Power of Soils to absorb Manure. 



135 



it may attack the ammoniacal compound, and, by driving away 

 the ammonia, impoverish the land. 



The action of heat on these silicates is peculiar. It has been 

 before stated, that they contain w^ater of combination, which is 

 driven off at a red heat, but after being strongly heated they lose 

 their property of acting on different salts. For instance, the 

 double silicate of soda, after being heated to redness, no longer 

 absorbs ammonia, or is decomposed by salts of potash. 



This is the reason why felspar and albite-substances, of the 

 same composition in all other respects as the artificial silicates, 

 are devoid of the power of absorbing ammonia. It is only in the 

 state of hydrates that the double silicates possess the property in 

 question ; and this, again, accounts for the fact which was 

 observed, that the retentive power of clay and soils in general for 

 ammonia was very much diminished, and in some cases entirely 

 destroyed, after the soils had been heated to redness. 



We have here a further proof of the identity of these double 

 silicates, artificially produced, with the substances in the soil, 

 which give it the power of absorbing the salts of m_anure. 



I do not think, indeed, that this point admits of much dispute. 

 We find that a power is possessed by soils, which is not referable 

 to the organic matter, the sand, or carbonate of lime which they 

 contain; and further, that pure clays, free from any of the ordi- 

 nary salts of lime or soda, possess this property in a high degree. 

 Believing that the activity of clay can only be due to some com- 

 pounds of silica, we are led to examine some of these compounds 

 anew; and whilst it is ascertained that the ordinary simple 

 silicates are not the salts we are in search of, and that the natural 

 double silicates of the type of the felspars, which are likely ta 

 exist in clay, are equally devoid of the requisite properties, a new 

 class of substances is discovered, having, though in a far higher 

 degree, all the qualities of the clay itself. It seems that this is, 

 on the whole, as conclusive as any evidence of the kind could be ; 

 and we may hereafter, with advantage, employ our knowledge of 

 these double silicates in elucidation of some of the questions 

 of practical manuring. 



It being, therefore, proved that the ammonia, potash, and other 

 alkaline ingredients of manure, are under the influence of the 

 soil separated from solution, and converted into double silicates, 

 the question arises, how do plants ultimately obtain their mineral 

 food from the soil ? If the compounds so produced are insoluble 

 in water, how is the ammonia or the potash liberated for the 

 purposes of vegetation ? The answer to these questions is, that 

 the double silicates are not altogether insoluble in water. It is 

 true, indeed, that in my account of the early experiments on the 

 filtrations through soils, I stated that ammonia was entirely 



