Water Transport From Roots to Soil 1

Numerous studies have indicated that water can flow from roots to soil (negative direction) but little can be concluded about the magnitude of such a flow. Therefore, experiments were performed on potted cotton plants ( Gossypium hirsutum L. ‘Auburn 7‐683’) to compare the magnitude of water transport in the positive (soil to roots) and negative directions. Experimental results were also compared qualitatively with the predictions derived from mathematical models simulating root water uptake. Each plant was left unwatered in an environmental chamber to dry the soil from field capacity to near the —15 bar percentage. After drying, the stem was severed under water and the resulting water flow from roots to soil recorded by weight changes. Results indicated that water flow in the negative direction is small (1:36 during a 110‐hour period) compared to flow in the positive direction. This difference is thought to be due to a hysteretic, unsaturated, and possibly non‐Darcian flow phenomenon or to the generation of growth‐related water potentials. Contemporary mathematical water extraction models ignore both phenomena and, therefore, would be expected to overestimate flows in the negative direction if “calibrated” to predict correctly flows in the positive direction.