130 FERTILIZERS 



This experiment shows in the most convincing manner the 

 tendency of burned lime to destroy fertility. In every case 

 it is noticed that the limestone rock produced higher yields. 

 This experiment incidentally shows that it often takes a 

 long time to bring even the most careful work to any con- 

 clusion that will stand the severest tests. Farmers should 

 therefore be cautioned against arriving at conclusions from 

 their home experiments carried on for a brief period only. 



Too much lime is harmful. The best practice is to 

 make small but frequent applications ; it is not well to try 

 to add enough lirne at one time to last a generation. 



88. What Fertilizers to Apply. A few years ago 

 nearly all farmers believed that the sole purpose of fer- 

 tilizers was to supply a plant food exhausted from the 

 soil. They reasoned as follows: "Fifty bushels of 

 corn remove from the soil about 74 pounds of nitrogen, 

 12 pounds of phosphorus and 35 pounds of potassium. 

 Growing corn on the same field for a few years, then, 

 must exhaust the supply of these food elements ; and so 

 fertilizers must be supplied to make good the loss." 

 While men argued in this way, they thought that a 

 chemical analysis of the soil was all that was needed to 

 show what fertilizers should be supplied. 



But now we have learned (Chapter VIII) that in an 

 average soil there is enough of all these plant foods to 

 last for two generations of yearly cropping. We have 

 also learned, however, that only a small part of this food 

 is available. That is, only a small part of it at any one 

 time is soluble, or in a form that can be used by plants. 

 The addition of a little fertilizer, however, may cause a 

 marked increase in a crop yield. This is because the fer- 

 tilizer may not only add plant food directly to the soil, 

 but may also release for use some of the store before 

 imprisoned in the soil in unusable forms. 



