The uptake of soil-applied chlorotriazines by seedlings and its prediction

¹⁴C-labelled atrazine or terbuthylazine was applied to the surface of soil columns 100 mm deep and the uptake by oat seedlings measured for up to 24 days after germination following two contrasting rainfall treatments. Attempts were made to predict measured uptake on the basis of mass flow theory given also measurements of transpiration rates, water, rooting and herbicide distribution profiles in the soil, and soil adsorption characteristics. Predictions over a tenfold range of uptake were close enough to suggest that the assumptions made to predict mass flow uptake were essentially correct. Discrepancies appeared to result principally from (i) shoot-zone uptake, and (ii) incorrect prediction of long-term herbicide adsorption in undisturbed soil. Greater attention to the effect of herbicide on transpiration rates and to the patterns of water uptake by roots should improve the accuracy of such predictions. The various factors that influence the mass flow uptake of chlorotriazines in the soil surface by seedlings are discussed as a basis for modelling herbicide uptake.