Water Loss From the Soil
There are several ways in which water is lost from the soil. Rain, melted snow, or water applied by the homeowner may percolate down through the soil beyond the root zone. This water is useless to growing plants.
Water also may evaporate from the soil surface, leaving it dry. Water from lower layers in the soil is drawn to the surface by capillary action and also evaporates. This continual evaporation may deplete water from quite deep in the soil.
Transpiration is the process by which a plant loses water through its leaves. This is a necessary process for plant growth. A large tree may lose hundreds of gallons of water a day in the summer. Water lost from the soil by evaporation and transpiration must be replaced by precipitation or supplemental irrigation.
Establishing the correct water-air relationships in the soil is essential for the best growth of all plant types. Oxygen in the soil is necessary for plants to grow. Watering too often or too much is likely to exclude the necessary oxygen from the soil pore spaces. Without enough oxygen, plant roots suffocate and die, preventing water uptake. Plant parts aboveground exhibit symptoms of this stress; wilting, yellowing, and drying foliage, leaf drop and twig dieback may all occur. Constant overwatering kills most plants.
Too little water, on the other hand, does not allow the roots to replace water lost by the plant through transpiration. The roots may dry up and die, and the top growth begins to show abnormal symptoms. In both cases, either too much or too little water, the plant suffers from lack of moisture in its tissues.
A good rule-of-thumb to follow in watering plants is to fill the entire root zone with water, and then allow the soil to dry out partially before the next irrigation. The amount of drying depends on the plant species and size. Large trees and shrubs can be allowed to dry several inches down in the soil before rewatering. A small or newly established plant will need watering before very much soil drying takes place.
It is essential that gardeners become familiar with how long it takes the root zones of the various plants in their gardens to become completely moistened, and then, how deeply they can allow the soil to dry before the plants begin to show stress and need rewatering. It is also necessary to understand that quick, light sprinkling will not do the job of wetting the entire root zone.
Soil type or texture is a major determining factor of how much water a soil will hold, or how quickly a soil can be irrigated. For example, one inch of water applied to a sandy soil will penetrate 12 inches. It will move anywhere from 6–10 inches into a good loam soil, and in a clay soil it will percolate down only 4–5 inches.
Sandy soils allow water to penetrate more quickly than will heavy, dense soils. Wetting the entire root zone of plants growing in heavy soils takes much longer than wetting plants growing in lighter soils. Sandy loams will accept from 1/2–3 inches of water per hour. A clay-loam may absorb only 1/10–3/5 inch of water in the same amount of time. A very dry clay-loam soil could take as long as 120 hours to completely wet to a depth of 12 inches. A sandy loam, however, might take as little as 4 hours.