This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
35 Trees to Acre.
Dry matter ...
At the end of twenty years (the Apple trees being then thirty-three years of age) it was computed that the amount of nitrogen, phosphoric acid, and potash taken from the soil during the period of trial was as follows: -
Total for 20 years ...
Average per year
These quantities are much less per acre per annum than those already given above. They serve, however, to indicate the small amount of food exhaustion that takes place, and incidentally the quantities of nitrogen, phosphoric acid, and potash that might have to be supplied to maintain the equilibrium of available foods in the soil. Assuming the figures to be fairly accurate, it would appear that the fallen leaves, if dug into the ground during the winter months, would supply, when rotted, about half the entire quantity of food taken out during the year, as shown, thus: -
Food taken from an acre of soil each year
Food supplied by fallen leaves per acre
The quantities of nitrates, phosphates, and potash taken out of an acre of soil each year naturally vary a good deal, according to the nature of the crop. It will be seen from the table below that some crops absorb much larger quantities of certain foods than others, and this fact should be borne in mind when applying manures.