“And what was the actual effect of the nitrogen?” questioned Mr. Thornton.  “How much did the wheat yield when they left out the nitrogen and applied all the other elements?”

“Only fifteen bushels,” was the reply.

“Only fifteen bushels!  Only two bushels increase for all the other elements, phosphorus, potassium, magnesium, and calcium,—­and I remember you said that sulfur also was applied.  Why didn’t they leave off all these other elements, and just use the nitrogen alone?”

“They did on another plot in the same field.”

“Oh, they did do that?  What was the yield on that plot?”

“Only twenty bushels.”

“Only twenty bushels!  Well, that s mighty queer.  How do you account for that?”

“Does Mrs. Thornton sometimes make dough out of flour and milk?” asked Percy.

“Another Yankee question, eh?” said Mr. Thornton.  “I told my wife once that I wished she could make the bread my mother used to make, and she said she wished I could make the dough her father used to make.  Yes, my wife makes dough, a good deal more than I do, and she makes it of flour and milk, when we aren’t reduced to corn meal and water.”

“Can she make dough of flour alone?” continued Percy.

“No,” replied Mr. Thornton.

“Nor of milk alone?”

“No.”

“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.”