The permanent wilting percentage as a reference for the measurement of soil moisture

Soil moisture as usually reported as a percentage on a dry‐weight basis without reference to some standard or base for comparison is of limited value. Arbitrary definitions of the kinds of water in soil convey only qualitative ideas. It is shown that the permanent wilting percentage is remarkably constant and that in regions of rainless summers this moisture condition is reached year after year in the soil below the surface layer which is affected by surface evaporation. Soil‐moisture extraction curves slope downward approximately uniformly until the permanent wilting percentage is reached. Then the curves continue nearly horizontal. The soil moisture is slowly reduced to a minimum which is slightly below the permanent wilting percentage. The permanent wilting percentage is a satisfactory reference point from which the amount of readily available water and also the amount needed to raise the soil to its field capacity may be calculated.