Hygroscopic Coeflicient

To have a more complete picture of soil moisture relations, let us take a soil sample and oven dry it for 24 hours at 11^oC. Now if this soil is kept in an atmosphere which is completely saturated with water vapour, the soil will absorb some water, which makes a thin film around the soil particles. The water is held tightly and can move only in the vapour phase. The moisture at this point, absorbed by a unit weight of dry soil is termed the hygroscopic coefficient . As might be expected, soils high in colloidal materials will hold more water under these conditions than the sandy soils. Soil moisture is sometimes expressed in terms of pF, which has a range from 0 to 7. The pF is the logarithm of capillary potential. The pF-0 represents saturated soil, 2.7 represents the field capacity, 4.2 the permanent wilting percentage, 5.5 the air dry level, and 7.0 the oven dry conditions. At the same pF, the actual amount of moisture content is higher in clay than in sand.