Site-specific soil hydraulic quality index to describe the essential conditions for the optimum soil water regime

This study was conducted in northeast Thailand (i) to identify soil hydraulic quality (SHQ) factors under four common cropping systems in a specific soil series, (ii) to configure indicators from these factors that can be used to develop SHQ indices for each cropping system, and (iii) to describe the essential conditions for the optimum soil water process in the study area. The cropping systems were (i) maize (Zea mays L.)-maize, (ii) mungbean (Vigna radiate L.)-maize, (iii) maize-fallow and (iv) cassava (Manihot esculenta Crantz). Ten representative farms under each system were used to develop location-specific SHQ indices. Soil hydraulic quality indices were determined for two soil layers, such as the Ap (0-20 cm) and the EB (20-65 cm) horizons at each farm. Principal component analysis (PCA) and factor analysis identified eight factors affecting soil hydraulic quality, such as pore size and distribution frequency, water-stable aggregates, aggregate shape and grade, shrinking-swelling, infiltration rate (IR) and percent sand particle for the Ap horizon. Aggregate shape, size and grade, pore distribution frequency, shrinking-swelling, packing density, soil organic matter (SOM), porosity and root density influenced water movement in the sub-soils (EB horizon). A comprehensive SHQ index was developed for both soil layers under each of the cropping systems. The mungbean-maize (Mn-M) system scored the highest total SHQ index (0.726) in topsoil followed by maize-fallow (M-F) (0.708), cassava (C) (0.663) and maize-maize (M-M) (0.623). Finally, SHQ indices were ranked into three distinctive classes under the selected cropping systems, i.e., I - Mn-M; II - M-F and C; and III - M-M. The Mn-M system is better compared with the M-M in sustaining SHQ due to favorable individual soil parameters such as pore distribution frequency, root density, SOM (2.80%) and IR (494.95 mm h -1 ). For the rooted subsoil, no significant differences could be identified irrespective of cropping systems. All SHQ indices for sub-soils scored higher index values compared with their respective topsoil scores indicating less deteriorating effect of crop husbandry practices in this layer. Our findings could be used to advance the assessment of valid location-specific SHQ indicators to describe the essential conditions for sustainable soil water processes.Key words: Site-specific, soil hydraulic quality index, soil water process, cropping system, essential conditions