Development of soil morphological descriptors to improve field estimation of hydraulic conductivity

The wide application of hydraulic conductivity data depends on linking hydr aulic parameters to soil morphology. Conventional morphology has been found to be a poor predictor of hydraulic conductivity. In this study we present new methods of characterising soil morphology to provide key morphological descriptors that can be related to hydraulic conductivity. Relationships were established between morphological properties (macrovoids , ped size, and associated surface features), and in situ consistence (degr ee of packing) with measured saturated and near-saturated hydraulic conduct ivity (K-s and K-40). These relationships were applied to estimate K-s and K-40 classes for soil horizons of pedal and apedal soils. In pedal soils, K is estimated from the area of macrovoids and from the proportions of diffe rent-sized peds in each horizon, their closeness of fit, and their degree o f packing. Low packing and fine peds with rough surfaces is indicative of r apid conductivity, whereas high packing and coarse peds with smooth/shiny p ed surfaces is indicative of slow conductivity. In apedal soils, K is estim ated from the area of macrovoids and from degree of packing and particle-si ze class. Low packing with coarse particles indicates rapid conductivity, w hereas high packing with fine particles indicates slow conductivity. Field characterisation of soils to determine functional morphology requires : (i) a dye to measure extent and continuity of cracks and macrovoids; (ii) a metal blade (Singleton Blade) and a hand-held penetrometer to measure so il consistence in situ; and (iii) weighing different-sized peds and describ ing their surface features.