Partial Percentage Composition

Source of soil Humid Arid
Number of samples analyzed 696 573
Insoluble residue 84.17 69.16
Soluble silica 4.04 6.71
Alumina 3.66 7.61
Lime 0.13 1.43
Potash 0.21 0.67
Phos. Acid 0.12 0.16
Humus 1.22 1.13


Soil chemists have generally attempted to arrive at a determination
of the fertility of soil by treating a carefully selected and
prepared sample with a certain amount of acid of definite strength.
The portion which dissolves under the influence of acids has been
looked upon as a rough measure of the possible fertility of the
soil.

The column headed "Insoluble Residue" shows the average proportions
of arid and humid soils which remain undissolved by acids. It is
evident at once that the humid soils are much less soluble in acids
than arid soils, the difference being 84 to 69. Since the only
plant-food in soils that may be used for plant production is that
which is soluble, it follows that it is safe to assume that arid
soils are generally more fertile than humid soils. This is borne out
by a study of the constituents of the soil. For instance, potash,
one of the essential plant foods ordinarily present in sufficient