Trichoderma koningii, originally isolated from a take-all-suppressive soil in Western Australia, has been shown to protect wheat against take-all disease and increase grain yield in field trials in Australia, China, and the United States. However, within a region, the level of protection provided by T. koningii can dramatically vary between field sites. We evaluated suppression of take-all by this fungus in eight silt loams from the Pacific Northwest of the United States and the influence of 21 abiotic soil parameters on biocontrol activity. While T. koningii significantly increased plant growth and reduced disease severity in all eight silt loams, the level of protection varied significantly among the soils. Disease suppression was not associated with the conduciveness of a soil to take-all, but rather to the supportiveness of a soil to biocontrol activity. Biocontrol activity was positively correlated with iron, nitrate-nitrogen, boron, copper, soluble magnesium, and percent clay, and negatively correlated with soil pH and available phosphorus. Principal component factor analysis using these eight variables resulted in a three-component solution that accounted for 95% of the variation in disease rating. Least squares regression analysis (R2 = 0.992) identified a model that included nitrate-nitrogen, soil pH, copper, and soluble magnesium, and described the variance in take-all suppression by T. koningii. Potential applications of these results include amending soil or inoculants with beneficial factors that may be lacking in the target soil and customizing biocontrol treatments for sites that have parameters predictive of a favorable environment for disease suppression.