Influence of leguminous plant additions on the organic matter content and available nutrient supply of southern soils

Studies dealing with the addition of leguminous plant materials to soil as green manures, hay mulches, and accumulated residues were made to determine the effects of these various methods of application on the organic matter content of soils, the available soil nutrient supply, and yields of succeeding crops. The results obtained warrant the following conclusions: The average organic matter content of the cultivated soils in the Piedmont section of South Carolina is approximately 1%, while plant materials supplied by growing lespedeza, vetch, and crimson clover have raised the percentage of organic matter to 1.5. Only where lespedeza is grown continuously on the same soil for a number of years from 2.5 to 2.7% have been attained, indicating that amounts as high as 2.7% would be difficult to maintain under ordinary farming practices. Analyses for nitrate nitrogen show that under field conditions check soils produced a maximum of 41 p.p.m., while 4 tons of crimson clover applied either as a mulch on the surface, or incorporated with the soil accumulated 80 p.p.m. The lespedeza accumulated residues treatment maintained approximately 10 p.p.m. and remained at this level until these residues were incorporated, when 61 p.p.m. of nitrate nitrogen developed after 120 days of incubation. The acid-soluble phosphorus tests showed that soils from nonlegume areas contained 16 p.p.m. of available phosphorus, whereas 25 to 30 p.p.m. were found in the soil receiving the leguminous plant material. The replaceable potassium content varied with treatment, giving 48 to 64 p.p.m. for nonlegume plats, while the legume-treated soils contained from 136 to 188 p.p.m. Significant yield differences were secured for sorghum and rye from the addition of plant materials to the soil over that produced on the adjacent soil receiving only commercial fertilizer. These results suggest that leguminous plant additions are beneficial for soil improvement not only for the nitrogen supplied, but also because they render phosphorus and potassium more available.