The hydrophobic organic skins on sand grains were resistant to removal by solvents such as cold water, concentrated acid, diethyl ether, ethanol, benzene, chloroform, and acetone. Prolonged treatment with hot diethyl ether, ethanol, and benzene removed part of the coating. Treatment with dilute solutions of alkali removed the skin as suspended particles. Compounds within the very stable humic fraction of the soil organic matter appeared to be mainly responsible for water repellence in soils. Deposits of fresh organic materials could also produce water repellent properties.

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Sorghums were grown on a coarse deep sand under various sprinkler irrigation regimes. Bulk soil water potentials under all treatments remained in the range of 0 to - 1000 cm throughout the experiment. Measurements of leaf water potential showed a rapid diurnal change from no plant water stress at dawn, to large negative potentials in mid-afternoon. It is concluded that, at least in part, this stress was a direct consequence of the unavailability of soil water to meet the potential demand of the plant. Theoretical and experimental evidence suggests that the rate of supply of soil water was limiting. Increased diurnal water stress reduced plant growth and tiller numbers in forage sorghum, and delayed time to flowering in grain sorghum. Less water was evaporated from the treatments showing the greatest levels of diurnal water stress.