An index for soil pore size distribution

An index for classifying soil pore size distribution is proposed. The arithmetic mean change in percent soil water content by weight as the tension changes from zero to 1.5 bars is used as the index. This number characterizes the size distribution of pores with a radius of one micron or greater. A simple equation is presented to calculate the index from soil water contents at pressure plate settings of zero, 0.2, and 1.5 bars. Moisture release curves from 3 different soils show that the index does tend to characterize the shape of the release curve and that it is sensitive to past management which affects the distribution of large soil pores. When all other conditions are optimum, it appears that there exists a specific value of the index which indicates when the soil pore size distribution may be expected to limit plant growth. It is further suggested that the index, together with penetrometer measurements made at the 1.5-bar water content, may be used as “soil test values” for making practical management decisions and for predicting the stability of soils under varying field conditions.