The Effect of Green Manure Crops of Varying Carbon‐Nitrogen Ratios upon Nitrogen Availability and Soil Organic Matter Content 1

T HE desirability of maintaining or increasing soil organic matter is well recognized, but the addition of the necessary plant materials to accomplish this aim may produce a marked decrease in nitrogen availability. Such a decrease almost invariably occurs when mature carbonaceous materials are added. Young succulent materials may produce no such depression, but the recovery from a unit of nitrogen added in these materials is commonly lower than from a similar quantity of nitrogen added as commercial fertilizer. The nitrogen that is tied up in the added green crops is slowly released later. Successive additions of green crops, or of mixtures of carbonaceous materials with fertilizer nitrogen, should therefore produce larger and larger yields as the inactivated nitrogen from the earlier additions is released. This might not, of course, occur if the nitrogen loss were large. The studies here reported were planned to furnish information on the points mentioned. More specifically, the objectives were (a) determine the efficiency of utilization of commercial nitrogen applied with and without green manure crops of various ages, (b) to study the rate of release to successive crops of nitrogen from green manure crops of varying carbon-nitrogen ratios, (c) to obtain quantitative data on the extent to which the added organic materials and fertilizer nitrogen affect the soil organic matter content, and (d) to determine if gaseous losses of nitrogen are appreciable. The present final report supplements the preliminary results already reported (6). a