SOIL

Revised by Brian Malin

The Purpose of Roots

In earlier chapters most of the discussion was about the plant’s activities above the soil level. Though hidden, the plant’s roots are just as complex as the canopy. The canopy and flower size have a lot to do with how strong the root system is.

It’s obvious that roots anchor a plant in place. The network of branched roots forms a tight relationship with the soil that firmly anchors the plant in the ground or its container. Although the canopy may endure winds that tug and pull it, the roots provide stability to the stem, keeping it upright while allowing it to bend.

The roots are the carrier of nutrients and moisture to the entire plant. The root system secretes fluids called exudates into the rhizosphere that help make nutrients available to the plant. Exudates are made up of amino acids, carbohydrates, sugars, other acids, and secondary plant compounds. Exudates help solubilize nutrients and send signals to rhizospheric microbes.

What Is the Rhizosphere?

The rhizosphere is the volume of soil or growing medium that is in the immediate vicinity of the plant’s root system. The rhizosphere is integral to the roots’ chemical (nutrient and water absorption) and microbial activities.

With proper oxygen concentration, among a few other conditions, microbial colonies form and thrive. These beneficial microbes help maintain the health of the plant throughout its life cycle (See Oxygen.)

Cannabis plants grow roots both vertically and horizontally. The horizontal roots can stretch out to a width equal to the plant’s canopy. This means that a plant that is 10 feet (3 m) wide has roots to match. The horizontal roots also grow down to a depth of 9-18 inches (22-45 cm), depending on the soil’s moisture. The vertical roots can stretch down to 4 feet (1.2 m) or more in search of water; in moist soil, the vertical roots may be quite short.

Native Soils

Native soil in the northern tier of North America and most of Europe forms layers of decomposed plant material in two distinct patterns. Forest soils develop from the trees’ leaf drop. Such soil forms by continually adding to the top layer. A typical soil of this kind has an undecomposed top layer. The contents are progressively more decomposed, with the lower portions forming a fine compost. The depth of this layer varies by locale and natural environmental conditions as well as the forest’s history. Some native soil is unsuitable for growing in.

Soil that has been covered by grasses and annual plants has accumulated organic matter in a different way. Known as “prairie soils,” they may have a thin layer of topsoil near the surface, but the nutrient-rich area is deeper than the topsoil. The root layer of annual plants dies off and decomposes each year, leaving an organic component that provides nutrients, holds moisture, improves texture, and hosts beneficial organisms. This layer, while sometimes shallow, can reach a depth of 10 feet (3 m).

Here are some things to consider when deciding to plant in native soil or to grow in a potting mix in a container:

Does the soil have drainage problems? Dig a hole two feet (60 cm) deep, fill it with water, and see if water seeps into the ground. If water is still in the hole one hour later, there will be drainage issues with this soil.
Is the soil too rocky? Can a hole be dug without hitting rocks every shovelful?
Have toxic or banned fertilizers and pesticides been sprayed on the growing site?
Were old vehicles or equipment parked on the site?

If the answer to any of the questions above is yes, the land should not be farmed. Instead, it should be covered with some kind of ground cover and the plants grown in raised beds or containers. Cannabis bioaccumulates everything from nutrients to pesticides to heavy metals and will absorb these toxic chemical residues.

Some native soil is perfect or can be slightly amended to create an excellent environment to grow cannabis.

Growing in native soil has many advantages:

The earth provides natural insulation against hot days and cold nights.
Using it is much cheaper than buying soil.
It takes much less work to use native soil than to prepare imported material.

Soils vary in quality, so there are few generalizations to rely on when discussing soil preparation. Anything done to prepare the soil must be customized to it.

The most important quality of any soil is its texture, which is determined by the size of the soil particles. Cannabis prefers soil that drains well but also holds moderate to large quantities of water, especially in warm dry regions. Depending on the type of soil composition, add materials to improve the texture and nutrients.

When North America was colonized in the 16th century, earthworms were introduced that changed the soils in the eastern half of the continent. Earthworms pull semi decomposed material down into their burrows, which have a depth of several feet. Duff, the large accumulated layer of decomposing organic matter that typified pre colonial forests, has largely disappeared. In its place, there is a smaller layer of decomposing compost soil. The mineral soil, which formed a distinct layer below the compost soil, is now laden with organic material and burrows, so it is more friable.

Soil Tips

Growing in soil has many advantages over other methods. It takes less labor and time to prepare the planting area. Adjusting the soil’s fertility with nutrients and amending it with additives such as compost, mulch, or fertilizer takes relatively little time and energy as compared with replacing a container’s soil.

Roots can stretch out and obtain water and nutrients from a larger area, and, as a result, they can support the needs of a larger plant.

There is no “perfect” soil for growing cannabis; different varieties each grow within a range of soil condition parameters. The soil must be well-drained, nutrient rich, and have a pH between 5.8 and 6.5.

Always test the soil to find out what amendments it may need before preparing for planting. The pH and fertility of soils vary, so there are few generalizations that can be made about preparing them. Only after soil qualities and nutrient values are determined can the correct adjustments be made to optimize the soil’s fertility. Also test for chemical residues and heavy metals.

For cannabis plants, the soil should test high in the three macronutrients: nitrogen (N), phosphorus (P), and potassium (K). (See Nutrients & Fertilizers.)

Grassland and prairie soils have a different history. The annual dieback of canopy growth deposits a small top layer that decomposes in the same way as in forest soils. However, most of the added organic material develops below the surface, fed by the dieback of annual plant roots. They compost in place, so the organic matter integrates with the mineral content of the soil, creating deep levels of nutrient-rich, composted material.

Soils in warmer climate zones contain much less organic matter and have fewer nutrients because microorganisms, whose metabolic rates increase with the temperature, are more active, so they digest new organic matter faster.

Where the canopy is less dense, and in zones with less rain, not as much decomposing material actually makes it into the structure of the soil.

Moist warm tropical soils can contain abundant organic matter and rich microbial colonies; however, the microbes break down dead plant matter and turn it into nutrients for other plant life at a rapid rate due to the warm, wet environment. The quick turnover of organic nutrients is a process called biomineralization, which typically results in soils that are poor in nutrients as they leach out of the soil from the frequent rains.

As soon as vegetation dies, bacteria and other microlife feast and render the nutrients water-soluble. They are absorbed into the soil and almost immediately taken up by the roots of higher living plants or leached out of the topsoil.

Underneath the layer of organic material in forest soils and the mixture of organic matter and minerals in grasslands lies the mineral soil, which contains little organic matter. The mineral portion of soils is usually composed of three main elements: sand, silt, and clay. Each of them lends properties to the mix, and each is needed in order to create a soil that promotes healthy growth.

The relatively small clay particles cling together and form a barrier to water percolation. Clay has a negative electrical charge that binds minerals with positive charges including calcium, magnesium, potassium, and sodium. It releases them in exchange for weak hydrogen ions released by plant roots and microorganisms. Soils that are more than 30-40% clay are considered clay soils because the other ingredients are absorbed into the clay dough.

SOIL TYPE PARTICLE

It is challenging to grow cannabis in clay soil. Drainage is difficult when too much clay is present. If the native soil is composed of more than 20% clay, consider growing in a container.

Loams are composed of sand, silt, and clay mixed with organic matter, or humus. As a result, the soil particles vary in size. This creates multiple paths for water to flow and also allows air pockets to remain, even when the soil is saturated.

Clay loams can be silty or sandy but are a little heavy and, in wet weather, can become saturated. Adding organic matter and sand increases porosity, although this may be an arduous undertaking.

Sandy soils leach water and nutrients. They need to be irrigated and fertigated more often because they hold less water and nutrients.

Sufficient amounts of organic compost, worm castings, coir, peat moss, and potting soil can add texture and increase water and nutrient-holding abilities.

Silty soils are composed of very small particles with a high surface-to-volume ratio that makes them chemically active. They hold nutrients in a buffer of charged particles, releasing them as roots draw them out using weak acids and exuded enzymes. This type of soil can be challenging in areas that don’t have warm temperatures or receive consistent rainfall throughout the growing season.

These soils become mud with sufficient water. They hold water so well that heavy rains can result in hypoxic or anoxic conditions, so the roots will be unable to get the oxygen they need. When amending silty soils, growers need to “lighten” the soil so that the root zone will not be in a soggy, overwatered environment. Sand and compost help increase soil porosity.

After growers receive the results of the soil test and analyze it for their specific needs, it’s time to prepare the soil. There are a few decisions to make, such as whether to plant in holes or rows, amend the entire garden, or use raised beds or containers filled with amended native soil or imported planting mix.

The budget should also be taken into consideration:

Amending the hole to plant in is probably the most economical and focused method when trying to grow large plants. The amendments and nutrients are delivered only to areas where they will be used.
Rows are a good way to reduce labor and focus the grower’s energy on care. Power equipment such as rototillers or tractors can be used.
Raised beds are an excellent way to maximize effort and dig less. Imported media or soil can be dumped into a framed-up raised bed if the soil condition is poor. A minimum of one foot (30 cm) deep is recommended for cannabis. This saves the energy of digging. Using raised beds can reduce both water and nutrient requirements.
Amending the entire planting area is beneficial because roots grow unimpeded in every direction and can cross paths with other plants’ root systems. However, this may require the most amount of material to be added, which increases initial costs.
Another option is to use a fabric pot. Fabric pots are available in many sizes. They allow the roots to “air prune,” which helps grow larger plants in smaller containers. Container size depends on how large the grower intends for the plants to grow. Irrigation should also be considered. Smaller containers need to be watered more often.

Seed Drilling

If the planting is to take place on soil that has been recently used for agriculture, the seeds can be drilled directly into the ground, at a depth of three eighths to a half inch (9.5-12 mm). Sometimes the ground is disced before the seeds are drilled.

Once the decision is made and the soil is in condition, it’s time to break ground. This can be done in several ways: plowing with a tractor, rototiller, hole digger, or simply a shovel.

Whether using a shovel, tractor, or rototiller, it is important to break up compacted soil one to two feet (30-60 cm) deep, more if possible or if bigger plants are desired. Some soils need only to be disced.

As the soil is loosened, amendments are added. These might include nutrients such as plant meals or other organic materials that release their nutrients over the season. If the soil has a bit too much clay, compost, used planting media, bark, and coir all help increase porosity.

Sometimes all the native soil in a hole is replaced. On the West Coast, it is not uncommon for growers to dig huge holes one cubic yard (0.75 m³) deep and fill them with a premium potting soil mix. This technique has been proved successful for nurturing giant plants. However, this technique has been replaced for the most part with using raised beds or giant cloth bags, eliminating the chore of digging.

Tilling, or turning the soil over, breaks up compacted soil to make root penetration easier and facilitates mixing in amendments such as compost, worm castings, bat guano, biochar, and fertilizers.

There is some controversy about tilling soil because it chops up earthworms, disturbs valuable microorganism communities, and makes the soil subject to erosion because it has been loosened. (See Sustainability.)

Most of the time soils need loosening to a depth of 10-15 inches (25-38 cm). However, soil with loose texture, sandy soils, and soils high in organic matter may have adequate aeration, porosity, and space for roots and may not have to be tilled at all. This is occasionally the case and is most likely to be found in fallow fields.

Texture

Regardless of the particular composition of the soil, its texture is critically important to healthy plants. Texture refers to a soil’s density, particle size, and ability to hold together— all of which affect the soil’s porosity, or ability to hold or drain water.

Root health, and ultimately plant health, relies mainly on the soil’s drainage ability. If a pool of water develops after rain, the soil is saturated. There are no air pockets, so the roots have restricted access to oxygen; plants fail because pathogens that cause root and stem rot attack roots weakened by the hypoxic conditions. Well-drained soil allows roots to be in contact with both water and air, the ideal condition for healthy growth.

Soils that drain well but can also hold an adequate amount of water are best for cannabis. Loams, silts, and sands drain well and are usually loose enough to encourage healthy root development. Clay is too compacted for roots to penetrate, and when dry, clay soils form hard crusts or clods in which cannabis plants cannot thrive.

There are a few simple tests that should always be done before planting to check both the moisture-holding and drainage characteristics of the soil. For best results, the soil is tested when it is moist but not wet.

A three-foot (1 m) deep hole can be dug so the soil’s profile, that is, the composition of its layers, can be examined. The method described in the box “Determining Soil Texture” is used to get a more accurate sense of the soil’s relative percentages of clay, sand, silt, and humus, but even a visual inspection of the soil can reveal its profile.

Soils that develop under trees or other overhanging plant cover have decaying plant matter at the top that decomposes into a layer of topsoil. The nutrients the plant needs to grow are found at the topsoil layer, though nutrients also leach to the next lower level.

Topsoil, which can be as shallow as an inch (2.5 cm) or several feet (a meter) deep, is usually the darkest of the layers. It is home to an abundance of life, including a variety of worms, bugs, and microorganisms such as yeasts, fungi, anarchia, and protozoa. If it’s easy to dig through the topsoil layer by hand, its texture is right for healthy root growth.

Whether forest or prairie soil, there is a layer of subsoil beneath the nutrient-rich upper layer. The subsoil can be composed of clay, sand, fine mineral particles, and small rocks. Sandy, rocky, and loamy subsoils provide excellent drainage without amending.

Below the subsoil, there may be clay or bedrock; both can create drainage problems, particularly if the area is in a spot with a high water table or where the soil tends to remain wet. If the clay or bedrock is within three feet (1 m) of the soil surface, consider alternatives such as building raised beds or mounds to ensure proper drainage and adequate root penetration.

After a visual inspection of the soil comes the squeeze test. A handful of soil from each layer is squeezed tightly, then released, and then the ball is poked with a finger. If it falls apart easily, the soil is either sandy or loamy. If the soil remains hard and stuck together, or if it feels sticky, there is quite a bit of clay present, and it needs to be amended.

Testing soil for drainage is very simple. The hole that was dug is filled with water. The water is allowed to penetrate the surrounding soil, then the hole is filled again with water. If the moisture drains easily, the soil is sandy. If the water has still not drained through after 24 hours, the soil has very poor drainage.

Determining Soil Texture

Fill two thirds of a quart jar with water and add soil until the water nears the top. Screw on the lid and shake thoroughly. Allow the soil to settle. The heaviest sand particles sink to the bottom and quickly become visible. It will take hours for the silt and clay particles to settle. The fine clay particles are so small that the molecular action of water will keep them in suspension indefinitely.

Types of Soil

Each type of soil has its own unique characteristics that determine how it interacts with plants. Soils can be mainly classified as sands, clays, silts, loams, or mucks. Most soil is a combination of two or more of these. Sand granules, bits of clay, organic matter, and fine silty material can all be found in a random handful of soil.

Sandy Soils

Sand is formed from ground or weathered rocks, including limestone, quartz, granite, and shale. Sand particles have a diameter between 2 and 0.5 mm and will feel gritty when rubbed between one’s fingers. Sand doesn’t form bonds easily, so little water is trapped. Sandy soils drain quickly, and they don’t hold moisture and nutrients very well. Some sandy soils are particularly fertile, however, because they contain significant amounts (up to 2%) of organic matter. This matter also helps the soil hold water.

Sandy soils are rich in potassium (K), magnesium (Mg), and trace elements, but are usually lacking phosphorus (P) and nitrogen (N). Nitrogen, the most soluble of the elements, is quickly leached from sandy soil. Yellow, pale, stunted, or very thin plants growing in sandy soil usually indicate low nitrogen.

Sandy soils are easily prepared for cannabis cultivation. First, clear the ground cover from the soil. Next, treat it with compost, humus, composted manure, chopped plant matter such as small wood chips, mineral powder, and other organic, nitrogen-containing fertilizers. Adding compost, peat moss, coco coir, worm castings, humus, or planting mix increases the water-holding capacity and fertility.

Sandy soil with some organic matter or loam that is supporting healthy plants doesn’t usually need to be turned or tilled because plant roots can penetrate easily. The row is hoed immediately before planting. Nutrients and water-soluble fertilizers are provided throughout the season.

If there are concerns about the ability of the soil to hold water, add more compost, worm castings, humus, and peat moss. There are also water-holding polymers that can be added to soil, but they are not organic.

Sandy soil can be amended with more organic matter. In preparation for planting, adding composting green waste on top of the sand will help leach nutrients and microbes into the sand. When the composting process is complete, the organic matter will improve the soil structure.

Silt

Silts are composed of fine particles of minerals such as quartz or other fine-grained rocks. The diameter of silt particles is between 0.05 and 0.002 mm. Dry silt has the consistency and feel of flour. Silty soils resemble a sort of mucky clay when wet and look like dark sand or brittle clods when dry.

Silts are the result of alluvial flooding, created by the deposits that remain after rivers and lakes flood. Alluvial silty soils are most commonly found in the Midwest, in valleys, and along river plains. The Mississippi Delta, for example, is a very fertile alluvial plain.

Silts drain well, but they also hold moisture well. They are rich in most nutrients unless they were leached out by bad gardening techniques. They are simple to work with when damp and are regarded as some of the most fertile soils for planting cannabis. Gardens using silty soils must irrigate frequently. Usually, silts support very healthy and hearty plants because they contain an excellent supply of nitrogen.

If the soil is clay-loam or compacted, it can be used for making raised beds. First, turn the soil in the whole area. Construct aisles by digging out pathways and adding soil from the aisles to the beds. As the beds are filled, amend the clay-loam by mixing in organic soil amendments and sand. Build the bed to two to three feet (0.6-1 m) tall.

Mucks

Mucks are composed primarily of humus from drained swampland or bogs and are found in areas that get plenty of rain. Though they are often very fertile and normally support dense vegetation, mucks are fairly acidic. They contain little potassium, so this nutrient needs to be added.

Mucks range from very dense to a lighter, sandy soil. Denser mucks need tilling so that healthy roots can develop. Lighter mucks are easily cleared for planting. The dense vegetation that mucks support may be helpful in that, when turned into the soil, the plant material becomes “green” manure.

Soil Tests

The best way to learn about the soil or planting mix is to have a professional laboratory perform a soil test. This is the only way to know exactly how much of each nutrient the soil contains. Using the results, the grower can target the best ways to amend the planting medium.

pH Proper pH is important because outside the range some essential nutrients become unavailable.

Electrical Conductivity A low EC means that there are not a lot of available nutrients. When EC is too high, water, although present, becomes unavailable to the roots.

Nitrate (NO₃) is the form of nitrogen that plants absorb most easily. Manure has a low nitrate rating. Most of its nitrogen is held in organic compounds and is released over time.

Ammonium (NH₄) is absorbed, to a limited extent, directly by the plants. However, it is used by soil organisms and converted to the more soluble and plant-friendly nitrate. It is not desirable because it is toxic to plants in high amounts.

Phosphorus (P) is one of the macronutrients that plants need. Soluble P is very acidic, so it brings down the pH of the water solution.

Potassium (K) is one of the macronutrients that plants need.

Calcium (Ca) is often considered a macronutrient because plants use so much of it. It is usually adequate in garden soils, but some planting mixes don’t contain enough. Coir, pure peat moss, bark, and wood have very little, so it must be added when making a planting mix with them.

There are also results for other plant nutrients including magnesium (Mg), sulfur (S), boron (B), zinc (Zn), copper (Cu), manganese (Mn), iron (Fe), sodium (Na), and chlorine (Cl).

The next two categories, moisture-holding capacity and air-filled porosity, are indications of how well the medium holds each. Media with high water-holding capacity need irrigation infrequently as compared with those that hold less. The roots need oxygen, which is found in air. Media with high air-holding capacity meet roots’ needs. Low air capacity leads to anaerobic conditions.

Cation exchange capacity (CEC) is the ability of the medium to hold nutrients. Too low a level indicates a soil or planting medium that has the capacity to hold few nutrients. It will have to be fertilized frequently for healthy plant growth.

A biological soil test will also help determine ingredients to use for amending.

The presence of a diverse microbial population can be the difference between a good and a great crop. Microbes also deter pathogenic microorganisms that could cause problems or even crop failure.

Drainage

No matter how well the soil is prepared, the groundwater level and the permeability of the lower layers are of utmost importance. Soils in areas with high water tables or underlying clay or hardpan do not drain well. In either case, the garden can be grown in raised beds filled with soil from the site that has been amended with additives and nutrients. If the local soil is too poor, use landscape or planting mix.

Clay Soils

Clays are made of fine crystalline particles formed by chemical reactions between minerals. These particles are so small (smaller than 0.002 mm) that they have no structure when wet, but react more like a very viscous liquid. Sticky and easily molded or shaped when wet, dry clay forms hard clods, normally observed as a grid of square cracks along the ground surface. Clays are rather difficult to work with, mainly because they drain so poorly.

Despite their disadvantages, clay soils are often very fertile. The success of a plant in clay soil depends on how well the soil drains. A reddish-colored clay soil (sometimes referred to as “red dirt”) indicates proper aeration and good drainage. Blue or gray clays mostly have insufficient aeration for growing cannabis; they must be tilled and amended in order to support healthy growth.

Since cannabis roots (particularly the vertical ones) must penetrate the soil, it is necessary to till clay soils thoroughly to loosen them. The addition of sand, biochar, used planting mix, compost, gypsum, manure, and fresh clippings helps loosen and aerate clay soils.

One method of growing in clay soils is to plow rows of mounds and furrows and plant in the mounds so that the water can drain.

Attempting to plant in clay has led many cultivators to containers or raised beds. Hydroponic systems are a water-saving alternative.

In low-lying areas, with vernal pools or stream banks, for example, the soil often retains too much water. This can lead to rot in both roots and stems. Planting mounds or raised rows and furrows helps the soil drain, so the stem stays dry and the root has access to oxygen in the soil.

Prepare workable clay soil in late autumn before frost by tilling it and adding conditioners such as bark, charcoal, compost, grass clippings, gypsum, leaves, manure, paper and cardboard, used planting mix, and wood chips.

Spread ground cover seed such as clover, vetch, or rye over the top of the soil after tilling. This creates green manure that provides soil texture as well as air and water pathways.

If the soil is still sticky, wait until it dries a bit and then use a tiller to break up large clods.

As the composts and green manure raise the organic level in the soil, it becomes less dense. With each passing year the soil becomes easier to work, and the roots will have an easier time penetrating the soil. After several years, it’s likely that the only thing to be done is to use fire to weed or disc the cover crop. No further tilling should be needed.

Loams

Loams are a combination of the soils and humus. They are usually made up mostly of sand and silt with about 20% clay. They are described as sandy silt, silty clay, sandy clay, or organic silty clay. Organic loams are made up of at least 20% organic matter. Loams range from easily worked fertile soils all the way to densely packed sod. Loams with a lot of organic matter produce an excellent cannabis crop with little soil modification.

Loams usually have good drainage, but hold water well. They are packed with nutrients and are excellent garden soils.

Humus & Composts

Decayed organic matter, including plant life, animal manure, and microbes, is referred to as humus or compost. Humus is plant or animal material decomposed to the point it can no longer be broken down any further.

Nutrient contents depend on the original ingredients, but most humus or compost contains bacteria, fungi, insects, worms, and microorganisms that are necessary for a complete conversion of important nutrients. During their life processes, many of these organisms convert insoluble chemicals to a soluble form that plant roots can absorb.

Humus and composts hold water well and are frequently added to condition soils.

Good composts have a rich, earthy smell and a dark brown color.

Humus and compost can be produced naturally as part of the soil’s life process, or they can be “manufactured” at the grow site simply by gathering vegetation and piling it up. It takes between one and three months to cure, depending on what type of matter is being used. Decomposition of the humus or compost can be achieved more quickly by chopping up the ingredients, turning the pile, and adding substances high in nitrogen. The art of making compost is not to be taken lightly; it is important to learn about proper composting before adding any material to the garden.

Dr. Elaine Ingham and Jeff Lowenfels have written informative books on DIY compost. A proper compost’s pH should be neutral to slightly alkaline and should smell earthy, not rancid or putrid. If compost has a putrid smell, it quite possibly has gone “anaerobic” and could cause more harm than good.

If it isn’t possible to make enough compost or there isn’t time to make it, there are many resources. When sourcing compost, ask for a copy of the lab report. Check the color, and of course the smell. It should be the color of dark chocolate (black compost has most likely gotten too hot and most of the microbes have been cooked off).

pH

The pH level is a measure of how alkaline or acidic the soil is. The pH scale runs from 0 to 14, with 7 considered neutral: a pH level below 7 is acid, above 7 is alkaline. The pH determines the solubility of nutrients and affects the plant’s regulation of its metabolism and nutrient uptake. Slightly acidic soils with a pH range from 5.8 to 6.5 are regarded as cannabis friendly.

The pH of the soil interacts with the soil’s composition to affect nutrient solubility. Soils with a high percentage of organic matter contain nutrients that are soluble between pH levels of 5.0 and 6.5. Phosphorus, manganese, and boron are less soluble at pH values above 6.5.

Dry western states have soil that usually range from slightly acidic to highly alkaline. Nutrients tend to be quite soluble in these types of soils, providing that the pH range is adjusted to between 5.8 and 6.5. Use a simple pH meter or a test kit, available at most gardening stores, to accurately test the soil pH. If it is alkaline or near neutral, adjust the soil accordingly before planting.

Adjusting the pH

Always test the pH of any soil before using. The pH of the garden’s soil may be different from other soils in the area, so don’t simply trust what a neighbor is doing. Developers and homeowners often truck in new soil if the native soil is poor in texture, lacking in nutrients, or both.

Keep in mind that soils vary in the amount of material needed to adjust the pH. Sandy soils require less material to change pH as compared to loam. Clays require the most because of their density and the electrical charge of the soil particles. Whether the soil is excessively alkaline or acidic, there are a number of materials that can be added to adjust the pH.

Something to keep in mind when considering soil pH is that in a 100% organic program, beneficial microbes naturally adjust the pH and the conditions in the rhizome to what the plant needs. Using synthetic salt-based fertilizers will inhibit the natural pH adjuster’s ability to help with that.

Adjusting Acid Soils

Limestone, also known as calcium carbonate (CaCO₃), is a good way to treat acidic soils. Quarried and powdered limestone contains large amounts of trace elements. It comes in three forms: ground limestone, quicklime, and hydrated and liquid lime (which is the fastest acting).

Dolomitic limestone is a limestone variety high in magnesium as well as calcium. It is a good choice for adjusting the acidic, magnesium-deficient soils often found in the northeast United States.

Seashells and eggshells are composed mostly of calcium, and both raise soil pH. Grind them into a fine powder using a blender. They affect soil pH gradually. By contrast, most wood ashes are alkaline and extremely soluble, so they affect soil pH very quickly.

All commercial lime products list their calcium carbonate equivalent, which is a measure of their neutralizing power, on the bag.

To determine how much lime to use, divide the total amount of limestone required by the pH test by the calcium carbonate equivalent. For example, a field may require 50 pounds (22 kg) of limestone, and the calcium carbonate limestone may have an equivalent of 1.78. If 50 pounds (22 kg) are divided by 1.78, the resulting figure, about 29 pounds (13 kg), is the amount required.

Grade, or particle size, of the powder is also listed on the package. The categories used to define the fineness of the powder are superfine, pulverized, agricultural grade, and fine meal. Finer grades result in faster soil-adjusting action but are more prone to washing away.

Add lime to a depth of one inch (2.5 cm) four to five months ahead of planting so it has time to adjust the soil. It can be watered into the soil. Water well afterward or spread it before a rain, providing that the soil is moist enough to absorb the water and lime, and it does not simply run off.

Adjust the medium in planting holes and raised beds. Soil can be adjusted with lime during or after planting, as long as the lime doesn’t come into direct contact with the plants. Hydrated or liquid lime can be added to soil with a hoseend sprayer. Lime can also be mixed in irrigation water; however, it can clog up drip lines, hoses, and pumps. Alternately a liquid hydroponic adjuster such as pH Up (usually a potassium salt) can be used to raise irrigation water pH; it has an immediate effect because it’s water-soluble.

The best way to adjust pH in acidic soil is to build the microbial population as well as add calcium carbonate. The best way to build the microbial population is to amend with high-quality compost, worm castings, and humus. A good compost tea recipe applied regularly is also recommended.

Adjusting Alkaline Soils

Alkaline soils have a pH higher than 7, outside the range for optimum cannabis growing. Amending alkaline soil with elemental sulfur, gypsum, and compost is a great way to get the native soil’s pH where it needs to be. Sulfur lowers the pH even more significantly when used in conjunction with an organic microbial active compost. The bacteria oxidize the sulfur and convert it to an acid that can be absorbed by plants.

Gypsum

Gypsum is a natural mineral composed of calcium sulfate (CaSO₄). It’s useful as a slow-release form of calcium, or sulfur that doesn’t affect the soil pH too much. It should not be added to soils with a pH below 5.5 because it interacts with aluminum (Al) in those acidic soils, making the Al soluble and poisonous to plants. Gypsum is available at garden centers under various local and store brands.

Gypsum is used to break up clay soils. It is a naturally occurring mineral that, like lime, is high in calcium and can be used to address deficiencies. It is important to note that gypsum and lime are different. Gypsum is CaSO₄, whereas lime comes in three forms: CaCO₃, CaO, and Ca(OH)₂. Both are sources of calcium, but only gypsum also provides sulfur.

Gypsum improves soil quality in a few ways:

It is fairly water soluble, so it improves soil fertility. Both calcium and sulfur penetrate evenly as the gypsum dissolves and seeps deep into the soil.
It increases a soil’s ability to store water. A soil’s ability to hold on to water helps prevent erosion, as the soil takes longer to reach saturation before runoff occurs. Adding gypsum to clay-dominant soil helps break up the soil particles, increasing the porosity and the amount of space between them. Additional water is held in these spaces.
Increased porosity also increases air penetration. Between irrigation events and as the soil loses moisture, air penetrates deeper into the soil, providing better oxygen delivery to the roots, which helps improve overall plant health.

Applying gypsum to the soil can be done through several methods; however, most farmers grind it to a fine powder so that it can dissolve in irrigation solution. It can also be applied as a top dressing before planting or after harvest and can be tilled under or just left alone to soak down as it rains. Although gypsum is fairly soluble, the sulfur delivered from a single application is available for several plantings.

Compost tea or compost extract applications also help speed up the process of lowering the pH in multiple ways:

Beneficial microbes in conjunction with the plant’s roots can adjust the pH to the plant’s preferred level.
Microbes interact with sulfur and gypsum to convert them to soluble nutrients.
Fish hydrolysate in the compost tea will help lower the pH as well.

Both iron sulfate (FeSO₄) and magnesium sulfate, commonly known as epsom salts (MgSO₄), act quickly to adjust soils and planting mixes.

Some acidic plant materials that can gradually lower pH include cottonseed meal, which is high in nitrogen; coffee grounds; pine needles; and pineapple and citrus rinds.

Use pH down when alkaline soils are being fertigated. Usually diluted phosphoric acid, pH down is used to adjust the nutrient-water solution to a pH in the low 5s. This will counteract the roots’ alkaline environment and create an acidic water-nutrient solution available to the roots.

Adjusting Alkali Soils

Alkali soils have a high sodium content (from sodium carbonate or sodium bicarbonate) and frequently a pH above 8.5. They are usually hard-packed and crusty, sometimes with white powdery salts on the surface. They don’t absorb water easily.

Alkali soils such as those found in western Colorado, parts of the US Southwest, Spain, and the Caspian Sea area of Central Europe can be frustrating, energy draining, and time-consuming to work with. Farmers usually prepare alkali soils for cultivation by leaching them of the toxic accumulation of salts.

This is achieved by tilling the soil to a depth of 30 inches (75 cm) and then flood-irrigating using 6-12 inches (15-30 cm) of water at least once or twice to flush the salt deep into the soil, out of contact with the roots. Next, the soil is tested to determine the amount of amendment needed. Phosphorus and sulpher fertilizers are acidic and quickly bring the pH of these soils down to a more suitable pH.

Another way to adjust alkali soils is to add a thick mulch layer, which interacts with the soil during the winter. The mulch layer should be a minimum of nine inches (23 cm) thick, or about 130 pounds (60 kg) per 100 square feet (10 m²). Thicker mulch layers neutralize more salts faster. Gypsum is also very effective in these applications, but with all things considered, growing in containers with imported planting soil may be a better option than investing the time to adjust the soil.

Outdoor Water-Saving Techniques

This section is devoted to customizing raised beds, planting holes, troughs, and hydroponic systems.

Plant size and yield are determined to a great extent by the development of the root system. Even plants that are receiving copious quantities of water and nutrients require enough room for their roots to grow and spread out in order to reach their full potential.

When limited by law to growing just 6 or 10 plants, the grower sometimes aims to increase the size of each plant. Creating a large area of prime soil helps the roots penetrate easily and absorb nutrients. Figure that the diameter of the plant is usually the same size to about one-third larger than the diameter of the root ball. A plant with a 10 foot (3 m) diameter canopy will have a root spread of at least a 7 foot (2.25 m) diameter.

With hydroponic systems the canopy diameter often exceeds the diameter of root colonization by 50%.

In dry areas that require irrigation, providing water to plants this size can be an arduous task that requires a lot of water, so conservation becomes a significant factor. Water that reaches the garden is used in one of four ways:

It is used by the plant. Plants use water for metabolic purposes in much the same way that animals do, as well as a raw ingredient in photosynthesis, and plants transpire water vapor from the leaves in order to regulate temperature.
It evaporates from the earth surface. Heat and sunlight speed the evaporation of water from the top surface levels.
It drains from the root level deeper into the earth and becomes unavailable to the roots.
It is delivered to the garden, but to areas where there are no roots. It eventually evaporates or sinks, unused.

Water-Saving Strategies

Organic matter holds more water than the mineral portion of the soil. Adding compost, peat moss, coir, biochar, or other organic matter increases the soil’s capacity to retain water. This is especially helpful in fast-draining sandy soils.
Organic mulch placed on top of the soil layer lowers the evaporation rate: the barrier between the moist soil and heat and light eliminates most evaporation. The topsoil stays moist longer.
White plastic film placed over the soil keeps it cool and lowers the rate of evaporation.
Drip irrigation, when installed properly, is a very efficient way to get water to the plants without wasting it.
Raised beds and containers are water-saving alternatives.

Cannabis grows very well in containers, both indoors and out, but requires more care and attention than plants growing in native soil. Rather than working with the natural environment, or at least a resemblance of it, gardeners can control the space the roots have when plants are growing in containers, as well as the water conditions and nutrients. Container size plays a role in determining the growth, final size, and yield of the plant as well as how much and how often it needs to watered.

Premade Soil Mixes

There are two categories of soil mixes to keep in mind: amended and inert.

Amended soil has macro- and micronutrients, minerals, and potentially microorganisms. There are many high-quality amended soil mixes on the market to choose from.

Inert mixes are basic blends that have little or no nutrient value. They are designed for growers who want to control the nutrient program and to have no variables.

Organic certification is another consideration. There are many reasons to choose organic when picking a brand of soil.

Organic products are safer for people, plants, animals, rivers, and streams.
Organically grown cannabis is more desired by users for health reasons as well as those who embrace organic lifestyles.
Organic soil companies tend to choose higher-quality ingredients, which leads to a better-quality finished product.

Enriched planting mixes usually contain compost. This component makes or breaks the quality of the potting mix. Compost varies greatly in quality, and poor-quality compost can kill plants.

Indoors

Indoors, the grower determines what size containers to use based on specific needs. Common sizes for indoor and greenhouse growing are 3-5 gallons (11-19 l) per square foot (0.09 sq. m) of canopy space the mature plant is to occupy. Sixteen of these containers fit in a four by four foot (1.2 by 1.2 m) space. Some gardeners opt to use as few as four 10-20 gallon (38-75 l) containers in the space.

In large commercial cultivation spaces with rolling benches, it is not out of the ordinary for growers to use 2 gallon (7.5 l) containers or even four inch (10 cm) rockwool cubes. Using this technique, the plants are also watered multiple times a day, controlled by a timer or based on sensors to make sure those small root zones don’t dry out.

These 3-5 gallon (11-19 l) pots provide the roots with enough space to spread out and down throughout the container without becoming “rootbound,” although the roots provide for a canopy of the same width. Sometimes the plant canopy spreads up to a third larger than the diameter of the roots. The plants stay in the container for only a few months because they are forced to flower as soon as they grow into the canopy, a matter of weeks. The plant may live 10-12 weeks after transplanting before being harvested.

Sea of Green (SOG) System

If plant limits are not relevant, plants can be grown close together so that each plant grows a single stem and is then forced to flower. This system, called “sea of green” (SOG), requires very little time spent in vegetative growth. One gallon (~4 l) pots and six inch (15 cm) rockwool cubes are excellent sizes for sea of green cultivation. The plants may require more than one watering/feeding a day. The plants quickly fill the space, increasing harvest frequency.

The advantage of SOG systems is that the plants spend little time or energy in vegetative growth. Rather than a few plants gradually filling out the canopy, in a large group each plant grows a little bit at the same time, so the canopy is quickly filled and the plants can be forced to flower.

Instead of spending months in vegetative growth, with all the energy and labor that goes with it, the plants are forced to flower with only one stem. This decreases time from transplant to flower from 30-45 to at most 10 days.

Another method is to use planting beds rather than containers, which are like raised beds, but indoors. The plants grow directly in the planting mix without the restrictions of containers. Each root system has more total space to explore and grow, allowing it to gather more water and nutrients. Once it’s set up, there is no need to pot or unpot the planting mix. Just freshen it up between crops, and it’s ready to go again. Make sure there is adequate drainage. Drilling holes in the bottom of the “bed” is important to make sure the soil drains well so it doesn’t get too wet and become anaerobic.

Container Facts

Fabric pots are an excellent choice and are available in many shapes and sizes.
Rectangular containers hold more than circular containers of the same size.
Containers must drain well. Avoid containers with holes on the bottom in favor of containers with holes on the side, especially if the containers are to be placed on a solid surface. Bottom holes are easily blocked.
Unconventional containers are fine to use. Adapt square and rectangular-shaped plastic trays and containers by creating drainage holes.
Plastic grow bags come in all sizes. They work well indoors and out, and are lightweight and inexpensive. Also, gusseted plastic reusable shopping bags with a rectangular bottom are inexpensive (often free). They’ll last at least one season. Drainage holes should be cut on the sides, near the bottom.
Bags and containers are much easier to move and make gardening much less fatiguing when they are equipped with handles.
Large containers can be moved with a container handler, hand truck, or heavy-duty wagon.
There are many potting soil manufacturers who make “perforated” bags, which are meant to be used for growing.
Large plants require large containers. The desired size of the plant should determine the size of container to be used.
When containers are on wheels, they are much easier to move around to reorganize a space.

Tips for Outdoor Containers

Fabric pots are excellent for areas that experience warm temperatures.

Boxes built from wood such as cedar or redwood will last a very long time, but even when built from pine or fir, they will last for many years.

Black plastic containers may be a good choice indoors, but in direct sun the roots can get cooked. White plastic containers are an option for outdoor gardens. A solution for black containers is to wrap them in white plastic.

When choosing a potting mix, growers have a few things to consider: quality ingredients, porosity (the soil’s ability to retain water and dry out), and texture (how light or heavy as well as how much air the soil holds).

The mixture should drain well so that oxygen, required by the roots, can fill the empty spaces.

The quality of ingredients greatly varies from company to company. Choose a brand with a good reputation. Soil is the foundation of the crop, so choose wisely.

The porosity is a key component in the plant’s environment. Some growers like a light mix for young plants and switch to a heavier blend when the plants are larger and more demanding. However, small containers dry out faster than larger containers, so it is important to have a mix that has the ability to hold water but can provide sufficient porosity so that air spaces, containing oxygen, are created.

A moist potting mix with a good texture should form a clump when squeezed into a fist; with a slight poke, the clod should break apart. If it sticks together, it should be amended with ingredients that loosen it up.

These include:

coir
compost
biochar
mini-sized horticultural clay pellets

High-quality commercial potting mixes almost always have good texture, so they provide plants with high amounts of water and air.

There is an incentive for both home gardeners and commercial growers to use ready-made planting mix. Whether it is a totally non-nutritive or a fortified mix enriched with organic nutrients, the manufacturer has adjusted its pH to ensure healthy growth.

Some mixes perform better than others, so it is worthwhile to test different brands in side-by-side experiments. Although the cost of enriched prepared mixes varies widely, the most important factor is not the cost of the mix but the yield. Although the initial cost might be higher, if the difference in yield or quality is significant, the difference in the cost of the soil will seem insignificant.

Most of the ingredients of planting mixes—bark, coir, and peat moss—are all carbon based and chemically active, so they act as a buffer for fertilizers and nutrients. They catch and hold excess nutrients dissolved in the water-nutrient solution and release them when the nutrient water solution becomes less concentrated. For this reason, they are more forgiving than some hydroponic methods, where there is little or no interaction between the medium and the water.

Another factor is the carbon-based ingredients that anchor the microbial complex found in the rhizosphere, the area surrounding the root and influenced by its exudates.

These organisms solubilize minerals into dissolved salts that are available to plant roots, which sip, but don’t eat, solid food. They mediate the peaks and valleys of nutrient availability, acting as buffers.

Perlite

Perlite is a useful ingredient that has been used for many years in horticulture. There is no questioning that its use has made life easier for horticulturalists, commercial growers, and home growers alike.

The problem is that it is extremely lightweight and creeps up to the soil surface, where the small pieces can be carried away with stiff winds. Large pieces also creep up to the top of the soil. Because perlite retains air pockets when it is irrigated, the granules float. These properties make planting mixes containing perlite less desirable when they are recycled.

Perlite is a mined volcanic glass-like mineral, similar to obsidian, that melts at 1560-1740°F (850-950°C). Water held in the rock turns to steam and causes the liquid to foam. It expands up to 20 times its original volume and has a structure with many tiny closed cells or bubbles. The resulting snow-white particles are sterile and inert, and have a neutral pH. The hard pieces remain stable for years. They wick water through capillary action.

The particle surface is covered with tiny craters that create an extremely large surface area that can hold the water-nutrient solution. Passages in the structure also hold pockets of air and promote drainage.

Perlite comes in various sizes, or grades, that range from very fine to pea-sized gravel. The amount of water adsorbed on the surface of perlite is a function of particle size. Coarse perlite particles absorb less water than the finer grades. Generally the smaller sizes are used in smaller containers. Larger sizes are used in larger containers and maintain porous spaces for air. However, various sizes are sometimes included in soil mixes to customize water-holding capacity.

Perlite’s bright white color protects seeds by keeping them cool and moist even when placed under bright sunlight. It reflects light back to plant foliage, which further enhances growth.

Propagation and Seed Cultivation: Fine-grade perlite makes a good medium for seed starting and cloning because it maintains a uniform moisture level. Excess water drains, so there is no waterlogging. Since the perlite doesn’t form a chemical bond with the water, water tension doesn’t increase as long as there is moisture present. This helps young plants overcome environmental stresses.

Perlite clings to roots and root hairs so they can form a root ball. This reduces transplant shock.

Planting Mixes: Perlite is often used as an ingredient in planting mixes. Different sizes are used to adjust water-holding capacity and provide aeration. It keeps the mix lightweight and provides structure, since it does not deteriorate.

Hydroponics: Perlite’s capillary action and fast drainage are ideal qualities for hydroponic media. It makes water management self-regulating. It is inert, so it doesn’t interfere with fertilizer and nutrient programs.

Perlite can be used in a single-ingredient planting medium in many hydroponic systems, including wick, reservoir, drip, and constant flow. Its ability to retain water and provide air spaces, along with its neutral pH and nonreactive qualities, make it an excellent medium. However, because the lightweight granules shift position in ebb and flow systems, plants must be supported using a ScrOG net, staking, or another technique.

Perlite is often used as an ingredient in hydroponic and soil-less media with vermiculite, peat moss, or bark. Part of its usefulness is that it retains its structure, in contrast to peat moss, which deteriorates as it reacts with soil and degrades into compost and vermiculite that gradually loses its structural integrity and collapses.

Perlite can be reused as long as it is rinsed and sterilized and old plant material is removed. It has been used to remediate clay soils, but this is expensive and doesn’t work as well as adding organic material.

Peat Moss

Peat moss is an ingredient with an advisory. Peat bogs are nature’s natural repository of plant-collected carbon. The world will be better when peat bog mining ends. For this reason, this wetlands-mined ingredient is listed with an advisory. However, the authors would be remiss in not covering it because it is still widely accepted as a media ingredient by the horticultural industry.

Peat moss consists of layers of sphagnum moss that died and were naturally buried in acidic, anaerobic water.

Peat moss is probably the most popular container planting material used in North America. It is a key ingredient in most mixes because it holds 15-20 times its weight in water, has excellent buffering ability, and is inexpensive. It comes in several sizes, or grades, ranging from very fine, which holds more water because the spaces between the particles are smaller, to larger pieces that provide more airspace.

Peat moss is very acidic and must be treated with lime to raise its pH. Most mixes have already adjusted the pH, but the grower should also give it a pH test. Although it has a high buffering capacity, it has no nutrients of its own, so an unenriched mix must be fertilized from the start.

Peat moss has a chemical relationship with water, and as it loses moisture, it holds on to the remaining water more tightly, increasing water tension and making it more difficult for the roots to absorb water and nutrients. For this reason, peat-moss-based media should never be allowed to dry out. Although the medium may feel moist, this water may be unavailable to the roots.

Peat moss is composed of dead plant matter, so it contains carbon. Carbon is used by microorganisms in the soil to build tissue, which degrades the peat moss over time. At the same time, it shrinks as it loses its structure. For example, home gardeners might notice that the soil in a houseplant container shrank over a few months; this happens as the microorganisms use the carbons in the soil. This presents a problem only if the plant is in the mix long-term. The compacted mix doesn’t provide enough air to the roots, the roots don’t have as much space to spread out, and the plant has less support to hold itself upright. Over a few months this shrinkage does not usually have a noticeable adverse effect. However, it becomes less suitable for reuse without adding structural ingredients to the mix.

Coir

Coir, or coco peat, is made from the soft fibers and pith that protect the inner kernel of the coconut. Horticultural coir is made from ripe coconuts. As the fruit ripens, the fibers, initially composed mostly of cellulose, increase their content of lignin, which is also found in wood. Lignin resists decomposition better than both bark and peat moss, so it can be used and reused far longer than bark or peat.

Coir holds between five and eight times its own weight in water. The air porosity, the amount of air the material holds, depends on the size of the particles and fibers. The finer the fiber, the more water it holds in relation to air. The larger the particle sizes, the lower the initial water retention.

Quality coir typically also has a medium cation exchange capacity; the fibers not only absorb water but also chemically hold nutrients and buffer nutrient swings. This depends greatly on how well the coir was rinsed and buffered prior to use. Buffering coco is a process used to replace the potassium and/or sodium ions that are bound to the coir particles, with calcium and/or magnesium ions. Soaking with a calcium and/or magnesium solution can help balance the ratio of cations held in the medium and avoid nutrient deficiencies when growing in coco coir.

Another advantage coir holds over peat moss and bark is that it has a higher pH and is in the acceptable range for growing media of 5.8-6.4. It is also a source of potassium as well as iron, manganese, zinc, and copper, which it releases gradually. The finer particles, the pith, can be thought of as organic potassium sources with a sponge-like molecular structure. It is combined with short fibers that act as an anticompacting texturizer.

There is anecdotal evidence that coir possesses fungicidal qualities, which may stem from the lignin or from its surface structure.

There are many grades of coir based on the particle size and which type of tissue is used. Long fibers have granular material, called pith, attached before processing. Sometimes the two materials are left together, but usually they are separated into fiber and pith.

Each coir processor has its own recipe for a particular combination of pith and fiber. The more pith that it contains, the better its water retention, but the less air it holds.

Coir chips are pieces of coir sliced into cubes. They range from 0.25-0.75 inches (6-20 mm). They are used in place of bark in some soil recipes, as a planting medium for orchids and as a hydroponic planting medium.

Bark

Bark is a filler ingredient and can cause issues if used in too high of a concentration. Chopped or ground bark is often used as an ingredient in planting mixes. Bark’s qualities, such as its water-holding capacity and pH level, differ by tree variety. The grade of bark and the size of the pieces affect water-holding capacity. The smaller the pieces, the more water they hold and the less airspace between them. Bark contains carbon, so it is subject to microbial disintegration. However, this occurs at a slower rate than with peat moss. It also buffers the water nutrient mix, absorbing some excess nutrients and releasing them when the solution becomes less concentrated.

If planning to grow a plant that will be kept for five months or more, select a bark or coir-based soil rather than one made from peat moss.

Bark is not usually used in hydroponics except in fertigation systems such as capillary mats.

Worm Castings

Worm castings are often an ingredient in a high-quality amended potting soil. Worm castings are the excrement of earthworms. Worm castings are packed with macronutrients, humic acid, beneficial microbes, and micronutrients.

Worm castings also improve the structure of the soil. Castings hold water well, and the roots weave around the little balls, which are filled with soluble nutrients and microbes.

Mycorrhizae

Mycorrhizae are fungi that live in soil in various forms and share a symbiotic relationship with plants. They are found mostly in the rhizosphere, the area surrounding the roots where plants absorb water and draw nutrients.

They benefit plants in a number of ways;

Increase nutrient uptake
Exchange of fungal hormones and other manufactures to plants
Protection from pathogens and root predators
Better water relations, including reduced drought stress This results in increased root vigor and growth with more resistance to disease organisms and drought. Bud quality and yield also increase.

Types of Mycorrhizae

Mycorrhizae are divided into two groups, endomycorrhizae and ectomycorrhizae. The latter is associated with forest trees in temperate climate zones and probably has no effect on cannabis growth.

There are several groups of endomycorrhizae, but cannabis growers are interested primarily in the arbuscular mycorrhizae (AM fungi, named for their “tree-like” structure). Their hyphae, the center of new growth in AM fungi, are root-like or branch-like structures. Hyphae grow through the root cell wall and position themselves between the cell wall and the cell membrane. The string-like structures, the arbuscles, colonize new space through growth. As the hyphae mature, vesicles containing stored lipids saved for leaner times develop along their length.

Once it has grown into the cell, it has parts both inside and outside the roots. Outside it grows a fine network of filaments that serve as an extension of the root hairs. They use electrochemical reactions to dissolve nutrients, especially phosphorus (P), but also the micronutrients, and bring them into the cells. Nutrients get delivered to plants inside the roots and absorb sugars and other plant products released by the roots.

The AM fungi produce and release abundant amounts of the glycoprotein glomalin.

A glycoprotein is a molecule that combines protein and sugar, which gives it many unique properties. Glomalin is almost 40% carbon, and with that much carbon it can host a lot of microorganisms and supply them with one of their needed sources of food.

Glomalin permeates all the ingredients of soil and organic matter (sand, silt, and clay) and binds them together so that they form little clumps of soil granules called aggregates. This adds structure to the soil and is the first step in nature’s erosion control program that keeps stored soil carbon from escaping.

Mycorrhizae increase absorption in two ways: physical and chemical. Mycelia are smaller in diameter than even the smallest root, so they examine the soil more closely, providing a larger surface area for absorption. Fungi use different methods than plant roots to make soil nutrients available. Their work is most beneficial in nutrient-poor soils.

AM fungi protect plants from microbial soil-borne pathogens by forming a protective shield around the roots, which keeps them healthier. They also produce warning chemicals that stimulate the plant to prepare for an attack, possibly with protective chemicals, making the roots more resistant to disease organisms and drought because of improved water and mineral uptake.

Plants grown in sterile soils and growth media don’t yield as much as compared with enriched planting media and inoculated soils. Add an AM fungi mix to new soils and planting media. AM fungi take several weeks to colonize a container holding a large plant. However, if the plants are inoculated when they are small, the microlife will grow with the roots and fill the rhizosphere with organisms that ward off pathogens.

AM fungi and other beneficial organisms may be present in soil or planting mixes that have been used to grow plants for a crop, especially if it was inoculated and the organisms had time to colonize the roots.

That is why uninfected, inoculated planting mix usually yields a larger harvest on the second and third crops. To change the planting mix, mix some of the old mix into the new medium to inoculate it with the microbes.

In soil and planting mixes, most of the phosphate is bound in water-insoluble minerals. The soil water contains very low concentrations, and roots often have a hard time obtaining enough of it. The root hairs and associated mycorrhizae have an active transport system to supply the canopy with orthophosphate (H₃PO₄). Most plants enhance their nutrient uptake capacity using AM fungi to extend the surface area of the roots by proxy. This increases the plant’s ability to transport nutrients and to obtain phosphate.

Trichoderma

Trichoderma fungi are found in nearly all soils. Some species live freely, while others colonize the roots. They are variable because cells often contain more than one nucleus and there is a lot of gene mixing, resulting in many unique combinations.

The fungus grows long filaments, or arms, in the search for food. Trichoderma attack, parasitize, and otherwise gain nutrition from other fungi. They use numerous mechanisms to attack other fungi and to enhance plant and root growth. Different strains of Trichoderma control almost every pathogenic fungus for which control has been sought. Each Trichoderma strain has its specialty and controls some pathogens better than others. A particular species may be ineffective against some fungi.

Some species such as T. harzianum are used in bioprotectants such as RootShield. Mixed species are often included in endo/ectomycorrhizal mixes.

There is some controversy on whether mycorrhizae and Trichoderma can cohabitate. Many growers only inoculate with Trichoderma if a root disease or pathogen is threatening the health of the plant. After the Trichoderma inoculation, they reapply mycorrhizae.

The mycorrhizae will not harm the Trichoderma’s ability to eradicate root-borne pathogens. Trichoderma are known to be a stronger species of fungi and kill most microorganisms. Many organic growers will also apply compost tea or extract after using the Trichoderma to help the microbial population get stronger.

Beneficial Bacteria

There are many theories about beneficial bacteria in soil. One thing is certain: there is a shortage of bacteria in most native soil as well as heavily farmed agricultural land. Inoculating a beneficial bacteria blend is a good way to water the garden and introduce new species of bacteria to help with plant health. A nitrogen-fixing bacteria helps make nitrogen more available, and a phosphorus-solubilizing bacteria helps break down phosphorus and make it more soluble to plants.

VOLUME OF SOIL EXPLORED BY A 1 CM LONG ROOT WITH AND WITHOUT VAM MYCORRHIZAE

Beneficial Microorganisms

Inoculating growing media or soil with beneficial microorganisms, sometimes referred to as plant growth-promoting bacteria (PGPB), benefits the productivity of plants through a number of methods (Souza et al. 2015):

Increasing efficiencies through reducing production costs and environmental pollution
Tolerance toward abiotic stress through modulation of ACC deaminase
Soil microorganisms that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase promote plant growth by sequestering and cleaving plant-produced ACC, and thereby lowering the level of ethylene in the plant. Decreased ethylene levels allow the plant to be more resistant to a wide variety of environmental stresses (Bernard et al. 2005).
Microbial protection against disease and pests
Inducing a systemic acquired response in the plants so they are better equipped to protect themselves against pathogens (e.g., beneficial siderophore production, which induces the plant’s natural defenses)
Predation or hyperparasitism, aka beneficial microbes preying on pathogenic microbes
Competitive exclusion or antagonism, which is best described as the beneficial microorganisms’ ability to outcompete pathogens for nutrients and/or producing compounds that either kill pathogens or make them less viable in the environment

Rhizospheric competency, or the physical blocking of access to the roots by the beneficial microbes
Increase of access to nutrients and increase of plant productivity
Microbial biological nitrogen fixation (BNF)
Microbial Indole-3-acetic acid (IAA) production
Increase of phosphate solubility through microbial activity in the root zone

Examples of Commercially Available Beneficial Microorganisms & Modes of Action Examples of organisms that have been shown to be effective in field studies (Gaskin et al. 2013).

Regenerative Growing

By Jeff Lowenfels

Given the state of the environment, it makes sense to grow cannabis in a regenerative manner, one that aids the environment. Most important, regenerative techniques build up farmed soils instead of depleting them.

Indoors and outdoors, there are two mainstays to rebuilding soils: the addition of organic matter and establishing and maintaining soil biodiversity.

These strategies can be implemented in a number of ways. Primary among them is the use of living soils. Conventional farming practices such as tilling and the use of synthetic fertilizers and pesticides results in soils with reduced microlife and diversity. Living soils contain thriving microlife communities composed of diverse populations of beneficial microbes. In sum, living soils are regenerative. Because they are alive and thriving, they add to the system while supporting plants.

By its very nature, farming anything can be detrimental to the environment. At the very least, cannabis growers violate the Law of Return: plant material that falls to the soil should be left so it decays, adding back to the soil.

Growing cannabis regeneratively requires adding life and organic matter to the soil as it is farmed. This is accomplished by sheet composting and adding composts, compost teas, and living and static mulches. They add organic matter and microbes to the soil in quantities that replace or improve what is removed at harvest.

Sheet Composting

Sheet composting, or “composting in place,” is a method that mimics the natural layers of decay found in soil from the forest floor. Sheet composting is spreading a layer of compost directly over the soil with a top layer of mulch. As the compost breaks down, it releases usable carbon dioxide to the plants.

Living Soil

Many cannabis growers, indoors and outdoors, have discovered the benefits of growing in soils that are full of beneficial microbes. These microbes support a healthy and functioning soil food web. They sequester carbon in their bodies, make nutrients available when they are consumed and excreted, and build soil structure by producing glues and threads and more.

Simply put, compared to hydroponics, living soils substitute compost for coco coir and other substrates, and microbes for chemicals. Living soils allow a grower to produce cannabis as nature does, naturally, organically, and with odors and tastes that are universally considered better than chemically grown products.

Best of all, using living soils is easier than maintaining a hydroponics system. A properly made living soil only needs the addition of water during the life of the crop. The balance of microbes and the metabolites they produce helps keep pathogens in check while feeding the plants.

As the name suggests, there are two obvious aspects to living soils: the soil part, of course, and then the microbial component. Each brings specific attributes to the mix.

Soil

Soil is very different from hydroponic substrates in lots of ways. Properly made, it contains all the nutrients plants need. All that is required is to add water as the plants grow.

The reason is clay and organic particles in soils hold negative electric charges. These negative particles are known as ions, and they attract and hold positively charged plant nutrients known as cations.

These positively charged cations include potassium (K+), calcium (Ca₂+), magnesium (Mg₂+), ammonium (NH₄+), sodium (Na+), hydrogen (H+), aluminum (Al₃+), iron (Fe₂+ or Fe₃+), manganese (Mn₂+), copper (Cu₂+), and zinc (Zn₂+). All of these are essential plant nutrients. They move from the soil to plants in a process known as cation exchange. Cation exchange potential can be measured and is known as cation exchange capacity. The higher the number, the better the soil’s ability to hold and transfer needed nutrients. (See Water.)

Soil Food Web

The second part of living soils is microbes, specifically bacteria and fungi that form the base of the soil food web. This is a complex population of diverse organisms that live in the soil and interact with each other.

Plants grown in living soil produce substances called exudates that are released out of roots to attract these microbes. Root exudates are full of carbon, which is needed by the bacteria and fungi. The fungi and bacteria, in turn, attract their predators, mostly nematodes and protozoa (think paramecium and amoebas).

Nematodes and protozoa graze on the bacteria and fungi attracted to the roots by the exudates, which include sugars, carbon, and other phyto products. The bacteria and fungi use these exudates as food because they cannot produce it themselves. Only chlorophyll-containing organisms can fix carbon using photosynthesis.

What is most important is that wastes produced as a result of this microbial grazing contain nitrogen in electrically charged forms that can move into plants. This is one way microbes help feed plants.

The second way is via specialized mycorrhizal fungi that enter roots and form a symbiotic relationship with the plant. Once these fungal-plant mycorrhizae are established, the fungi feed phosphorus, nitrogen, zinc, copper, and even water to their plant partners. In return, the fungi receive plant exudates.

The microbial diversity attracted by exudates results in a balance of populations, including pathogens, which keep each other in check.

Bacteria attracted to exudates produce a slime as a protective covering, which also glues soil particles together. Then they are literally woven together by fungal hyphae. What results is an aggregate of soil particles replete with pores in which microbes hide from predators and nutrients accumulate and are reservoirs for water and air.

Finally, the microbes attracted to the root system by the exudates in turn attract other, bigger life forms looking for a meal. These keep populations in balance while they travel through the soil and create more structure and excrete substances that are useful to plants.

Benefits of Living Soil

There are several benefits to using living soils to grow cannabis. First, and perhaps most important, the grower does not have to add chemical fertilizers. If made properly, all the nutrients that plants need will be in the soil, and there will be ample life to ensure these nutrients are made available to the plants.

Since it is the microbes that make the nutrients available to the plants, as long as there are microbes and there is enough organic matter for them, living soil can continue to be used from grow to grow.

This is quite an advantage. The addition of a bit of compost to refresh things between several grows should be all that is needed with a good living soil.

A second benefit to using living soils is that the soil food web tends to police itself. The diversity of microbes and micro and macro arthropods results in a delicate predator-prey balance. This is impossible when using synthetic chemicals, which alter or destroy the soil food web and thus its natural protections, leading to the use of more, and often more dangerous, chemicals.

A third benefit is that the gardener knows where the soil comes from and what is in it. When it’s made on the premises, the blend’s quality is assured.

Fourth, living soils should contain the proper fungi to form mycorrhizae with cannabis. Fortunately, there is only one, currently known as Rhizophagus intraradices. If purchasing living soil, make sure this is included. If it has not been added by the manufacturer, it can be added by the grower.

Fifth, cannabis grown in living soil smells and tastes better than that grown using non-soil methods. Many claim they can taste chemical fertilizers. Others insist these affect the quality of the smoke. Not so with cannabis grown in living soils.

How to Get Living Soil

Living soils can either be made on the site or purchased. Each has its advantages and disadvantages. The end products should be pretty similar.

Obviously, buying living soil is the easier of the two. There is no need to expend time or energy sourcing ingredients and mixing them, just open the bag and use.

Making living soil requires ingredients in well-made compost to provide the living component, an additive such as biochar, coir, or pumice to provide drainage, and nutrient ingredients to feed the microbes so that they can feed the plants. These include alfalfa, crustacean meal, gypsum, kelp, rock dust, ground sea shell, and soy meal. All these ingredients should be organic; one reason to source materials is to know what is in it.

A Recipe For Living Soil

There are myriad recipes on the internet for making living soils. Some growers insist that indigenous microbes from locally sourced compost are better.

Composts are great for use in living soil mixes. Worm compost is even better. In fact, comparing vermicompost, as worm compost is called, to thermally made compost is like comparing cream to skim milk when fat is desired. Up to 30% of compost used for making living soil should be vermicompost. It complements the thermal compost. Why? Because vermicompost has higher concentrations of nutrients than regular composts. Properly made, it contains 10 times more potash, 5 times more nitrogen, 7 times more phosphate, 1.5 times more calcium, and 3 times more magnesium.

Vermicompost is made by feeding worms organic matter in specialized containers. They ingest it in their search for fungi, bacteria, and protozoa, their real food sources. Added to regular compost or soils, it enriches them.

Worms can also be added to regular compost piles, where they will add their excrement to the mix. One reason many recommend composting systems with bottoms open to the ground is to allow worms to move up into the pile when it cools. It also allows them to move away from too hot areas.

Many growers swear by putting worms into their living soil mixes. Worms eat mostly bacteria like those attracted to plant roots by exudates. They do not eat roots. They help create and maintain soil structure, which ensures aeration, water filtration, and water drainage. They also help feed the plant through their droppings.

DIY Living Soil

Before planting, let this mix sit for at least two weeks to enable the microbes in the compost to start cycling the nutrients.

Start by mixing:

1/3 sphagnum peat moss or coco coir

1/3 pumice, perlite, or lava rock

1/3 compost

For each cubic foot (0.02 m³) of the above, add:

1/2 cup (118 ml) organic neem meal

1/2 cup (118 ml) organic kelp meal

1/2 cup (118 ml) crab meal

1 cup (237 ml) glacial rock dust

1 cup (237 ml) gypsum (calcium sulfate, CaSO₄)

1 cup (237 ml) oyster shell flour

1 cup (237 ml) basalt flour

This recipe can be adjusted to suit local prices and material availability. A vegan alternative to crab meal is alfalfa meal, and dolomite instead of oyster shell flour.

Keep Living Soil Alive

It is easy to kill life in the soil. The most obvious ways to accomplish this would be activities that deprive the soil life of oxygen, like compacting the soil or letting it get waterlogged. Much less obvious are the impacts of rototilling an outdoor grow or disturbing the soil of an indoor one.

The rule is to disturb the soil as little as possible. When the soil is tilled, the network of fungi that feeds plants and provides soil structure is broken and destroyed. The burrows and passages used by micro and macro arthropods and worms are also destroyed. Soil food web organisms are severely displaced and moved into new environments that are often not suitable for them, where they may miss symbiotic partners and proper foods. Most important, tilling can kill worms.

Once a garden bed is established, there is no good reason to till it. Nutrients and organic materials placed on the surface will be moved into the soil by members of the soil food web. Plants have no trouble moving roots through non tilled soil, which is why it is common to see plants growing up through pavements and driveways.

The same is true with container soil. The least amount of disturbance possible is the rule here, too. Having built up exudate carbon in it, having established good soil structure, and having the old roots serve as organic material all make sense. Not only should it be reused, but if possible, simply replant without disturbing the remnants of the last grow. New roots will find it easier to grow in the old root channels, and the right bacteria and fungi should be nearby in the old rhizosphere.

Mulches

Mulches do several things. They provide cover, which helps conserve water use. More important, they provide organic matter to soil and serve as a food source to the soil food web.

There are two kinds of mulches to use for growing soils indoors and outdoors: living and static mulches. Each adds to the soil to help grow better cannabis.

Living Mulches

Plants can serve as mulch. There are two kinds that make the best living mulches. The first, nitrogen-fixing mulches, or legumes, are mulches that take nitrogen from the air, which cannot be used by plants, and fix it so that it is available through the roots. When growing cannabis, nitrogen-fixing legumes shouldn’t compete for light with the plants they are helping, so they should be pruned to stay out of the path of the crop plant’s direct relationship with the sun.

The second kind of living mulches is known as bioaccumulators. These plants can draw nutrients from the soil. These are released back when the plant dies and is decayed by the bacteria and fungi in the soil. The regenerative impact is obvious.

There are many plants a grower can use as a living mulch, but the most popular (especially for indoor gardens) and the easiest to obtain is Dutch white clover (Trifolium repens). This nitrogen-fixing legume is a very low grower. It forms dense mats of very low green growth so it does not interfere with cannabis growth. It acts as an annual indoors and a perennial outdoors.

There are reports that Trifolium repens is a bioaccumulator. It is said to accumulate phosphorus, one of the necessary elements for growth. Bioaccumulator or not, when it does, the accumulated nutrients that grew the clover are returned to the soil.

Every area is different in terms of invasive plants. For this reason, outdoor growers should consult their local agricultural agents to ensure a proper living mulch is used. Cannabis has enough problems, and there is no sense in being blamed for introducing an invasive. Some other common living mulches include alfalfa, cow peas, and vetch.

Outdoors, living mulches also serve as green manure. When the season is over, they are cut and left on top of the soil as fresh sheet compost or worked back into the soil, where they feed the food web microbes.

Static Mulches

Static mulches are not alive per se. They just used to be alive. They include things like straw, leaves, or stalks. Static mulches support soil food web life, slow water from evaporating, and sometimes act as barriers to pests. Static mulches are also great at keeping weed growth in check when applied thick enough.

Straw is covered with protozoa, which work their way into the soil and consume bacteria, ultimately releasing usable nitrogen. Thus straw is a great static mulch for growing cannabis, indoors or outdoors. Straw is available at feed stores as well as many grow supply stores. Just make sure to use straw and not hay, which contains seeds.

Composts, Compost Teas & Extracts

The same compost that is used to make living soil can also be used alone to freshen soil. Adding vermicompost makes it an even better mulch.

Compost mulch is so useful because it has a tremendous population of bacteria and fungi that solubilize the basic nutrients needed by plants. Compost also contains ample numbers of nematodes and protozoa. These consume the bacteria and fungi in the compost, thereby releasing and making available the nutrients that the bacteria and fungi held.

Compost can be made into living soils or mulch. There are many great books that discuss how to do this. Good composts can also be purchased, but it is important to know what’s in them.

In either case, compost is used as one would use living and static mulches. Layer an inch (2.5 cm) or so onto the surface of the soil after each crop cycle. Both the life in the compost as well as the organic matter in it will be worked into the soil by the soil food web that utilizes it.

Compost Teas

Compost teas are liquid compost mixes. They are made by putting a relatively small amount of compost into water and growing the microbes, feeding them, and allowing them to grow over a few days.

The mixture is kept aerobic if properly made. The nutrients are designed to feed the compost’s microbes released into the tea. The mixture is fermented using heavy aeration for 24 to 48 hours.

The aeration of the tea provides the energy to strip the microbes from the compost. The oxygen should keep the mixture aerobic and free of harmful pathogens that live under anaerobic conditions. The added nutrients keep feeding the microbes as they multiply or grow.

The final brew should smell sweet, not sour, which would indicate unwanted anaerobic microbes. It is best to use within 24 hours because the microbes use up the oxygen and quickly turn it anaerobic. One gallon (3.75 l) of compost tea can be diluted up to five times or used straight.

The tea can be applied as a soil drench or sprayed on leaves. The microbes become part of the soil microbial biomass and coat leaves to outcompete pathogens for food, space, and entry locations.

DIY Compost Tea

Fill a five gallon (19 l) bucket with non chlorinated water or allow the chlorine to evaporate over 12 hours. Then add:

1 to 2 cups (128 to 256 g) compost

1 tablespoon (15 g) unsulphured blackstrap molasses*

1 tablespoon (15 g) liquid kelp

There is no such thing as applying too much compost tea; it is not like using fertilizers and will not burn the plants. Instead, the process adds beneficial microbes to the soil. *Molasses can become pathogenic. Good alternatives include honey, refined cane sugar or syrup, and beet juice.

Compost Extracts

Compost extracts differ from compost teas. They are made by stripping the microbes from compost as one does when making tea, but nutrients are not added, and the process takes about 15 minutes.

These extracts don’t have as much microbial life, but they are much quicker to make. Good compost will provide great benefits, and extracts will help keep soils alive and functioning.

Compost extractors can be made at home or purchased. A simple method is to wrap up compost in cheese cloth and knead it into the water for 15 minutes, or let it soak like a teabag for half an hour.