172 



SOIL PHYSICS AND MANAGEMENT 



supplied with organic matter the rotation should be quite different 

 from that on soils deficient in this constituent. In the former 

 case much of the residues might be sold from the farm, while in 

 the latter much the larger part should be returned to the soil. One 

 essential of a rotation for soil improvement is at least one legume 

 crop during the cycle. Soils deficient in organic matter should 

 have a more frequent recurrence of this crop, as the value of the 

 rotation in improving the soil depends primarily on the use of it. 

 The legume should be turned back into the soil whenever possible. 

 If it is removed and nothing returned in its place very little or 

 nothing is gained for permanent soil improvement and maintenance. 

 f ' " 



Fid. 86. Adding organic matter to the soil in the form of sweet clover. 



Clover and cowpeas are commonly grown. The best one to grow 

 on the grain farm is that which provides the largest amount of 

 material to turn under. Medium red clover is most common in the 

 northern states, but alsike or sweet clover is better adapted to 

 somewhat poorly drained soils. Mammoth or English and sweet 

 clover probably furnish the largest amount of material to plow 

 under, and both plants will furnish a fair crop of seed, upon which 

 the farmer must depend for his immediate returns. It requires 

 the very best conditions for red clover to produce three tons per 

 acre for both crops, which is at least one ton above the average. 

 Sweet clover is an excellent legume for soil improvement because of 

 its large growth (Fig. 86) and deep rooting characteristics. 



