Deposition of Pesticides on the Soil Surface

Deposition of Pesticides on the Soil Surface

Summary and conclusions

A decision tool used by the Danish Environmental Protection Agency to evaluate the risk of transport of pesticides to water bodies such as streams and lakes is being developed. The development of this tool is co-ordinated by DHI, Water & Environment in the project “Model based tool for evaluation of exposure and effects of pesticides in surface water”. This report describes the results from the subproject “Investigations of the importance of the plant cover for the deposition of pesticides on the soil”.

The subproject has included 3 activities:

Measurements of the plant cover during the growth season in 8 crops. Deposition of spray liquid on the soil surface after treatment at different growth stages during the growth season in winter wheat, spring barley, sugar beet and potatoes Investigations on the influence of spray quality and pesticide formulation on the deposition of spray liquid on the soil below winter wheat and spring barley.

Due to limited resources measurements of deposition of spray liquid on the soil was limited to four crops. In order to obtain data on a further number of crops measurements of plant cover was carried out in 8 crops during the growth season in order to deliver data to the Danish Environmental Protection Agency that can be used to evaluate the amount of pesticide that will be deposited on the soil at different growth stages. The results from this activity is presented together with results from a review which summarises corresponding plant cover assessments from a large number of experiments under Northwest European conditions.

The primary activity in the subproject consisted of measurements of deposition of spray liquid on the soil surface after treatment at different growth stages in winter wheat, spring barley, sugar beet and potatoes. This part was carried out during 3 years. In the first year a spray liquid without any surface-active ingredients was used. Such a spray liquid has a high surface tension and the deposition of spray is reduced on difficult to wet targets. The results from this year therefore must be considered as a worst-case situation concerning deposition of spray liquid on the ground below the crops. The spray liquid used in the two following years included a surfactant and the properties of the spray liquid are expected to be more corresponding to spray liquids with formulated pesticides. The result, which shows which percentage of the spray liquid that is deposited on the ground, is presented in the report. The results are compared to literature values estimated from plant cover measurements in a large Northwest European dataset from pesticide trials. (Becker et al, 1999). The values from Becker et al (1999) are together with values from Ganzelmeier (1997) and from van der Zande (not published) the basis for the values on deposition of spray on the ground mentioned the FOCUS values.

On the basis of the experimental deposition values obtained in this subproject with the formulated spray liquid mean values for deposition of spray liquid on the ground under the four investigated crops at different growth stages are given. These values are used in the decision support model developed in the overall project. In the report the experimental values are compared to the FOCUS deposition values. The main deviations between the experimental values found in this project and the FOCUS values are the values in the two cereal crops, but especially in winter wheat. The percentage of spray liquid on the ground found during stem elongation (31-39 BBCH) in this work is significantly below the FOCUS values. The explanation for this is that FOCUS uses only one value covering both the stem elongation and the earlier tillering growth stages.

The last activity in the subproject consisted of experiments with the purpose to investigate the influence of spray quality and pesticide formulation on the deposition of spray liquid on the ground. Two experiments in winter wheat and two in spring barley were carried out during two years. The general conclusion on these experiments was that both factors had highly a significant influence on the percentage of spray liquid that was deposited on the soil below the two cereal crops.