GREEN MANURES 621 



523. Quantities of plant constituents added by green- 

 manuring. — In an average crop of green manure, from 

 five to ten tons of material is turned under. Of this, 

 from one to two tons is dry matter, and from four to eight 

 tons water. Of this dry matter a great proportion is car- 

 bon, hydrogen, and oxygen — a clear gain to the soil in 

 so far as these constituents are concerned. The amount 

 of nitrogen added to a soil if the green manure is a legume * 

 is a difficult question to decide. Much depends on the 

 virulence of the organisms occupying the nodules. These 

 bacteria are in turn much influenced by plant and soil 

 conditions. Hopkins 2 estimates that about one-third of 

 the nitrogen in a normal inoculated legume comes from 

 the soil and two-thirds from the air. He also considers 

 that one-third of the nitrogen exists in the roots. It is 

 evident, therefore, that in general the nitrogen found in 

 the tops will be a rough measure of the nitrogen fixed by 

 the soil organisms. If this is returned to the soil, there 

 is a clear gain of just that amount. 



If the preceding assumption is correct, clover 3 would 

 actually add to every acre about 40 pounds of nitrogen 



1 Smith, CD., and Robinson, F. W. Influence of Nodules 

 on the Roots upon the Composition of Soybean and Cowpea. 

 Mich. Agri. Exp. Sta., Bui. 224. 1905. 



Hopkins, C. G. Alfalfa on Illinois Soil. Illinois Agr. Exp. 

 Sta., Bui. 76. 1902. 



Hopkins, C. G. Nitrogen Bacteria and Legumes. Illinois 

 Agr. Exp. Sta., Bui. 94. 1904. 



Shutt, F. T. The Nitrogen Enrichment of Soils through the 

 Growth of Legumes. Canadian Dept. Agr., Rept. Centr. Exp. 

 Farms, 1905, pp. 127-132. 



2 Hopkins, C. G. Soil Fertility and Permanent Agriculture, 

 p. 223. Boston, 1910. 



3 Penny, C. L. The Growth of Crimson Clover. Delaware 

 Agr. Exp. Sta., Bui. 67. 1905. 



