Comparison of laboratory- and field-derived soil water retention curves for a fine sand soil using tensiometric, resistance and capacitance methods

The approximate range from 100 to 50% of plant-available water in Apopka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult) is 0.08-0.04 cm(3) cm(-3) soil water content (theta) or -5 to -15 kPa of soil water matric potential (phi). This narrow range of plant-available soil water is extremely dry for most soil water sensors. Knowledge of the soil water retention curves for these soils is important for effective irrigation of crops in fine sand soils of subtropical and tropical regions of the world. The primary objective of this study was to compare sandy soil water retention curves in the field as measured by tensiometer and resistance block phi values and capacitance sensor theta. The second objective was to compare these curves to one developed on a Florida fine sand soil using a pressure plate apparatus. Tensiometer and resistance block phi values were compared to theta values from capacitance sensors calibrated gravimetrically. The effective range of both tensiometers and resistance sensors in fine sand soils is between -5 and -20 kPa phi. Soil water potential values for both sensors were within 2 kPa of the mean for each sensor. Change in phi was similar over the range of 0.04-0.08 cm(3) cm(-3) theta. Curves for the two sensors were different by 4 kPa at 0.04 cm(3) cm(-3). The relationship between phi and theta were similar at 10-20, 20-30 and 40-50 cm depths. This was not true for a laboratory determined soil water retention curve for the same soil type. These differences are significant in soils with very low water holding capacities. Differences between laboratory- and field-determined retention curves could be due to a combination of entrapped air in the field soil and/or alteration in bulk density in the laboratory samples.