THE USE OF FERTILIZERS 215 



exceeds 300 or 400 pounds to the acre, it is better to apply 

 it before seeding. Grass seed and other small seeds should 

 be planted only after the fertilizer has been mixed with the 

 soil for several days. 



273. The limiting factor. — Attention has been called to 

 the important influence that any condition unfavorable to 

 plant growth is sure to exercise in curtailing yield of crops. 

 If poor drainage is the difficulty, crop yields may be reduced 

 to almost nothing, while if this be corrected a very productive 

 piece of land may result. The same principle holds true 

 when there is a deficiency of any one of the fertilizing sub- 

 stances. There may be present in a soil an abundant supply 

 of available phosphoric acid and potash, but if nitrogen is 

 deficient the crop yield is limited to the size of crop that the 

 quantity of available nitrogen present will produce. Each 

 of the essential plant-food materials exercises this control. 

 It is, therefore, a requisite in the economical use of ferti- 

 lizers to have a well-balanced mixture of plant nutrients. 

 The balance must be adjusted to the needs of each partic- 

 ular soil, and to each kind of crop. Of course it is impossible 

 to work out any fertilizer mixture that will fit these condi- 

 tions exactly. These relationships are best worked out by 

 field tests with fertilizer mixtures (see § 269). 



274. The law of diminishing returns. — A small applica- 

 tion of fertilizer usually effects a greater percentage increase 

 of crop than does a larger application. This is unfortu- 

 nate, because it means that there is a limit to the profit- 

 able use of fertilizers, for although the cost of the fertilizer 

 rises in direct proportion to the quantity used, the rate of 

 yield decreases after a certain point has been reached, and 

 consequently the value of the product finally becomes less 

 than the cost of the fertilizer. This law of diminishing 

 returns may be illustrated by an experiment in which floats 

 were applied in several different quantities to plats of land, 



