RELATION OF CROP GROWTH TO EXHAUSTION. 153 



Third. Some substances appear to enter into the circu- 

 lation of plants, not so much as actual and necessary con- 

 stituents but more as agents by which other compounds 

 may be conveyed into them. Salt for instance appears to 

 enter into the substance of plants chiefly for'' supplying 

 chlorine in some cases, and soda in others. In such cases 

 when these substances are found to exert any marked ef- 

 fect upon the vegetation, it is to be concluded that the soil " 

 is deficient in them, and that their use necessarily causes a 

 larger draft upon the soil for other kinds of plant food to 

 supply the larger growth of the crops. 



Fourth. That while the soil may contain a very large 

 quantity of the substances required for the growth of vege- 

 tation, yet the most of these may be in an unavailable con- 

 dition for the use of the crops. 



Fifth. That every soil possesses a certain amount of 

 natural fertility, which has been accumulated during past 

 ages, and that this stock is exhausted in a comparatively 

 few years, and during this time it will produce full crops 

 in proportion to the amount of plant food which it con- 

 tains. 



Sixth. That when this store of accumulated fertility is 

 exhausted, or any one element of it, the crops fail and final- 

 ly refuse to grow. 



Seventh. That the soil then is able to afford a certain 

 amount of plant food, which is derived from its natural re- 

 sources; and that these consist of certain contributions from 

 the atmosphere and from the mineral compounds which ex- 

 ist in the soil : but these are wholly inadequate for the pro- 

 duction of crops. 



Eighth. That when the soil has been reduced to this low 

 condition of natural fertility, the farmer is obliged to sup- 

 ply an adequate amount of available plant food for the 

 growth of crops, in the form of manures, composts, or fer- 

 tilizers. 



Ninth. That it is not safe for the farmer to depend 

 wholly upon the analyses of the various crops as to the 

 -amount of plant food required by them; but should supply 



