Proposing and Evaluating a Laboratory Method for Quick Determination of Different Quantities of Soil Available Water to Plant

A laboratory method was proposed and its capability was evaluated for quick and reliable determination of soil available water (SAW) for plant using concepts of plant available water (PAW), least limiting water range (LLWR) and integral water capacity (IWC). Water retention (θ) of 20 agricultural soils was detemined at 13 matric suctions (h) using sand box and pressure plate apparatuses. These data were fitted by the van Genuchten model and considered as the reference method. We used the soil water retention at h values of 0, 330 and 15000 hPa for the model fitting in the proposed method. Penetration resistance (Q) was measured on 10 undisturbed cores at different water contents for each soil using a lab micropenetrometer. Except for PAW (with h of 100 hPa considered as field capacity), there were not significant differences between the mean values of SAW determined by the reference and proposed methods. Strong regression lines were obtained between corresponding SAW values calculated by the two methods. Therefore, it is not needed to measure soil water retention and penetration resistance curves at several h values; instead water retention measurement at h values of 0, 330 and 15000 cm and quick measurement of penetration resistance at different water contents could be used for determination of different quantities for SAW. The PAW, LLWR and IWC were negatively and significantly regressed with relative bulk density (RBD) in the studied soils. These findings show that the SAW values calculated by the proposed method are susceptible to soil structure and might be considered as soil physical quality indices. The results showed suitability of the proposed method for quick and reliable determination of soil available water quantities which are useful for applied purposes and irrigation scheduling.