"Of course the surface soil is by far the most important," Percy explained. "It represents just about the depth of earth that is turned by the plow in good farming on normal soils; and it weighs about two million pounds per acre. The subsurface stratum extending from six and two-thirds to twenty inches in depth represents the practical limit of subsoiling; and this stratum weighs about four million pounds; while the subsoil stratum weighs about six million pounds, where the soil is normal, such as loam, silt loam, clay loam, or sandy loam. Pure sand soil weighs about one-fourth more, while pure peat soil weighs only half as much as normal soil." "I wish you would tell me," said Mr. Thornton, "what the fertilizers cost that have been used on that Rothamsted wheat field." "The annual application of nitrogen has been one hundred twenty-nine pounds per acre," said Percy. "What will it cost?" "Well, at twenty cents a pound, it would cost $25.80," was Mr. Thornton's reply after he had figured a moment. "But why didn't they grow clover and get the nitrogen from the air?" "For two reasons," replied Percy. "First, when those classic experiments were begun by Sir John Lawes and Sir Henry Gilbert in 1844, it was not known that clover could secure the free nitrogen from the air; and, second, the experiment was designed to discover for certain whether wheat must be supplied with combined nitrogen, by ascertaining the actual effect upon the yield of wheat of the nitrogen applied." "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."