Greenhouse Experiments on the Effect of Green Manures upon Nitrogen Recovery and Soil Carbon Content

'•T-HE most efficient method of utilizing, fertilizer J. nitrogen is ordinarily by direct application to the growing crop if efficiency is measured in terms of immediate crop yields. If an indirect fertilization system is used where some or all of the commercial nitrogen is used to produce a green crop that is later' turned under, then the initial crop yields are almost invariably less. This is necessarily true since a portion of the added nitrogen remains in the soil in a slowly available condition combined with carbon. With successive additions of green manure crops the yields should gradually increase as the nitrogen tied up by earlier crops is released. This might not be true if leaching were a major factor, or if large gaseous losses of nitrogen occurred. The aims of the present experiments were (a) to compare the efficiency with which commercial nitrogen is used by successive crops when applied with or without plant materials of different stages of maturity, (b) to determine the extent to which soil organic matter is increased by successive additions of organic materials with and without fertilizer nitrogen, and (c) to obtain evidence of any gaseous losses of'nitrogen from the soil.