The first experiments carried out on this subject were those by Huxtable and Thompson in 1850. The liquid portion of farmyard manure was filtered through soil and subsequently examined, when it was found to have not only lost its colour, but also to have lost its smell. Ammonia and ammonia salts were also experimented with, and it was found that soils possessed the power of fixing ammonia.
To Thomas Way, however, we are indebted for the most valuable contribution on this important subject made by any one single investigator. His experiments were not merely carried out with regard to ammonia, but also with regard to other bases—such as potash, lime, magnesia, soda, &c. Since Way's experiments much work has been done by Liebig, Stohmann, Henneberg, and Heiden, as also by Voelcker, Eichhorn, Knop, Rautenberg, Pochwissnew, Warington, Beyer, Bretschneider, Sestini, Laskowsky, Strehl, Pillnitz, Peters, W. Wolff, Lehmann, and Biedermann.
Bases and Acids fixed by Soil.
From these experiments it may be taken as proved beyond doubt that soils have the power of fixing, to a greater or less extent, the following bases: ammonia, potash, lime, magnesia and soda; as well as the two acids, phosphoric and silicic. The order in which the different bases are fixed is an important point. It would seem that the soil has a greater affinity for the more valuable manurial substances, such as ammonia, potash, and lime, and that these substances are first fixed. That in fixing any one of the above-mentioned bases from its solution, it can only do so at the expense of another base. Thus, in fixing potash, either lime, magnesia, or soda must be given up. Further, when a base in solution, as sulphate or chloride, is absorbed by a soil, the base is alone fixed, while the sulphuric acid or chlorine is left in solution. Lastly, the amount of base absorbed by a soil depends on the concentration of its solution, on the nature of its combination, and the temperature. Way found in his experiments that a clay soil has more power than a peaty soil, and that a peaty soil has more power than a sandy soil.
Causes of this Fixation.
So much for the fact of soil absorption; as to the cause or causes of this absorption, a great number of theories have been put forward. Those may be divided into two classes—those accounting for it as due to physical properties of the soil; and those, on the other hand, explaining it as due to chemical action.
To the latter class Way's belonged. He explained it as due to the formation in the soil of hydrated double silicates, consisting of a silicate of alumina, along with a silicate of the base fixed. Brüstlein and Peters, on the other hand, were of the opinion that it was purely physical in its nature. A theory has been advanced that it is due to the formation of insoluble ulmates and humates, formed by the union of ulmic and humic acids, along with the bases fixed. Among others who devoted investigation to this interesting question, may be mentioned Rautenberg and Heiden.
On reviewing the evidence, it seems to be pretty well established that it really is mainly a chemical act, due chiefly to the formation of double silicates, and doubtless to a certain extent to the formation of insoluble humates and ulmates. Heiden's experiments would seem to indicate, however, that it is also partly of a physical nature.
With regard to the absorption of phosphoric acid, this has been shown to be a chemical act, and depends on the formation of insoluble phosphates of calcium, iron, aluminium, and magnesium, the percentage of iron especially determining this.
Much analytical work has been accomplished of late years with a view of ascertaining the amount of ash in different kinds of plants, and in the different parts of the plant.