rank I. Mann has produced a 70-bushel average yield of
corn for a five-year period, and with 200 acres of land in corn
annually. It cost him only $1 an acre a year in fine-ground natural
rock phosphate to produce increased yields of 16 bushels more corn,
23 bushels more oats and 1 ton more clover than the average yields
secured without adding phosphorus.

But this progressive, practical farmer is only putting into
profitable practice the results of the long-continued careful
investigations with raw phosphate conducted by such public-service
institutions as the agricultural experiment stations of
Pennsylvania, Maryland, Rhode Island, Massachusetts, Ohio and
Illinois. He knows also that on four different fields of typical
Corn-Belt land in McLean county, Illinois, the total crop values per
acre for a period of ten years were $148.75 $151-30, $149.43 and
$149.96, respectively, and that on four other adjoining or
intervening fields, which differed only by two liberal additions of
phosphorus during the ten years, the respective crop values for the
same time were $229,37, $221.30, $229.20 and $225.57.

Of course, Mr. Mann does not buy nitrogen, but he takes it from the
inexhaustible supply in the air by means of clover and alfalfa or
other legumes. He does not buy potassium because he knows how to
liberate it from the inexhaustible supply contained in the soil, and
because he knows that in the Illinois investigation just cited the
crop values from four different fields not receiving potassium were
$148.75, $151.30, $229.37 and $221.30; while four other adjoining
fields, which differed only by liberal applications of potassium,
produced during the same ten years $149.43, $149.96, $229.20 and
$225.57, respectively.

Thus, as a general average, phosphorus increased the crop values by
$76.50 an acre, which amounts to more than 300 per cent on the
investment, and at the end of the ten years the soil on the best
treated and highest yielding land was 10 per cent richer in
phosphorus than at the beginning; while the crops from the
unfertilized land removed an amount of phosphorus equal to nearly
one-tenth of the total supply in the plowed soil. But a similar
general average shows that potassium produced increased crop values
worth only 86 cents, or 3 per cent of its cost.

What other results should be expected from land containing in the
plowed soil of an acre less than 1200 pounds of phosphorus and more
than 36,000 pounds of p