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 herbic ide analysis. Adsorption was rapid, with 40-80% of the final (24 h) so rption being achieved within 2 min. These equilibria were achieved mor e rapidly for diuron, which was also the more highly adsorbed. Adsorpt ion of both herbicides was favoured by low soil moisture initially, bu t 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. Regar ding 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 ti me (about 30 min).