26 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
EFFECT OF OTHER 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 their full quota of the third. While this is to a certain 
extent true, 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, 1. 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- 
1 The Soil Solution, pp. 105-109 (1911). 
2Fred and Hart. Research Bul. No. 35, pp. 35-66, Wis. Expt. Station (1915). 
