Characterizing soil spatial variability with apparent soil electrical conductivity

Geospatial measurements of apparent soil electrical conductivity (ECa) are recognized as a means of characterizing soil spatial variability at field and landscape scales. However, inconsistencies in the measurement and interpretation of field- and landscape-scale geospatial ECa measurements have resulted in data sets that are unreliable and/or incompatible. These inconsistencies are, in part, a consequence of the lack of ECa-survey protocols that provide standardized guidelines to assure reliability, consistency, and compatibility. It is the objective of this paper to apply ECa-survey protocols to a soil quality assessment to demonstrate their utility in characterizing spatial variability. The soil quality assessment was conducted on a 32.4-ha field on the westside of central California's San Joaquin Valley where a mobile electromagnetic induction (EM) survey was performed following outlined protocols. The EM survey consisted of ECa measurements taken at 22,177 locations in April 2002. A response-surface sampling design was used to identify 40 sites where soil-core samples were taken at 0.3-m increments to a depth of 1.2 m. Duplicate samples were taken at eight sites to evaluate the local-scale variability. Soil samples were analyzed for a variety of physico-chemical properties associated with soil quality for an arid zone soil. Analysis characterized the soil as montmorillonitic, saline, and sodic with ECe (electrical conductivity of the saturation extract) varying from 4.83 to 45.3 dS m−1, SAR (sodium adsorption ratio) from 5.62 to 103.12, and clay content from 2.5 to 48.3%. Spatial trends showed high areas of salinity and SAR in the center of the southern half of the study area. Strong correlation was obtained between ECa and the soil properties of the saturation extract (ECe; Cl−, HCO3−, SO42−, Na+, K+, and Mg2+), exchangeable Na+, and SAR. Other properties were poorly correlated, including: volumetric water content (θv), bulk density (Db), percent clay (% clay), saturation percentage (SP), exchangeable sodium percentage (ESP), Mo, CaCO3, gypsum, total N, Ca2+ in the saturation extract, and exchangeable cations (K+, Ca2+, and Mg2+). The spatial distribution of the poorly correlated properties is not as well represented with a response-surface sampling design suggesting the need for a complementary stratified random sample design.