Cover crops are plants grown primarily to improve soil health rather than for harvest. Planted during off-seasons or between cash crops, they shield the soil surface from erosion and nutrient loss. The strategic use of cover crops helps to increase soil organic matter, cycle nutrients, suppress weeds, and improve water infiltration. Successfully incorporating this practice requires matching the specific needs of your soil to the appropriate plant species and management techniques. This guidance outlines how to select, plant, manage, and integrate cover crops effectively.
Matching Cover Crop Types to Specific Soil Goals
Selecting the right species is based on the primary soil health goals you intend to achieve. Cover crops are generally categorized into three functional groups, each providing distinct benefits to the soil ecosystem. Often, a mixture of two or more types is used to gain multiple benefits simultaneously.
Legumes, such as clovers, vetches, and peas, are primarily chosen for nitrogen (N) fixation. They form a symbiotic relationship with Rhizobia bacteria, converting atmospheric nitrogen gas into a plant-available form within root nodules. This fixed nitrogen is released into the soil when the residue decomposes, reducing the need for synthetic fertilizer for the subsequent cash crop. Legumes are generally lower in carbon, meaning their residue breaks down relatively quickly after termination.
Grasses and small grains, including cereal rye, oats, and barley, are exceptional at nutrient scavenging and building soil structure. Their dense, fibrous root systems capture residual soil nitrate that might otherwise leach out of the root zone during the off-season. These species also produce large amounts of carbon-rich biomass, which contributes significantly to increasing soil organic matter and providing long-term soil cover for erosion control.
Broadleaves, specifically the Brassica family like forage radish and mustard, are used for breaking up soil compaction and pest suppression. Forage radish develops a deep, fleshy taproot that penetrates compacted soil layers, creating channels for water infiltration. Certain brassicas, such as mustards, release biofumigant compounds called glucosinolates when their tissues are broken down, which can inhibit the growth of soilborne pathogens, nematodes, and weeds.
Essential Timing and Planting Methods
Successful cover crop establishment relies heavily on appropriate timing and achieving sufficient seed-to-soil contact. Planting typically occurs immediately after the main crop harvest, aiming to establish the cover crop while soil temperatures are still warm enough for germination and early growth. Planting too late results in poor establishment, severely limiting the amount of biomass produced and the overall benefits realized.
Drilling the seed with a no-till drill generally provides the most reliable establishment because it ensures precise seed depth and firm seed-to-soil contact. This method is effective for larger seeds and allows for planting at lower seeding rates compared to broadcasting. Small-seeded cover crops, like clovers or brassicas, often require very shallow planting, sometimes less than half an inch deep, to emerge successfully.
Broadcasting involves scattering seed onto the soil surface, either by hand, with a ground spreader, or aerially. This method is quicker and requires less specialized equipment but demands an increased seeding rate (often 20% to 50% higher) to compensate for reduced seed-to-soil contact. For successful germination, the seed must be lightly incorporated into the soil, such as with a light harrowing or roller, or planted just before a predicted rain event. Overseeding, which is broadcasting the cover crop into a standing cash crop shortly before harvest, maximizes the growing season when the post-harvest window is too short.
Management and Termination Strategies
Termination balances maximizing biomass and nutrient accumulation with preparing the field for the next cash crop. Cover crops must be killed effectively to prevent them from becoming weeds that compete with the subsequent crop for light, water, and nutrients. The timing of termination is critical: delaying it until the cover crop reaches its reproductive stage (e.g., flowering or early pod set) maximizes the accumulation of biomass and nitrogen.
Physical termination often uses a roller-crimper, an implement with angled blades that flattens and crimps the stems without cutting them. This creates a dense mat of residue on the soil surface, effective for weed suppression and moisture conservation in no-till systems. For successful roller-crimping, cereal grains must typically be at the milk or dough stage, and legumes like hairy vetch at the full bloom or early pod stage, to prevent regrowth.
Chemical termination using herbicides is a reliable and widely used method, particularly in large-scale operations, as it leaves the residue undisturbed on the soil surface, maximizing soil-protective benefits. Systemic herbicides, such as glyphosate, are often used for a gradual kill, while contact herbicides result in a faster breakdown of plant tissue. To conserve soil moisture, it is recommended to terminate the cover crop at least two weeks before planting the cash crop, especially in dry conditions.
Mechanical termination involves incorporating the cover crop residue into the soil using tillage implements. While tillage breaks down the residue quickly and prepares a fine seedbed, it negates many soil health benefits by disturbing soil structure and accelerating the loss of organic matter and carbon. A simpler strategy is selecting species that naturally winter-kill, such as oats or forage radish, which simplifies spring preparation but provides soil protection for a shorter period.
Integrating Cover Crops into Different Rotation Systems
Cover crops can be integrated into virtually any growing system by tailoring the species and management to the specific rotation schedule and operational scale. In a small-scale home garden, cover crops are often planted in unused beds during the fallow period to prevent erosion and build fertility. Gardeners might use a mix of a cereal grain, like winter rye, with a legume, such as crimson clover, and then turn it under in the spring before planting spring vegetables.
For large-scale annual cropping, such as the common corn-soybean rotation, cover crops are strategically planted to address specific nutrient and weed management challenges. Cereal rye is frequently planted after corn harvest to scavenge residual nitrogen and suppress weeds before planting soybeans. Conversely, a winter-killed mix of oats and forage radish is often planted after soybean harvest to scavenge nutrients and break compaction before planting corn.
In perennial systems like orchards and vineyards, cover crops are planted in the row middles between the trees or vines to manage soil health and competition. Low-growing perennial grass or clover mixes are typically used to reduce erosion on slopes, improve water infiltration, and provide habitat for beneficial insects. In high-vigor vineyards, a competitive cover crop, such as a grass blend, can be maintained to draw down water and nutrients, effectively reducing the main crop’s growth rate.