Wheat, 32 bushels in 1851 and 37.8 in 1911.

Here we have data which span a period of sixty years and which show
that where mineral plant food has been provided the clover in
rotation and the manure produced by the feeding of only one of the
four crops have maintained the yield of all crops except the
barley-the third crop after clover-and without the application of
nitrogen in any other form. If the clover and straw had been
returned to the land either directly or in farm manure the
additional nitrogen thus provided would have been sufficient both to
maintain the yield of barley and to prevent the moderate decrease
which has occurred in the nitrogen content of the soil.

CHAPTER III

PHOSPHORUS: THE MASTER KEY TO PERMANENT AGRICULTURE

THE greatest economic loss that America has ever sustained has been
the loss of energy and profit in farming with an inadequate supply
of phosphorus. Phosphorus is a Greek word which signifies
"light-bringer"; but it is a light which few Americans have yet
seen, else we should not permit the annual exportation of more than
a million tons of our best phosphate rock, for which we receive at
the mines the paltry sum of five million dollars, carrying away from
the United States an amount of the one element of plant food we
shall always need to buy, which if retained in this country and
applied to our own soils would be worth not five million but a
thousand million dollars for the production of food for the oncoming
generations of Americans.

For five million dollars we export to Europe each year enough
phosphorus for 1,400,000,000 bushels of wheat, or twice the average
crop of the entire United States. Meanwhile our ten-year-average
yield of wheat is 14 bushels an acre, while Germany's yield has gone
up to 29, Great Britain's to 33, England's to 37-1/2 and Denmark's
to more than 40 as the average for a decade.

Potato Yield Twice Doubled

There is only one place in the world where we can go for the results
of soil improvement for more than a quarter of a century in
connection with the growing of potatoes. Of course this place is
Rothamsted, England, where as an average for twenty-six years the
yield of potatoes was 51 bushels an acre on unfertilized land and
exactly 102 bushels where only a phosphate fertilizer was applied.
Where the same amount of phosphorus--29 pounds of the element per
acre per annum--was used in connection with other minerals--300
pounds of potassium