222 PLANT-FOOD AND AGRICULTURE. 



The natural fertility of a soil is measured by the crop that may 

 be grown out of the plant-food, which is liberated each year by 

 the atmospheric and other natural actions in the soil, and which is 

 supplied to the soil through the rainfall. It may often be measured 

 by a fallow. 



The maximum fertility of a soil is measured by the crop which 

 can be grown from plant-food present in as groat a quantity as the 

 soil can retain without unnecessary waste. It may often be 

 measured by that amount of crop which, labor being equal, can be 

 raised most economically by an ample supply of fertilizer or 

 manure. 



In ascertaining the value of a mauurial application, it must be 

 recognized that much of the effect depends on the season, on the 

 culture, and on the seed. In practice, therefore, in order to deter- 

 mine the limit of production to our land, which we have defined as 

 the maximum, we must take the average produce of a series of 

 years, under well-defined and similarly-conditioned circumstances. 

 Let us illustrate by supposed cases. I have a field which, with the 

 ordinary rainfall of mj' localit}', will retain during the season of 

 growth sufficient plant-food for the formation of a given crop of 

 corn, and this fertility is thoroughly distributed through the soil, 

 in such a manner that the roots of the plants will always be in 

 contact at the proper tinle with the fertilizer, and can absorb 

 suflBcient to produce one hundred bushels of grain. Then the one 

 hundred bushel yield may be assumed as the maximum fertility of 

 my field. My neighbor has a field, of a different character of soil, 

 and applies the same quantity of fertilizer that I do, but his field 

 has not the retaining power on the plant-food, and much is carried 

 downward out of reach of the roots with the spring rains. He 

 therefore harvests but fifty bushels of grain, and this represents 

 the maximum fertility of his field. Now, if he fertilizes beyond 

 the needs of this fiftj' bushels, he is meeting with a loss ; if I fer- 

 tilize beyond the one hundred bushels, I am meeting with a loss. 

 If either of us fertilize below this respective maximum capacity of 

 our fields, we are equally suffering a loss. 



Very likely in practice we both might get larger yields by apply- 

 ing more fertilizer than this maximum quantity, but in my case, all 

 obtained over the one hundred bushels would be at a cost greater 

 than the cost of the first one hundred bushels ; in his case, all he 

 obtained beyond fifty bushels would be at an enhanced cost over 

 that of his first fifty bushels. In practice, however, no farmer 



