Computation of Integral Water Capacity Using Plant Green Leaf Temperature at Different Soil Compaction Levels

Integral water capacity (IWC) is the integral of differential water capacity functions in the range of 0 to infinity soil matric potential multiplied [H1] by weighting functions each taking into account the effect of varioussoil limitations that may develop at a given soil matric potential domain and restrict soil water availability to plant roots. The domains selected for development of weighting functions in most studies have seldom been based on plant response, but rather arbitrary. The purpose of this study was implementing midday green leaf temperature (TL) as a plant-response-based variable to compute integral water capacity. For this purpose, a sandy clay loam soil passed through 4.76 mm sieve was evenly compacted to three bulk densities of D1=1.35, D2=1.55, and D3=1.75 g cm-3, each replicated thrice, in PVC tubes (called pots hereafter) with 30 cm diameter and 70 cm height. Sunflower (Helianthus Annuus L) seedlings were planted in the pots and, after their full establishment, two periods of wetting and drying cycles were imposed. By monitoring daily soil moisture content at the three depths in the pots and converting them to soil moisture suctions along with the midday TL measurements, a plant-response-based weighting function was developed and integral water capacity (IWCP) was computed. Integral water capacity (designated as IWCG) was also computed by adopting the weighting functions proposed by Groenevelt et al. IWCP and IWCG in D1 treatment were obtained as 0.187 and 0.229 cm3cm-3, respectively. At the highly compacted D3 treatment, the corresponding values diminished to 0.152 and .038, respectively, equivalent to 19% and 84% reduction in soil water availability and reflecting the dominant effect of soil compaction on water availability. Averaged over the three compaction levels, IWCP and IWCG were 0.169 and 0.14 cm3cm-3,indicating that water availability determined on the plant response basis was 17% greater than that predicted by IWCG. This difference and over-susceptibility (84%) of IWCG to soil compaction imply that the soil suction domains proposed for the various soil limitations and the experimental relations employed in Groenevelt et al. approach to quantify their restricting effects as weighing functions need to be modified according to each particular plant needs or response.