Influence of topsoil tilth and soil moisture status on losses of pesticideto drains from a heavy clay soil

Twelve lysimeters with a surface area of 0.5 m(2) and a length of 60 cm wer e taken over mole drains from a Denchworth heavy clay soil and divided into two groups with either a standard agricultural tilth or a finer topsoil ti lth. The influence of topsoil tilth on leaching of the herbicide isoproturo n and a bromide tracer was evaluated over a winter season. The effect of va riations in soil moisture status in the immediate topsoil on leaching of is oproturon, chlorotoluron and linuron was investigated in the following wint er season. Here, water inputs were controlled such that lysimeters received 50 mm at a maximum intensity of 2 mm h(-1) over a 4-week period with herbi cides applied on day 15. Three treatments received the water either all pri or to application, all after application, or evenly spread over the 4-week period. Leaching losses of the three herbicides were monitored for a subseq uent drainage event. Analysis of covariance showed a significant effect of topsoil tilth and total flow on both the maximum concentrations (P = 0.034) and total losses (P = 0.012) of isoproturon in drainflow. Both concentrati ons and losses were c 35% smaller from lysimeters with the finer tilth. How ever, generation of the fine tilth in the field was restricted by a wet aut umn and this is not considered a reliable management option for reducing pe sticide losses from heavy clay soils. In the second experiment, variation i n soil moisture content prior to and after application did not have any sig nificant effect (P < 0.05) upon subsequent losses of the three herbicides t o drains. (C) 2001 Society of Chemical Industry.