"In the general process of decay and oxidation of the organic matter
of the soil, the nitrogen thus passes through the forms of ammonia,
nitrous acid, and nitric acid, and at the same time the carbon
passes into various acid compounds, including the complex humic and
ulmic acids, and smaller amounts of acetic acid (found in vinegar),
lactic acid, oxalic acid (found in oxalic), and tartaric acid (found
in grapes). The final oxidation products of the carbon and hydrogen
are carbon dioxid and water, which result from the decomposition of
the carbonic acid.

"Now the various acids of carbon and nitrogen constitute one of the
most important factors in soil fertility. They are the means by
which the farmer can dissolve and make available for the growing
crops the otherwise insoluble mineral elements, such as iron,
calcium, magnesium, and potassium, all of which are contained in
most soils in great abundance. These elements exist in the soil
chiefly in the form of insoluble silicates. Silicon itself is a
four-handed element which bears somewhat the same relation to the
mineral matter of the soil as carbon bears to the organic matter.
Quartz sand is silicon dioxid (SiO2). Oxygen, which is present in
nearly all substances, including air, water, and most solids,
constitutes about one-half of all known matter. Silicon is next in
abundance, amounting to more than one-fourth of the solid crust of
the earth. Aluminum is third in abundance (about seven per cent),
aluminum silicate being common clay. Iron, calcium, potassium,
sodium, and magnesium, in this order, complete the eight abundant
elements, which aggregate about ninety-eight per cent. of the solid
crust of the earth.