Assessment and field-scale mapping of soil quality properties of a saline-sodic soil

Salt-affected soils could produce useful forages when irrigated with saline drainage water. To assess the productive potential and sustainability of using drainage water for forage production, a saline-sodic site (32.4 ha) in California's San Joaquin Valley was characterized for soil quality. The objectives were (1) to spatially characterize initial soil physicochemical properties relevant to maintaining soil quality on an arid zone soil and (2) to characterize soil quality relationships and spatial variability. An initial mobile electromagnetic (EM) induction survey was conducted in 1999, with bulk soil electrical conductivity (EC_a) readings taken at 384 geo-referenced locations, followed by an intensive mobile fixed-array survey with a total of 7288 geo-referenced EC_a readings. Using the EM data and a spatial statistics program (ESAP v2.0), 40 sites were selected that reflected the spatial heterogeneity of the EC_a measurements for the study area. At these sites, soil-core samples were taken at 0.3-m intervals to a depth of 1.2 m. Duplicate samples were taken at eight sites to study the local-scale variability of soil properties. Soil-core samples were analyzed for a variety of physical and chemical properties related to the soil quality of arid zone soils. Soils were found to be highly spatially heterogeneous. For composite soil-core samples taken to a depth of 1.2 m, EC_e (electrical conductivity of the saturation extract) varied from 12.8 to 36.6 dS m^-1, SAR from 28.8 to 88.8, and clay content from 2.5% to 48.3%. B and Mo concentrations varied from 11.5 to 32.2 mg l^-1 and 476.8 to 1959.6 μg l^-1, respectively. CaCO₃, NO₃^- in the saturation extract, exchangeable Ca²⁺, Se, and As consistently had the highest coefficients of variation (CV) while pH_c, ρ_b, and Ca²⁺ in the saturation extract consistently had the lowest CVs at all depths. A one-way analysis of variance (ANOVA) was used to spatially partition the local- and global-scale variability. Local-scale variability was greatest for pH_e. Laboratory measurements of saturated hydraulic conductivity (K_s) were very low (0.0000846-0.0456 cm h^-1), whereas field measurements were considerably higher (0.49-1.79 cm h^-1). Based on the Cl^- data, the leaching fraction (LF) for the entire study area was estimated to be 17%. Soil quality was reflected in yield and chemical analysis of forage. Forage Mo contents determined from newly established Bermuda grass varied from 1 to 5 mg kg^-1 on a dry matter basis, and Cu/Mo ratios averaged 3.3, while forage yield in the establishment year declined with EC_e, and failed to grow above EC_e levels of approximately 22 dS m^-1. The initial soil quality assessment of the research site indicated that the sustainability of drainage water reuse at this location would depend upon maintaining a sufficient LF with careful consideration and management of salinity, boron, molybdenum, and sodium levels.