318 Manures, Composts, Green Manures 



the land is free from a crop, the nature of the soil, and the rotation 

 system. (2) To prevent the nutrient elements of the soil, especially 

 the nitrates, from leaching out during the part of the year when no 

 cultivated crops are being grown on the soil. (3) To increase the 

 supply of organic matter in soil. (4) To protect the soil against 

 erosion. 



The plants used for green manuring are high in water-soluble 

 constituents, in nitrogen, and in minerals; they are comparatively 

 low in cellulose and in lignin. As a result, decomposition of a green 

 manure crop plowed into the soil sets in very rapidly. This is accom- 

 panied by rapid liberation of the nitrogen and the minerals in avail- 

 able forms; comparatively little humus is produced. Figure 48 

 illustrates the difference between the chemical composition of young 

 plants and of mature plants and the influence that this has upon the 

 decomposition of the various organic constituents of these materials. 

 As the plants grow older, their ash and nitrogen contents decrease, 

 and their cellulose and lignin contents increase. Decomposition of 

 younger plants results in liberation of some of the nitrogen as am- 

 monia; the younger the plant and the higher the nitrogen content, 

 the more rapidly is ammonia liberated and the greater is the amount 

 liberated. Beyond a certain stage, no ammonia will be liberated; 

 actually, nitrogen may have to be added to hasten decomposition, 

 as already shown. 



The plants grown for green manuring and plowed into the soil 

 can be divided into three categories : ( 1 ) those that contain a cer- 

 tain balanced proportion of available carbohydrates to nitrogen; (2) 

 those that contain an excess of nitrogen, or more than is required for 

 decomposition of the carbohydrates; and (3) those that contain an 

 excess of carbohydrates and lignin over nitrogen. The third group, 

 comprising both legumes and nonlegumes, decompose more slowly 

 than the plants of the other two categories. 



Table 71 shows the relative amounts of total plant material and 

 nitrogen in a number of plants used for green manuring, and the 

 effect of such manures on crop growth and loss of humus from soil, 

 as compared to the plowing under of weeds only. Vetch and crim- 

 son clover produced the largest amounts of growth, contained the 

 highest amounts of nitrogen, and gave the highest crop yields of 

 corn after plowing under of the green manure. Wheat and rye used 

 as green manures gave the highest percentages of plant material in 

 the roots, the lowest nitrogen contents in the plant material, and the 

 poorest effects upon the corn crop; they also resulted in the smallest 



