Green Manures 



319 



loss of humus. Whereas the liunuis loss for the weed plot was 0.08 

 per eent of the carbon, losses lor tlu- wheat and r\e plots were only 

 0.01 pi-r eent. It is important to note that, even with heavy apph'- 

 eations of green manures for 5 years, the humus content of the soil 

 decreased. 



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When comparatively young plant materials are used as green 

 manures, there is danger of a loss of nitrogen through volatilization 

 as ammonia, the loss depending upon the amount of total nitrogen, 

 as well as of readily decomposing nitrogen compounds in the green 

 plant material. Young plants, low in lignin and in cellulose, but 

 high in water-soluble substances and in nitrogen, decompose much 

 more rapidly than do mature plants; they leave a much smaller resi- 

 due in the form of humus; and only a small part of the original nitro- 

 gen is stored away in this humus. In the case of more mature plants, 

 a considerably larger amount of humus is left in the soil, because of 

 slower decomposition of the plant material and because of the higher 

 lignin content; smaller quantities of the plant nutrients are liberated 

 in decomposition of these materials. In many instances, consider- 

 able time may elapse before the nutrient elements, especially the 

 nitrogen, are liberated in forms available for plant growth. The age 

 of the plant used for green manuring exerts an important influence 

 upon the amount and rapidity of liberation of the nutrient elements 



