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