HOW 8OIL8 LOSE NITROGEN 107 



themselves in their turn die, decay, and pass back into 

 the soil; and by this process of gradual growth and 

 decay but little nitrogen is lost. But the conditions 

 .ire \«rv different in cultivated fields. Here quantities 

 of nitrogen-containing organic matter are each year 

 taken from the soil, and but little returned. A crop 

 of hay, for instance, weighing about 2,000 pound-, 

 takes from the ><>il about 1,700 pounds of organic mat- 

 ter, which contains about twenty-five pounds of nitro- 

 gen. And so each crop taken from the soil removes 

 a certain amount of nitrogen. Nearly all of this nitro- 

 gen comes from the decay of organic matter in th 

 and unless fresh organic matter be added to replace 

 that used up, the supply must in time beOOUfc 

 hausted, and the bacteria of decay be left without 

 material from which to manufacture compound! of 

 nitrogen. Suppose a crop removes from the -oil twenty- 

 ti\<- pounds of nitrogen; if the organic matter in the 

 soil contains about 1 per cent of nitrogen. >uch a crop 

 would require 3,500 pounds to supply it< nitrogen. The 

 roots and stubble from crop- do -ay in the -<>:l, :ind ire 

 converted into humus and some compounds of nitro- 

 gen, hut they cannot supply enough nit rog en to replace 



that removed by the crops. After each crop is removed 



the soil should he supplied with organic matter which 



may be converted by the bacteria into fresh supplies 



of humus. After some crop-. >u« h M wheat and 

 oats, crops of weeds spring up, which, when plowed 

 under, supply fresh quantities of organic matt. r. Bui 

 other « rops, such as tobacco and cotton, lea\ 

 with only a few scattered roots and stubble. When 



