210 GEOLOGY AND CHEMISTRY OF SOILS. 



any magnesia, and that contained 3 per cent. It must, how- 

 ever, exist in all fertile soils, as it enters into the composition 

 of many varieties of grain. The kernel of corn could not 

 be formed without the presence of the phosphate of magnesia. 

 Magnesia, like lime, does not generally constitute a suffi- 

 ciently large portion of the soil to affect its texture. "When 

 it does it has the properties of clay, absorbing moisture and 

 imparting its adhesive properties. It acts as an alkali, to 

 convert vegetable matter into food, and constitutes a part of 

 the vegetable structure. When applied in its caustic state, 

 it has been found injurious to vegetation ; but as a carbonate, 

 it is highly useful. Like lime, it must be regarded as an im- 

 prover of the soil, as a manure, rather than an earth. 



The union of these four earths in a soil, give to it gener- 

 ally the properties of each. But as they are combined in 

 the soil with each other, and with other substances, in the 

 form of salts, we cannot infer with certainty the exact char- 

 acter of the soil, by knowing in what proportions they ex- 

 ist ; but we must know in what state of combination they 

 are found. It will be seen that silica and alumina constitute 

 almost the entire mass of the earthy ingredients of all our soils, 

 and the qualities of a good soil will depend upon the right 

 proportion of these substances. But there are other inorganic 

 bodies found in soils, as essential to fertility as any that have 

 been described. 



2. Alkalies and metallic oxides, contained in the soil. The 

 most important, and almost the only substances under this 

 head, are ammonia, potash, soda, oxide of iron and manga- 

 nese. 



Ammonia has been shown by Liebig to exist in the atmos- 

 phere, in very small quantities; of course, inconsequence of 

 its solubility in water, it is found in all soils. It has been 

 supposed, that ammonia was a product of the putrefaction of 

 animal and vegetable substances, containing nitrogen. But 

 Liebig believes, that it belongs to the original formation of 



