y fine, assuming that
the entire product is used, including the finer dust produced in
grinding, and it is very possible that final investigations will
show that the entire product from a quarter-inch screen is even more
economical and profitable in permanent systems.

Limestone is quite easily soluble in soil water carrying carbonic
acid. It is thus readily available; in fact, it is too available to
be durable if very finely ground; and in humid sections the loss by
leaching far exceeds that removed by cropping. In practical economic
systems of farming about two tons an acre of ground limestone should
be applied every four years, or corresponding amounts for other
rotation periods.

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.

CHAPTER II

THE NITROGEN PROBLEM AND ITS ECONOMICAL SOLUTION

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 l