aREEN-MANURES 539 



310. Gain, of constituents by green-manuring. — In an 



average crop of green-manure, from five to ten tons of mate- 

 rial are turned luider. Of this, from one to two tons are dry 

 matter, and from four to eigiit tons water. Of this dry matter, 

 a great proportion is carbon, hydrogen, and oxygen. It might 

 seem at first thought that such an addition is pure gain as 

 far as carbon and carbonaceous matter are concerned. Such 

 is not the case. Large amounts of carbon are lost continu- 

 ously in drainage, to say nothing of that removed by crops or 

 that which is respired by the soil as carbon dioxide. It has 

 already been shown, from results obtained with the Cornell 

 lysimeters, that a heavy soil will yearly lose over 250 pounds 

 of carbon, in drainage alone (see par. 220). This is approxi- 

 mately equivalent to a 2-ton application of green-manure. 

 Although the loss of carbonaceous material is considerable, 

 even during the period that the green-manuring crop is being 

 grown, nevertheless the practice offers a rapid as well as a 

 natural means of increasing the soil organic matter. 



The mineral parts of the turned-under crop came from the 

 soil originally and they are merely turned back to it again 

 and represent no gain. As they return, however, they are in 

 intimate union with organic materials, and are thus readily 

 available as the decay processes go on. Indeed they are prob- 

 ably more readily available than they previously were, when 

 the green-manuring crop acquired them. 



The amount of nitrogen added to a soil if the green-manure 

 is a legume ^ is an uncertain quantity. Much depends on the 

 virulence of the organisms occupying the nodules. These bac- 



^ Smith, G. D., and Robinson, F. W., Influence of Nodules on the Boots 

 upon the Composition of Soyhean and Cowpea; Mich. Agr. Exp. Sta., 

 Bui. 224, 1905. 



Hopkins, C. G-., Alfalfa on Illinois Soil; 111. Agr. Exp. Sta.^ BiiL 76, 

 1902. 



Hopkins, 0. G., Nitrogen Bacteria and Legumes; 111. Agr. Exp. Sta., 

 BuL 94, 1904. 



Shutt, F. T., The Nitrogen Mnriehment of Soils through the Growth 

 of Legumes; Canadian Dept. Agr., Kept. Centr. Exp. Farms, 19 05^ pp. 

 127-132. 



