Due to increasing pressure from consumers and government regulations, a push towards more sustainable and low impact farming techniques have garnered more interest from the agricultural community. Coupled with these ideals are practices that reduce salt based fertilizers, pesticide and herbicide usage, as well as reduce soil erosion and loss of structure. The use of cover crop by a farmer or gardener indicates a dramatic step in that direction. Cover crops have an abundance of benefits for a soil physical characteristics, including physical and chemical characteristics which have a pronounced cascade of positive effects on ecosystem functioning. The USDA lists 109 species that have potential as and characteristics of cover crops (https://plants.usda.gov/java/coverCrops) and using them can seem like a daunting and challenging practice. Understanding key differences and ecological niches for each species can drastically reduce costs of a farming operation, as well as create a more sustainable organism of production.
In reality almost any plant can act as a cover crop, if an ecological niche can be found in which it does not out compete surrounding species and acts to achieve a beneficial equilibrium in the community that it inhabits. Adding to the complexity of the strategy, often times multiple species of these crops are planted at the same time or in rotation of one another, which can lead to a mis-attribution of success or failure. Sometimes cover crops are allowed to mature and die directly where they were planted. Other times they are harvested for animal feed and even other times they are plowed back into the soil during seedling or early stages of maturity. Some cash crops prefer certain cover crops to precede it. Each situation that manifests itself will have its own positive and negative aspects on unique aspects of the ecosystem, from the type or amount of nutrients in the root zone to the bacterial community that get sustained in the rhizosphere. Understanding the way in the which the synergistic effects of the biological, geochemical, and physical processes catalyzed by cover crops effect the overall ecosystem equilibrium remains of the utmost importance for achieving proper functional condition of a natural ecosystem.
The universal ability of cover crops to mitigate soil erosion and losses due to wind, water, and human activity manifests itself as the ubiquitous beneficial characteristic imparted by this strategy on a landscape. Mitigating these erosive factors focuses essentially on one essential focal point, minimize the amount of energy that comes in contact with the soil surface and individual soil peds. The most common source of energy that comes in contact with the soil surface is from rain falling from the atmosphere, as well as water running over the soil surface on its way to the lowest point on the landscape. In arid conditions the most common source of energy is wind, but even so the infrequent rains in these areas create extremely large storm surges. During November 2015 in Armagosa Valley, Nevada, there was a storm that created a flash flood with the capacity of the Mississippi River. Despite an annual rainfall average of 2 inches per year, this flood allowed water to regain control of the the landscape expression and its behavior indefinitely; with a much greater magnitude that constant wind pressure. So the strategy of attempting to reduce the duration and volume of water on the soil surface by increasing infiltration of water into the subsoil will have the greatest net benefit to mitigating soil erosion on a landscape. It is also important to understand that nutrient and soil runoff/erosion is a natural process and will always happen, regardless of land management strategies. Natural soil erosion and runoff values are influenced by a variety of factors, including soil series, precipitation and climate, plant and animal communities, landscape position, land management strategies, as well as numerous other factors.
Normally soil loss occurs in the 3 – 5 tons per hectare per year on 3 – 8% grassland slopes. Tillage and bare fields can increase this number to 20 tons per hectare per year on some soil types and landscape positions. Canada bluegrass and downy browne decrease soil erosion 96% and 95% respectively (Langdale et. al.) indicating that cover crops can exert a significant influence over the landscape expression. A single cover crop species has the ability to essentially negate the harmful effect of a fallow field. Alfalfa had the ability to reduce rain runoff rates up to 28% (Shufang et. al.). In addition, alfalfa reduced sediment loadings by 78%. Interestingly enough, compared to the bare soil plots, rainfall runoff in alfalfa fields remained in a subcritical flow state, while bare soil plots exhibiting supercritical flow rates. This crucial concept gets reinforced with the observation of a 1.77 times increase in infiltration in the plots, with a decreasing infiltration rate at a lower slope position, contrary to a bare soil plot. Ryu et. al. found that hairy vetch and rye grass maintained soil erosion rates at .7 and .4 MT/ha, while bare ground lost 1.4 MT/ha of soil each year over the course of a 3 year studies. This study was done in South Korea on 7% slopes. Also, not to be underestimated as a factor in above ground soil erosion, soils depth to bedrock ranged from 10 cm to 60 cm on these slopes. These soil are already more susceptible to natural soil erosion due to this fact. Reducing rates significantly will express itself in economic gains for the farmers in this region. Olsen et. al. found that cover cropping decreased soil erosion by 25% a year over an eight year long period under no till, chisel plow, and moldboard plow management strategies in a fine-silty, mixed, mesic, Typic Fragiadulf in southern Illinois on 5% to 7% slopes. They found that only the no till, with cover crop scenario was able to keep erosion to a level close to natural soil erosion (6.2 Mg ha-1 yr-1). Jaehoon et. al. demonstrated that under vetch and wheat hydraulic conductivity values were consistently double than the hydraulic conductivity value for a field left in fallow. It is also of importance to note that under a no till management strategy versus a conventional tillage strategy the hydraulic conductivity doubled as well, indicating the importance of allowing mature root systems to establish and influence soil physical properties over time. Cover crops are an instrumental aspect of this strategy and knowing which cover crops will have what effect on your soil greatly will help land managers maintain soil fertility.
Cover crops have an seemingly infinite potential for different combinations and rotations, all with distinct and complex resulting characteristic changes for a field, some expected and some paradoxical. For instance, Mupumbwa illustrated that an oat monoculture decreased soil strength by 48.5%, while monocultures of vetch and lupin raised soil strength by 41% and 35%, compared to a weedy fallow. Soil strength is a metric that measures the response of soil to sheer stress. A soil with high soil strength will not physically react or shift as much as a low strength to say a vehicle driving over it. So a soil with low soil strength is actually more beneficial for agricultural purposes, due to high porosity, transitivity, hydraulic conductivity, low compaction etc. The loss in soil structure from the oats versus the oats and lupin was attributed by the authors to the fact that legumes need more water than perennial grasses, leading the soil to hard set more often than the grass species. Also root mass from oats and cereals are more pervasive in the soil-atmosphere interface, as well as more dense, than legumes, leading them to build organic carbon at higher rates, relative to water consumption. Also consider the root structure of oat versus a vetch or lupin
Vetch Root System
Oat Root System
Lupin Root System
Oats have an markedly more dense root system, while vetch and lupin have a less dense root system – the lupin specifically exhibits a more taproot like structure. Oats do not go as deep as the lupin and vetch, its root system spreading on the surface, breaking up soil peds and opening up pore space to the surface. Lupin and vetch go deeper, instead of outward, reducing their positive effects on soil structure and infiltration. Grasses, in particular, can attribute their benefit as a cover crop to a dense root system and overall biomass. Ekeren et. al. found that the percent of granular ped structures in 0 – 10 cm fraction was the same for a grass cover crop and a grass/white clover mix. Additionally the grass cover crop contained twice as many granular ped structures than the white clover in the same fraction. Clover, like vetch and lupin, is a legume which tend to have a less dense, taproot dominated root structure. In another comparison of 14 cover crops (3 grasses, 9 legumes, and 2 cover crops after soybean summer crop), Barber et. al. demonstrated grasses (Panicum maximum var. Tobiatd, Panicum maximum var. Centenario, Brachiaria brizantha) overwhelming had the ability to increase biomass, soil organic matter, and increase subsoil structure above legumes, as well as had greater root densities than legumes. Perhaps this benefits nodule creation for nitrogen fixation, but as a result they remain inferior to grasses in their contribution to promoting soil physical properties that benefit water quality.
Bicultures, however, of oats and vetch increased soil infiltration rates from 10.3 mm h-1 (oats) and 9 mm h-1 (vetch) to 16.7 mm h-1 for an even application of oats and vetch (Mupumbwa et. al.). The control infiltration rate for the weedy fallow was 6.33 mm h-1. This a substantial increase in water infiltration that also lead to an increase in water hold capacity and available water in the root zone. Interestingly enough the positive effects of the cover crop strategy become dampened as the balance of diversity shifts towards a single species’ dominance, although even adding 10% of a different species into the mix increases the infiltration rate of a soil compared to the infiltration rates under a monoculture. Sainju et. al. found that above and below ground biomass was greater by 2.0 – 3.0 Mg ha-1 yr-1 when a rye/vetch biculture was planted versus a rye monoculture over the course of a 3 year study. The importance of the biomass of a plant cannot be understated. That is the physical and chemical process in which the majority of nutrients become bound to organic matter, making them available to the plant the following season. Wang et. al. noted an increase in biomass from a okra/cow pea biculture of 11.5 Mg ha-1 compared with 2.0 and 5.3 Mg ha−1 in respective monocultures. This demonstrates the synergistic effect of cover cropping. In addition, Subedi-Chalise found in South Dakota that a cover mix of hairy vetch and rye usage decreased soil penetrometer resistance on both the surface, as well as up to a depth of 15 centimeters. This indicates that the infiltration effects of cover crop when applied in mixture, does not stop at the surface, but becomes translocated downwards into the soil horizon. Also it indicates that complexity and diversity can mitigate dominating effects of specific species, allowing species to negate the shortcomings of each other in creation of an ecosystem.
Soil organic matter (SOM) encompasses a variety of forms. Organic matter refers to the amount of lignin, cellulose, hemicellulose, or partial decayed plant matter that can be made available to plants, as well as the biological community that is breaking down these complex molecules into ions and metabolites that plants can use. Organic matter plays many roles in the health of the soil ecosystem. SOM increases soil structure and aggregation. Its ability to perform this task stems from the large, complex molecules that encompasses organic matter. These large molecules, compared to the salt ions of fertilizer, need more applied energy to begin to percolate out of the root zone. Also by virtue of having multiple charge sites, both positive and negative, on each molecule are able to bind to cation exchange sites more frequently and in some cases more tightly than ions. This increases the amount and decreases the size of soil peds, and encourages them to adhere to each other, both of which contribute to increasing connected porosity of a soil. Also these large, complex organic molecules increase effective cation exchange capacity by being able to bind ions in solution from running off through two different mechanisms. Polar regions on the organic molecules chemically attract charged ions in solution. Also the large organic structures act as a catcher’s mitt and preventing the direct percolation of ions traveling in solution. It should be noted that cation exchange capacity contribution from organic matter should not be taken into consideration when analyzing soil cation exchange capacity as organic matter will break down over time, making its contribution variable and unreliable. But depending on the amount of organic matter in the soil, it can be the largest contributor to cation exchange capacity.
In any case, one of the most important contributors to soil fertility is soil organic matter, more specifically organic carbon. Organic carbon in the soil emerges on crop land as one of the most fragile nutrient reserves involved in plant production. Carbon is the most abundant element in plant biomass, both above and below ground. The overwhelming amount of biomass gets removed from crop land on a yearly basis and without cover crops the only additions every year is fractional leaf litter, root mass, fauna scatting, and whatever weeds are able to sprout in the inter crop areas and can die back. But normally herbicides ensure that root mass is the only annual contribution to soil organic carbon stores. Plants fix their own carbon, but soil bound carbon remains an important food source for bacterial and fungal communities, as well as serves as a matrix for the building up of other plant available nutrient reserves, such as nitrogen, phosphorus, boron, manganese, molybdenum, and silicon. In natural situations, soil organic matter has a multitude of inputs, but just as many outputs. Building carbon reserves takes time, and getting them to incorporate to depth is a pedologic process that requires certain scenarios to unfold. Just as an illustration to the paradoxical nature of this build up, rain forest soils are notoriously infertile, as almost 100% of its soil organic matter is contained in organisms. In fact only about 2 – 3% of its organic carbon is contained in rain forest soils. In addition 95% of detritus is decomposed within a year (Powers et. al.). Although this is an atypical situation, rain forests illustrate the dynamic nature of carbon accumulation and cycling in ecosystems.
Cover crop usage, in part, looks to offset carbon loses from crop removal by allowing root biomass to accumulate in soil horizon as well as by building up surface soil carbon stores from detritus generated by plant senescence. Ideally, land managers would like to see net increases of soil organic matter, but often times this will not be the case. The amount of carbon removed from the soil each year is nearly impossible to replicate with most cover crops due to the sheer size and density of cash crops. Also often times the ecological equilibrium has shifted in such a way that multiple scales of carbon storage must be filled before organic matter can be built up as stable organic matter. Bacterial communities need to be regenerated, clay-humus complexes need to be reestablished, etc. In effect in the majority of high intensive farmland is playing a game of catch up. Olson et. al. found that over an eight year period in no-till, chisel plow, and moldboard plow situations, the addition of cover crops damped the carbon loss effect of crop removal, but that only the no-till cover crop situation maintained a level of soil organic carbon that existed in the first year. Each year, the upper 75 cm exhibited a net loss of organic carbon for all other plowing situations, regardless of cover crop presence. Clearly management style was the dominating factor here, but the use of cover crops still demonstrated themselves as a tool for slowing the depletion of organic carbon from the soil. Interestingly enough, although the moldboard plow scenario did not exhibit an overall net increase in soil organic matter, it increased soil organic matter at depth greater than that of the no till situation. No till still performed better in terms of building organic matter, but the aggressive plow style is able to push organic matter at depth quicker. Also the moldboard plow scenario increased overall soil organic matter on the overall plot – if you incorporate the runoff caught in the barrier plants surrounding the plots (personally though I wouldn’t really consider this a successful accumulation of organic matter in the soil subsurface).
Chu et. al. also demonstrated that after a four year study in Tennessee that cover crops effects on soil organic carbon, although relatively uniform between treatments (wheat, rye, rye/vetch, rye/oats/daikon/turnips/crimson clover), soil organic carbon did not increase over the course of the study. They attributed it to the duration of the study as well as the climate favoring a net mineralization of carbon. Interestingly enough, despite the similarities in effects on soil organic carbon, only the rye/oats/daikon/turnips/crimson clover resulted in an increased yield in the cash crop, indicating once again that diverse communities have synergistic effects on producing a more healthy ecosystem over all and that not anyone aspect of a community should be highlighted, but more so the structure and function of the ecosystem as a whole.
Clearly cover crops have an extremely important role to play in mimicking a natural situation on a farm and should be utilized, both outdoors in fields, but also indoors in beds and pots. Understanding what the limiting factors in your situation and how each species falls into their ecological niche are key to correctly and successfully integrating this strategy. Many strategies that are applied, although they increase the overall fertility and production of the system, can actually limit the yield of the cash crop.
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