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origin and composition of agricultural clays in general, and pointed 

 out on what substances chiefly the fertilising powers of clay depend. 

 Clays generally result from the disintegration and degradation 

 of granitic and felspatic rocks. Felspar, a mineral composed of 

 silicate of potash or soda, and silicate of alumina, exposed for a 

 long time to the united action of the atmosphere and water, suffers 

 a gradual decomposition, and falls altogether to powder. Silicate 

 of potash, a soluble salt, is washed out by the rain falling on the 

 decomposed rock, and converted, in its turn, by the carbonic acid 

 of the atmosphere into carbonate of potash and silica. Part of the 

 silica remains behind with the insoluble silicate of alumina, the 

 chief constituent of clays. Agricultural clays, however, are never 

 pure silicate of alumina, but mixtures of pure clay (silicate of alu- 

 mina) with more or less of sand, undecomposed fragments of felspar 

 and other minerals, lime, magnesia, free alumina, oxide of iron, 

 soluble silicate of potash and soda, and traces of phosphoric acid, 

 chlorine, and sulphuric acid. The state of combination in which 

 these different constituents occur, varies in different clays. The 

 complex nature of agricultural clays will become apparent by the 

 subjoined analyses of three samples of clays from Dumbelton, in 

 Gloucestershire, made in my laboratory. 



As alumina or silicate of alumina is not found in the ashes of 

 cultivated plants, the chief component part of clay cannot be said 

 to contribute in itself to the direct nutrition of plants, and we have, 

 therefore, to look amongst the accessary ingredients of clay for 

 the fertilising agents or substances which are used as direct 

 articles of food by plants. Lime, magnesia, sulphuric and phos- 

 phoric acid, and chlorine substances which, in larger or smaller 

 quantities, occur in clays are, indeed, essential to the growth of 

 plants ; but the value of an agricultural clay chiefly depends on 

 the proportion of potash and soda which it contains. Potash is an 

 essential element in all ashes of plants, and acts as a most powerful 



