Chapter 16

The Use of Cover Crops to Manage Soil

Taxonomy of Cover Crops

Cover crops are classified by taxonomy: (1) legumes—clovers, vetches, and alfalfa; (2) grasses—barley, rye, and wheat; (3) Brassicas—radish, turnips, and rapeseed); or (4) as forbs—sunflower, composites, and buckwheat.

Legumes

Legume cover crops may include clovers, cowpea, faba bean, medics, peas, soybean, sunn hemp, and vetches (Appendix A). The primary advantage of legumes compared to grasses, brassicas, and forbs is their ability to obtain the nitrogen they need through a symbiotic relationship with Rhizobium bacteria in the nodules of the roots that fix nitrogen from the atmosphere. When the proper bacteria and conditions exist in the soil, nodules form on legume roots indicating that nitrogen fixation is taking place. Legume seeds must be inoculated with the proper strain of bacteria to effectively fix nitrogen.

Termination

The amount of nitrogen fixed and/or taken up by legumes is proportional to the amount of legume biomass produced. A traditional recommendation for maximizing legume cover crop nitrogen fixation and supply to subsequent crops is to terminate at early to mid-bloom.

Carbon-to-Nitrogen Ratio

Legume-based cover crop systems can also play an important role in the short- and long-term cycling of soil nitrogen and carbon in an agroecosystem. Legumes are generally lower in carbon and higher in nitrogen than grasses. This lower carbon-to-nitrogen ratio (C:N) results in faster breakdown of legume residues. Initially, this can result in a faster release of mineralized residue nitrogen (converted from organic to plant-available forms) to the system that is available for the subsequent crop.

Cool-Season and Warm-Season Legumes

Cool-season (winter) annual legumes are generally planted in the fall and provide forage in late fall and spring. These plants flower and produce seed in late spring and die soon after seeds mature. Cool-season annual legumes differ substantially in their preferred soil characteristics, growth distribution, cold tolerance, bloat potential, and reseeding potential.

Grasses

Grass cover crops include many species traditionally grown for grain (wheat, barley, etc.) and others commonly grazed or harvested for forage (annual ryegrass, sorghum-sudangrass, etc.) (Appendix A). Grass cover crop options also include perennial forages such as tall fescue and orchardgrass. Grasses are not able to “fix” or take nitrogen directly from the air for their nutrition. Thus, grasses require an ample supply of soil nitrogen for fast growth and high yield. Consequently, grass cover crops tend to be very good scavengers of soluble soil nitrogen. Grasses typically produce more above-ground biomass than either legumes or broadleaf species.

Carbon-to-Nitrogen Ratio

Grass-based cover crop systems can also play an important role in the short- and long-term cycling of soil nitrogen and carbon in an agroecosystem. Grass cover crops generally contain relatively high levels of carbon compared to nitrogen (i.e., they have a high C:N ratio). This higher C:N ratio results in slower breakdown of grass residues. The C:N ratio determines the general timing of nitrogen release from these residues.

Cool Season and Warm-Season Grasses

Grasses are often described by their growth cycle through a year: cool season and warm-season grasses. The main growing period for cool-season grass is in spring and fall when soil temperature is 50 to 65 degrees F (10–18°C), and the air temperature is 60 to 75 degrees F (16–24°C).

Brassicas

Brassicas used as cover crops include rapeseed, mustards, forage turnip, and forage radish (Appendix A). The two main benefits from this group of cover crops are weed suppression and breaking up soil compaction. Brassicas are also useful for controlling erosion, scavenging nutrients, and for producing a large amount of biomass in a short period of time which adds organic matter to the soil. Brassicas need lots of soil nitrogen to fuel their rapid growth and generally have deep root systems, so they can be excellent nitrogen scavengers. Forage radish and rapeseed in particular are known for their fast-growing, cylindrical taproots that can “biodrill” four or more feet down in a short-time and produce impressive macropores.

Forbs

The forb subgroup is a catch-all category for non-legume, non-brassica broadleaf cover crop species with many differing characteristics (Appendix A). Common forbs include buckwheat, black oilseed sunflower, and phacelia. Like brassicas, forb cover crops do not fix atmospheric nitrogen and need ample soil nitrogen to thrive. However, they do not generally have the deep roots or very fast growth rate of brassicas. Therefore, the forb subgroup has much lower nitrogen scavenging potential. The C:N ratio and durability of forb residues vary.

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