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Plant Cover Crops

2 Spoke WheelThe second spoke is cover crops. Primarily grown for non-commercial purposes at times when soil would otherwise be without living vegetation, cover crops are used to:

Provide soil erosion protection. This is especially important if crop residue cover is minimal after harvest of the main crop.Leachate nitrate-N

Absorb, retain, and recycle nutrients. Some nutrients, such as nitrate, are soluble and can easily leach below the root zone. Cover crops take up soluble nutrients, protecting them from leaching. In a survey of cover crop biomass on working farms in different parts of Pennsylvania, every ton of dry matter (mostly rye) contained on average 57 pounds of nitrogen. Without cover crops, much of this nitrogen would have leached to groundwater and been lost to streams and rivers.

Maryland research confirmed that nitrate concentrations in soil water decreased remarkably when a cover crop of rye was used (See figure at right).

Cover crops can make nutrients available from soil and release them to following crops upon decomposition. Some cover crops release organic acids that solubilize nutrients from soil particles. Research Maryland showed that phosphorus was concentrated in the taproots of forage radish. Available phosphorus concentrations were increased where the root had decomposed in the spring (See figure below).

Phosphorus ConcentrationsFix atmospheric nitrogen. Leguminous cover crops fix large amounts of nitrogen from the atmosphere because of a unique bacterial infection in their roots that is beneficial to lhe legume and the bacterium. Because of this symbiotic relationship, no nitrogen has to be applied to these crops. The bacteria are called rhizobium, and different legumes are adapted lo different rhizobium species. These bacteria can survive for many years in the soil. However, when the legume has not been grown in a field for a long period, it may be necessary to re-introduce the bacterium by mixing it with the seed prior to planting. Examples of legumes are alfalfa, soybeans, snap beans, hairy vetch, peas, red, white and crimson clover, cowpea, sunn hemp, and fava bean. While direct transfer of nitrogen from the living legume to companion crop has been shown to be small, when part or all of the legumes dies, the proteins in their cells decompose, releasing nitrogen that can be absorbed by another crop. A hairy vetch cover crop can supply all the nitrogen needed by a following corn crop, while a crimson clover cover crop can supply roughly 80-100 pounds per acre of nitrogen-equivalent to the following crop.

Provide weed control. There is no practice that provides complete weed control - even herbicides provide weed control only for a short period of time, after which the growing crop provides weed control by out-competing them. Cover crops also provide weed control by competing with weeds when they grow. Some cover crops release chemicals that inhibit germination and early growth of certain weeds, and the mulch left behind supplies a physical barrier and light control mechanism that inhibits weed emergence.

Provide forage. Although cover crops are not grown primarily for commercial purposes, they can be used tor grazing, green chop, silage, or hay if needed. The use of cover crops for forage is beneficial because more feed is produced on the farm and fewer nutrients are imported in purchased feeds. This helps address the nutrient imbalance in Pennsylvania (nutrient importation in feed). Another important benefit of using cover crops for forage is that farmers are motivated to plant and manage the cover crops carefully to grow feed while providing environmental benefits at the same time. When cover crops are used for feed, it is recommended to leave a portion standing and provide enough time for regrowth to help feed soil microbes and provide soil armor.