Spatial Variability of Soil Properties In Agricultural Fields of Southern New Mexico.

Estimating spatial variability of soil physical and chemical properties is the prerequisite for site specific management. The objectives of this study were to determine the degree of spatial variability and variance structure of soil physical and chemical properties on a 40-ha agricultural field in Las Cruces, NM and to utilize it for making better management decisions and future sampling designs. Soil bulk samples (n = 572) were collected during Nov. 2008 and 2009, and soil core samples (n = 286) were obtained during Nov. 2009 from 0-15 cm depth. 151 soil samples were collected at the center of a regular grid of 50 x 50 m and rest 135 were obtained on the grid line at a mean separation distance of 8 m. The software package GS+ (Gamma Design Software, Plainwell, MI) was used to model the variance structure of sand, silt, clay, soil bulk density (ρ_b), saturated hydraulic conductivity (Ks), pH, Electrical conductivity (EC), Chloride (Cl), Nitrate-N (NO₃- N) and volumetric water content (θ) at six pressure potentials ( Ψ_a) (–33, –100, –300, –500, –1000 and –1500 kPa). The coefficient of variation (CV) ranged from 4% (pH) to 141% (Ks). The semivariograms showed that range varied from 33 m (Cl, 2009) to 563 m (Ks) for all measured soil properties. Cross-semivariograms showed that NO₃- N and EC were spatially correlated; therefore, kriging or cokriging can be used to estimate NO₃- N values throughout the growing season from relatively easily available EC data. Correlograms with Moran's I, indicated a distance of 140 m was sufficient to yield independent samples for measured soil properties. The kriged contour maps showed positional similarities. These contour maps of soil properties along with their spatial structures can be used in making better future sampling designs and management decisions.