ared for in the same
manner as the cowpea.

The _Canadian Field Pea_ is sometimes grown in the north as a green
manure crop.

_White Sweet Clover_, white melitot or Bokhara clover, grows as a weed
from New England to the Gulf of Mexico. In the Gulf States it is
regarded as a valuable forage and green manure plant. One or two pecks
of seed per acre are sown in January or February.

_Alfalfa_, or lucern, though grown more for a forage crop than for
green manuring, should be mentioned here, for wherever grown and for
whatever purpose, its effects on the soil are beneficial (Fig. 82).
This plant requires a well prepared soil that is free from weeds.
Twenty to twenty-five pounds of seed are planted per acre. In the
north the seeding is generally done in the spring after danger of
frost is past, as frost kills the young plants. In the South fall
seeding is the custom in order to give the young plants a long start
ahead of the spring weeds. One seeding if well cared for lasts for
many years. Alfalfa is pastured or cut for hay, four to eight tons
being the yield. Many fields run out in five or six years and the sod
is plowed under. This plant sends its roots thirteen, sixteen, and
even thirty feet into the soil after water and food, and when these
roots decay they furnish the lower soil with organic matter and their
passages serve as drains and ventilators in the soil. Alfalfa is grown
extensively in the semi-arid regions of the country.

NON-LEGUMINOUS GREEN MANURE PLANTS

Among the non-leguminous green manure plants are rye, wheat, oats,
mustard, rape, buckwheat. Of these the rye and buckwheat are most
generally used, the rye being a winter crop and the other a warm
weather plant. They are both strong feeders and can use tough plant
food. They do not add new nitrogen to the soil though they furnish
humus and prepare food for the weaker feeders which may follow them.

CHAPTER XXI

COMMERCIAL FERTILIZERS

THE RAW MATERIALS

Next to the soil itself, the farmer's most important sources of plant
food are the farm manures. But most farms do not produce these in
sufficient quantities to keep up the plant food side of fertility.
Therefore the farmer must resort to other sources of plant food to
supplement the farm manures.

There is a large class of materials called Commercial Fertilizers,
which, if judiciously used, will aid in maintaining the fertility of
the farm with economy.

We learned in a previous ch