Nitrogen, which is the most important plant-food that may be absent
from dry-farm soils, may be secured by the proper use of leguminous
crops. All the pod-bearing plants commonly cultivated, such as peas,
beans, vetch, clover, and lucern, are able to secure large
quantities of nitrogen from the air through the activity of bacteria
that live and grow on the roots of such plants. The leguminous crop
should be sown in the usual way, and when it is well past the
flowering stage should be plowed into the ground. Naturally, annual
legumes, such as peas and beans, should be used for this purpose.
The crop thus plowed under contains much nitrogen, which is
gradually changed into a form suitable for plant assimilation. In
addition, the acid substances produced in the decay of the plants
tend to liberate the insoluble plant-foods and the organic matter is
finally changed into humus. In order to maintain a proper supply of
nitrogen in the soil the dry-farmer will probably soon find himself
obliged to grow, every five years or oftener, a crop of legumes to
be plowed under.
Non-leguminous crops may also be plowed under for the purpose of
adding organic matter and humus to the soil, though this has little
advantage over the present method of heading the grain and plowing
under the high stubble.
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