HOW BURNED CLAY ACTS. 261 



action of burning upon clays, the chemical composition of a clay 

 must, in a great measure, determine how far by burning it is 

 likely to become useful as an improver of the soil, and ought to 

 indicate when and where experiments should or may profitably 

 be made with it. 



3. How burned clay acts when applied to the soil, becomes, 

 after this explanation, very plain. By my experiments, a clay 

 which, in its natural state, as taken from the field, yielded with 

 difficulty three-quarters of a per cent (0.77) of soluble salts, of 

 which one-third (0.24) was organic or combustible matter, gave 

 readily to water, after being burned, one and three-fourths of a 

 per cent (1.77) of soluble salts, of which only one-thirtieth part 

 (0.06) was combustible matter. 



Thu's, while one ton of the natural clay yielded with difficulty 

 11 pounds of mineral matter, the burned yielded readily 36 

 pounds. The natural clay was also liable to become hard, 

 baked together, and impervious to water, air, or roots, while the 

 burned is always open, pervious to rains, and accessible to the 

 roots. 



This soluble matter consists of potash, soda, lime, magnesia, 

 chlorine, sulphuric acid, silica, nearly all the mineral ingre- 

 dients, in short, which are necessary to the growth of plants, 

 in proportions which, of course, vary with each specimen of 

 clay examined. 



An idea will be formed of the importance of this greater solu- 

 bility, and of the striking effects which it ought in some cases 

 to produce, if it be recollected 



a That fifty or a hundred cart-loads or tons is not an unusual 

 application of burned clay to an acre of land. 



b That a hundred tons of some clays will yield not less than 

 one and three-fourth tons, or 3900 pounds, of soluble mineral 

 matter. 



c That the natural clays often contain, or, for the purpose 

 of burning them, are mixed with much vegetable matter, the 

 ashes of which will increase still further the quantity of soluble 

 mineral matter which they will be able to yield when burned. 

 And lastly, 



d That the whole mineral matter carried off by the grain, 



