CROP ROTATION 21 



one time became so foul with weeds that it was necessary 

 to devote a year to fallowing before another crop could be 

 grown. 



(3) Legume crops help out on the nitrogen supply. 

 A rotation of crops gives an opportunity to include some 

 legume, especially clover or alfalfa, which will leave the 

 soil richer in nitrogen than before the crop was grown. 

 If one-fourth or more of the rotation is devoted to one of 

 these crops and if a part of the crop, as well as the roots 

 and stubble, are turned under, and perhaps also an occa- 

 sional catch crop of some other legume is plowed under, 

 it is possible to supply in this way sufficient nitrogen for 

 all the other crops in the rotation, and thus the supply 

 of this element be maintained indefinitely. It must be 

 remembered, however, that legumes as well as any other 

 plants can feed upon the nitrogen already in the soil and 

 in fact always take a part of their supply in this way, so 

 that if the crop is entirely removed and only the roots and 

 stubble plowed under, the soil is seldom being enriched in 

 nitrogen and may, in fact, be somewhat reduced. Clover, 

 for example, is known to take, under average soil condi- 

 tions, about one-third of its nitrogen from the atmosphere. 

 Now the roots and stubble of clover seldom amount to 

 more than one-third of the crop, and, as nitrogen is con- 

 stantly being lost from the soil by leaching, it may often 

 happen that a clover crop in the rotation does not increase 

 the nitrogen supply at all, though, of course, it does not 

 deplete this supply to the extent of a non-legume crop. 



(4) Heavy sods supply organic matter. Crops such as 

 grasses, and clovers, which keep the surface soil well 

 filled with a mass of fibrous roots, increase materially the 

 organic matter content of the soil. When this sod is 

 plowed under, the soil is much benefited in respect to 



