152 DECOMPOSITION OF THE ROCKS. 



Prof. Voelcker, of the Agricultural Institution at 

 Cirenchester, said, in a lecture on this subject: 



" Into the composition of sandy soils but a very 

 small amount of those substances enter which are 

 food for plants. Clay soils contain insoluble sili- 

 cates ; and undecomposed materials, from which the 

 clay is formed, furnish, on gradual decomposition, 

 potash, which is one of the most fertilizing constit- 

 uents. By working the land, fresh quantities of 

 granite are brought under the influence of the 

 atmosphere, and the felspar contained in it is grad- 

 ually decomposed, and furnishes soluble potash. 

 This benefit is not reaped from stirring a sandy soil. 

 However much it is worked we cannot eliminate 

 such a result from it, because there is not potash 

 present in any considerable quantity ; but in many 

 clay soils the supply is almost inexhaustible; and 

 they possess, to a remarkable degree, the power of 

 absorbing moisture and fertilizing gases from the 

 atmosphere. They also have the power of absorb- 

 ing carbonic acid gas and ammonia, — two constitu- 

 ents which furnish the materials on which the plant 

 builds up its organic structure. 



" The more we plough the glebe, the more readily 

 we give the atmosphere access to it, the more it is 

 pulverized, the greater is the absorbing surface, and 

 the more beneficial is the result. The advantages 

 of deep ploughing, as a mode of improving clay 

 soils, become more and more apparent from year 



