Site-Specific Crop Management Using a Geophysical Proximal Sensor.

Key components of site-specific crop management are (i) identifying the site-specific factors that influence within-field crop yield variation and (ii) spatially characterizing those factors.  Geo-referenced measurements of apparent soil electrical conductivity (EC_a) provide a potential means of characterizing the spatial variability of edaphic properties that influence crop yield.  It is the objective (i) to utilize an intensive EC_a survey to direct soil sampling, (ii) to identify soil properties that influence cotton yield, and (iii) to use this spatial information to delineate site-specific management units (SSMUs).  A 32.4-ha field in California’s San Joaquin Valley was used as a study site.  Cotton yield monitoring data were collected in August 1999 followed by an intensive EC_a survey of 4000+ measurements.  Sixty soil sample sites were selected based upon a response-surface sampling design utilizing the spatial EC_a measurements.  Scatter plots were obtained and correlation and regression analyses were performed to assess the relationship between cotton yield and the properties of pH, boron (B), nitrate-nitrogen (NO₃-N), chloride (Cl^-), salinity (i.e., electrical conductivity of the saturation extract; EC_e), leaching fraction (LF), water content (θ_g), bulk density (ρ_b), % clay, and saturation percentage (SP).  A site-specific response model of cotton yield based on ordinary least squares (OLS) and adjusted for spatial autocorrelation using restricted maximum likelihood was developed.  The response model indicated that leaching fraction, salinity, water content, and pH were the most significant soil properties influencing cotton yield: cotton yield (Mg ha^-1) = 19.28 + 0.22 EC_e – 0.02 EC_e² – 4.42 LF² – 1.99 pH + 6.93 θ_g.  From the crop yield response model SSMUs were delineated, which provided spatial information for site-specific variable-rate technology useful for producers, ag extension specialists, irrigation district managers, irrigation specialists, and ag consultants.