MINERAL INDUSTRIES OF THE UNITED STATES FERTILIZERS, 5 
The rest of its substance the plant obtains from the air, from mois- 
ture, and from the bacteria present in the soil, although the last 
may be regarded as part of the soil itself. 
The chemical material contributed by the soil focuses our interest 
here — the calcium, the phosphorus, the sulphur, the nitrogen, the 
potassium, the sodium, the magnesium, the iron, and some others. 
These substances are not present in their elemental or simple con- 
dition, but as compounds available for transfer into the body of 
plants. 
These elements are likewise present in the solid rocks that every- 
where underlie the soil, but in combinations wholly unfitted for sus- 
taining plant growth. They must first be rendered suitable by 
the general process of natural rock decay called weathering, which 
mechanically reduces the solid rocks to the consistency of soil and 
chemically prepares their components for the sustenance of life. 
The soil then becomes the laboratory in which the gases of the at- 
mosphere, with water, and certain constituents of decayed rock are 
combined into the organic products, or plants, upon which human 
life ultimately depends. 
In the normal course of events, plants spring up, live their course, 
and die, giving back to the atmosphere and soil the elements em- 
ployed in their life cycle. Thus it is that the rank growth of the 
jungle is unremitting. But where plants are removed by artificial 
means, the balance is upset and the soil is depleted in respect of some 
of its essential compounds. But soil is not static; the processes of 
weathering are continuous, and If the plant removal be stopped for 
a sufficiently long period, the deficiency will be naturally remedied 
by the transformation of a further quantity of needed compounds 
into an available or soluble condition. This result, however, can not 
be waited for; in populous sections the soil must be cultivated con- 
tinuously; the elements removed too rapidly must be replaced arti- 
ficially, that is, by the use of fertilizers. This is an absolute, basic 
necessity. Fertilizers, indeed, play a more fundamental part than 
that of merely increasing yields and profits ; no soil can be cultivated 
continuously without their use. 
Fertilizers, moreover, have an application beyond that of making 
possible continuous and intensive cultivation; by their use, many 
soils can be rendered fertile which are deficient in one or more essen- 
tial constituents by virtue of peculiar geologic history. Thus fer- 
tilizers extend the possibilities of food production to great areas 
unfitted by nature for productive yield. 
These considerations of the social value of fertilizers are aside 
from the economic advantage of their use as leading to greater 
profits to the acre and greater yields to the unit of labor expended. 
Yet this economic advantage is the great motive force which, under 
