Cover Crops

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Cover crops are plants used to improve conditions in and around the soil. They increase organic matter content thereby enhancing water availability and beneficial soil biology, which typically leads to increased yields. They control pests and diseases, smother weeds and generally increase biodiversity. Cover crops help bring other nutrients back into the upper soil profile from deep soil layers. Due to these abilities to reincorporate nutrients, cover crops are also referred to as ‘green manure’.

Before modernized agriculture, cover crops were an important tool in farming. As industrial agriculture developed new technology, the need for these crops diminished. But now, with heavy chemical use of synthetic fertilizers and pesticides impacting ecosystems, the need for environmentally responsible agriculture has grown. Cover crops represent a highly integrated component to ecologically sound agriculture.

choosing the right crop

It’s important to understand that there is an element to getting your feet wet. There are quite a few nuances to optimizing yields and quality. Cover crops can provide a variety of services.  They can be biofumigants, soil builders, weed suppressors and nitrogen fixers.  It really is a highly dynamic science. Applying cover crops for cannabis is largely similar to that of traditional crops, however, it will require ingenuity of the modern farmer to maximize its benefits.

1. The first step is to determine the principal objective for adding cover crops to your system. Do you want to add nitrogen to your soil, increase organic matter, manage nutrients or deter soil pests and pathogens?

2. Then, schedule in your crop rotations.  Are you looking for winter cover crops to scavenge and/or fix nitrogen or just for building organic matter in the soil and blocking weeds?  Careful considerations for accommodating your long-term objectives will need to be mapped out.

3. Finally, think through the schematics of fieldwork.  Locate the supplier for the seeds.  Outline a plan to determine when you will plant and terminate the cover crop. When in doubt, always confer with local experts and consultants, including other farmers. Having soil tests run and consulting with knowledgeable professionals can make a big difference.  It is pertinent that you outline your objectives for growing a cover crop and take as much care in selecting and managing cover crops as you would your cannabis crop.  

The most important tool a farmer can have is familiarity regarding the different types of cover crops and how they interact with the soil. For simplicities sake, they are broken into two broad groups, legumes and non-legumes.

role of legumes

Legume cover crops are mainly used to fix atmospheric nitrogen, effectively extracting free nitrogen from the surrounding air and depositing it into your soil.  They are also used to prevent erosion, add some organic matter to the soil and attract beneficial insects. Legume cover crops include various types of clover, vetch, peas and beans can fix quite a bit of nitrogen for succeeding crops. Legumes are good at fixing nitrogen but not scavenging it.  For optimal results, the legume seeds should be inoculated with the appropriate Rhizobium bacteria before planting. The bacteria colonize and form nodules on the roots that store the nitrogen.
Legumes especially, encourage populations of beneficial fungi and other microorganisms that help bind soil aggregates. The fungi, called mycorrhizae, produce a water-insoluble protein known as glomalin, which catches and glues together particles of organic matter, plant cells, bacteria and other fungi. Glomalin may be one of the most important substances in promoting and stabilizing soil aggregates.

Just to be clear, when we talk about scavenging for nutrients, that means the plants root system pulls up nutrients from the soil and stores it in the stalks, leaves and flowers. This is important on outdoor farms when nutrients will move out of the root zone into subsurface flow or runoff. The scavenging traps the nutrients in the plant, and once the plant is composted back into the soil, the nutrients are recycled.

So generally speaking, legumes are higher in nitrogen and lower in carbon than grasses. This ratio causes the legume residues to break down faster into the soil. That means that the nutrients in the broken down residue are more rapidly assimilated in the soil. The faster break down time means that legumes may not be as effective for weed control than other types of grasses.

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Legumes

  • Berseem Clover
  • Cowpeas
  • Crimson Clover
  • Field Peas
  • Hairy Vetch
  • Medics
  • Red Clover
  • White Clover

Role of Non-Legumes

Non-legumes greatest benefits are their ability to scavenge nutrients, add organic matter, suppress weeds and provide erosion control. They include a much wider range of plants, including the broadleaf species like buckwheat, mustards and brassicas and the forage radish, the cereals like rye, wheat, barley, oats, and forage grasses.

Non-legume cover crops are higher in carbon and lower in nitrogen than legume cover crops. The higher carbon values mean they take longer to break down, but they contribute large amounts of organic matter back to the soil. Non-legumes, such as grasses, increase soil organic matter more effectively than legumes.

Brassicas are a non-legume showing some interesting dynamics as cover crops, especially for cannabis. The Sustainable Agriculture Research & Education (SARE) has shown that: “All brassicas have been shown to release bio-toxic compounds or metabolic byproducts that exhibit broad activity against bacteria, fungi, insects, nematodes, and weeds. Pest and pathogen suppression is believed to be the result of glucosinolate degradation into biologically active sulfur containing compounds call thiocyanates.” Sulfur is known to increase oil production in cannabis flowers. The incorporation of brassicas could lead to increased terpene and oil production in the flowers. However, due to the bio-toxic or "allelopathic" compounds in brassicas root exudates, this might result in a reduction of the growth stimulation of mycorrhizal fungi. This aspect is still under discussion and research. A common sense approach is to provide a healthy balance of other legumes and non-legumes if brassicas are used.

SARE also noted that: “Some brassicas, notably rape, can scavenge phosphorus by making insoluble phosphorus more available to them via the excretion of organic acids in their root zone.”  So while legumes are highly adept at fixing nitrogen, it looks like non-legumes hold a lot potential for managing other nutrient levels in the soil. 

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non-Legumes

  • Annual Ryegrass
  • Barley
  • Brassicas & Mustards
  • Oats
  • Cereal Rye
  • Sorgham and Sudangrass
  • Winter Wheat
  • Buckwheat

Mixtures

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Cover crops mixtures offer the best of both worlds. By combining the benefits of multiple species, you can create specialized blends to fit your needs.

These mixtures are generally hardier than standalone cover crops.  They consistently generate more overall biomass and nitrogen, tolerate more adverse conditions, increase winter survival, provide ground cover and improve weed control.

Farmers can also more successfully influence nitrogen cycling with mixed cover crop species. By mixing grasses with legumes, you will have a crop decomposing at different intervals. The early decomposition of the nitrogen rich legumes will accelerate more rapid decomposition of the carbon rich grasses. This will facilitate the release of stored nitrogen more quickly for uptake by cannabis crops.

Interesting research reviewed by The American Society of Agronomy highlights that: “Poultry litter mixed with cover crop residues increased decomposition and nitrogen release.” So mixing some chicken manure in with the tilled residue could be a key in optimizing these systems. Still, test plots would be advised. The addition of bokashi (fermented grains) is another methodology to further accelerate this decomposition process.

Increased benefits of mixtures can also come with increased responsibilities. Because mixtures generate more residues, they can clog tilling equipment faster if not properly managed. This industry is going to require the leaders in these fields to run some research and come up with optimal ratios for cannabis cover crops.

Cover Crop Economics

The science of cover crops is clear. They boost yields, increase soil nitrogen levels, improve soil biology and reap many agronomic benefits.  Careful, long term planning is paramount for success. Cover crops can take several years for you to see strong returns.

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Analyzing management costs such as labor, fuel, fertilizer and equipment will help provide the right framework to determine the efficacy of your model. Careful adjustments of existing fertilization regimens will prove an important consideration.

There are also economic variables between outdoor farms and greenhouse systems. Outdoor usually has reduced maintenance costs associated with watering and tilling management. Greenhouse has the advantage that nutrient washing from the rain will not occur.

Any farm that is interested in moving away from chemical and pesticide use and into sustainable and regenerative management practices should highly consider using cover crops.  The living soils article in the last issue of Sun Grower clearly delineates that by simply adopting a more scientific approach to the biological sciences, one can begin transitioning into ecological management practices.

 

soil fertility and tilth

SOIL EROSION - Soil erosion is only appropriate when considering outdoor crops. The reduction in soil erosion due to cover cropping will be roughly proportional to the amount of cover on the soil. The Revised Universal Soil Loss Equation developed by the Natural Resources Conservation Service predicts that a soil cover of just 40 percent when winter arrives can reduce erosion substantially until spring. It’s worthwhile to get covers established early, to ensure that maximum soil cover develops before winter rains. Consider planting as soon as possible after harvest. It’s always a good idea to maintain year-round soil cover whenever possible.

ORGANIC MATTER - The benefits of organic matter include improved soil structure, increased infiltration and water-holding capacity, increased cation exchange capacity (the ability of the soil to act as a short-term storage bank for positively charged plant nutrients) and more efficient long-term storage of nutrients. There is some argument over how to classify the various soil organic components. Most will agree, however, that there is a portion that can be called the “active” fraction, and one that might be called the “stable” fraction, which is roughly equivalent to humus. 

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The active fraction represents the most easily decomposed parts of soil organic matter. It tends to be rich in simple sugars and proteins and consists largely of recently added fresh residues, microbial cells and the simpler waste products from microbial decay. Because microorganisms, like human organisms, crave sweet stuff, compounds containing simple sugars disappear quickly. Proteins also are selected quickly from the menu of edible soil goodies. When these compounds are digested, many of the nutrients that they contain are released into the soil. Proteins are nitrogen-rich, so the active fraction is responsible for the release of most N, as well as some K, P and other nutrients, from organic matter into the soil. The easily decomposed proteins and sugars burn up almost completely as energy sources, and don’t leave much behind to contribute to organic matter building.

After the microorganisms have devoured the portions of the active fraction that are easiest to digest, a more dedicated subset of these microorganisms will start munching on the more complex and tough material, such as celluloses and lignins, the structural materials of plants.
Since cellulose is tougher than simple sugars, and lignin breaks down very slowly, they contribute more to the humus or stable fraction. Humus is
responsible for giving the soil that rich, dark, spongy feeling and for properties such as water retention and cation exchange capacity. Plant materials that are succulent and rich in proteins and sugars will release nutrients rapidly but leave behind little long-term organic matter. Plant materials that are woodier or more fibrous will release nutrients much more slowly, perhaps even tie up nutrients temporarily, but will promote more stable organic matter, or humus, leading to better soil physical conditions, increased nutrient-holding capacity and higher cation exchange capacity. In general, annual legumes are succulent. They release nitrogen and other nutrients quickly through the active fraction, but are not very effective at building up humus. Long-term use of annual legumes can increase soil humus, however, some research suggests (429). Grains and other grasses and nonlegumes will contribute to humus production, but won’t release nutrients very rapidly or in large quantities if incorporated as they approach maturity. Perennial legumes such as white and red clover may fall in both categories—their leaves will break down quickly, but their stems and root systems may become tough and fibrous and can contribute to humus accumulation.

SOIL AGGREGATION - As soil microorganisms digest plant material, they produce some compounds in addition to the active and stable fractions of the organic matter. One group of these by-products is known as polysaccharides. These are complex sugars that act as glues in the soil to cement small soil particles into clusters or aggregates.

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Cover crops can promote good aggregation in the soil through increased production of these and other microbial glues. See Cover Crops Can
Stabilize Your Soil (p. 19). Well-aggregated soils also are less prone to compaction, which has been shown to reduce yields of vegetables such
as snap beans, cabbage and cucumber by 50 percent or more (451). As they decompose, leguminous cover crops seem to be better than grasses for production of polysaccharides (9). However, polysaccharides will decompose in a matter of months, so their aggregation effect is likely to last only the season after the use of the cover crop. Grass species also promote good aggregation, but by a different mechanism. Grasses have a
‘fibrous’ root system—made of numerous fine roots spreading out from the base of the plant. These roots may release compounds that help
aggregate the soil between roots.

Your tillage method is an important consideration when using cover crops to build soil, because tillage will affect the rate of organic matter accumulation. It is difficult to build up organic matter under conventional tillage regimes. Tillage speeds up organic matter decomposition by
exposing more surface area to oxygen, warming and drying the soil, and breaking residue into smaller pieces with more surfaces that can be
attacked by decomposers. Like fanning a fire, tillage rapidly “burns up” or “oxidizes” the fuel, which in this case is organic matter. The resulting
loss of organic matter causes the break down of soil aggregates and the poor soil structure often seen in overtilled soil. When adding cover crops to a system, minimize tillage to maximize the long-term soil benefits. Many of the cover crops are also ones you can seed into growing crops or no-till plant into crop residues. Otherwise, the gain in organic matter may be counteracted by higher decomposition rates.

Nutrient Cycling

In addition to reducing topsoil erosion and improving soil structure, cover crops enhance nutrient cycling in your farming system by taking up nutrients that otherwise might leach out of the soil profile. These excess nutrients have the potential to pollute groundwater or local streams
and ponds, not to mention impoverishing the soil they came from. Of the common plant nutrients known to leach, nitrogen in the nitrate form is the most water-soluble and therefore the most vulnerable to leaching.

Cover crops reduce nitrate leaching in two ways. They soak up available nitrate for their own needs. They also use some soil moisture, reducing the amount of water available to leach nutrients. The best cover crops to use for nitrate conservation are nonlegumes that form deep, extensive root systems quickly after cash crops are harvested. For much of the continental U.S., cereal rye is the best choice for catching nutrients after a summer crop. Its cold tolerance is a big advantage that allows rye to continue to grow in late fall and put down roots to a depth of three feet or more.

Research with soil high in residual N in the mid-Atlantic’s coastal plain showed that cereal rye took up more than 70 lb. N/A in fall when planted by October 1. Other grasses, including wheat, oats, barley and ryegrass, were only able to take up about half that amount in fall.

Cover crops help bring other nutrients back into the upper soil profile from deep soil layers. Calcium and potassium are two macronutrients with a tendency to travel with water, though not generally as rapidly leached like nitrogen. These nutrients can be brought up from deeper soil layers by any deep-rooted cover crop. The nutrients are then released back into the active organic matter when the cover crop dies and decomposes.
Although phosphorus (P) doesn’t generally leach, as it is only slightly water-soluble, cover crops may play a role in increasing its availability in the soil. Some covers, such as buckwheat, are thought to secrete acids into the soil that put P into a more soluble, plant-usable form. Some cover crops enhance P availability in another manner. The roots of many common cover crops, particularly legumes, house beneficial fungi known as mycorrhizae. The mycorrhizal fungi have evolved efficient means of absorbing P from the soil, which they pass on to their plant host. The filaments (hyphae) of these fungi effectively extend the root system and help the plants tap more soil P.

Selected References

Managing Cover Crops Profitably 3rd Edition, Sustainable Agriculture Research and Education (SARE)

J Chem Ecol. 2003 Jun;29(6):1337-55. Growth stimulation of ectomycorrhizal fungi by root exudates of Brassicaceae plants: role of degraded compounds of indole glucosinolates.