84 THE SOIL. 



"What is here asserted of all the mineral nutritive 

 substances without distinction applies equally to every 

 soil constituent required by the plant. The exhaustion 

 of a field may often simply depend upon a deficiency of 

 available silicic acid for the coming crop of cereal 

 plants, while the other food elements may be super- 

 abundant. 



It is evident from the nature of the process, that if 

 the soil is altogether deficient in disintegrate silicates 

 or soluble earthy phosphates, the action of time, the 

 plough, and the weather in fallow will not restore fer- 

 tility to a field, and that the effect of disintegrating 

 causes will vary with the time they are in operation, 

 and with the composition 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 w r ork. 



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 trans- 

 pose the soil-constituents when rendered soluble ; 

 water, with the co-operation of carbonic acid, decom- 

 poses the silicates, and makes the undissolved phos- 

 phates soluble and diffusible through the soil. 



The organic remains decaying in the ground afford 

 feeble but long-continued sources of carbonic acid ; but 

 without 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 re- 

 mains, a field exhausted by culture will regain its fer- 

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



