26 BULLETIN 699, U, S. DEPARTMENT OF AGKICULTUEE. 



EFFECT OF OTHEE FERTILIZERS. 



It is maintained by many agricultural investigators that it is not 

 possible to ascertain the phosphoric acid requirement of a soil and 

 crop until nitrogen and potash have been supplied in optimum quan- 

 tities. In other words, since these three fertilizer elements are taken 

 up in a definite ratio by various crops, an adequate supply of any 

 two elements must be present in the soil solution before the plants 

 can utilize t^ :av full quota of the third. While this is to a certain 

 extent tru^! the methods employed in bringing about this end by no 

 means show conclusively that only the optimum quantities of the 

 fertilizer elements are present. For instance, the application of 

 potash to a soil until a crop no longer responds to further additions 

 of this element simply means that there is a limit above which potash 

 when applied alone gives no increase in yield. But potash may be 

 present in the soil solution in far greater quantities than can be 

 utilized by the plant. The excess over and above that required to 

 grow a maximum crop may be performing any or all of several 

 functions just as important as the direct supplying of plant food. 

 Moreover, the addition of a phosphatic or nitrogenous fertilizer may 

 entirely alter the function of the excess potash and will certainly 

 affect the nutrient qualities of the soil solution and the feeding 

 powers of the crop. 



It has been pointed out by Cameron^ that fertilizers not only 

 affect the chemical composition of the soil by the actual addition of 

 salts or organic compounds, but they alter the nature and solubility 

 of the minerals already present. They also have an important in- 

 fluence on the physical condition of the soil, its bacterial content,^ 

 and upon other active biological agents which directly or indirectly 

 affect plant growth. 



One of the most conclusive proofs that the plant food theory is 

 inadequate to explain entirely the action of fertilizers is the fact 

 that stable manure, which is recognized generally as the most effec- 

 tive of all fertilizers, contains the three elements, nitrogen, phos- 

 phorus, and potassium, in quantities entirely insufficient to account 

 for the increase in yields obtained from ordinary applications of this 

 substance. 



No matter which view is taken concerning the action of fertilizers, 

 the conclusions are in a general way the same, i. e., the application of 

 one fertilizer has an important influence on the action of another. 

 In the case of raw rock phosphate it has been pointed out that most 

 fertilizer salts exert a solvent effect upon this material, and there- 

 fore it is reasonable to expect its effectiveness to be somewhat in- 



»Tlie Soil Solution, pp. 105-109 (1911). 



•Fred and Hart. Research Bui. No. 35, pp. 35-66, Wis. Bxpt. Station (1915). 



