Very few plants will grow on wet swampy soils. Most plants need soil conditions that give free movement of water and air. Their roots will not penetrate into soil where the minute spaces between soil particles are filled with stagnant water. Should their roots be planted in such a layer, they will soon perish.
In all fertile soils certain soil bacteria are present and of vital importance. They are of many kinds, the chief being those concerned with the breaking down of organic matter, those concerned with food absorption by plants, and those living on the roots of leguminous plants-such as clovers, peas, and beans-and enabling those plants to absorb nitrogen from the air. All these helpful bacteria require air, available only in well-drained soil. Faulty drainage fosters bacteria that break down organic matter by putrefaction, with consequent formation of products toxic to plants, instead of plant food.
As water percolates through soil, air follows it into the soil interstices to the full depth of drainage. As roots absorb moisture, air will be drawn into the soil to replace it. The deeper the drainage, the deeper will roots and air penetrate. With deep drainage, plants can draw on deep soil for moisture and food, and consequently be better able to withstand drought and heat.
With inadequate drainage the root systems develop near the surface. During the summer such soil quickly dries by evaporation and plant absorption, and though there may be-ample moisture in the subsoil it is brought up by capillarity too slowly for the plants. With better drainage the roots go deeper and capillarity is more efficient. These two factors provide for more rapid, continuous, and prolonged growth of the plants. Sodden soils are cold; aerated soils are warm. Soil warmth stimulates plant growth, because heat increases plant cells' absorption of moisture together with plant foods in extreme dilution.
