SOILS AND FERTILIZERS 461 



necessary in the rational use of fertilizers to study the needs of the 

 crop as well as of the soil. On certain soils and under certain condi- 

 tions of crop rotation it may be advisable to provide for a better cir- 

 culation of soil fertility by the use of the more slowly available, and 

 therefore cheaper, forms of nitrogen and of phosphoric acid. Thus, 

 there may be conditions, as already indicated, where finely ground 

 phosphate rock, when used on soils well supplied with rapidly de- 

 composing organic matter, may serve as a more economical source of 

 phosphorus than superphosphate. 



The fact that fertilizers may now be easily secured and are 

 easily applied has encouraged careless use rather than a thoughtful 

 expenditure of perhaps an equivalent amount of money or energy 

 in the proper preparation of the soil for them. Of course it does 

 not follow that no returns are secured from plant food applied under 

 unfavorable conditions, but it needs to be emphasized that full re- 

 turns can not be obtained under such circumstances, either with or 

 without fertilizers. 



It should be the aim in the use of commercial fertilizers to sup- 

 plement the plant food derived from the soil itself in such a manner 

 as to make possible the most profitable returns. The farmer should 

 be in a position to decide whether the increase produced by the ap- 

 plication of any given fertilizer is sufficient to pay for the extra ex- 

 penditure of labor and money. He will be in a better condition to 

 decide correctly by remembering that the relation of the cost of the 

 fertilizer to the value of the increased crop is a variable factor, de- 

 pending, aside from the climatic conditions, upon the availability of 

 the constituents, the character and composition of the crop, and upon 

 the market value of the latter. It may happen, for instance, that the 

 liberal application of certain nitrogenous materials to crops of a low 

 market value may result in a maximum yield of plant substance, and 

 yet fail to return a profit simply because the cost of the nitrogen ap- 

 plied is so great as to exceed the value of the increase secured by its 

 application. On the other hand, a similar application of the same 

 material to a crop possessing a high commercial value may, without 

 being as thoroughly utilized or without producing an increase pro- 

 portionately as large, lead to much greater profits, mainly because the 

 cost of the nitrogen, however considerable, is relatively small when 

 compared with the money value of the crop increase secured from 

 its use. 



To make heavy application of fertilizer is costly. The increase 

 in the yield of crops from the use of heavy applications of fertilizer 

 on soils deficient in humus does not always correspond to the in- 

 crease in the quantity of fertilizer applied. By increasing the humus 

 content of the soil the yield or crops will be nearer in direct propor- 

 tion to the quantity of fertilizer applied. Because of the frequent 

 failure to observe this fact there is often a great deal of waste in the 

 present method of using fertilizers. To prevent this waste the farmer 

 needs to know more about the fertilizer constituents in fertilizers and 

 the factors which influence their use. 



