Influence of soil moisture on short-term adsorption of diuron and isoproturon by soil

The adsorption of diuron and isoproturon by a clay loam soil at 35% (3.16 kPa) and 62% (1 kPa) soil moisture content was studied by means of glass microfibre filters capable of sampling soil solution for herbicide analysis. Adsorption was rapid, with 40-80% of the final (24 h) sorption being achieved within 2 min. These equilibria were achieved more rapidly for diuron, which was also the more highly adsorbed. Adsorption of both herbicides was favoured by low soil moisture initially, but was enhanced by higher soil moistures at sorption times greater than 30 min. However, increasing the soil moisture from 31% (10 kPa) to 62% (1 kPa) had little effect on the final soil sorption capacity. Regarding the water status in the soil, it is thought that adsorption took place in small pores (<3 mu m). Herbicides diffused rapidly into small pores and adsorption by wet soil was delayed for a short period of time (about 30 min).