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The Story of the Soil; from the Basis of Absolute Science and Real Life, by Cyril G. (Cyril George) Hopkins
page 158 of 371 (42%)

"If they rotate their crops, they would probably yield as well as
ever," said the Chief.

"No, that, too, has been tried," replied Percy. "The Illinois
Experiment Station has practiced a four-year rotation of corn,
cowpeas, wheat, and clover on an experiment field on the common
prairie soil down in 'Egypt,' and the average yield of wheat has
been only twelve bushels per acre during the last four years, but
when legume crops were plowed under and limestone and phosphorus
applied, the average yield during the same four years was
twenty-seven bushels per acre."

"Probably the increase was all produced by the green manure,"
suggested the Chief. "Organic matter has a great influence on the
control of the moisture supply."

"That was tested," said Percy. "The green manure alone increased the
average yield to only fourteen bushels while the green manure and
limestone together raised the average wheat yield to nineteen
bushels, the further increase to twenty-seven bushels having been
produced by the addition of phosphorus."

"Well, Sir," said the Chief, "we have made both extensive intensive
investigations concerning the chemistry of the soil solution by very
delicate and sensitive methods of analysis we have developed, and we
have also conducted culture experiments for twenty-day periods with
wheat seedlings in the water extract of soils from all parts of the
United States, and the results we have obtained have changed the
thought of the world as to the cause of the infertility of soils."
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