Weed control with atrazine and chlorsulfuron is determined by herbicide availability and persistence in soils

Effectiveness and length of weed control with atrazine and chlorsulfuron can be variable in the field. While some of this may be due to climatic variations, differences in soil properties may also be important. We tested this by recording changes in control of mintweed ( Salvia reflexa Hornem.) and turnip weed ( Rapistrum rugosum L.) with time in different soils, and comparing these results with the measured changes in plant-available herbicide in the soils. Length of weed control with the same herbicide rate varied from 0 to >15 weeks. Mintweed and turnip weed were controlled (85-100%) only when the soils had ¸ 0·1µ#9839;g available atrazine/g and 0·8 ng available chlorsulfuron/g, respectively. This agreed with the sensitivity data for these weeds when grown in a soil-free system. The herbicides were initially more available in grey clays than in black earths, and soil pH accounted for most of the variations in the persistence of the available residues. Thus, the efficacy of these herbicides in different soils could be estimated if the available residues in the root-zone could be predicted and the sensitivity of different weeds was known.