"Well, wheat cannot be made of nitrogen alone, nor can it be made
without nitrogen. On Broadbalk field at Rothamsted, where the wheat
is grown, the soil is most deficient in the element nitrogen. In
other words, nitrogen is the limiting element for wheat on that
soil; and practically no increase can be made in the yield of wheat
unless nitrogen is added. However, some other elements are not
furnished by this soil in sufficient amount for the largest yield of
wheat, and these place their limitation upon the crop at twenty
bushels. To remove this second limitation requires that another
element, such as phosphorus, shall be supplied in larger amount than
is anually liberated in the soil under the system of farming
practiced."

"Yes, I see that," said Mr. Thornton, "it's like eating pancakes and
honey; the more cakes you have the more honey you want. I think I
can almost see my way through in this matter; we are to correct the
acid with limestone, to work the legumes for nitrogen, and turn
under everything we can to increase the organic matter, and if we
find that the soil won't furnish enough phosphorus, potassium,
magnesium, or calcium, even with the help of the decaying organic
matter to liberate them, why then it is up to us to increase the
supply of those elements."

"You must remember that the calcium will be supplied in the
limestone;" cautioned Percy. "And, if you use magnesian limestone,
you will thus supply both calcium and magnesium. Keep in mind that
_magnesian _only means that the limestone contains some _magnesium._
and that it is not a pure calcium carbonate. The purest magnesian
limestone consists of a double carbonate of calcium and magnesium,
called dolomite."