Green manures are crops that are grown to enhance soil fertility and include legumes such as red clover, sweet clover, and hairy vetch. These legumes are an important source of nitrogen because they have the ability to fix nitrogen to the soil when they are plowed down and are therefore an economical way to provide nitrogen. With the advent of the synthetic nitrogen industry and the resulting availability of inexpensive nitrogen, many farmers discontinued using green manures to supply their nitrogen needs. In a sustainable system, green manures are used instead of a synthetic chemical form of nitrogen, such a anhydrous ammonia, due to the concerns about the effects of anhydrous on the soil. Research has shown that anhydrous is toxic to soil microbes and earthworms.
Soil testing is an important component in an sustainable system.
The soil is sampled and then tested for pH, organic matter, phosphorous, and potassium. Additional tests can be requested for calcium, magnesium, sulfur, boron, manganese, and zinc. Recommendations are based on the average soil test values for all samples from a given field. These recommendations are then used to make decisions about present and future fertilizer applications. Soil testing is the most important single guide to the profitable application of fertilizer and lime.