"It is worth while to know that about two and one-half per cent. of
the earth's crust is potassium, while about one-tenth of one per
cent. is phosphorus; also that when a hundred bushels of corn are
sold from the farm, seventeen pounds of phosphorus, nineteen of
potassium, and seven of magnesium are carried away.

"The acids formed from the decaying organic matter not only liberate
for the use of crops the mineral elements contained in the soil in
abundance, but they also help to make available the phosphorus of
raw phosphate, when naturally contained in the soil, as it is to
some extent in all soils, or when applied to the soil in the
fine-ground natural phosphate from the mines.

"Now the increase or decrease of organic matter in the soil is
measured with a very good degree of satisfaction by the element
nitrogen, which is a regular constituent of the organic matter of
the soil; and you are already familiar, Mr. Thornton, with the
amounts of nitrogen contained in average farm manure and in some of
our most common crops."

"Yes, Sir, I have some of the figures in my note book and I mean to
have them in my head very soon. But, say, that organic matter seems
to be a thing of tremendous importance, and I'm sure we've got
mighty little of it. I think about the only thing we'll need to do
to make this old farm productive again is to grow the vegetation and
plow it under. As it decays, it will furnish the nitrogen, and
liberate the phosphorus, potassium, calcium, and magnesium; and we
may have plenty of all of them just waiting to be liberated."

"That is altogether possible," said Percy; "but it must be