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 fi ne 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 mat ric potential (phi). This narrow range of plant-available soil water is ext remely dry for most soil water sensors. Knowledge of the soil water retenti on curves for these soils is important for effective irrigation of crops in fine sand soils of subtropical and tropical regions of the world. The prim ary objective of this study was to compare sandy soil water retention curve s in the field as measured by tensiometer and resistance block phi values a nd capacitance sensor theta. The second objective was to compare these curv es to one developed on a Florida fine sand soil using a pressure plate appa ratus. Tensiometer and resistance block phi values were compared to theta v alues from capacitance sensors calibrated gravimetrically. The effective ra nge of both tensiometers and resistance sensors in fine sand soils is betwe en -5 and -20 kPa phi. Soil water potential values for both sensors were wi thin 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 diff erent 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. Th ese differences are significant in soils with very low water holding capaci ties. Differences between laboratory- and field-determined retention curves could be due to a combination of entrapped air in the field soil and/or al teration in bulk density in the laboratory samples.