LEGUMINOUS CROPS AND FERTILITY 297 
peas, beans, vetch, clover, and lucern, are able to 
secure large quantities of nitrogen from the air 
through the activity of bacteria that live and grow 
on the roots of such plants. The leguminous crop 
should be sown in the usual way, and when it is well 
past the flowering stage should be plowed into the 
ground. Naturally, annual legumes, such as peas 
and beans, should be used for this purpose. The 
crop thus plowed under contains much nitrogen, 
which is gradually changed into a form suitable for 
plant assimilation. In addition, the acid substances 
produced in the decay of the plants tend to liberate 
the insoluble plant-foods and the organic matter is 
finally changed into humus. In order to maintain a 
proper supply of nitrogen in the soil the dry-farmer 
will probably soon find himself obliged to grow, every 
five years or oftener, a crop of legumes to be plowed 
under. 
Non-leguminous crops may also be plowed under 
for the purpose of adding organic matter and humus 
to the soil, though this has little advantage over the 
present method of heading the grain and plowing 
under the high stubble. The header system should 
be generally adopted on wheat dry-farms. On 
farms where corn is the chief crop, perhaps more 
importance needs to be given to the supply of organic 
matter and humus than on wheat farms. The 
occasional plowing under of leguminous crops would 
be the most satisfactory method. 
