Characterization of Soil-water Retention of a Very Gravelly Loam Soil Varied with Determination Method

Soil-water characteristic curves were determined for Krome calcareous very gravelly loam soil in the laboratory and in situ in an orchard. In the laboratory, soil-water retention was determined with a pressure plate and pressure Tempe cells for soil collected from vegetable fields. In the orchard, soil-water suction measured with tensiometers was compared to volumetric soil-water content (E) determined with neutron or multi-sensor capacitance probes. Before field measurements, calibration equations were developed for neutron and multi-sensor capacitance probes for this soil. Krome calcareous gravelly loam soil was found to have two distinct solid fractions with 51% coarse particles and 49% loam particles that resulted in a peculiar soil-moisture retention pattern. Two soil-moisture retention regions were identified, each corresponding to one of the soil solid fractions. As shown by a large number of observations, rapid drainage occurs in the gravel fraction corresponding to soil-water suction less than 75 cm. In an orchard, suction values rarely exceeded 125 cm even when there was no rainfall and irrigation was withheld for three weeks. In the orchard, E measured with a capacitance probe was considerably more variable and less correlated with soil suction than E measured with a neutron probe. In very gravelly loam soils such as Krome, results from capacitance sensors may be too variable and inconsistent for reliable monitoring of soil-water content. (Soil Science 2006;171:85–93)