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.