Abstract: Information extracted from water retention properties are commonly used to evaluate soil structure. The objective of this work was to derive and test a soil entropy measure of pore size distributions (SH) from the parameters of a water retention model that assumes a log-normal distribution of soil pores. The SH function uses information on saturated and residual water contents and geometric mean pore size and geometric standard deviation. The SH function was tested with water retention data taken from separate published studies on soil textural class averages, soil compaction, and structural development in the rhizosphere. Values of SH ranged from −0.07 to about 4. Lower values were found in textural classes dominated by clay and in compacted soils; SH values were between 48% and 100% greater in soil aggregates from the rhizosphere of three crops than in aggregates sampled away from the roots. Compaction decreased the values of entropy by decreasing the values of geometric mean and/or standard deviation characterizing a pore size distribution. Soil entropy SH was correlated to saturated hydraulic conductivity from compaction studies (P < 0.001). Similar trend was followed by Ks and SH estimated from textural averages. The proposed SH function is a potentially useful measure of soil structure that is not restricted to a particular mathematical model of soil-water retention. Further improvement of SH should distinguish between porosity related to texture from porosity associated to soil structure.