The following six facts will furnish a comprehensive basis for the
solution of the nitrogen problem in practical general agriculture:

(1) To produce 100 pounds of grain requires about 3 pounds of
nitrogen, of which 2 pounds are deposited in the grain itself and 1
pound in the straw or stalks.

(2) In live-stock farming one-fourth of the nitrogen in the food
consumed is retained in the animal products--meat, milk, wool, and
so on--and three-fourths may be returned to the land in the
excrements if saved without loss.

(3) When grown on soils of normal productive capacity legumes secure
about two-thirds of their total nitrogen from the air and one-third
from the soil.

(4) Clover and other biennial or perennial legumes have about
two-thirds of their total nitrogen in the tops and one-third in the
roots, while the roots of cowpeas and other annual legumes contain
only about one-tenth of their total nitrogen.

(5) Hay made from our common legumes contains about 40 pounds of
nitrogen per ton.

(6) Average farm manure contains 16 pounds of nitrogen per ton.

Question: How many tons of average farm manure must be applied to a
40-acre field in order to provide as much nitrogen as would be added
to the soil by plowing under 2-1/2 tons of clover per acre? Answer:
400 tons.