PRACTICAL AGRICULTURE. 27 



in the process. Plants, also, by producing carbonic acid 

 during their decay, and by means of the acids emitted 

 by their hving roots, contribute no less powerfully to de- 

 stroy the coherence of solid minerals. Air, water, and 

 changing temperature prepare the difTerent species of 

 rocks for yielding to plants the potash or soda they con- 

 lain. 



Changing temperature is a most important agent in 

 nature. It not only assists in the original formation of 

 soils, but exerts a most powerful influence over those al- 

 ready in existence. In wet soils the temperature rises 

 slowly, and never attains the same height as in one that 

 is sandy and dry. When the heat of the atmosphere 

 rises no higher in the shade than 60 or 70 degrees, a dry 

 soil may become so warm as to raise the thermometer 

 to 90 or 100. Hence, though the expression be used 

 figuratively, it is in this instance strictly correct to say 

 that wet soils are cold. 



The exhaustion of alkalies in a soil by successive 

 crops is the true reason why practical farmers suppose 

 themselves compelled to suffer land to lie fallow. It is 

 the greatest possible mistake to think that the tempora- 

 ry diminution of fertility in a field is chiefly owing to 

 the loss of the decaying vegetable matter it previously 

 contained : it is principally the consequence of the ex- 

 haustion of potash and soda, which are restored by the 

 slow process of the more complete disintegration of the 

 materials of the soil. It is evident that the careful till- 

 ing of fallow land must accelerate and increase this fur- 

 ther breakitig up of its mineral ingredients. Nor is this 

 repos^e of the soil always necessary. A field, \vhich has 

 become unfitted for a certain kind of produce, may not, 

 on that account, be unsuitable for another ; and upon 

 this observation a system of agriculture has been grad- 

 ually formed, the principal object of which is to obtain 

 the greatest possible produce in a succession of years, 

 with the least outlay for manure. Because plants re- 

 quire for their growth different constituenls of soil, 

 changing the crop from year to year will maintain the 



