Tallow alcohol (TA) affects the properties of water in soil by forming compressed monolayer films on the water‐filled pores of the soil. Besides directly effecting a small evaporation reduction, the films lower the soil‐water tension. This tension‐lowering leads to several other effects, including reduction of the flow rate of water through uniform soils and, in layered soils, lowering of the soil‐water concentration in the TA‐containing layer. The last effect, which leads to self‐mulching, accounts for most of the substantial evaporation reduction from soils with TA in the surface layer. This effect also aids the infiltration of water into such soils. The full effects of TA are realized only when it is solvent‐dispersed; with powdered TA only partial effects are obtained. Attention is directed to the possible effects on the soil‐fatty alcohol‐water system of soil micro‐organisms growing on fatty alcohol. (Key words: Evaporation; infiltration; soil moisture; water.)