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

Abstract

The wide application of hydraulic conductivity data depends on linking hydraulic 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 (degree of packing) with measured saturated and near-saturated hydraulic conductivity (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 different-sized peds in each horizon, their closeness of fit, and their degree of packing. Low packing and fine peds with rough surfaces is indicative of rapid conductivity, whereas high packing and coarse peds with smooth/shiny ped surfaces is indicative of slow conductivity. In apedal soils, K is estimated from the area of macrovoids and from degree of packing and particle-size class. Low packing with coarse particles indicates rapid conductivity, whereas 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 soil consistence in situ; and (iii) weighing different-sized peds and describing their surface features.