Monitoring management-induced spatio–temporal changes in soil quality through soil sampling directed by apparent electrical conductivity

Abstract

Characterizing spatial and temporal variability of soil properties at field and landscape scales is tremendously important for a variety of agronomic and environmental concerns including solute transport modeling of non-point source pollutants in the vadose zone, site-specific crop management, and soil quality assessment, to mention a few. Currently, there is a global need to develop tools that evaluate the overall quality of soil to determine the effectiveness and sustainability of farm-management practices. The knowledge now exists for characterizing the spatial variability of soil quality with non-invasive geophysical measurements of apparent soil electrical conductivity (EC a) using mobile GPS-based systems. The objective of this research was to evaluate EC a-directed soil sampling as a means for monitoring management-induced spatio–temporal changes in soil quality. Appraisal was made of a specific management practice, the reuse of irrigation drainage water applied to a saline–sodic soil in central California. A soil quality assessment study was conducted on a 32.4-ha saline–sodic field comprised of 8 rectangular paddocks in California's San Joaquin Valley from August 1999 to April 2002. The study evaluated the spatio–temporal changes that had occurred as a result of irrigation with drainage water over that time period. Using geospatial electromagnetic induction (EMI) measurements of EC a and a spatial response surface sampling design, 40 sites were selected that reflected the spatial variability of the EC a measurements. Duplicate samples were taken at eight selected sites (one randomly selected site from each paddock) to study local-scale variability. At each site soil-core samples were taken at 0.3-m intervals to a depth of 1.2 m and analyzed for 28 physical and chemical properties. Maps created from a geographic information system (GIS) show spatio–temporal changes of four dynamic soil properties (salinity, sodium adsorption ratio, boron, and molybdenum) critical to soil quality, which were strongly and significantly correlated with EC a. Data from 1999 indicate the presence of high salinity, which increased with depth, high sodium adsorption ratio (SAR), which also increased with depth, and moderate to high B and Mo, which showed no specific trends with depth. The application of drainage water for 32 months resulted in leaching of B from the top 0.3 of soil, leaching of salinity from the top 0.6 m of soil, and leaching of Na and Mo from the top 1.2 m of soil. The leaching fraction over the time period from 1999–2002 was estimated to be 0.10. The level of salinity in the reused drainage water (i.e., 3–5 dS m − 1) allowed infiltration and leaching to occur even though high sodium and high expanding-lattice clay levels posed potential water flow problems. Preliminary spatio–temporal analyses from 1999–2002 indicate at least short-term feasibility of drainage water reuse from the perspective of soil quality when the goal is forage production for grazing livestock. The implications of this research extend beyond the provincial applications of assessing drainage water reuse in central California to the global potential of EC a-directed soil sampling for evaluating farm-induced management ramifications on soil quality.