To determine the effect of the pH of the soils on the persistence of the toxin, the pH of the K and K6K soils, in which there was high persistence in the 6 month study, was adjusted to ca. pH 7.0 by the addition of CaCO3 to 1% (wt wt−1) with the dilutions of the toxin and a freshly prepared supernatant from the garden soil, as in the 6 month study, and the soils were periodically assayed for insecticidal activity during 234 d of incubation.