Soil Moisture and Absorption of Water by Tree Roots

Shade trees undergo periodic dehydration because the rate of absorption of soil water lags behind the rate of transpirational water loss from tree crowns. The rate of absorption of water from wet, warm, and well-aerated soil is controlled largely by the rate of transpiration. However, absorption of water often is impeded by low soil moisture content, a small or slow-growing root system, poor soil aeration, low soil temperature, a high concentration of the soil solution, or combinations of these. As the soil dries down from field capacity, the rate of absorption of water is reduced because of increased resistance to water movement in the soil and within the tree as well as loss of soil-root contact. Poor soil aeration in compacted or flooded soils decreases water absorption by inhibiting root growth, inducing decay of roots, and suppressing development of mycorrhizae. Low soil temperature reduces absorption of water by decreasing the permeability of roots, increasing the viscosity of water, and inhibiting root growth. High concentrations of deicing salts and fertilizers in the soil solution may reduce absorption of water by osmotic effects.