils with a low aluminum content.

In a state where recommendations are based upon the fixing power of the soil, and where the soil and pH are characteristic for that state, then the fertilizer recommendation may be very good. Otherwise one has to make a choice based on other, usually average considerations.

One rule of thumb, when planning to use a soluble phosphorus fertilizer, is to determine the amount of phosphorus needed for growing the crop and then increase this by about 50%. Tables 3. Estimated Fertilizer Requirements – Field Crops and 5. Average Nutrient Requirements For Vegetables , for example, can be used to make an estimate of the amount required.

The rock phosphate choices are usually used for long-term benefits rather than to meet an immediate demand. A rate of approximately 1 ton/A is customary. This application is based upon the notion that such a quantity will supply crops for 4 years, which is about as long as one can plan in advance. But the amount is arbitrary, and more or less could be spread with the same results, unless, of course, that 2% immediate availability is essential.

Bone meal is intermediate between rock phosphate and the soluble synthetics, and so intermediate rates are appropriate.

Table 20. Comparison Of Phosphorus Fertilizers also shows how much of each of the fertilizers is needed to supply a given amount of phosphorus.

1 Under extreme conditions, leaching of phosphorus can be significant, for example in very coarse soils with little organic matter or clay, or in peat soils with little aluminum or clay. Some loss also occurs from the leaching of soluble organic substances containing phosphorus.

2 see chapter 10. Nitrogen – Other Considerations for an argument against the use of ammonium phosphate or any fertilizer containing ammonia or an ammonium salt

Garden Myths – Learn the truth about gardening

Rock dust is a very popular soil additive especially with organic and permaculture groups. It is full of nutrients and adding it to soil will replenish all of the nutrients that agriculture has taken out of our soil. This process of adding nutrients back to soil is known as mineralization.

This seems to make a lot of sense. We remove food from the land, and the food contains lots of minerals. At some point we need to put them back into the soil or else we will have soil that won’t grow anything. This seems logical but is it really true? Is our soil losing fertility? If it is deficient, can rock dust be used to solve the problem? How effective is rock dust and which type of rock works the best? Time to crush some myths about rock dust.

Azomite – a common brand of rock dust

What is Rock Dust?

The simple definition is that rock dust, also known as rock powder and rock flour, is pulverized rock. It can be man-made or occur naturally. Cutting granite for commercial use produces granite dust. Glaciers naturally produce glacial rock dust. Rock dust is also found near ancient volcanoes and consists of basalt rock.

To be effective the rock needs to be ground into a very fine powder. That way it is more easily used by microorganisms and decomposed by environmental elements.

Two common forms of rock, namely limestone and phosphate rock have been used for a long time to amend soil. Although these products are correctly called rock dust, they are usually not included when gardeners talk about rock dust, and I will exclude them from this post.

Is Rock Dust a Fertilizer?

Some commercial products call themselves a fertilizer and I even found one that was labeled like a fertilizer showing an NPK of 0-0-1, but by most legal definitions rock dust does not contain enough NPK to qualify as a fertilizer.

Claims Made for Rock Dust

Rock dust is claimed to add all kinds of minerals back to soil. These are the nutrients that plants need to grow. Because of this, rock dust products make all kinds of claims for growing bigger plants, producing higher yields, increasing disease resistance, etc. These are all valid claims if the soil is deficient of one or more nutrients and rock dust adds the missing nutrient.

There are two clear questions we must answer to validate these claims and I’ll do that in the rest of this post.

Does rock dust add plant available nutrients to soil?

Is soil deficient of nutrients?

If the answer to either question is no, rock dust will not help plants grow.

Before answering these questions, let’s look at some other claims made for rock dust.

Helps restore the correct mineral balance in soil

To be true, this would mean that soil has some kind of “correct balance” to begin with and that this balance is important for plant growth.

It turns out that there are many different kinds of soil, and they vary widely in their mineral composition. There are plants that are adapted to and grow on just about any soil. There is no such thing as a “correct mineral balance”.

When the correct balance is achieved organic matter is turned into humus

I have news for these companies, microbes turn organic matter into humus in all kinds of situations. In leaf mold it is done without any soil. This is just nonsense from a marketing person reaching for straws.

Plants can complete their life cycle without the full range of minerals but will not produce at their full potential

If plants don’t have the nutrients they need, they will not complete their life cycle – instead they die.

Analysis reports show Lanthanum (La), Cerium (Ce) and Praseodymium (Pr) at 644 ppm

These are rare earth elements, which makes it sound as if you would want them in your soil – who does not want rare stuff? I have heard of the first two, but not praseodymium – I must have been away the day we did experiments with it!

The claims go on to say, “These elements act as cofactors for the methanol dehydrogenase of the bacterium Methylacidiphilum fumariolicum.” So what is this important bacterium?

Methylacidiphilum fumariolicum is an autotrophic bacteria, first described in 2007 growing on volcanic pools near Naples, Italy. It grows in mud at temperatures between 50 °C – 60°C (about 130 °F) and an acidic pH of 2–5.

I guess if you are gardening in hot acidic mud, you might need these rare earth elements to keep your autotrophic bacteria alive. For the rest of us, we don’t need these elements in our soil!

Basalt, an igneous rock wasn’t processed or transformed by the environment, so the plant nutrients in it, are just as they were when they came out of the center of the Earth

This marketing person seems to be unaware of the fact that the minerals in rocks can’t be used by plants until the environment, or life forms convert them into usable nutrients. “Transformed by the environment” is a good thing.

The other desirable quality of the best rock dust powders is that they are paramagnetic

That may be true, but there seems to be no published research to show that paramagnetic rock has any affect on plant growth. However, many pseudoscience groups do make such claims.

Mineral Content of Rock Dust

Rock dust does contain a lot of minerals. I have seen claims ranging from 60 up to 90 different minerals. Azomite is a common product and their analysis list of 74 minerals can be seen here.

I don’t dispute the claims, but there is no evidence that plants need all of these minerals. They use about 20 minerals – that’s it. The other 40 to 70 are not needed by plants.

How Much Should You Use?

I find that this question can tell you a lot about a product. If rock dust is good for gardens, how much should you use? What happens if you use too much?

One site had this recommendation;

3 tons/acre = 14 lb/100 sq. ft. = 1.25 lb/sq. yd.


7.5 tons/ha = 750 kg/1000 sq.m = 75 kg/100 sq.m = 750 grams/1 sq.m

But a rate even 8x higher can be used, although it would have to be incorporated into the soil.

You can add anywhere from 3 tons/acre to 24 tons/acre. If 3 was the right number, would 24 not be way too much? Would 24 not burn plants due to the high nutrient load? Only if the product actually added nutrients to soil.

Rate of Decomposition of Rock Dust

rock dust mine

Earlier in this post, I posed the question, does rock dust add nutrients to soil. There is no doubt that adding rock dust adds the minerals, but I can also do that by laying a big bolder on top of the garden. The bolder will not help plants grow but it does add minerals to the garden. Unless the minerals in the rock decompose to release the nutrients in a form plants can use, there is little point in adding the rock dust.

For this reason I think that one of the most important questions we need to ask is, how quickly does rock dust decompose?

Some of my early reading on the matter indicated time frames of a hundred years. I have searched on many web sites selling rock dust and none have any claims or data to show decomposition happens even after 100 years or more. No one in the industry wants to put a number on this important property.

My recent visit to the Guelph Organic Conference allowed me to discuss rock dust with two suppliers. Neither one has been able to supply any details about decomposition. One never claimed to have such data, and the other only has it available in French only – but they did not provide it.

Google Scholar produced no research paper that looked at the rate of decomposition of rock dust.

The best information I have is a casual comment that it is about 100 years. At that rate the product is essentially useless.

If you find some numbers on this please post them in the comments, or even better post them on our Facebook Group, called Garden Fundamentals.

Are Soils Nutrient Deficient?

This is also an important question to ask. Do we have a problem that needs to be fixed?

I had a closer look at this question in a previous post called Is Soil Fertility Decreasing? My conclusion was that our soils are not losing fertility. They are not nutrient deficient. Therefore, rock dust, assuming it actually works, is a product that tries to solve a problem that doesn’t exist.

What Does Research Say?

Some papers report some improvements in plant growth with some soils but many show no change. There is limited field work done – it is almost all lab work. I did not find a single paper that measured the chemical characteristics of soil before and after adding rock dust to the field – maybe you can find one for me.

There is some evidence that rock dust may provide an important source of potassium in regions like Africa that tend to have soils which leach nutrients quickly and where fertilizer costs are very high.

The science does not support the use of rock dust for most agricultural areas and even the suppliers of rock dust suggest it has no value in alkaline soil.

What about some citizen science results? This trial is interesting.

If this video does not play, try this link:

Summary for the Gardener

Most garden soil is not deficient of nutrients, so there is no point in adding more. If you do have a deficiency as shown by a soil test, add the nutrient that is needed.

For home gardeners, rock dust is a waste of money and natural resources.

If you like this post, please share …….

What Is Rock Phosphate: The Use Of Rock Phosphate Fertilizer In Gardens

Rock phosphate for gardens has long since been used as a fertilizer for healthy plant growth, but exactly what is rock phosphate and what does it do for plants? Read on to learn more.

What is Rock Phosphate?

Rock phosphate, or phosphorite, is mined from clay deposits that contain phosphorus and is used to make organic phosphate fertilizers that many gardeners utilize. In the past, rock phosphate was used alone as a fertilizer, but due to a lack in supply, as well as low concentration, most applied fertilizer is processed.

There are a number of types of rock phosphate fertilizer available on the market, some are liquid, and some are dry. Many gardeners swear by using rock-based fertilizers such as rock phosphate, bone meal and Azomite. These nutrient-rich fertilizers work with the soil rather than against it as chemical fertilizers do. The nutrients are then made available to plants at a steady and even rate throughout the growing season.

What Does Rock Phosphate Do for Plants?

These fertilizers are commonly called “rock dust” and provide just the right amount of nutrients to make plants strong and healthy. The use of rock phosphate for gardens is a common practice for both flowers as well as vegetables. Flowers love an application of rock phosphate early in the season and will reward you with big, vibrant blooms.

Roses really like rock dust and develop a stronger root system and more buds when it is used. You can also use rock phosphate to encourage healthy tree and lawn root system development.

If you use rock phosphate in your vegetable garden, you’ll have fewer pests, greater yields and richer flavor.

How to Apply Rock Phosphate Fertilizer

Rock dusts are best applied in early spring. Aim for 10 pounds per 100 square feet, but be sure to read about application rates on the package label as they may vary.

Adding rock dust to compost will add available nutrients for plants. Use this compost heavily in your vegetable garden and the nutrients will make up for what is removed when you harvest.

rock phosphate

How to achieve high brix tomatoes.

Customer, BillSF9c writes:

“Any suggestions to increase the brix level to at least 8 for tomatoes next season?”



Would 10-13 brix for large tomatoes and 15-20 brix for the smaller grape tomatoes be okay? Of course it would.

Prepare your soils using good organic practices. Dig a slightly larger hole for the plant then you normally do.

Although mixing BioMinerals into the soil is really unnecessary now with our growing system it will make more energy available to the plant roots down the line in its growth as the plants roots move into it and as the bacteria in our tea reach it, you can mix one to two cups of BioMinerals into the soil you dug out when making this hole. However, it will not become available to the plants until the microbe tea comes in contact with it and the bacteria make the nutrients and energy available.

Next, put up to 1-tablespoon of BioVam on your plant roots and plant your tomato plant.

Every three days to weekly brew up our microbe tea for 24 hours, filter it, add ¼ cup of yucca extract per gallon of tea, then stir in 2 cups BioMinerals. Use our hose end sprayer (we modify them to dilute the tea properly) and apply the undiluted tea to the plants and soil around the plants.

Set up a drip irrigation system to deliver water periodically during the day. ½ hour every 2 hours seems to work fine for the drip line.

We grew some tomato plants in ½ wine barrels that held 3 cubic feet of soil with a 2 square foot surface area. The plants can go as high as 10 feet and then will loop back down to the ground.

It’s important with this system to apply the microbe tea mixed with BioMinerals and yucca extract every three days to weekly. We are using bacteria as a resource to make minerals bio-available in the tea so when it is sprayed on the plants the nutrients go directly into the plant, and the plant can draw on them from the soil as the plant needs them in between spraying throughout its life cycle.

This method is making the minerals bio-available so the plants can uptake them immediately and as it needs them to produce the energy plants live on.

The compound mineral colloids from the hard rock phosphate in our microbe tea and added BioMinerals feed bacteria which make the nutrients used by the plants to build up their cellular structures. Only compound mineral colloids will allow plants to become healthy so bugs and diseases will not eat up the plants. Ordinary mineral compounds will go into plants but they result in weak plants without the compound mineral colloids. Truly healthy plants are possible only when the compound mineral colloids are present to build most of the plant.

In our tea the acids from the bacteria and fungi and plant roots solubalize the minerals so the minerals can react and generate the energy the plants live on.

Compound mineral colloids are important for plant, animal and human health. These colloids can quickly be placed on the plants frequency and immediately be used to build plant tissues. You will see your plants grow larger and faster when the compound mineral colloids in hard rock phosphate are made available to the plants. Mycorrhiza fungi work well to transfer the compound mineral colloids into the plants from the soil.

It is not unusual to see annual production levels of 200 to 400 tomatoes per plant for the growing season. It’s also not unusual to see tomatoes, peppers, cucumbers, crook neck squash, and cucumbers produce ripe fruit in under 30 days. High brix results is normal for this method of growing plants. We have also used this method to raise the brix of our turf grasses to 14 brix.

When sufficient minerals are reacting in the soils and the ergs approach 450, we find that most weeds will not grow in the soils with our plants. When the ergs are driven from calcium reacting with other minerals in the soil, the weeds do not like to live in such soils. They won’t start growing in those soils until the minerals start dropping into the soil profile and diluting out at the surface.

If you are doing soil tests, they can cause you to do the wrong things in your soils. No soil test will tell what kind of mineral compounds are present and no soil test will tell you if compound mineral colloids are present in your soils. If the compound mineral colloids are low in the soil, the plants will be weak and bugs and diseases will be there to consume your plants. And eating such food will result in animals and humans that will become weaker and unhealthy. This is the main reason why we have so much illness that is derived from eating low quality food which fills the stores all over the world.

Visit our web site for more details or . You may also email me directly at [email protected]

Rock Your Garden With Rock Dust!

Q. I’ve been enjoying your program on our local NPR station, WSCL in Salisbury, MD. I ran across an intriguing article in the UK’s Independent about an apparently miraculous growth medium for plants called rock dust. Sounds like this stuff is right up there with corn gluten meal. I hope you have an opportunity look into this and perhaps comment on it in future broadcasts. Best regards,


During one of your yearly talks at The Philadelphia Flower Show, you mentioned the use of rock dust for the garden. Could you recommend any quarries where I might get some?

    —Harry in Levittown, PA

On one of your broadcast you mentioned the positive effects of stone dust on plants. Where can I get some?

    —Ed in West Philadelphia

A. What a treat to be able to re-introduce this concept to our listeners! Spreading rock dust is a technique that literally began the legendary J. I. Rodale’s lifelong journey of Organic Gardening—including his launch of the magazine of that very same name in 1942. But it’s one of those things that seems to easily fade from the collective memory and fall by the wayside. I’m pleased that I’ve remembered to mention it on the show and at some of my appearances.

My introduction to rock dust came from Carol Keough, one of my first editors when I was a brand new health writer at Rodale Press in the early 1980s. Like many of us, Carol was a hybrid—working in the health side of the company, but a serious gardener at home. When I became Editor and began preparing the 50th Anniversary issue of ORGANIC GARDENING magazine, I got Carol to write about J.I.’s passion for dust, and had an absolute ball revisiting her story for this little condensed treatise.

The theory is simple and elegant. Plants constantly suck minerals out of the soil, slowly depleting these ancient reserves. And restoring those minerals reaps many agricultural rewards: Plants with an abundance of minerals in their soil grow bigger, faster and healthier; and many studies show, contain more nutrients in the form of minerals for the people who consume them.

Now, it’s easy to put the basic plant food, Nitrogen, back into the soil via compost, composted manures and other forms of organic matter. And it’s pretty easy to naturally supply the next two biggest plant-essential elements, Phosphorus and Potassium. But minerals like calcium and trace elements like iron and manganese can be tough to replace once they’re depleted.

So, in the late 1930s, budding organic expert J. I. Rodale took geology classes at Muhlenberg University to learn which rocks might supply the best and most minerals, later wowing the members of the National Academy of Science with his own large-scale comparisons of asparagus grown with organic matter alone (in the form of compost, of course) and with added rock dust. The compost-alone plants looked and tasted great, but the ‘dusted’ plants were even healthier—and, the academics agreed, tastier.

But rock dust alone won’t do it. As J. I.’ s geology professor, Dr. Richmond Myers explained, many of the nutrients in even the most finely ground rock dust are locked up tight. An active soil teeming with microbes is the best way to get those minerals into a form that can be used by plants. And compost is the ideal medium for such activity.

Some people add a little rock dust to their compost piles, either in the beginning or along the way. Some spread rock dust on their soil and then cover it with compost. Others mix the dust into their finished compost right before application. All ways seem to work; the important thing is to combine the dust with lots of organic matter.

You’ll find some packaged rock dusts for sale mail order and at hipper garden centers. But many organic growers buy it in bulk from local quarries and gravel pits—the places where the raw material of the stuff our gardens used up long ago is mined. Our OG article sources recommended that you call up a few local ‘sand, rock and gravel’ suppliers and tell them you want “a very fine material you can add to your soil as a source of plant mineral nutrients.” They might call it pond sand, pond silt, pond fines or swamp sand; crusher screenings, crusher fines, bug dust, float, fill sand, or flume sand. Most gravel workers, we were told, will not call it ‘dust’.

The most important thing is the size of the particles—the closer to it feeling like flour between your fingers the better. Specifically, the dust should fit through a fine 200-mesh screen. Oh, and although the quarry workers probably won’t call it ‘dust’, it IS dusty, so wear a dust mask when you work with it.

Different rocks contain different minerals, and so serious gardeners and farmers have a soil test done that includes mineral needs, and then seek out the specific kinds of rock that can help. Granite dust, for instance, contains lots of potassium—the K in the NPK fertilizer ratio. Rock phosphate, a ‘single dust’ that’s available in packages at most garden centers, just supplies phosphorus. But ground basalt (pronounced “ba-salt”) contains a nicer mix—phosphorus, potassium, calcium, magnesium and iron.

An application of 10 to 15 pounds of mixed rock dust will re-mineralize a hundred square feet of garden for the next three or four years. Don’t use more than that and don’t add rock dust every season—just once every three to five years.

Glacial Rock Dust

Why Glacial Rock Dust?

Glacial Rock Dust contains a broad range of trace minerals, many of which have been slowly lost through the ages on commercial farmland through erosion, leaching and farming practices. Glacial Rock Dust is a natural mineral product which is produced over many thousands of years by glacial action.

Both plants and animals, including humans, benefit from a regular supply of the trace minerals present in Glacial Rock Dust, which are best transferred to the human body through foods grown in soil rich in trace minerals.

Glacial Rock Dust has minerals essential to humans.

Glacial Rock Dust is organic and loaded with essential nutrients that it releases slowly so your plants can be healthy throughout the growing season. It will encourage the root systems of trees, lawns, roses, and vegetable gardens. Add as a soil amendment to your garden, raised beds, or container garden to improve the quality of your soil and add essential minerals to your harvested fruits and vegetables.

Glacial Rock Dust is made from a wide variety of rocks which contain a broad spectrum of trace minerals that are collected and pulverized by the expansion/contraction action of the glacier. As the glacier recedes, it leaves behind deposits of “glacial moraine”. These deposits are mined, dried, and screened for agricultural and horticultural re-mineralization.

Benefits of Glacial Rock Dust

Glacial Rock Dust isn’t just for vegetable gardens. It can also be used in flower beds for increased number of blooms, larger blooms, and healthier growth. Glacial Rock Dust will improve root structures of trees and lawns as well. Vegetables will have fewer pests and increased yields. The mineral nutrients in Glacial Rock Dust work with the soil rather than against it like chemical fertilizers, building healthy soil full of plant-available nutrients and life. Glacial Rock Dust should be applied during the spring once per year, but can be applied seasonally for intensive growing.

Glacial Rock Dust is an excellent source of readily available calcium, iron, magnesium and potassium plus trace elements and micronutrients. It also increases phosphorous availability to plants.

Glacial Rock Dust improves soil structure, moisture holding properties, nutrient availability and bacterial action. When the correct balance is achieved, organic matter is turned into humus and the soil becomes a favorable environment for a host of beneficial molds, fungi, bacteria and earthworms. When this happens, the living soil becomes a buffer to the many negative variables gardeners must contend with. Minerals contained in Glacial Rock Dust are an essential part of the soil food web.

How to use Glacial Rock Dust

Transplanting and pre-mixing soil for potted plants: 8 tablespoons into 1 gallon of soil or growing medium.

Top dressing potted plants: 4 tablespoons per 1 gallon of soil or growing medium. Apply up to once per month or as desired.

Composting: Mix liberally into compost pile or bin as microbial enhancer during processing.

Glacial Rock Dust can be used in potting mixes to improve plant health.

Find it here among other helpful soil amendments in our store.

Soft Rock Phosphate

Soft Rock Phosphate is an important phosphate source that is actually a byproduct from the old time hard rock mining. In order to increase the purity of the higher analysis of the hard rock, the soft rock was a clay impurity which was washed and then it was sent in this water through to a settling pond. So then it filled this settling pond up and so these are huge huge settling ponds and the surface dries out and the soft rock is skimmed off the top of the settling pond and then it can either be processed or sold as is.

Hard rock phosphate, in contrast, is mined just as it is and is a tri-calcium phosphate. Sometimes it has a little carbon. But it is extremely insoluble in the soil. It requires a hyperactive bacterial system and a tremendous amount of soil acidity in order to derive much benefit. Soft rock, on the other hand, is quite a bit more available. A lot of times we’ll see availability showing up on the soil test within 1-2 years, even the first year if a soil’s got some acidity.

There are a tremendous amount of trace minerals in soft rock phosphate and there’s a certain electromagnetic energy or field with soft rock phosphate so that it’s extremely sticky. If you get it wet, you can feel how sticky it is. This electromagnetism helps to hold calcium up in the root zone. So it’s a real critical part of building the soil so that you can hold the calcium.

Soft rock is also a pretty good supplier of silicon, it has a decent amount of boron in it, and then a whole bunch of other trace minerals, about 60 trace minerals all together. And we find it to be an absolute critical ingredient in building soil phosphorus levels for organics. And any time the goal is quality, soft rock phosphate should be part of what is used to build and maintain soil phosphate levels.

We supply this from three sources. One source is from Idaho, and this is not a processed product so it’s pretty much just skimmed off the settling pond and put into bulk truck loads or super sacks in powder form. And we sell it by the semi load, so either a 22-25 ton semi load in bulk, and if we ship it on a flatbed, 24 tons in super sacks. These bulk bags are not standardized in their weights, but overall the semi load will be 22-24 tons.

We also sell Florida soft rock phosphate from two sources. Both are available as either powder or granular. The granular makes it a little easier to work with since can be used in a fertilizer spreader, but it is dusty, it’s not a perfect pellet and it will coat the beater blades on the back of a spreader, so even though it is granulated you still eventually have to go back and clean those blades as you’re spreading soft rock phosphate. Otherwise it just builds up and coats the blades.

The advantage of the granular, of course, is that it can be spread with a fertilizer spreader. The Idaho source needs to be spread with a lime spreader and even in that situation sometimes the soft rock will still bridge in the spreader and someone’s going to have to knock down those bridges so you can get it to spread. It is a little cumbersome product. Another way it could be done is if the powdered product is added to a liquid manure and then mixed in with the manure and then spread out as liquid as part of the manure. This would be a very good way to do that.

Soft rock phosphate, again, will hold the calcium, and that’s really key if you want to build available calcium and also to build a soil that is capable of producing nutrient dense foods. So it’s kind of a foundation point.

Soft rock phosphate is not recommended automatically, you need a soil test to determine if you need phosphates. Many soils, especially those with composted manure have excessive phosphates. And consequently soft rock phosphate is not recommended on those soils. So make sure that the soil is tested beforehand.

The Idaho soft rock has a higher carbon content, that gives it a nice chocolate brown color. The Florida sources have less carbon but have a little higher analysis of phosphorus and calcium. So they’re all good products, we like them equally. They all fit into a good biological program to grow quality.

Organic Soft Rock Phosphate Powder from Idaho

Download Analysis (PDF)

Organic Granular or Powder Mineral Grow from Florida

Info Sheet (PDF)
Download Analysis (PDF)
Download MSDS (PDF)
OMRI Listed Canada (PDF)
OMRI Listed US (PDF)

Organic Granular or Powder Soft Rock Phosphate from Florida

Download Analysis (PDF)
Download MSDS (PDF)

Soft Rock Phosphate

NOTE: Product varies between granular and powder depending on availability. Please call us if you have any concerns or want a particular version.

Calphos Soft Rock Phosphate:

Soft rock phosphate, also known as colloidal phosphate is a clay material that is surface mined from the old settling basins of former hard phosphate rock mining operations in Florida. It contains about 20 percent P205 as well as over 25 percent lime and other trace minerals. It is a very fine material, but can be applied with all common fertilizer spreaders. Natural phosphate stays where it’s put when applied and does not move or dissolve into the soil solution. It needs to be plowed under or tilled into the soil. Unlike chemically made phosphates, soft rock phosphate is insoluble in water, will not leach away, and therefore is long-lasting. Has 18% phosphorous and 15% calcium as well as trace elements. Florida is the primary source. As annual plant takes up to 60 percent of its total phosphates needs the first few weeks of its life. If it doesn’t get phosphate then it is always behind and never catches up. Soft rock phosphate used directly under the seed or transplant at planting time is the very best method of application, especially in low acid or high alkaline soils. It is not as critical but still beneficial in slightly acid to neutral soils. It is almost impossible to overuse soft rock phosphate, you can grow beautiful plants directly in it without any harmful effects.

Soft phosphate rock should be applied at a rate about two tons per acre with other organic amendments. This phosphorous source will soon be gone. It is a byproduct of the making of 0-18-0 and only one company still bags it. In alkaline soils, apply the phosphate directly under the seed or transplants so the small roots don’t have to search for it. This is especially important in the spring.

*information derived from

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