LIME AND MAGNESIA. 
9 
just referred to. This reaction is upon the silicates of potash, and other alkalies. It has 
been shown by Prof. Fuchs of Munich, that the silicates of alumina and the alkalies are 
decomposed by mixture with milk of lime in a subcaustic state. The same action takes 
place when lime is mixed with the soil. There is, therefore, a liberation of silica and of 
potash, or the alkali with which the silica is combined. Besides this chemical reaction, 
the texture of clay soils is loosened, and the whole mass becomes porous and friable. 
The functions of lime, then, it will be seen, consist, first, in supplying an element neces¬ 
sary to the plant; secondly, in liberating the alkalies in combination with silica ; thirdly, 
in rendering the freed silica soluble; and fourthly, in giving porosity to argillaceous soils. 
Too much importance, therefore, can scarcely be given to lime as an agricultural agent. 
There are probably other actions and uses of a minor importance, which may be passed 
over without particular notice. Some of the uses attributed to lime seem rather problema¬ 
tical, or not well sustained by facts. 
I have already intimated that all plants require lime. I do not know that this intimation 
requires modification or reserve. It is important, however, to state that the different parts 
and organs of plants require it in different proportions. The wants of a tree for lime can 
not be determined by an analysis of its wood or leaves, nor by that of its bark. In the 
outer coverings it is always greater than in the internal parts. Its presence is therefore 
functional and necessary. 
In the foregoing remarks, I have had in view one variety of lime only, the subcaustic 
or subcarbonate in a hydrous condition. It is in this condition only that it is usually em¬ 
ployed. I except here of course the phosphate of lime, as contained in bones, which exerts 
more of a physical effect upon the clay soils than the hydrous subcarbonate does. 
Many farmers have used lime without benefit. In view of this fact, it may be stated 
that at the time the lime was used, it was not required, there being already sufficient in 
the soil for the purposes of vegetation, or for the use of the crop to be raised. Whenever 
there is a deficiency of the silicated alkalies and of organic matter, the effects of lime are 
not so apparent. 
MAGNESIA. 
The uses of magnesia, either as an element of soil or as a constituent of plants, are not 
so well known as the use of the salts of lime. It can exert but little influence mechanically 
on the soil, its proportion being very much smaller than that of lime ; and though magne¬ 
sian rocks are not unfrequent, still it never occurs in quantities sufficient to produce any 
perceptible effect in the texture of the soil. Magnesia, however, appears to be an essential 
constituent of the grains, as indian corn, wheat, etc., in which it even exceeds lime in 
quantity ; thus, in the ash of rye it amounts to 12 per centum, while the lime is only 2'61. 
The carbonate requires 2000 parts of water for its solution, and is more insoluble than 
carbonate of lime. All the rocks furnish magnesia by their decomposition. 
The most common form in which magnesia enters into the composition of plants, is that 
[Agricultural Report — Vol. ii.] 2 
