A moist soil.

A ventilated soil.

A warm soil.

A soil supplied with plant food.

Decaying organic matter or humus is one of the most important ingredients of our soils.  Because: 

It greatly influences soil texture and therefore the conditions necessary for root growth.

Its presence or absence greatly influences the attitude of soils toward water, the most important factor in plant growth.  Its presence helps light, sandy soils to hold more water and to better pump water from below, while it helps close, heavy soils to better take in the water which falls on their surface.  Its absence causes an opposite state of affairs.

The presence of organic matter checks excessive ventilation in too open, sandy soil by filling the pores, and improves poor ventilation in heavy clay soils by making them more open.

Humus, on account of its color, influences the heat absorbing powers of soils.

The organic matter is constantly undergoing more or less rapid decay unless the soil be perfectly dry or frozen solid.  Stirring and cultivating the soil hasten this decay.

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.

How does cotton growing affect soil texture?

Cotton growing wastes soil humus and therefore injures soil texture by making the lighter soils more loose and open, and the heavier soils more dense and compact.