The toxicity to Drosophila melanogaster Meigen of dieldrintreated soils was measured by bioassay. Under adequate standardization and control of light, temperature, humidity, and soil moisture, the variation in insecticidal activity was closely related to the colloidal properties of the soils. As organic carbon content, specific surface area of the minerals, and soil reaction increase, higher levels of dieldrin were required to maintain the killing power. Cation‐exchange capacity interacts with soil organic carbon content to increase dieldrin effectiveness. The dieldrin activity equation developed shows that often the simplest practical mathematic model of soil pesticide activity should be quadratic.

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Synopsis Optimum response is occasionally sought while maintaining a minimum quality factor level. Mathematical analysis may need to be confined within the range of factor levels observed. An example of such an applied problem is worked. It involves searching for the nitrogen, phosphorus, and potassium fertilizer treatment which will produce the maximum potato yield and yet not cause the specific gravity of the potato tubers to fall below a minimum value.