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. How does cotton growing affect soil water? By wasting humus cotton growing injures soil texture and so weakens the water holding and water pumping power of light soils and weakens the water absorbing power of heavy soils. Therefore the continuous cultivation of cotton weakens the power of the soil over water, that most important factor in crop growth. How does cotton growing affect soil ventilation? Continuous cotton culture, by wasting humus, injures texture and therefore injures soil ventilation, causing too much ventilation in the lighter soils and too little in heavier soils. How does cotton culture affect plant food in the soil? Continuous cotton growing wastes plant food: Because it wastes organic matter which contains valuable plant food, particularly nitrogen.