6 
SOILS AND ALKALI. 
stance, Sachs showed that the amount of silicic acid in 
the ash of Indian corn could be reduced from 18 per cent, 
to .7 per cent., without injurious effect upon the plant. 
The virgin soil is generally productive, but when in time 
it comes under tillage, the crops raised upon it are con¬ 
sumed by animals and removed to a distance, so that the 
mineral food contained in the soil is by degrees exhausted, 
and unless it is restored the soil becomes barren. The 
object of manuring is to restore the fertility of the soil, 
which consists in adding to the soil some substance which 
shall itself serve directly as food for the plant, or shall so 
modify, by chemical action, some material already pres¬ 
ent in the soil, as to convert it into a state in which the 
plant may take advantage of it. The length of time 
before fertilizers must or should be used, depends upon 
the character and amount of the soil. The character of 
the soil is caused by the different geological formations 
disintegrated, the rule being that primitive and igneous 
rocks yield soils rich in potash, while the fossiliferous 
rocks yield soils rich in phosphoric acid. 
The amount of soil that covers the rocks will depend 
upon the slope of the ground, and the activity of those 
natural agencies that cause the disintegration. The soil 
may be so thin as barely to cover the rocks, or in other 
cases verv thick, as in the drift material; the former are 
called soils of disintegration, the latter soils of transporta¬ 
tion. A soil is commonly named from the preponderance 
of one of its constituents—as, if composed of sand, it is 
called sandy soil. Soils rest upon a sub-soil, which is 
tougher, more compact, and contains rubbly and stony 
debris. Soils differ not only in chemical composition, but 
also in physical characteristics, as coarse or fine, its power 
to absorb or retain the volatile and soluble parts of 
manure, etc. 
PHYSICAL PROPERTIES. 
The physical properties of the soil have a great deal 
to do with the growth and nutrition of the plants. 
