Cover crop
[4] Although cover crops can perform multiple functions in an agroecosystem simultaneously, they are often grown for the sole purpose of preventing soil erosion.
Often, green manure crops are grown for a specific period, and then plowed under before reaching full maturity to improve soil fertility and quality.
Green manure crops are commonly leguminous, meaning they are part of the pea family, Fabaceae.
[9] Another quality unique to leguminous cover crops is that they form symbiotic relationships with the rhizobial bacteria that reside in legume root nodules.
Bradyrhizobia are encountered as microsymbionts in other leguminous crops (Argyrolobium, Lotus, Ornithopus, Acacia, Lupinus) of Mediterranean origin.
[13] Such widespread environmental impacts include nitrogen fertilizer losses into waterways, which can lead to eutrophication (nutrient loading) and ensuing hypoxia (oxygen depletion) of large bodies of water.
An example of this is in the Mississippi Valley Basin, where years of fertilizer nitrogen loading into the watershed from agricultural production have resulted in an annual summer hypoxic "dead zone" off the Gulf of Mexico that reached an area of over 22,000 square kilometers in 2017.
[18] Catch crops are typically fast-growing annual cereal species adapted to scavenge available nitrogen efficiently from the soil.
An example of green manure use comes from Nigeria, where the cover crop Mucuna pruriens (velvet bean) has been found to increase the availability of phosphorus in soil after a farmer applies rock phosphate.
It is noted that if soil quality is properly managed and maintained, it forms the foundation for a healthy and productive environment.
[27] Just before cover crops are killed (by such practices including mowing, tilling, discing, rolling, or herbicide application) they contain a large amount of moisture.
[29] Furthermore, even when weed seeds germinate, they often run out of stored energy for growth before building the necessary structural capacity to break through the cover crop mulch layer.
[33][34] This occurs when certain biochemical cover crop compounds are degraded that happen to be toxic to, or inhibit seed germination of, other plant species.
Some well known examples of allelopathic cover crops are Secale cereale (rye), Vicia villosa (hairy vetch), Trifolium pratense (red clover), Sorghum bicolor (sorghum-sudangrass), and species in the family Brassicaceae, particularly mustards.
[32] In a 2010 study released by the Agricultural Research Service (ARS),[37] scientists examined how rye seeding rates and planting patterns affected cover crop production.
The results show that planting more pounds per acre of rye increased the cover crop's production as well as decreased the amount of weeds.
In sum, the results from this study support an understanding that phylogenetic relatedness can be harnessed to significantly suppress weed growth.
[41][42] Species in the family Brassicaceae, such as mustards, have been widely shown to suppress fungal disease populations through the release of naturally occurring toxic chemicals during the degradation of glucosinolate compounds in their plant cell tissues.
In some organic systems, farmers drive over the trap crop with a large vacuum-based implement to physically pull the pests off the plants and out of the field.
[46] Another example of trap crops is nematode-resistant white mustard (Sinapis alba) and radish (Raphanus sativus).
Researchers found that the planting of several different leguminous cover crops (such as bell bean, woollypod vetch, New Zealand white clover, and Austrian winter pea) provided sufficient pollen as a feeding source to cause a seasonal increase in E. tularensis populations, which with good could potentially introduce enough predatory pressure to reduce pest populations of citrus thrips.
