Percy had opened the letter from the chemist, but said to Mr. West that it would take him an hour or more to compute the results to the form of the actual elements and reduce them to pounds per acre in order to make possible a direct comparison between the requirements of crops, on the one hand, and the invoice of the soil and application of plant food in manure and fertilizers, on the other hand.

“Please let me help you make the computations,” said Adelaide, much to the surprise of her parents, who knew that she took no interest in affairs pertaining to farming.  “I like mathematics and will promise not to make any mistakes if you will tell me how to do some of the figuring.”

“Thank you,” said Percy.  “With your help it will take only half the time that I should require alone.”

This proved to be correct, for in half an hour after supper they had the results in simplified form.  Even the mother and grandmother joined the circle as Percy began to discuss the results with Mr. West

“Now here is the invoice,” said Percy, “of the surface soil from an acre of land where we collected the first composite sample,—­the land which you said had not been cropped since you could remember.  This soil contains plant food as follows: 

1,440 pounds of nitrogen 380 pounds of phosphorus 15,760 pounds of potassium 3,340 pounds of magnesium 10,420 pounds of calcium

“I’d like to know how these amounts compare with what your Illinois soil contains,” said Mr. West.

“We have several different kinds of soil in Illinois,” replied Percy.  “The common corn belt prairie soil is called brown silt loam.  It contains, as an average, 5000 pounds of nitrogen and 1200 pounds of phosphorus, or nearly four times as much of each of those elements as this Virginia soil which you say is too poor to cultivate.

“I wrote to the Illinois Experiment Station before I left Washington to see if I could get the average composition of the heavier prairie soil, which occupies the very flat areas that were originally swampy, and one of the letters you had received for me gives 8000 pounds of nitrogen and 2000 pounds of phosphorus as the general average for that soil.  That is our most productive land, and it contains about five times as much of these two very important elements as your poorest land.

“Our more common Illinois prairie contains about 35,000 pounds of potassium, 9,000 pounds of magnesium, and I 1,000 pounds of calcium.  This is more than twice as much potassium and nearly three times as much magnesium as in your poorest land, but the calcium content is about the same in your soil as in ours.  However, as you will remember, your soil is distinctly acid and consequently markedly in need of lime, the magnesium and calcium evidently being contained in part in the form of acid silicates with no carbonates; whereas, our brown silt loam is a neutral soil and our black clay loam contains much calcium carbonate, the same compound as pure limestone.”