86 



FIG CULTURE 



have run out at last. Yet at the end of such periods 

 chemists find the same quantity, and proportion, of 

 potash and phosphorus that existed when the fields 

 produced abundantly. 



To illustrate again: potatoes failing, a normal 

 yield of wheat may be obtained from the same land. 

 "Wheat uses potash and phosphorus in the same 

 forms potatoes do. The relative proportion of min- 

 erals consumed by the former varies slightly from 

 that used by the latter, but in either case the two 

 minerals are essential to support plant growth. 

 That wheat will thrive where potatoes failed shows 

 that potash and phosphorus are not only abundant, 

 but. further, are present in forms suited to vege- 

 table uses. If abundant, soluble and available, then 

 we must look for some other fact besides the sup- 

 posed absence of plant food to account for soils 

 wearing out by continuous cropping. Scientists 

 have been searching for that result a long time; 

 they have looked fpr some substance gradually in- 

 creasing in quantity while one crop is grown ex- 

 clusively, deleterious to it. and yet not injurious 

 to other plants; that substance has been found to 

 be their excreta. 



"If there are toxic substances thrown off by 

 plants which the soil is not in a condition to re- 

 move, or change at once, we try to change it at 

 once by cultivation, by aeration, by oxidation." 

 (U. S. Bull.. 257.) This idea, though recently 



