Green Manures 



317 



moisture prevents absorption of the heat. A low moisture, a high 

 teuiperaturc, and a lack of oxjgen penetration may thus create con- 

 ditions favorable for spontaneous heating. Losses of organic matter 

 during spontaneous heating of alfalfa were found to be largely at 

 the expense of the fats, sugars, and hemicelluloses, and to a lesser 

 degree of the cellulose and protein; lignin suffered no loss at all. 

 Absorption of oxygen by the lignin, accompanied by a rise in tem- 



100 



90 



80 

 2 70 

 >• 60 

 '^ 50 

 "^ 40 



30 



20 



10 





 1870 1880 1890 1900 1910 



Fig. 127. Residual effects of liea\y applications of manure (from Hall). 



perature to the ignition point, was believed to lead to the actual 

 ignition. It has been suggested that addition of salt ta moist alfalfa 

 hay will inhibit bacterial development and thus delay the process of 

 decomposition long enough to permit curing. 



Green Manures 



Green manures comprise plant crops grown in a given soil to a 

 certain stage of development and plowed under while still green. 

 Both leguminous and nonleguminous plants are utilized for this 

 purpose. The nature of the plant to be selected for green manuring 

 depends upon the soil and the climate and upon farming practice. 



Green manures serve several distinct purposes for plant growth 

 and soil improvement: (1) To increase the supply of total and avail- 

 able nitrogen in the soil. Various leguminous plants are utilized for 

 this purpose. The nature of the legume thus selected depends 

 largely on the geography of the region, the season of the year when 



