Abstract
Summary. Effective dosages of picloram (4–amino‐3,5,6–trichloropicolinic acid) required to reduce fresh weight of sunflowers (Helianthus annuus L. var. Menonite) were determined for seven Saskatchewan soils under controlled environmental conditions. The relationships between ED50 value and clay content, organic matter content and cation exchange capacity were evaluated by correlation and regression analysis for possible usefulness in predicting dose requirements. ED50 values were also determined for Weyburn loam and in culture solutions adjusted to various pH levels.There Was no significant correlation between ED50 values of picloram and soil clay content or cation exchange capacity. ED50 values were highly correlated with soil organic matter content, and they increased as the soil pH was lowered or raised from pH 6.5. The increase in ED50 values in the acidic range was attributed to adsorption of the unionized molecules of picloram on the organic matter in the soil. The increase in ED50 values in the alkaline range may be mainly due to reduced uptake of the ionized acid by plant roots.
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