FERTILIZERS. 79 



Tn the process of agriculture, by removing crops we 

 take away a quantity of these elements, year by year. 

 If this is continued, and nothing is returned to the soil, 

 in the course of time it becomes impoverished. The 

 supply of plant food is exhausted, and not enough is 

 changed to an available form, year by year, to produce 

 a crop. 



Land that is " run out " in this way may still contain 

 large quantities of some elements of fertility, being de- 

 ficient only in a few. By supplying the latter we may 

 still keep up the fertility of the land for many years. If, 

 for instance, the soil of a certain field contains enough 

 available nitrogen to support a crop two years, enough 

 phosphoric acid and potash for five years, enough lime 

 for ten years, and enough of other substances for a longer 

 period, it is evident that after two years we must supply 

 nitrogen, after five years phosphoric acid and potash, 

 and so on, unless some of these elements have been lost 

 in the mean time, or some have been added by natural 

 causes. 



The Soil a Storehouse of Plant Food. Ordinary soil ap- 

 pears to be composed of simple, inactive, unchanging 

 substances, but in reality it is like a vast chemical labo- 

 ratory, in which plant food is continually prepared, and 

 either furnished immediately to the plant or kept in store 

 for the future. 



A portion of the rocky or mineral parts of soil con- 

 tain substances which, when they have been changed by 

 chemical action, become food for plants. Among the 

 most fertile kinds of soil are those which have been pro- 

 duced by the crumbling and decay of granite and lime- 

 stone. Vegetable mold, which results from the decay 

 of plants and leaves, and which is found to a certain ex- 



