20 per cent of calcium carbonate makes a soil unfitted for plant
growth. There are, however, a great many agricultural soils covering
large areas and yielding very abundant crops which contain very much
larger quantities of calcium carbonate. For instance, in the Sanpete
Valley of Utah, one of the most fertile sections of the Great Basin,
agricultural soils often contain as high as 40 per cent of calcium
carbonate, without injury to their crop-producing power.

In the table are two columns headed "Soluble Silica" and "Alumina,"
in both of which it is evident that a very much larger per cent is
found in the arid than in the humid soils. These soil constituents
indicate the condition of the soil with reference to the
availability of its fertility for plant use. The higher the
percentage of soluble silica and alumina, the more thoroughly
decomposed, in all probability, is the soil as a whole and the more
readily can plants secure their nutriment from the soil. It will be
observed from the table, as previously stated, that more humus is
found in humid than in arid soils, though the difference is not so
large as might be expected. It should be recalled, however, that the
nitrogen content of humus formed under rainless conditions is many
times larger than that of humus formed in rainy countries, and that
the smaller per cent of humus in dry-farming countries is thereby
offset.

All in all, the composition of arid soils is very much more
favorable to plant growth than that of humid soils. As will be shown
in Chapter IX, the greater fertility of arid soils is one of the
chief reasons for dry-farming success. Depth of the soil alone does
not suffice. There must be a large amount of high fertility
available for plants in order that the small amount of water can be