Determination of soil available water for plants: Consistency between laboratory and field measurements

- There were significant differences among lab and field PAW, LLWR, IWC and S values.
- Soil water retention of “SB/FC/PWP” and “0/FC/PWP” agreed well with “Total” scheme.
- Lab penetration resistances were considerably greater than field-measured values.
- “SB/FC/PWP” and “0/FC/PWP” schemes are suggested for quick SAW determination.

This study was conducted to investigate the consistency between laboratory and field measurements of plant available water (PAW), least limiting water range (LLWR), integral water capacity (IWC) and soil physical quality index (S). It was also intended to propose quick and reliable method(s) for calculation of these indices. The S, PAW, LLWR and IWC of twenty soils were calculated using laboratory and field measurements. The consistency among four laboratory and two field schemes was evaluated to calculate soil available water (SAW). In the laboratory, soil hydraulic properties were determined using sand box and pressure plate apparatuses. Soil penetration resistance (Q) was measured using a cone micro-penetrometer. In the field, soil hydraulic and mechanical properties were determined and/or predicted using the data collected by tensiometers “Tens” or tension/disk infiltrometer “Disk”, and field cone penetrometer, respectively. The SAW and S values were calculated using all laboratory data “Total” which was regarded as benchmark for evaluation of the other schemes. The “SB/FC/PWP” scheme represents laboratory measurements of soil water retention by sand box (SB), field capacity (FC) and permanent wilting point (PWP), and “0/FC/PWP” scheme represents only saturated water content (0), FC and PWP measurements. In general, differences were observed between the water retention curves of different schemes. Also the laboratory-measured Q values were considerably greater than those measured in the field at similar water contents. These dissimilarities caused significant differences between SAW and S values calculated by different schemes. However, there was good compatibility between SAW and S values calculated by “SB/FC/PWP”, “0/FC/PWP” schemes and “Total” scheme suggesting these schemes as the reliable and quick schemes for SAW determination. We also found good compatibility between LLWR and IWC values calculated by “Tens” and “Total” schemes. Using “Tens” scheme, LLWR and IWC could be determined/calculated in the field in about ten days.