The essential facts relating to potassium, magnesium
and calcium and to the use and value of different
forms of lime have been stated above, and they may
be accepted with confidence for use in economic systems
of farming on normal soils.
THE NITROGEN PROBLEM AND
In the previous chapter emphasis has been laid upon
the fact that plants as well as animals must have
food, and that the neglect or ignorance of this factor
in American agriculture has led to soil depletion
and land ruin on vast areas, especially in the older
states.
It has been shown that of the ten essential elements
of plant food, five are provided by natural processes
without the intervention of man; that, of the remaining
five, potassium is the most abundant in normal soil,
but requires liberation by good systems of farming;
that ground natural limestone is the ideal material
with which to supply calcium and to prevent or correct
soil acidity; and that if dolomitic limestone be used
magnesium is also supplied in suitable form for plant
food, Thus only nitrogen and phosphorus remain for
consideration.
Keeping in mind that systems of permanent profitable
agriculture in America must be founded upon an intelligent
understanding of the foundation principles involved,
let us pray for strength to acknowledge the truth
and cease trying to deceive ourselves. The truth
is that by soil enrichment alone the average crop yields
of the United States could be doubled, with the same
seed and seasons and with but little more work than
is now devoted to the fields; and we should cease
trying to deceive ourselves in the hope or belief
that the fertility of our soil will be maintained if
we continue year after year to take crops from the
land and fail to make adequate return.
Nitrogen is both the most abundant agriculturally
and the most expensive commercially of all the elements
of plant food; and yet there is a method by which
it can be secured not only without money but with
profit in the process. The percentage of nitrogen
in normal soils decreases with depth, so that subsoils
are almost devoid of nitrogen. This would be
more generally understood if it were known that the
supply of soil nitrogen in humid countries is contained
only in the organic matter.
This organic or vegetable matter consists of the partly
decomposed residues of plants, including the roots
and fallen leaves which may accumulate naturally,
and the green manure crops, crop residues and farm
manure which may be supplied in farm practice.
Thus the nitrogen of a soil is measured approximately
by its content of organic matter; and, vice versa,
the percentage of nitrogen is an approximate measure
of the organic matter, because nitrogen is a regular
constituent of the organic matter normally contained
in soils. Consequently if the organic matter
of a soil is reduced the supply of nitrogen is also
reduced.
