The potash and
phosphoric acid supply can probably be maintained for ages by proper
methods of cultivation, though the phosphoric acid will become
exhausted long before the potash. The nitrogen supply, however, must
come from without. The nitrogen question will undoubtedly soon be
the one before the students of dry-farm fertility. A liberal supply
of organic matter In the soil with cultural methods favoring the
growth of the nitrogen-gathering bacteria appears at present to be
the first solution of the nitrogen question. Meanwhile, the activity
of the nitrogen-gathering bacteria, like azotobacter, is one of our
best explanations of the large presence of nitrogen in cultivated
dry-farm soils.
To summarize, the apparent increase in productivity and plant-food
content of dry-farm soils can best be explained by a consideration
of these factors: (1) the intrinsically high fertility of the arid
soils; (2) the deep feeding ground for the deep root systems of
dry-farm crops; (3) the concentration of the plant food distributed
throughout the soil by the upward movement of the natural
precipitation stored in the soil; (4) the cultural methods of
dry-farming which enable the weathering agencies to liberate freely
and vigorously the plant-food of the soil grains; (5) the small
annual crops; (6) the plowing under of the header straw, and (7) the
activity of bacteria that gather nitrogen directly from the air.
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