As the organic matter decays it adds available plant food to the soil, particularly nitrogen.
As it decays, it produces carbonic acid and other acids which are able to dissolve mineral plant food not soluble in pure water and thus render it available to plants.
Plants, although they require the same elements of plant food, take them in different amounts and different proportions.
Plants differ in the extent and depth of root growth and therefore take food from different parts of the soil. Some are surface feeders while others feed on the deeper soil.
Plants differ in their power to take plant food from the soil; some are weak feeders, and can use only the most available food; others are strong feeders, and can use tougher plant food.
Plants vary in the amount of heat they require to carry on their growth and development.
THE ONE CROP SYSTEM
We are now ready for the question. What effect has the continuous cultivation, year after year, of the same kind of crop on the soil conditions necessary to the best growth and development of that crop or any other crop? Suppose we take cotton for example.
How does cotton growing affect soil humus?
During the cultivation of cotton, the organic matter or humus of the soil decays in greater quantities than are added by the stalks and leaves of the crop. Therefore, cotton is a humus wasting crop, and the continuous cultivation of this crop tends to exhaust the supply of organic matter in the soil.