SOIL IMPROVEMENT. 99 



either case, a sale crop, such as wheat, corn, oats, barley, 

 potatoes, or other non-legume, should be grown, to be 

 followed by a legume again, either the following season, 

 or the second season after. In this way less nitrogen 

 fertilizer need be bought, and this comprises practically 

 the whole value of legumes as a fertilizer. 



PART II. 



HOW TO EXPERIMENT. 



In Part I is given in detail the general nature of 

 legumes or nitrogen gatherers, and the best use to be 

 n. ade of them. At best, however, legumes are only an aid> 

 and on many farms and plantations legumes cannot be 

 gr )wn regularly in such manner as to supply most of the 

 nitrogen fertilizer needed. There also remains to be 

 determined on all farms the natural richness of the soil, 

 that is, its contents in available potash, phosphoric acid 

 and nitrogen. Chemical analysis informs us how much 

 plant food a soil contains, but unfortunately it does not 

 indicate what portion of this plant food is in an available 

 or useful form. A soil, as we have shown in the preceding 

 chapter, may contain potash enough for a hundred crops, 

 and still fail for lack of available potash plant food. 



