650 
Journal of Agricultural Research 
Vol. xxvn. No. 9 
Our information as to the effect on the solubility of one salt of the 
presence in the solution of another constituent is far from complete. It 
is well known that carbon dioxid dissolved in water increases the solu¬ 
bility of calcium carbonate except when the solution contains sodium in 
excess of the strong acids. Likewise it is known that chlorin in solution 
increases the solubility of calcium sulphate unless the calcium in solution 
exceeds the equivalent of the carbonate and sulphate ions in the solution 
otherwise the solubility of calcium sulphate is decreased. These examples 
are cited merely to show that in a system as complex as the soil solution 
it is not possible to state definitely the limits of solubility. Not only is the 
system complex with respect to its composition but it is never in equilib¬ 
rium. It is either in the way of being made more dilute by irrigation 
or rainfall or of being concentrated by evaporation and withal it is 
participating in those reactions that are continually taking place between 
the substances in solution and those combined in the soil. 
A consideration of the factors influencing the solubility of the soil 
material can not well be confined to a discussion of the reactions of 
inorganic acids and bases. The soil of a cultivated field is teeming with 
organisms including fungi, bacteria, and nematodes, to name only a few 
of the groups. It contains also the organic residues of plants and of 
animals m various stages of decomposition. These organisms and the 
products of the decomposition of the organic matter have a most impor¬ 
tant bearing on the character of the soil solution in its relation to crop 
growth. They doubtless have, also, a bearing on the physical condition 
of the soil and consequently on its permeability to water. But it is not 
within the scope of the present paper to deal with these organisms or with 
the subject of organic matter in relation to the condition or productivity 
of the soil. - 
There remains to be considered in some detail one other constituent 
of the soil and of the soil solution which is believed to play a most important 
r 61 e in the matter of the movement of water. This is silica. Although 
silica is the most abundant substance in the soil, we have as yet very 
little knowledge of it and of the nature of its reactions. Silica, or the 
oxid of silicon (Si0 2 ), was first identified by Berzelius in 1823, though 
its molecular weight was not established definitely until many years later. 
It occurs in nature in a great variety of forms, sometimes pure, as in 
quartz or rock crystal, sometimes combined with water, as silicic acid, 
but more commonly in combination with one or more of the earthy or 
alkaline bases. It is the most universal cementing material known. 
Notwithstanding the fact that silica is not ordinarily recognized as a 
constituent of the soil solution, it appears to be unquestionable that it 
exists in and is often abundant in the solution. The fact that it con¬ 
stitutes an important part of the mineral matter of certain plants and that 
it is utilized by a great variety of small organisms, such as the infusoria 
in forming their shells or skeletons, justifies the belief that it exists in a 
soluble form. Furthermore, it is sometimes identified as an important 
dissolved constituent of the waters of mineral springs and deep wells. 
Some water analysts hold the view that the silica found in water is to be 
regarded as suspended matter rather than as a true solute; in other words, 
that it should be reported as Si 0 2 rather than as an acid ion Si 0 3 . Yet 
in some cases, at least, the evidence seems conclusive that it may exist 
as an acid ion. It combines with certain bases to form definite salts. 
As calcium silicate it is sparingly soluble in water, 95 parts per million. 
