114 ELEMENTS OF AGRICULTURE 



cannot tell how much of this the plant is able to get. A 

 soil may contain enough phosphoric acid for a hundred 

 crops, and yet the addition of phosphoric acid may be 

 beneficial, because the plant may be unable to get this 

 food in soluble form. 



A chemical analysis is of some value. It shows the 

 maximum limitations of a soil. It is quite desirable to 

 know how great a store of the plant-foods there is in a 

 soil, in order to provide a permanent agriculture. If 

 there is potash enough for a thousand years, we may still 

 add it in the fertilizers, if it pays, but we should certainly 

 try to find some way of unlocking that which is already 

 in the soil. But, if a soil contains potash enough for only 

 50 crops, we may well plan to add this food every year. 



Some of the peaty soils in Illinois contain only enough 

 potash for 41 crops of corn, each yielding 100 bushels. 

 These soils give greatly increased yields when potash is 

 added. In general, muck soils are deficient in potash. 



The gray silt loams of southern Illinois contain in the 

 surface soil enough phosphoric acid for 70 such crops of 

 corn, and enough potash for 1,900 such crops. 1 Evi- 

 dently one should try to draw on the supply of potash that 

 is in the soil, and should add phosphoric acid. These 

 soils were once so productive that southern Illinois was 

 called "Egypt," but they are now very unproductive. 

 By the use of lime, phosphoric acid and legumes, these 

 soils are easily made to produce good crops once more. 

 Most of the soils in Illinois are deficient in phosphoric 

 acid. 



113. Materials Used as Fertilizers. The oldest and best 



Illinois, Bulletin No. 123 



