CONDITIONS FOR RENDERING FOOD AVAILABLE. 75 



witli the time tliey are in operation, and witli the com- 

 position of the different soils. 



It clearly results from the foregoing observations, that 

 one of the principal requirements of the practical farmer 

 is to know the causes as well as the means whereby the 

 useful nutritive substances present in his field, but not in 

 a form available for nutrition, may be rendered diffusible 

 and capable of doing their work. 



The presence of moisture, a certain degree of heat, and 

 free access of air, are the proximate conditions of those 

 changes by which the nutritive substances in chemical 

 combination are made available for the roots. A certain 

 quantity of water is indispensable to transpose the soil- 

 constituents when rendered soluble ; water, with the co- 

 operation of carbonic acid, decomposes the silicates, and 

 makes the undissolved phosphates soluble and diflusible 

 through the soil. 



The organic remains decajang in the ground afford 

 feeble but long-continued sources of carbonic acid ; but 

 Avithout moisture no process of decay can take place. 

 Stagnant water, again, which excludes the access of air, 

 prevents the generation of carbonic acid ; and the pro- 

 cess of putrefaction is attended with the generation of 

 heat, whereby the temperature of the soil is perceptibly 

 increased. 



By the aid of putrescent vegetable and animal remains, 

 a field exhausted by culture will regain its fertihty in 

 a shorter time, and the use of farm-yard manure in 

 time of fallow will promote the process. The dense 

 shadow cast by a leafy plant tends to retain moisture 

 longer in the ground, and thus increases the action of 

 the disintegrating agencies during the fallow season. 



In a porous soil abounding in lime the putreftictive 

 process of organic matter proceeds much more quickly 



