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Water, and the decomposing animal and vegeta- 

 ble matter existing in the soil, constitute the true 

 nourishment of plants ; and as the earthy parts of the 

 soil are useful in retaining water, so as to supply it 

 in the proper proportions to the roots of the vegeta- 

 bles, so they are likewise efficacious in producing the 

 proper distribution of the animal or vegetable matter ; 

 when equally mixed with it they prevent it from de- 

 composing too rapidly ; and by their means the solu- 

 ble parts are supplied in proper proportions. 



Besides this agency, which may be considered as 

 mechanical, there is another agency between soils and 

 organizable matters, which may be regarded as che- 

 mical in its nature. The earths, and even the earthy 

 carbonates, have a certain degree of chemical attrac- 

 tion for many of the principles of vegetable and ani- 

 mal substances. This is easily exemplified in the in- 

 stance of alumina and oil ; if an acid solution of alu- 

 mina be mixed with a solution of soap, which consists 

 of oily matter and potassa ; the oil and the alumina 

 will unite and form a white powder, which will sink 

 to the bottom of the fluid. 



The extract from decomposing vegetable matter 

 when boiled with pipe-clay or chalk, forms a combina- 

 tion by which the vegetable matter is rendered more 

 difficult of decomposition and of solution. Pure silica 

 and siliceous sands have little action of this kind ; and 

 the soils which contain the most alumina and carbon- 

 ate of lime, are these which act with the greatest che- 

 mical energy in preserving manures. Such soils 

 merit the appellation which is commonly given to them 



