amount, is found in humid soils to the extent of 0.21 per cent,
while in arid soils the quantity present is 0.67 per cent, or over
three times as much. Phosphoric acid, another of the very important
plant-foods, is present in arid soils in only slightly higher
quantities than in humid soils. This explains the somewhat
well-known fact that the first fertilizer ordinarily required by
arid soils is some form of phosphorus:

The difference in the chemical composition of arid and humid soils
is perhaps shown nowhere better than in the lime content. There is
nearly eleven times more lime in arid than in humid soils.
Conditions of aridity favor strongly the formation of lime, and
since there is very little leaching of the soil by rainfall, the
lime accumulates in the soil.

The presence of large quantities of lime in arid soils has a number
of distinct advantages, among which the following are most
important: (1) It prevents the sour condition frequently present in
humid climates, where much organic material is incorporated with the
soil. (2) When other conditions are favorable, it encourages
bacterial life which, as is now a well-known fact, is an important
factor in developing and maintaining soil fertility. (3) By somewhat
subtle chemical changes it makes the relatively small percentages of
other plant-foods notably phosphoric acid and potash, more available
for plant growth. (4) It aids to convert rapidly organic matter into
humus which represents the main portion of the nitrogen content of
the soil.

Of course, an excess of lime in the soil may be hurtful, though less
so in arid than in humid regions. Some authors state that from 8 to