When crops are removed from the land
year after year, without any return being made, it naturally follows
that under ordinary conditions the amount of available plant-food is
diminished, with a strong probability of a corresponding diminution
in crop-producing power. In fact, the soils of many of the older
countries have been permanently injured by continuous cropping, with
nothing returned, practiced through centuries. Even in many of the
younger states, continuous cropping to wheat or other crops for a
generation or less has resulted in a large decrease in the crop
yield.
Practice and experiment have shown that such diminishing fertility
may be retarded or wholly avoided, first, by so working or
cultivating the soil as to set free much of the insoluble plant-food
and, secondly, by returning to the soil all or part of the
plant-food taken away. The recent development of the commercial
fertilizer industry is a response to this truth. It may be said
that, so far as the agricultural soils of the world are now known,
only three of the essential plant-foods are likely to be absent,
namely, potash, phosphoric acid, and nitrogen; of these, by far the
most important is nitrogen.
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