236 



SOILS AND FERTILIZERS 



and lime, in drainage water. If, therefore, green-manure crops 

 cover the soil, when otherwise nothing would be growing on it, 

 they exercise a protective action. In the case of orchards a 

 green-manure crop saves much nutriment as compared with 

 clean cultivation. A catch-crop, like rye, that is sown in the 

 fall after a summer crop has been harvested and is plowed 

 under in the spring, saves some plant-food material. 



295. Materials supplied by green-manures. — Probably 

 the most beneficial effect exerted by green-manures is the ad- 

 dition of organic matter to soil. Practically the only source 

 of organic matter is in the form of farm manure or of plant 

 residues. Farm manure is yearly becoming more scarce 

 and expensive. Some substitute must be found. 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 is water. This would 

 correspond to a dressing of four to eight tons of farm manure, 

 so far as the organic matter alone is concerned. 



Legumes add nitrogen as well as organic matter. The 

 nitrogen contained in a ton of the green crop, when in a con- 

 dition to plow under, is as follows : 



Table 55. — Quantities of Nitrogen in Some Leguminous 

 Green-Manure Crops 



Crop 



Red or mammoth clover 

 Crimson clover . 

 Alsike clover .... 



Alfalfa 



Cowpeas 



Soy beans 



Canada field peas . . 



Nitrogen 



per Acre, 



Pounds 



60 



54 



50 

 112 

 48 

 60 

 55 



