100 THE PRINCIPLES OP AGRICULTURE 



157. The cheapest fertilizer is the one in 

 which one dollar purchases the greatest amount 

 of plant-food. In No. 1, $29 obtained $13.80 

 worth, which is at the rate of 48 cents worth 

 for $1. In No. 2, $38 buys $24.90 worth of 

 plant-food, or at the rate of 65 cents worth for 

 the dollar. The difference between the commer- 

 cial value, as calculated, and the selling price, 

 is to cover expenses of manufacture, bagging, 

 shipping, commission fees, and profits. 



66. Advice as to their use 



158. We have seen that plants must have 

 all three of the general fertility elements nitro- 

 gen, phosphoric acid, potash in order to thrive. 

 It frequently occurs, however, that the soil is 

 rich enough in one or two of them ; and in that 

 case, it is not necessary to apply all of them. 



159. If a liberal application is made of one 

 element, the plant must use more of the other 

 elements which are already in the soil, in order 

 to balance up its growth. It may result, there- 

 fore, that the addition of one element exhausts 

 the soil of some other element. For example, 

 if heavy growth is obtained by the addition of 

 nitrogen, the plant may need to draw so 

 heavily upon the stores of available phosphoric 

 acid as to deplete the soil of that material. 



