Field‐Measured Limits of Soil Water Availability as Related to Laboratory‐Measured Properties

Abstract

Accurate evaluation of the soil water reserves available for plant use is vital in developing optimum water management for crop production in marginally dry regions. Laboratory estimates of the upper and lower limits of soil water availability used to calculate the soil water reservoir often deviate significantly from the limits measured in the field. To make a unified and broad assessment of the accuracy of laboratory measurements for estimating field soil water, we obtained and evaluated a comprehensive data base of field‐measured upper and lower limits of the soil water reservoir. The field‐measured upper limit was taken as the water content at which drainage from a prewetted soil had practically ceased. The lower limit was taken as the water content of the soil at which plants were practically dead or dormant as a result of the soil water deficit. These field‐measured limits were compared to laboratory measurements at −0.33 and −15 bar made on samples removed from each field site. A total of 401 observations were available for the comparisons of −15 bar measurements to the field‐measured lower limits and 282 observations of −0.33 bar measurements were available for comparison with the field‐measured upper limit. Variation often existed within a soil series at a particular site for the field‐measured upper and lower limits. However, the differences between the field‐measured limits, the total available water reservoir, were relatively constant. Crop species caused only minor differences in the lower limit water content for the upper part of the soil profile where root length density was apparently above some critical limit. However, some annuals extracted water to greater depths than others. The laboratory estimates of the upper limit obtained by −0.33 bar water contents were significantly less than the field‐measured drained upper limit for sands, sandy loams, and sandy clay loams and were significantly more than field measurements for silt loams, silty clay loams, and silty clays. The laboratory estimates of the lower limit obtained by −15 bar water content measurements were significantly less than field lower limit measurements for sands, silt loams, and sandy clay loams and significantly more than field observations for loams, silty clays, and clays. Because our study included relatively few measurements of loamy sands, silts, sandy clays, and clays, it was difficult to generalize about differences in field‐measured and laboratory‐estimated water limits for those textures. The results suggest that if absolute accuracy is necessary in water balance calculations, laboratory‐estimated soil water limits should be used with caution and field‐measured limits, if available, would be preferred.

Silvicultural treatments, microclimatic conditions and seedling response in Southern Interior clearcuts

“…These were placed on pressure plates in a pressure chamber apparatus (Soilmoisture Equipment Corp., Santa Barbara, CA), and allowed to equilibrate at given levels of applied air pressure (Klute 1986). Soil water contents for these cores were calculated on a fine soil volumetric basis (coarse fragmentfree), and then used to develop soil water characteristic functions and to calculate total available water capacities based on the difference between volumetric water contents at matric potentials of -0.033 and -1.5 MPA (Ratliff et al 1983). …”

Revisiting Field Capacity (FC): variation of definition of FC and its estimation from pedotransfer functions

“…To our knowledge, Cassel et al (1983) were the first to apply pedotransfer functions (PTFs) to evaluate field-determined FC. Based on the same data set used by them (312 soil samples from the USA), which was well described by Ratliff et al (1983), Ritchie et al (1999) presented a simple PTF, here called the Ritchie equation, based only on soil bulk density and two granulometric fractions, which gave similar or better results than the model of Cassel et al (1983). Using particle size distribution, bulk density, and the Arya & Paris (1981) model to predict water retention curves, Gerakis & Zalidis (1998) proposed a physical method to estimate fielddetermined FC.…”

Modeling responses of dryland spring triticale, proso millet and foxtail millet to initial soil water in the High Plains

“…Amount of plant available water was determined by subtracting lower limits of volumetric water from the total volumetric water content at each sampling depth. Lower limits at each site were determined from field observations of the lowest volumetric water content observed for these crops (Ritchie, 1981;Ratliff et al, 1983).…”