THE SOIL. 37 



capable of furnishing food to plants. Neither pure 

 silica nor pure clay can be used as food by plants. 



By far the largest part of the substances in the soil 

 which are capable of being made plant food is not 

 available for this use at any given time. Probably 

 not one per cent, of even the most fertile soils is avail- 

 able plant food. It is fortunate this is the case. In 

 order that the mineral ingredients of the soil may be 

 used as food for plants they must be soluble in water 

 or in weak acid solutions. Were the mass of them 

 in such condition there would be much waste and loss. 

 The air, the water, the frost; the action of worms and 

 burrowing animals; the tillage of the soil by man; the 

 growth and decay of plants — all these are continually 

 changing the dormant, unavailable, potential plant 

 food of the soil into forms in which it may be active or 

 available in the nourishment of plants. Sometimes 

 the processes of manufacture of available food go on 

 faster than the food is used by the crops grown. In 

 such case the fertility of the soil increases. Some- 

 times the new supplies of available food are furnished 

 more slowly than the crops grown and removed by 

 the farmer make use of such food. In such cases the 

 land becomes less fertile; tends to become exhausted. 



Value of Chemical Analysis of 8oil. — An 

 examination of a soil by a chemist will show, with 

 great exactness, of what it is composed, and the^rela, 

 tive proportions of the elements. It may show that 

 there is evidently a too small supply of some essential 

 ingredient. Or it may show that there is some sub- 

 stance or some combination present which will be in- 

 jurious to plants. In these ways such an examina- 

 tion may give most valuable suggestions as to manur- 



