Well drained soil potting mix

If and when to fertilize pre-fertilized potting soil
May 8, 2016 2:48 PM   Subscribe

Hold up, while I agree that you don’t need to bust out the Miracle Gro for the time being, container gardening is a little different than in-the-ground gardening in terms of water and fertilizer schedules.
That “9 months” of slow-release isn’t going to last 9 months in containers. You need to water a LOT more frequently in containers because there’s limited earth (so to speak) for the roots to draw upon for moisture, and the extra water being poured through the pots means that fertilizers wash out faster. (Mind you, you don’t want to over-fertilizing in pots and burn your plants in the short term, because, again, your plants are in a constrained amount of potting mix.)
Later in the summer (maybe end of July-ish) when your plants are busting forth with ripening vegetables and everything is humming along like crazy, you’ll probably need to start supplementing fertilizer again to keep your plants happy. Just take it easy, don’t overdose ’em, dilute your Miracle-Gro a little more than the package instructions say.
posted by desuetude at 10:41 PM on May 9, 2016

If you’ve ever bought a bag of potting soil, you may have been astounded to see it labeled “soilless.” Huh? If it’s not soil, what is it?

Most potting soils are made up largely of peat moss, bark, and perlite. (Perlite looks like tiny white pellets; it’s actually heat-puffed volcanic glass, included to keep the mixture from being too dense.) Often, potting soils are sterilized by steam to kill potentially disease-causing microorganisms.

So potting soil isn’t really soil, because it lacks both minerals and humus. Lifeless and devoid of minerals, potting soils “do no harm” but generally deliver few nutrients. You’ll want to fertilize early and often if growing plants in potting soil.

Actually, “soilless” potting soil helps make an interesting point: You don’t really need soil to grow plants. Hydroponic gardeners grow plants using only nutrient-rich water. In place of soil, inert substrates such as perlite or rock wool may be used to provide aeration and structural support for roots.

What about pH?
What is potting soil?
What about fertilizer?
What’s your soil type?

The Scoop on Fast Draining Potting Soils

I wasn’t born with a green thumb, all of my gardening success is a result of persistence, patience, and my willingness to educate myself in everything gardening. Potting soil, also commonly called potting mix or potting compost, is simply a medium in which people grow plants, vegetables, and herbs in a pot or other container.

Potting soil is composed of materials like peat, sand, composted bark, perlite, and recycled mushroom compost. Plants thrive in various potting soil mixtures, but for now, we’ll explore fast draining potting soils, the plants that thrive with this soil type, and the brands I trust with my home garden.

The Facts on Fast Draining Potting Soil

Fast draining potting soil is exactly what it sounds like, soil that drains water at higher rates. If you read just about any gardening article, blog, or book it will tell you that fast draining soil is needed.

The reason well drained soil is important is because the best soil is comprised of about half air space and half solid materials, preferably with up to 5% consisting of organic matter. Plants existing in these conditions will have about half of the air space filled with water. If the soil is too wet, the plants root system will fail to get the oxygen it needs to thrive.

Test Your Soil

I actually came upon a test you can use to find your soil drainage rates. First, dig a hole 1 foot wide by 1 foot deep. Next, fill the hole with water and make a note of the time. Let 10 minutes elapse and check the water in the hole. If the water has drained from the hole in 10 minutes or less you have fast drainage. If it takes an hour or more to drain the water, you have poor draining soil.

Plants that Prosper in Fast Draining Potting Soil

Okay, I’ve given you the dirt on potting soils, but let’s be honest, no one gardens just to dig around and get dirty. We all want beautiful and bountiful gardens that respond to our hard work.

Most plants will do fine in a basic potting soil mix of peat moss, pine bark, and perlite, however some plants require special mixes to reach their full potential. So now I’m going to tell you about some of the varieties of plants that will require Fast Draining Potting Soils.

  1. Succulents and Cacti: Both these plants need better drainage than annual flowers and often times these plants do well in clay pot containers. Many fans of succulents use Fast draining potting soils like Hoffman Organic Cactus and Succulent Soil Mix. Hoffman has created a special formula to provide proper drainage for succulents, and cacti of both the jungle and desert variety. Hoffman provides a great Fast drain potting soil mix that I personally use for my indoor cactus plants.
  2. Orchids: These precious flowers must have excellent drainage. Most potting soil mixes that you’ll find on store shelves are too heavy and hold too much water. If you want to have truly healthy, beautiful orchids, then you need to try a specialty fast drain potting soil mix. I have tried growing Dendrobium Orchids in regular potting mix and in specialty mixes. There is a noticeable difference when I chose to use the specialty mix. I gave Better-Gro Special Orchid Potting Mix a try with great results. They have worked with highly trained orchid growers and formulated a specialty mix of western fir bark, hardwood charcoal, and sponge rock to ensure optimal orchid growth. Orchids were never truly designed for soil, so using organic compounds such as fir bar and charcoal may work best for orchids.

Well, now I have given you all the dirty details on dirt, or in this case Fast Draining Potting Soil. Go ahead and take the test I mentioned to analyze your soil drainage rates. Plants do best when they have a potting soil that is designed for their individual needs. When your soil is too wet, the plants root system will fail to get the oxygen it needs to survive. If you want to give your succulents, cacti, or orchids the best shot at succeeding, try my recommendations and watch your garden grow!

Potting mixes

There are as many potting mix recipes as there are plants to grow in them. Many recipes have evolved to use local waste such as brewery sludge or jarrah woodchips. Such products may or may not be suitable as components of mixes in various proportions.

Use such a product only if it has the following characteristics:

  • desirable physical properties – wood chips may be suitable for large pots but would be too large for bedding plants
  • does not contain excessive levels of salt or nutrients as some local peats or animal wastes do
  • does not contain any toxic chemicals, such as copper chrome arsenate, PCP or boric acid from timber treatments, phenols as in uncomposted pinebark, or heavy metals as in sewage sludge.

If it might carry disease, sterilise material before use. Gravels, soils and sand from dieback-infected areas are likely sources of Phytophthora.

Characteristics of a good mix

The characteristics of a good potting mix are:

  • well drained, which means an air-filled porosity of at least 15%
  • re-wets easily – some peat and bark media are difficult to re-wet if they dry out
  • does not shrink away from the side of the pot as it dries
  • optimum weight – not too heavy to lift, not so light as to blow over easily
  • suitable pH, between 5.0 and 6.5 is satisfactory for most plants (all pH values quoted are measured in water)
  • free of pests, for example weed seeds, fungal pathogens, or can be sterilised without producing harmful by-products
  • can be stored for short periods without significant changes in physical or chemical properties
  • readily available
  • not expensive.

There is an Australian Standard (AS3743-2003) for bagged retail potting mix. For further information contact SAI GLobal. The standard contains specifications for a range of mixes in both regular and premium grades.

Porosity

Porosity is one of the most important properties of a potting mix. It is the space available within a mix for water, air or root growth. Small pores contribute to water retention whereas large pores promote aeration.

The shape of the container also affects the porosity of a mix. A soil mix in a shallow container will hold more water than an identical volume of mix in a deep container.

Calculating air-filled porosity

A measured volume of mix is saturated with water, then allowed to drain and the drainage water measured.

A milk carton is a suitable container for holding the mix. The container should be equal in volume to the plant pot in use and its shape should approximate that of the pot, since the air-filled porosity of the mix will increase as the height of the container increases. A length of 90mm stormwater pipe can also be used.

Table 1 Interpretation of air-filled porosity (AFP)

AFP%

5

Too low for all but wetland plants

5–10

Suitable for large plants in containers or where watering is infrequent, for example indoor plants

10

An absolute minimum for new mixes

10–15

Bedding plants and those that will receive little attention after planting out

15-20

General nursery lines

20–25

A better starting point for most mixes that allows rapid growth but needs more frequent watering

30

For propagation mixes, indoor mixes and some seed propagation mixes

30–40

Promotes rapid growth, but frequent watering is needed

40–50

For epiphytes

Remove the top of the carton and mark the inside at the same height as the mix will be in the plant pot. Make four drainage holes in the base of the carton.

Fill the carton to the mark with thoroughly moistened mix and leave where it can be watered overhead for several days. The mix should have been at potting moisture content for 24 hours before it is tested. If necessary top up the mix. Immerse the carton in a bucket of water so the water level in the carton can be seen just below the surface. Lift the carton vertically from the water and allow it to drain.

Repeat the immersion and draining twice more. This will compact the mix as would happen in normal practice. Finally, place the carton in another bucket of water so that the mix is saturated just to the surface.

Carefully remove the carton vertically, holding your fingers over the drainage holes. Remove your fingers and allow the carton to drain over a tray or bucket. Do not tilt the carton at any stage as this will permit additional drainage and give a false reading.

Cover the carton and tray or bucket to prevent evaporation and leave until drainage is finished.

Measure the amount of drainage water in millilitres, divide this by the volume of mix used (in millilitres) and multiply by 100. This gives the air-filled porosity in percentage. It is found generally useful to take the average of three readings as you will find that they can vary quite markedly until your technique improves. Table 1 gives a guide to appropriate levels of AFP for a range of nursery situations.

Buffering capacity

Potting mixes that have a higher buffering capacity will resist pH change. This can be useful if you are growing plants that will be in containers for lengthy periods of time. Spagnum peats generally have the best buffering capacity (sedge peats are poor) followed by composted materials and clay minerals such as attapulgite and bentonite. Bark, sand and perlite have little or no buffering capacity.

Cation exchange capacity (CEC)

Mixes with a higher CEC retain fertilisers better. Less will be wasted through leaching and nutrition will be more even. The more decomposed (and generally finer) a material is, the higher its CEC. Humus has a very high CEC whereas raw bark is low. The highest CECs are found in minerals such as zeolite. Clay minerals vary widely. Spagnum peats are moderate.

Potting mix components

Peat

Peat is one of the most common components of potting mixes, and can be quite variable. Peats are graded according to their state of decomposition and particle size.

Broadly speaking, peat falls into three categories: spagnum or light peat (most European peats fall into this category); reed/sedge peat which is darker in colour and finer; and peat humus which is even darker and finer.

The spagnum peats have the highest total porosity. Coarse, light peat has a lower water-holding capacity than fine light peat. Those from England, Scotland and Ireland tend to be similar with a pH of about 3.8 to 4.5 whereas those from Finland and Russia are younger, less decomposed and more acidic with a pH of 3.2 to 4.2. These younger peats can break down more quickly during use.

The reed/sedge peats are brown or reddish-brown to black. The more highly decomposed the peat, the darker the colour. These peats have a higher water-holding capacity than light peat with subsequently lower aeration and a pH of 4.5 to 7.0. Local peat is this type.

Peat humus is a substance derived from the two types of peat in an advanced stage of decomposition. It is dark brown to black and has the least aeration.

Peats should not be allowed to dry out as they can be difficult to rewet. The addition of a wetting agent will help.

Coir peat

Coir peat is a relatively new product derived from coir fibre dust. It is marketed in compressed bales to which water is added. A 30 litre bale reconstitutes to about 150 litres. Coir peat has a pH of about 5.0 making it slightly less acidic than spagnum peat. It has a high water retention capability and rewets easily, unlike spagnum and sedge peats.

Its AFP is 10-12%. Although manufacturers maintain it can be used in high rates (up to 100%) in mixes, it is generally used at about 10% v/v in a similar manner to sedge peats as it will degrade over the long term in a mix thus reducing aeration.

Sand

The particle sizes of sand used in potting mix have important effects on the mix.

To increase the water-holding capacity of the mix, the particle size should be between 0.05 and 1.0mm with less than 10% between 0.5 and 1.0mm.

Particles from 1.0 to 3.0mm increase the aeration of the mix. Sand may also be used to provide weight to the mix.

Perlite

Perlite is a natural mineral of volcanic origin that, when crushed and heat-treated to 1000°C, expands to produce white lightweight particles. The pH is usually neutral to slightly alkaline but may be as high as 9.0.

There are several grades of perlite. The finest has a higher total porosity and holds a reasonable amount of water while the coarser grades hold correspondingly less water but provide greater aeration.

Perlite usually contains dust which should be sieved out where aeration is important, as in propagation, especially when it is being mixed with peat. Perlite dust is highly irritating to lungs so the correct respirator should always be worn when handling the dry product. A dust mask is not sufficient.

Vermiculite

Vermiculite is produced by heat treatment of mica. It is porous and light and has a water-holding capacity of three to four times its weight. Care is needed when handling the material as it is easily compressed, which leads to poor drainage and poor aeration. Vermiculite is a useful material for small container plants.

Zeolite

Zeolite is a mineral with a moderate cation exchange capacity and improves nutrient retention. Its ability to supply nutrients in useful quantities is limited. It can be used in quantities up to 10% v/v.

Jarrah sawdust

Jarrah sawdust is one of the most widely used potting mix components in Western Australia. It is cheap, readily available and relatively sterile if clean.

Jarrah sawdust is best aged or composted before use. When fresh, it has a high carbon to nitrogen ratio and in a mix can cause nitrogen deficiency in plants because of the activity of micro-organisms.

Care is needed not to age sawdust excessively as it will break down to fine particles. In a container, these can move downwards and eventually clog the drainage holes. Six to eight weeks ageing is sufficient.

Although pine and karri sawdusts are available, they degrade too quickly and are not good potting mix components.

Wood chips

Wood chip materials from Eucalyptus diversicolor and E. calophylla are readily available in Western Australia in a range of grades. Experiments by DAFWA have shown these to be suitable for use in potting mixes. These materials should be composted before use, as they contain toxins that may stunt the growth of many plants.

As a compromise, wood chips can be aged for at least six weeks. Many nurseries use fresh material in mixes without apparent problems, but plant growth may not be optimal. A test to calculate the nitrogen drawdown index should be performed and additional nitrogen added to compensate. Given the supply problems with pinebark in Western Australia, it is likely that woodchips will become a standard component of potting mixes in the future.

Pine bark

Pinus radiata and P. pinaster bark are used extensively in the nursery industry both overseas and interstate. Unfortunately, Western Australian supply can be erratic and grades can be inconsistent. Often both species are mixed together. This presents problems as the relative amounts of each vary from batch to batch.

Although many nurseries use fresh and aged bark, this is not generally advisable. Both of these materials contain toxins which can cause problems in sensitive plants, such as the Gesneriads, including African violets and gloxinias. If composting is not done, the nitrogen drawdown index should be calculated and additional nitrogen added to compensate.

Experiments by DAFWA have shown clearly the advantages of composting pinebark (see Table 2). Unlike sawdust, ageing is of little benefit. The procedure for composting pinebark is:

  • Moisten the pinebark thoroughly. If the pinebark is dry and moistening is difficult, it may be necessary to add some soil wetting agent.
  • Add urea at the rate of 3.75kg/m3 of moist bark and mix well. Other sources of nitrogen may be used, but urea has proved to be best.
  • The heap should not be much more than 1m high.
  • Keep the heap moist and turn it regularly during the composting period, to ensure that all the material is composted evenly.
  • Composting will take from four to eight weeks, depending on the temperature. The warmer the heap, the quicker the composting process. The bark is ready when it is almost black and has lost its resinous odour.

To monitor the composting process, use a thermometer and pH test kit. Use the thermometer to ensure the middle of the heap is warm enough to promote the composting process. Temperatures of up to 50°C can be reached in summer but 30 to 35°C is more realistic in winter.

The pH of raw pinebark is usually about 4.5. As the material composts pH rises to a peak of about 8.0 and then falls back to about 6.0 to 6.5, which signals the end of the process.

When using composted pinebark in soil mixes it may be necessary to reduce the amount of lime and/or dolomite in the mix, since it is less acid than most other wood products.

Table 2 Fresh weight of tops (g) of selected plants grown in a potting mix containing 40% sand and 60% wood waste

Material Radish Petunia Gardenia Grevillea Geraldton wax
Pinus radiata bark

fresh

2.6 5.2 95 291 87

aged

4.6 6.5 118 214 105

composted

9.6 13.4 212 489 197
Jarrah sawdust

fresh

4.4 6.8 139 242 78

aged

5.6 8.9 165 163 111

composted

5.0 8.9 252 320 218

Milled marri and karri bark

Milled bark another waste product from the timber industry in the South-West. When composted it has been shown to have useful properties for suppression of plant pathogens, however, it is suitable for use in small (10% v/v) amounts only in potting mixes due to its rapid degradation. Often it is used as a direct substitute for the local sedge peat.

In large proportions, especially in longer-term (more than six months) mixes, as it breaks down it moves to the sides and the base of the pot as a silty layer, reducing aeration and causing waterlogging. The raw product is acidic (pH 4.0) but after composting the pH rises to about 6.0-6.5.

Phenols in the bark are phytotoxic but composting removes these within about 11 weeks. To completely compost marri and karri bark takes about 14 months so is usually not done.

Polystyrene foam

Polystyrene beads are often used to improve drainage and aeration and reduce the weight of potting mixes. They do not retain any water or fertiliser. The beads tend to blow about in windy weather or float to the surface under heavy watering. They are not biodegradable.

Soil

Soil is a less frequent component of potting mixes. If used at more than 30%, the mix is often heavy and prone to waterlogging. The physical and chemical properties of soil can be variable, so pasteurise it to ensure it is free from weed seeds and fungal pathogens.

Animal manures

Animal manures are generally not suitable components of potting mixes. They are variable in mineral analysis and frequently contain weed seeds and other contaminants. The pH can be extremely high, especially when the manure is fresh when it can often burn the plants. Pasteurisation is necessary to guarantee the manure is free from pests and diseases.

Other materials

Depending on location, a range of other materials may be available. Attapulgite (perhaps most familiar to people as ‘kitty litter’) is useful in small containers to retain nutrients at the rate of about 10% v/v. It comes in a range of grades – 1630 has a particle size of about 1mm and is suitable for potting mixes.

Vermicomposts used at the rate of about 10% can supply a range of nutrients for several months. Larger quantities will reduce aeration and cause waterlogging.

Examples of mixes

Propagation

The main criterion setting a propagation mix apart from a general potting mix is degree of aeration, which should be quite high (see Table 1). Consequently, almost anything can be used in a propagation mix provided it has adequate aeration.

Perlite or coarse sand tend to be the most common ingredients. Make sure you use a coarse grade of perlite and ideally sieve any dust out. Although this seems tedious you will find much better results. Some experimentation may be needed to find the best mix for each individual situation, and for each plant species involved.

Different mixes with different physical characteristics will create roots with differing forms. Some of these forms, for example, more or less branched, may be more difficult to work with when transplanting and this can influence survival rate.

Many growers do not incorporate fertilisers since cuttings do not take up nutrients until they have roots, however with quick rooting species it can be advantageous to use controlled release fertilisers in the mix so rooted cuttings can get an early start. If using relatively high temperatures on the propagating bed though, be aware that fertilisers may dump.

Perlite: peat

3:1

Perlite: peat

9:1

Perlite:peat: coarse sand

1:1:1

Pinebark fines: coarse sand

1:1

Bedding plants

The main parameter that needs to be satisfied for a bedding plant mix is to fit into small plug trays.

Peat: sand 1:1
Fine pinebark: peat 1:1
Peat: sand: vermicultie 2:1:1

General

These mixes are suitable for most nursery crops. The exception is epiphytes such as orchids where the media is primarily a means of support.

Orchids

Mixes can be made from a wide variety of components, but must be extremely well draining (see Table 1).

Coarse pinebark

100%

Composted wood chips: charcoal: coarse river sand

6:1:1

Peat moss: composted wood chips:charcoal: coarse river sand: perlite

1:1:1:1

Further reading

  • Growing media for ornamental plants and turf, K Handreck & N Black, 2010, NSW University Press (Sydney), 4th edition.
  • Nursery management – administration and culture, H Davidson & R Mecklexburg, 1999, Prentice Hall (USA), 4th edition.
  • Foliage plant production, J Joiner, 1991, Prentice Hall (USA).

Plant Care: Potting Mix 101

Not all soil is created equally.
What’s the deal with soil?

Most plants need soil to live. It’s where their roots are and where they get their water from. But it may surprise you to learn that not all soil is soil. That is, although we see indoor plants potted in what looks like brown, soily, dirt, they may actually be potted in something else. Generally known in the horticulturist community as “potting mixes” or “artificial potting media”, this 1960’s invention has been in wide use since then for it’s lightweight, weed-free properties and its ability to sustain seemingly any plant.

You may be thinking “why not just use outdoor soil for my indoor plants?” The truth is there are issues with that (see next paragraph), making artificial potting media superior. Allow us to explain.

When soil is taken from outdoors to indoors, you’re taking in all the critters that live in that soil too — critters that would love to eat your plants. You also run the risk of tracking in a soil-borne disease that may kill your plant. On top of all that, outdoor soil is primarily composed of clay, sand, and silt which are not only quite heavy but also prone to congealing and hardening when they dry out completely. Doesn’t sound so great for your indoor plant, does it?

Who came up with potting media/mix?

While we don’t know the first, we do know the best. The invention of Cornell Mix in the 1960’s by James Boodley & team at Cornell University catapulted the houseplant industry into life. The Cornell Mix formula is the basis of most horticulture mixes today. It provides the lightweight, weed-free media that allowed large-scale production of not just food crops, but any crop. Suddenly, large scale plant propagation was possible. Ornamental tropical foliage gained popularity soon after.

What’s in potting media/mix?

Most general potting mixes contain mostly peat, along with perlite and compost. Different mixes may have vermiculite, wood chips, sand or other materials added.

Breakdown by component:

Peat — the basis of most mixes and used in high proportions. Spongy and holds water.

Perlite — white, light, pebbles formed from superheated volcanic glass. Aids in aeration and water control.

Sand — silicon dioxide. Aids in water drainage.

Vermiculite — helps hold water and provides a slow leak of micronutrients and places for fungi/microbes to aid the plants growth.

Wood chips/Bark — decaying organic matter that provides a slow release of macronutrients and is a “denser sponge” than peat. If cut coarse, can aid in drainage.

Compost — nutrient-rich and microbe rich matter that aids in plant growth. Smells earthy.

Glass/Rocks — cheap filler found in not-so-great mixes. Although the rocks could provide a trickle of micronutrients, their weight is enough to make them a lousy ingredient.

What media should you use?

What media you use really depends on where you grow your plants. For example, you will want to use a potting mix to grow plants, herbs and vegetables that are indoors. Soil is best for any outdoor planting in your herb or vegetable garden. Why? Soil is heavier than potting mix and will add unnecessary weight to your containers. It could even be detrimental to your plants health; indoor plants need good air circulation in their roots system and using soil in a planter that is often too heavy and compact makes it virtually impossible for plant roots to spread and blocks moisture from penetrating the soil. As a result, diseases and bacterias can easily creep on your plant and attack it — essentially, your plant could die.

In addition, different plants will sometimes prefer different potting mix make up. For example, a succulent, snake plant or aloe will like a media that is more porous, such as perlite, that water can run through quickly and not hold as much water. (We all know how they prefer to be on the dry side, right?) On the other hand, ferns and mini terrarium plants will prefer a media with more peat, since it helps the soil to stay uniformly moist, which is what most tropical plants prefer.

Special mixes have been developed for different classes of plants. For example, succulent mix was developed with extra sand and coarser materials to aid in drainage for succulent growth. Orchid mix is mostly douglas fir bark, perlite and sphagnum moss. Although succulents can be planted in general potting mix without much fuss, orchids are epiphytes and must be planted in orchid mix, which resembles the trees that they grow on in the wild — they’re a little more high maintenance.

Bottom Line

Potting mix is different from outdoor soil. It’s best to use potting mix for any indoor plants. Use one that gives your plant roots the preferred air, moisture and nutrition balance it needs. Soil from the outdoors is heavy and is best used for outdoor gardening.

Keep growing your plant knowledge.

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Potting Mix: What Is It?

Potting Mix- Just what is it?

Well, for a start, it ain’t soil! That’s right, there is no soil in potting mixes sold in Australia today. Potting mix usually contains fractions of sand and organic matter like bark, coconut fibre and peat.

These elements are included in varying amounts to get a mix that is free draining and sterile. If you used soil there is a high chance there will be harmful fungal and bacterial spores included.

Better to start with disease-free media, no?

Let’s check out the science behind soil….

Soils are made over thousands of years from rocks. The rocks like granite or basalt gradually degrade through the action of temperature, wind, chemical interaction, living organisms and pressure differentials from coarse to fine particles. Into this material, plants begin to grow. When they die, their roots, bark and leaves (what we call peat, humus or charcoal) are left and are combined along with bacteria, fungi and other microorganisms to form soil as we know it.

Soil is great stuff for trees and shrubs and the dizzying variety of natural vegetation we enjoy on our planet but maybe not right for our containerised plants.

Ours is a modified landscape. Each genera, species and variety has it’s own favoured conditions in the natural world that they do best in, but to help them along in our climate modified and the containerised environment we need the right aeration and moisture conditions.

Plant roots need open pore space in soils to get oxygen to perform normal metabolic functions. Limited oxygen means poor root function (moisture and nutrient harvesting. No oxygen means plant death.

Waterlogged soils ‘drown’ roots and lead to invasion of cells by harmful bacteria.

Fine particles, like clay, hold water tightly and restrict air so are therefore excluded from commercial mixes. Bark, peat and coconut fibre are coarse; they prevent compaction but at the same time hold water and make it available to the plant as required.

Sand does provide some minerals but are basically a structural element; they hold the plant up and their coarse characteristic prevents compaction.

Each land plant species have different water requirements and so differing potting media is called for. Mix for arboreal orchids is really open (chunky lumps of coir) whereas mix for general indoor plants may contain more perlite (expanded rock like pumice). Outdoor species do better in a general purpose mix which may have more sand (and is therefore heavier).

It’s all about pore space and water holding capacity. Get it right and you are well on your way to luxuriance, horticulturally speaking!

Potting Soil Mix Explained: Ingredients and Labels

What Does “Natural “and “Organic” on a Potting Soil Mix Really Mean?

The USDA does not regulate the labeling of garden center products like it does organic food.

Not much, as it turns out. And on the whole, the ingredients of most off-the-shelf potting mixes are neither local nor good for the planet.

Most potting soil is “natural” in the sense that its ingredients are derived from naturally occurring substances. Seeing this label on a bag of potting soil by no means indicates it is been produced in an environmentally friendly manner. Similarly, many store-bought mixes are “organic,” but only in the sense that they are made from carbon-based substances, as opposed to inorganic – that is, inert – substances, like rocks.

These terms are plastered on a great number of potting mixes and soil amendments these days, but it’s important to realize that the USDA does not regulate the labeling of garden center products like it does organic food. The USDA plainly states that such products may be “labeled as ‘organic’ even if they do not comply with the standards to produce organic food.

Decoding Potting Soil Mix Labels

It’s difficult, if not impossible, to ascertain exactly what is in a potting soil mix, and the environmental implications of each ingredient, simply by reading the label.

“Compost,” for example, a common potting mix ingredient, doesn’t indicate what materials were composted. The term is often used for composted sewage sludge, which, as Mother Jones has noted, “can contain anything that goes down the drain – from Prozac flushed down toilets to motor oil hosed from factory floors. A 2009 EPA survey of sludge samples from across the US found nearly universal contamination by 10 flame retardants and 12 pharmaceuticals and exceptionally high levels of endocrine disruptors such as triclosan, an ingredient in antibacterial soap that scientists believe is killing amphibians.”

Luckily, even though the USDA doesn’t regulate how potting soil mixes are labeled, there are some third-party organizations that provide certification for potting mixes (along with fertilizers, soil amendments, and a wide variety of farm and garden products) that are free of synthetic substances and thus compatible with USDA organic standards.

These include the Organic Materials Review Institute (OMRI), the California Department of Food and Agriculture (CDFA), and the Washington State Department of Agriculture (WSDA). Look for the logo of one of these organizations when shopping for potting soil to know that the words “organic” and “natural” actually mean something.

Still, these designations only tell you that the ingredients do not include synthetic substances. It doesn’t mean they are sustainably-produced, and it certainly doesn’t tell you where they came from. Many potting soil ingredients, including those approved by the organizations above, cause some pretty horrific environmental damage and travel thousands of miles on their way to your garden.

Potting Soil Mix Ingredients and Their Environmental Implications

These ingredients are commonly found in commercial potting soils and recipes for DIY mixes. It’s next to impossible to produce potting soil that is 100 percent environmentally benign, but the following guide will help you weigh the pros and cons of each.

Coco Coir: Coir (pronounced kwaher), This is simply shredded coconut husks from plantations in southeast Asia. While it is technically a waste product, the fact that coco coir is shipped from overseas gives it a high carbon footprint. Often used as a main ingredient in potting soil, coco coir is highly effective at retaining moisture.

Compost: Homemade compost, comprised of kitchen scraps, garden trimmings, and manure, is probably the most eco-friendly and effect potting soil ingredient there is. The compost found in commercially available potting soils is often made from the waste products of forestry operations, and is often sourced regionally, making it a fairly low impact ingredient. Compost, while it is a spongy material, does not retain moisture as effectively as coco coir or peat moss, though it is much higher in nutrients than both of these ingredients.

Kenaf: Kenaf is the waste product of a type of hibiscus grown for industrial fiber applications. Like rice hulls, it is a sustainable alternative to peat moss.

Peat Moss: Long a primary ingredient in commercial potting soil, peat moss is increasingly being replaced with coco coir (both substances have similar water retention properties) as public awareness of its environmental impacts grows. Peat moss is harvested from deposits in northern bogs, where it accrues very slowly over centuries, and is being removed for horticultural uses faster than it can be replenished. Peat bogs are important habitats for plants and animals wherever they occur, but they also hold tremendous quantities of carbon dioxide, which is released into the atmosphere when the bogs are disturbed.

Perlite: This is a volcanic mineral that has been heated to make it puff up like tiny bits of popcorn or Styrofoam. Perlite is added in small quantities to potting soil to improve drainage – it’s the little white flecks found in almost every commercial potting mix. It is extracted from open pit mines, with all the associated environmental impacts.

Rice Hulls: This byproduct of rice production is an excellent alternative to peat moss for improving drainage, though it is not widely used in commercial potting soils. Specialty garden suppliers sometimes carry it as an ingredient for homemade potting mixes.

Sand: Used to improve drainage, sand is rarely used in commercial potting soils because it is so heavy, but is a common ingredient in homemade potting mix recipes where weight is less of an issue. Large-scale sand mining is highly destructive, though if you have access to a river or creek, it’s easy to harvest your own without causing damage (avoid beach sand, as the salt it contains may damage your plants).

Shredded Bark: This spongy material is largely used as filler in commercial mixes. A byproduct of the forestry industry (usually from pine plantations), this is a relatively low impact ingredient, especially if it comes from a local source.

Soil: Commercial potting mixes are often advertised as soilless simply because dirt is heavy and inhibits drainage. Though when incorporated in limited quantities, garden soil is a useful and highly sustainable ingredient for homemade mixes, especially if you use fertile, crumbly topsoil.

Vermiculite: This mica-like mineral is sometimes added to improve drainage. But like perlite, it comes with all the environmental implications of mined materials.

Worm Castings: This compost-like material is essentially worm droppings, and may be produced in a home worm bin or through commercial-scale worm composting. While quite expensive to buy, worm castings are an ideal potting soil ingredient, with minimal environmental impacts.

DIY Potting Soil Recipes for Mixing Potting Soil at Home

Buying and mixing your own potting soil ingredients saves money and empowers you to choose a mix that aligns with your ethical shopping priorities. Different plants, and different gardening situations, call for different mixes, however. Here is a sampling of recipes that should cover almost any potting soil need you might have. All of the ingredients listed should be available at a well-stocked garden center or may be purchased online.

How to Mix Potting Soil at Home

All of the following recipes provide ratios, rather than specific amounts – 1 part this, 3 parts that, etc. Thus you can choose the size of the container for measuring each “part” based on the size of your potting mix project and what is conveniently available to measure with. Almost anything will do: measure by coffee can, Tupperware, plastic plant pot, 5-gallon bucket, trowel scoop, shovel-full, or wheelbarrow load.

Mix small quantities in a bucket or plastic bin; medium-size batches in a wheelbarrow; and large loads on a flat concrete surface. Use your hands, a trowel, or a shovel to do the mixing. There is no science to it, just keep mixing until you have a uniform finished product. Store your potting soil inside plastic bins, bags, or under a tarp.

Basic Mix

This mix works for the vast majority of outdoor potted plants, including most annuals and perennials.

1 part peat moss

1 part compost

1 part perlite

Houseplant Mix

Most houseplants are tropical species that prefer a spongy, nutrient-rich mix.

1 part peat moss

1 part ground bark

1 part compost

1 part perlite

1 part sand

.1 parts blood meal

Succulent Mix

Cacti, succulents, and other species that hail from arid environments prefer a free-draining, low-nutrient mix.

1 part perlite

1 part sand

.5 parts peat moss

.1 parts compost

Tree and Shrub Mix

Fruit trees, berry bushes, grapevines and other woody plants do well on a compost-heavy mix.

2 parts compost

2 parts ground bark

1 part peat moss

1 part sand

1 part perlite

Deluxe Organic Vegetable and Herb Mix

This mix delivers abundant fertility with a balanced array of nutrients suitable for most food crops.

3 parts compost

2 parts peat moss

1 part sand

.5 parts perlite

.1 parts bone meal

.1 parts blood meal

.1 parts ground limestone

.1 parts kelp meal

.1 parts worm castings

Seed Starting Mix

Seedlings sprout best in an ultra-fine mix with equally spongy and free-draining qualities.

2 parts compost

2 parts perlite

1 part peat moss

.5 parts sand

Orchid Mix

Orchids need an ultra-spongy, low-nutrient bark-based mix.

2 parts ground bark

1 part peat moss

.5 parts perlite

Eco-Friendly Mix

Common potting soil ingredients like perlite and peat moss come with significant environmental implications. This recipe emphasizes ingredients that are likely to be of local origin and unlikely to involve egregious environmental impacts.

2 parts compost

1 part rice hulls

1 part ground bark

1 part sand

Budget Mix

All of the above mixes are made without actual soil, a necessity for providing suitable drainage in small pots. But it is okay to use small quantities of dirt (ideally good-quality topsoil from your garden) when filling large planters and raised beds. Doing so cuts down on costs considerably. It’s also possible to source the other two ingredients below for free.

1 part garden soil

1 part compost

1 part sand

A Guide to Homemade Potting Soil

Whether you use it with houseplants indoors, or for window boxes outside, potting soil is an essential element in any garden container. That’s because potting soil is different than gardening soil: It is lighter and airier, so it helps to keep water moving from top to bottom and keep plant roots as healthy as possible. Garden soil, on the other hand, moves water to the bottom and holds it there.

But pre-packaged potting soil can be expensive, particularly if you have lots of containers and flower boxes. Fortunately, you can make homemade potting soil quickly and easily with readily available ingredients.

Related: Propagating Houseplants

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What is in Homemade Potting Soil?

Potting soil is different than soilless potting mix; the latter is used only to germinate seeds. The best homemade potting mixes have three ingredients: a growing medium, something to help retain moisture and nutrients, and something to promote drainage.

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Recipe #1 for Homemade Potting Soil

There are several recipes to make homemade potting soil. To closely mimic pre-packaged potting soil, you’ll need

  • Growing medium: Garden soil from a home center, which is pre-sterilized to remove weeds or disease.
  • Moisture retention: Spaghnum peat moss. It is harvested from bogs that have been drained, so the moss has dried and turned a light brown color; you may need to lightly moisten before mixing the potting soil.
  • Drainage: Perlite, vermiculite, or sand. Perlite is made by heating bits of a glasslike mineral until they expand into puffy, lightweight particles. It holds no water, aside from the little that clings to the surface of each particle.

Mix those three ingredients in equal proportions, adding more of any ingredient until you have a loose, but clump-able, mix.

Related: Get the Lowdown on Soil Amendments

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Recipe #2: Homemade Potting Soil

There’s a second way to make homemade potting soil that involves fewer ingredients and is favored by some organic gardeners. To make compost-based potting soil, simply mix equal parts sterilized garden soil and compost (pre-packaged or homemade); add sand or pebbles as needed to increase drainage.

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Fertilizing Homemade Potting Soil

Any potting soil will, over time, leach out nutrients that plants need. So while homemade potting soil is a great growing medium, your plants won’t thrive unless you regularly amend the potting soil with fertilizer.

You can do this in a number of ways. You can amend your homemade potting soil mixture with limestone before using it. You can also top-dress plants occasionally with any number of types of compost, such as recycled mushroom compost. You can also rely on a fertilizer that offers slow-release nutrients in order to help your plants retain their growing vigor.

Related: Types of Fertilizer

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PHOTO: Susy Morris/Flickr by Jessica Walliser February 18, 2009

Although many container-vegetable gardeners eventually find a reliable, favorite brand of potting soil, buying farm-sized amounts is not cheap and shipping is a nightmare if you can’t find a local source.

Mixing your own custom blends of potting soil with readily available ingredients allows you to develop a soil mixture suited specifically to your farm’s needs. It also allows you to pick and choose your nutrient sources—especially important to farmers looking to cut out the chemicals. All good-quality potting soil is easy to handle, is well-draining and contains ample organic matter. It should provide physical support to your plants as well as nutrients and sufficient water and air. Making your own potting soil mix is easy, and it gives you complete control of one of the most critical steps in the growing process.

Like all good recipes, quality ingredients are key to making healthy potting soil. Here are the ingredients you should look for.

1. Garden soil

For homemade potting soil mixtures, garden soil adds density and is a cheap source of bulk. Don’t use soil that contains pesticides, chemical fertilizer residues or other environmental pollutants. Using your farm’s topsoil or soil from a certified organic grower is best. Solarize the soil first by covering the pile with clear, plastic sheeting for at least four to six weeks to kill weed seeds, pests and pathogens. Sterilization is also possible in an oven or microwave, but this method leaves the house smelling, well, earthy.

2. Compost

Containing billions of beneficial microbes and with great water-holding capacity and nutrient content, compost is a must for quality, homemade soil mixtures. And if you make compost yourself, it’s free. It should be fully decomposed and screened into a small, consistent particle size. An added benefit: Recent studies note a decrease in foliar diseases on plants grown in soil mixes containing 20 to 30 percent compost.

3. Sand

Coarse builder’s sand improves drainage and adds weight to the mix, providing ample physical support for growing plants.

4. Sphagnum peat moss

A very stable ingredient, peat takes a long time to break down and is widely available and inexpensive. It bulks up mixes without adding a lot of weight and, once wet, holds water fairly well. The environmental impact of current peat harvests is a factor for some farmers, many of whom prefer to turn to coir fiber products instead. Organic farmers cannot use peat moss treated with a wetting agent, and most are treated. Limestone must be added to mixes containing sphagnum peat moss to help balance the finished product’s pH.

5. Coir fiber

A byproduct of the coconut industry, coir looks and acts a lot like sphagnum peat. It has more nutrients than peat moss and lasts even longer, but it’s more expensive to purchase. Coir is sold in compressed bricks.

6. Composted pine bark

Composted pine bark lightens up soil mixes by increasing pore sizes and allowing air and water to travel freely in the potting soil mixture. It is slow to break down, but might rob nitrogen from the soil as it does. The addition of a nitrogen fertilizer is necessary when using composted pine bark as an ingredient. It is most commonly found in mixes designed for potted perennials and shrubs.

7. Perlite

A volcanic rock, perlite is heated and expanded to become a lightweight, sterile addition to potting soil mixes. It holds three to four times its weight in water, increases pore space and improves drainage. With a neutral pH, perlite can be used in place of sand when a lighter mix is required.

8. Vermiculite

Vermiculite is a mined mineral that is conditioned by heating until it expands into light particles used to increase the porosity of soil mixtures. It also adds calcium and magnesium to the soil and increases the water-holding capacity. Select medium grade for seed-starting mixes and coarse grade for older, potted plants. Use caution when handling vermiculite, as it naturally contains asbestos. The EPA recommends growers use substitute products, such as peat, sawdust or perlite, whenever possible to avoid excessive exposure.

9. Limestone

Calcium carbonate or dolomitic limestone are used to adjust the pH of soil mixes containing acidic ingredients, such as sphagnum peat or composted pine bark.

10. Fertilizers

Additional nutrient sources are especially important when using soil mixtures that don’t contain compost. Choose natural fertilizers derived from mined minerals, animal byproducts, plant materials or manures. A combination of these natural fertilizers provides a long-term, stable and eco-friendly source of nutrients. Such a blend can include combinations of any of the following: alfalfa meal, blood meal, bone meal, cottonseed meal, crab meal, feather meal, fish meal, greensand, kelp meal, dehydrated manures and rock phosphate.

Use newly mixed potting soil as quickly as possible. Try to estimate exactly how much you’ll need on a given day to avoid storing it.

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