KKirriMZKRS. 79 



111 thv ]»r()C('ss of ag'riciilturc, l)y nMuoviiig- crujis we 

 take away a (jiiantity of these elenieuts, year by year. 

 Jf this is eoutiuued, and nothing' is returned to the soil, 

 in the eourse of time it ])econies ini|)overished. The 

 sui)j)ly of })hint food is exliausted, and not enougli is 

 changed to an avaihible 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 

 })lios[)horic 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 

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

 ratory, in which })lant food is continually prepared, and 

 either furnished immediately to the j>lant or kept in store 

 for the future. 



A portion of the rocky or mineral })arts of soil con- 

 tain substances which, when they have been changed by 

 chemical action, become food for ])lants. Among the 

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

 duced liy 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- 



