Further, this power of the soil was found not to extend to the whole salt of ammonia or potash, but only to the alkali itself. If, for instance, sulphate of ammonia were the compound used in the experiments, the ammonia would be removed from solution, but the filtered liquid would contain sulphuric acid in abundance—not in the free or uncombined form, but united to lime; instead of sulphate of ammonia we should find sulphate of lime in the solution; and this result was obtained, whatever the acid of the salt experimented upon might be.
It was found, moreover, that the process of filtration was by no means necessary; by the mere mixing of an alkaline solution with a proper quantity of soil, as by shaking them together in a bottle, and allowing the soil to subside, the same result was obtained. The action, therefore, was in no way referable to any physical law brought into operation by the process of filtration.
It was also found that the combination between the soil and the alkaline substance was rapid, if not instantaneous, partaking of the nature of the ordinary union between an acid and an alkali.
In the course of these experiments, several different soils were operated upon, and it was found that all soils capable of profitable cultivation possessed this property in a greater or less degree.
Pure sand, it was found, did not possess this property. The organic matter of the soil, it was proved, had nothing to do with it. The addition of carbonate of lime to a soil did not increase its absorptive power, and indeed it was found that a soil in which carbonate of lime did not exist, possessed in a high degree the power of removing ammonia or potash from solution.
To what, then, is the power of soils to arrest ammonia, potash, magnesia, phosphoric acid, etc., owing? The above experiments lead to the conclusion that it is due to the clay which they contain. In the language of Prof. Way, however,
“It still remained to be considered, whether the whole clay took any active part in these changes, or whether there existed in clay some chemical compound in small quantity to which the action was due. This question was to be decided by the extent to which clay was able to unite with ammonia, or other alkaline bases; and it soon became evident that the idea of the clay as a whole, being the cause of the absorptive property, was inconsistent with all the ascertained laws of chemical combination.”
After a series of experiments, Prof. Way came to the conclusion that there is in clays a peculiar class of double silicates to which the absorptive properties of soil are due. He found that the double silicate of alumina and lime, or soda, whether found naturally in soils or produced artificially, would be decomposed when a salt of ammonia, or potash, etc., was mixed with it, the ammonia, or potash, taking the place of the lime or soda.
Prof. Way’s discovery, then, is not that soils have “absorptive properties”—that has been long known—but that they absorb ammonia, potash, phosphoric acid, etc., by virtue of the double silicate of alumina and soda, or lime, etc., which they contain.
Soils are also found to have the power of absorbing ammonia, or rather carbonate of ammonia, from the air.