WEED-CONTROL WITH ATRAZINE AND CHLORSULFURON IS DETERMINED BY HERBICIDE AVAILABILITY AND PERSISTENCE IN SOILS

Effectiveness and length of weed control with atrazine and chlorsulfur on can be variable in the field. While some of this may be due to clim atic variations, differences in soil properties may also be important. We tested this by recording changes in control of mintweed (Salvia re flexa Hornem.) and turnip weed (Rapistrum rugosum L.) with time in dif ferent soils, and comparing these results with the measured changes in plant-available herbicide in the soils. Length of weed control with t he same herbicide rate varied from 0 to >15 weeks. Mint Need and turni p weed were controlled (85-100%) only when the soils had greater than or equal to 0.1 mu g available atrazine/g and 0.8 ng available chlorsu lfuron/g, respectively. This agreed with the sensitivity data for thes e weeds when grown in a soil-free system. The herbicides were initiall y more available in grey clays than in black earths, and soil pH accou nted for most of the variations in the persistence of the available re sidues. Thus, the efficacy of these herbicides in different soils coul d be estimated if the available residues in the root-zone could be pre dicted and the sensitivity of different weeds was known.