Green manuring and related practices

Among the important points that determine the suitability for use as a green manure, aside from climate, season, and soil, are availability and cost of seed, rate and amount of growth, how well it fits into the cropping system, primary purpose for which it is grown, type of root system, ability to compete with weeds, ease of incorporation of the crop into the soil, probable effect on the subsequent crop, and many others. 

These subjects are discussed rather fully by Piper and Pieters (1925), Pieters (1927), McKee (1935), McKee and McNair (1948) and Terman (1949). 

From an agricultural viewpoint. Piper and Pieters classify leguminous green manure crops as (1) summer annuals, chiefly soybeans, cowpeas, peanuts, beans, velvet beans, common vetch and field peas (2) winter annuals, including hairy vetch, common vetch, crimson clover, bur clover, field peas and red clover (in the south) and (3) biennials or perennials, comprising red clover, alsike clover, white clover, alfalfa and sweet clover. Each of these crops is adapted more or less definitely to a limited region. 

Pieters mentions numerous other legumes that are grown less commonly in the United States and also several that have proven to be especially suited to conditions in various European countries, China, Japan, India, and the Tropics. 

Extensive experience in the use of green manures over many centuries, especially during the last two, has shown that they are usually very beneficial. Occasionally harmful effects may be experienced but these can usually be avoided by following good farm management practices. 

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