316 



THE SMALL GRAINS 



These not only improve the physical condition of the soil, 

 add humus, and make certain existing mineral foods avail- 

 able, as stable manure does, but they are much the cheap- 

 est means of supplying nitrogen, which they obtain from 

 the air (Fig. 92). With green-manures in a good rotation, 

 and returning straw, cornstalks, and other plant residues, 

 not fed, to the land, the humus supply can be fairly 

 maintained without stable manure. However, all of the 



FIG. 92. Plowing under cowpeas to improve the soil in Virginia. 



latter that it is possible to obtain should be used. In a 

 complete cropping system such as conducted by the 

 Ohio Experiment Station previously described (339), 

 if stable manure is unavailable, a green-manure is readily 

 provided by turning the clover under, and the phosphate 

 used in reenforcing the stable manure may be applied 

 directly to the clover sod. 



342. Crop rotations. All the various soil amend- 

 ments previously discussed are only made thoroughly 

 effective when joined with a proper rotation of crops into 

 a complete cropping system. Continuous cropping of 



