116 The Potato 



of a fertilizer for any crop is determined by the chemical 

 analysis of the crop itself is that it does not sufficiently 

 take into account the great natural stores of plant-food 

 in the soil, nor the biological conditions. The plant-food 

 needs of the growing plant at any time are not necessarily 

 based on its final composition. The presence of a par- 

 ticular number of pounds of an element in soluble form 

 somewhere among the millions of pounds of soil on an 

 acre is no guarantee that the plant roots will be able to 

 find and use it at the time when it is most needed. 



Growers in specialized potato-growing regions like Aroos- 

 took County and the early truck potato section apply more 

 plant-food than the potato can be expected to remove in 

 the crop, in order to make certain that it will not have to 

 suffer for a lack at any time. The effect of this practice 

 is seen in the subsequent crops. In southern trucking 

 regions, the choice of the crop to follow heavily fertilized 

 potatoes is largely governed by its ability to make good 

 use of the fertilizer left by the potatoes. The longer ex- 

 perience of European farmers with fertilizers has resulted 

 in England in laws by which the value of the fertilizers left 

 in a soil by crops is computed. Under the tenant system, 

 by which most of the farms of England are worked, an 

 outgoing tenant must be paid by his successor for the 

 unused fertilizer left. This proportion of the value of the 

 fertilizers applied varies with different materials, being 

 lowest with the soluble nitrate of soda and highest with 

 such slowly acting materials as ground bone and lime. 



' MARKET FORMS OF FERTILIZERS 



Commercial fertilizers are found on the market in the 

 form of many different materials which are also manu- 



