SOILS AND FERTILIZERS 



497 



It has been found that all plants vary widely in the relative 

 amount of these materials they contain. The same plant varies 

 when grown on different soils or in different seasons on the same 

 soil. The cause for the variation in nitrogen has been fully ex- 

 plained. The reason for the variation as regards the other sub- 

 stances which are obtained wholly from the soil are obscure. In the 

 accompanying table it will be noted that the variation in legumes as 

 regards phosphorus and potash is nearly as great in any one legume 

 as it is between different legumes. It has been suggested by several 

 writers that certain green-manure crops by feeding heavily on phos- 

 phorus and potash make these substances more available to subse- 

 quent crops. It is very doubtful whether this is true, but if it is there 

 is little apparent advantage of any one legume over another in this 

 respect. 



Fertilizing Materials in 100 Pounds of Dry Substance. 



[Compiled from various sources.] 



Green Manuring Compared with Feeding. Barnyard manure 

 contains from 75 to 90 per cent of the total fertilizing substance in 

 the feeds used. It is beyond question a better practice from all 

 standpoints to feed a crop, whether leguminous or nonleguminous, 

 and then distribute the manure over the land, than it is to plow 

 under the whole crop. This practice utilizes the whole feed value of 

 the crop. Further, it returns an important part of it to the soil in a 

 highly available form for plant food. Unquestionably this is the very 

 best farm practice in maintaining soil fertility. Were it possible on 

 any farm to feed the entire product to live stock, returning the 

 manure to the soil, the productivity of that farm would be main- 

 tained indefinitely. The only depletion that would occur would be 

 the comparatively small amount of potash and phosphorus sold in 

 the live-stock product. The loss of nitrogen would be more thap 

 maintained by the activity of the soil bacteria, 



