How to Build a Super Soil

In some ways, super soil represents a radical departure from “conventional” horticulture and may be regarded as an exciting new development.  Viewed from a different perspective however, super soil may be seen as a return to a more traditional approach based on centuries-old cultivation techniques.  Some growers are attracted to this approach because of their values around sustainability and regenerative agriculture.  Others may be drawn by the perception of increased product quality that can be achieved through these methods.  For others, it is simply a calculation of economic value; the promise of being able to use the same growing media through multiple crop cycles over several years can provide a considerable reduction in input and labour costs that positively impacts the bottom line. 

Whatever the motivations, super soil has become an increasingly hot topic in the cannabis community.  Today, there are several excellent super soil products available from a number of different companies which may provide appropriate off-the-shelf options for your garden.  This article attempts to provide a primer for those growers who may be considering building their own super soils but do not know where to begin. 

The success of any super soil will come down to three main factors; physical structure, chemistry, and biology.

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Physical Structure:

The structure of any high-quality growing media must provide sufficient aeration and water retention properties in the root zone to produce healthy plants.  A poor soil matrix that is too dense and heavy will tend to compact and lead to anaerobic conditions in the root zone which will cause endless headaches.  Conversely, a mix that is overly light and porous will dry out too quickly to maintain adequate soil moisture required for optimal nutrient cycling in this type of system.  In general, super soils tend to be have a higher bulk density than the high porosity-type potting mixes which have traditionally been utilized in horticulture.

Chemistry:

When it comes to “soil” growers, the norm has always been to use low-nutrient soilless growing media in conjunction with bottled synthetic nutrients.  Super soil growing turns this relationship on its head by preloading the growing media with all of the essential plant nutrients required to produce a bumper crop and foregoing the bottled nutrients for the most part.  This approach can work well, but nutrients must be properly balanced in order to be successful.

The Law of the Minimum states that growth is dictated not by the total resources available, but by the scarcest resource.  17 essential plants nutrients have been identified, but many plant scientists suggest that there may be as many as 23 elements essential to plant growth.  Some, like nitrogen, phosphorus, potassium, and calcium, are required by plants in large quantities.  Others, such as boron, zinc, and chloride, are required at very low levels.  But all are essential for plant growth and if any one of them is present in soil at levels below the requirements of the plant, growth will be limited.  It is therefore important to think beyond N, P, and K if we are to successfully build a super soil.

It is also important to realize that any plant nutrient can become toxic if too much is present in the soil.  And since super soils make use of organic fertilizer products which become mineralized in soil, it is not possible to flush excess nutrients out of the soil in the way that may be possible with synthetic nutrients.  Therefore, a cautious approach is recommended when adding fertilizers to a super soil.

The other important consideration regarding the chemistry of your super soil is pH.  Although super soils perform well under a wider pH range than the fairly narrow ranges typically associated with hydroponic growing, it is nonetheless a crucial parameter to building a proper super soil.  If the pH is too low (below 5.6 for example) certain micronutrient toxicities may become apparent.  If it is too high (say, above 7) availability of some nutrients may be reduced to the point where growth and plant health is diminished.  Keep in mind that it is always easier to raise soil pH than it is to lower it, so erring on the low side is best when initially mixing your soil.

The best way to assess the chemistry of your soil is to send off a sample to a soil lab for testing.  These services are widely available, and are usually quite affordable.  Although the labs in your area may not provide nutrient recommendations for the particular crop you are growing, they typically provide targets geared toward field ag crops which can be useful in providing an overall picture of your soil chemistry and pH.

Biology:

In order to achieve effective nutrient cycling, it is imperative to develop a healthy and diverse microbial population within your super soil.  The beneficial bacteria, fungi, archaea, protozoa, nematodes, and microarthropods that make up the basis of the soil food web are essential partners in the garden, and an effective super soil will provide an excellent habit and abundant food resources to support these desirable microorganisms.

If we have done everything right so far, we will have created a soil with a physical structure that maintains aerobic soil conditions which will favour beneficial microbes.  If the soil structure is poor and significant areas of the soil become anaerobic, the soil environment will favour undesirable and pathogenic microbes.  Under these conditions, the bad guys will begin to outcompete the good guys, and our plants will pay the price.

Similarly, if we have done a good job of building our soil chemistry, there will be ample food resources in the microbial ecosystem for beneficial microbes to survive and thrive.  This means not only providing a good balance of essential plant nutrients, but also providing a full range of trace minerals that may be required for a diverse community of microorganisms.

Finally, in order to inoculate our super soil with adequate populations of soil microbes, we need to include significant quantities of biologically active ingredients.  These include high quality compost products, as well as certain organic fertilizers such as alfalfa meal and insect frass.

Now, let’s turn our attention to some of the ingredients that are typical in super soil recipes.               

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Peat Moss:

Canadian sphagnum peat moss forms the backbone of most growing media including super soils.  It typically makes up between 30% and 60% of a super soil recipe depending on the other ingredients included and the desired structure of the soil matrix.  Typical horticultural grade peat moss is screened to produce a fairly uniform texture with only low to moderate amounts of chunks and woody material.  This represents the most common peat moss available at garden centers and greenhouses across the country.  Some growers prefer the soil structure benefits that can be achieved by using chunkier peat moss, but these products are somewhat more specialized, can be harder to source, and are not typically suitable for smaller containers. 

Canadian sphagnum peat moss is fairly acidic, with pH ranging from 3 to 4.5.  Since this range is too low for most crops to perform well, it is often necessary to adjust the pH with the use of calcium carbonate products or other pH buffering materials.  Peat provides excellent cation exchange capacity, which is a measure of nutrient holding potential.  And although it is technically a renewable resource, the time-scales involved in the growth of peat bogs make it a difficult resource to manage sustainably.

Coconut Coir:

Coco coir is an excellent product, though somewhat less common than peat moss for use in super soils.  If it is used at all, it will typically only make up 5-15% of a super soil recipe.  Made from the coir and pith of coconut husks, one of the attractions of coir is that it represents a captured waste stream from commercial coconut production, and is therefore a renewable resource.  Perhaps the greatest benefit of coco coir as a super soil ingredient is its excellent structure.  Good quality coir features lots of long fibers, much longer than those found in peat moss.  This can improve the soil matrix and help to promote even water distribution throughout the growing medium.  It also has excellent cation exchange capacity, and will therefore hold onto nutrients well. 

Coco coir is typically fairly pH neutral, with an average range between 6.5 and 7.  The addition of coir, therefore, tends to reduce the pH buffering requirements in a peat-based super soil.   One potential drawback of coir is the high levels of potassium and sodium that are often present in the unprocessed product.  Manufacturers of high-quality coir products get around this by buffering out the excess potassium and sodium with a mixture of calcium and magnesium.  These buffered coco products are more expensive than raw coco, but they provide a better starting point in terms of chemistry.  And this brings us to perhaps coco coir’s biggest drawback as an ingredient in super soil; cost.  Coco coir tends to be considerably more expensive than peat moss and can increase the overall cost of your super soil project.  One must weigh the benefits against the cost to determine how much coir, if any, to use in a super soil mix.

Perlite:

Perlite is used in most potting mix formulations as a reliable source of aeration.  It is made from an amorphous volcanic glass which is heated to a high temperature in industrial kilns so that it puffs up to a low-density popcorn-like particle.  The chief purpose of perlite is to prevent soil compaction and improve aeration.  It has relatively low water-holding capacity and is a fairly low-cost way to increase oxygen in the root zone.  A super soil may have as much as 33% perlite, depending on the other ingredients used and the desired soil density a grower is looking to achieve.

There are a few drawbacks when it comes to utilizing perlite in a super soil.  First, its porous nature often causes any particles in the upper soil horizons to separate from the mix and float to the surface where they do not contribute to the soil structure and can be a mild annoyance to the grower.  Second, because processed perlite is a soft and delicate substance, the particles tend to be slowly ground down over several crop cycles as the soil is turned and mixed.  This leads to a steady reduction in aeration and necessitates the periodic addition of more perlite or perlite alternatives over time.  

Vermiculite:

Vermiculite is a hydrous phyllosilicate mineral which, similar to perlite, expands when subjected to high heat.  Vermiculite is very similar to perlite and can serve as a fine alternative.  However, there are some key differences that should be considered when building a super soil.  Vermiculite holds more water than perlite does, and retains it for longer periods of time.  This increased moisture retention may be desirable if the overall mix is fairly light and porous, but it may not be desirable if the super soil is made with large quantities of heavier, wetter materials like compost or manure.  Vermiculite also provides a bit less aeration than perlite.

Vermiculite is also subject to the drawbacks of perlite.  Namely, it tends to float to the surface, although perhaps less than perlite does, and it will break up over time as the soil is worked and turned with a shovel. 

Pumice:

Horticultural pumice has gained popularity in recent years as an additional source of aeration for super soil recipes.  It is a volcanic rock that consists of rough-textured volcanic glass that is filled with cavities and crevices.  Pumice is much heavier than either perlite or vermiculite, and therefore will not float to the soil surface when water is applied.  It is also a harder, tougher substance that can stand up to a shovel and will not noticeably grind down or degrade over time.

The largest drawback with pumice is that it is a bit of a niche product and it can be a real challenge to source high quality horticultural pumice.  Many garden centers do not carry it, and if you can find it, it may prove to be a costly addition to your recipe.  It may be advantageous to use a mixture of perlite and/or vermiculite and/or pumice in order to balance the benefits of each while minimizing the drawbacks.

Compost:

Compost is a very large and complicated topic.  It represents one of the most important ingredients in any super soil formulation, and will often comprise up to 33% of the mix.  Compost serves several functions in super soil; It provides organic matter and humic and fulvic acids, it is a source of plant nutrients, and it is an inoculant of beneficial microorganisms upon which the success of your garden depends.  Some of the major categories are compost those made from food waste and plant biomass, earthworm castings and vermicompost, composted manures from various livestock, mushroom compost, fish compost, and others.   

It is important to note that not all composts are created equal and it is crucial to use only the highest quality compost products if your super soil is going to be a success.  You cannot assess the quality of a compost based solely on the input material.  The nature and length of the composting process must also be considered.  One of the simplest ways to assess the quality of compost is to use your nose.  Unpleasant odours are an indication of undesirable and pathogenic microbes that are likely to lead to poor results.  When you stick your nose into finished compost, you should not detect ammonia, vinegar, alcohol, or the putrid odours you might associate with rotten meat or garbage.  Good compost should have an earthy smell similar to healthy soils.  It should smell clean and attractive.

Another simple way to assess compost quality is by sight.  Many municipal compost operations turn out terrible products contaminated with plastic and other kinds of garbage.  Do not use these products.  There should not be flies or gnats or other insects buzzing around your compost on a hot day.  These are signs of poor quality and should be avoided.  If you are using manure of any kind, it must be thoroughly composted.  Fresh manure or partially composted manure will cause problems and should be avoided.  All of this may seem obvious, but it is a key element in building a super soil, so it seems worthy of discussion.

Another important consideration when selecting compost products is pH.  There can be a wide range in pH between different compost products, and it is important to understand how those may affect the finished product.  For example, worm castings are often fairly acidic, and some may have a pH as low as 3.5.  Plant biomass composts tend to be more alkaline, and some may have pH levels as high as 9.  Neither of these ranges will necessarily present any problems, but it will influence the amount of pH buffering material required to achieve the desired range in a finished super soil.

Although a respectable super soil can be produced with only one type of compost, it is often a good idea to use a variety of different compost products.  This way microbial diversity may be enhanced, and a more varied nutrient profile may be achieved.         

Biochar:

Biochar is a charcoal-like material made from organic biomass that has been processed through pyrolysis.   Due to the variety of feedstocks possible and differences in pyrolysis time and temperature, there are wide varieties in physical characteristics and chemical properties among different biochar products.  pH can range from as low as 5 to as high as 11, and nutrient levels can vary greatly, so care must be taken when working with biochar.  Many may be suitable for inclusion in a super soil, typically at a rate of 5% to 10% of the mix by volume.

Biochar is a great way to boost the organic carbon content of soils and is a fine method of soil carbon sequestration.  Its value in super soil recipes is largely due to its porous nature.  The structure of biochar is characterized by microscopic pores, tunnels, and crevices which give the material an immense amount of surface area relative to its mass.  This makes biochar great for absorbing and storing water and nutrients, and creates habitat for soil microorganisms which helps to boost beneficial microbial populations.    

Fertilizer:

Please see our Organic Nutrients 101 article for a complete run-down on organic fertilizer options.

A Note About Recipes

Although soil recipes like the one below can be useful starting points for those new to building these types of soils, it should be noted that the final product depends entirely on the specific inputs that went into it.  Different growers in different regions will have access to different inputs from different manufacturers which will have highly variable physical, chemical, and biological quantities.  Therefore, two growers can follow the same recipe and end up with soils that seem to have very little in common with each other.  So, unless you are using all of the same inputs as the grower who made the recipe, you should not expect to end up with their soil.  Each grower must alter and finetune their own recipe based on the inputs available and the performance of their plants.

 

Nevertheless, I will offer a general recipe that may be helpful in getting started.  

Super Soil Recipe

Soil Mix:

(Measured by Volume)

40% Peat Moss

30% Perlite

5% Coco Coir

5% Biochar

10% Worm Castings

5% Farm Compost

2.5% Well-Composted Chicken Manure

2.5% Well-Composted Horse Manure

Fertilizers per cubic foot of soil mix:

1 1/2 cups 4-4-4 or 5-5-5 blended organic fertilizer

½ cup glacial rock dust

½ cup basalt rock dust

½ cup kelp meal

½ cup oyster shell flour

½ cup gypsum