TRANSPORT OF ANIONS AND HERBICIDES IN A LOAMY AND A SANDY FIELD SOIL

A field experiment was conducted to investigate preferential transport of herbicides and to explore processes that cause rapid movement. Bro mide, chloride, and three herbicides (triasulfuron, atrazine, and terb uthylazine) with different mobility characteristics were applied to si x 1.4 X 1.4 m field plots on a loamy and a sandy soil. At both sites, three of the plots were covered with a plastic roof 1 month before the beginning of the experiment to achieve different initial water conten ts between the plots. Two days before the beginning of the tracer expe riment, crops were removed, and the soil surface was homogenized with a spade to a depth of 15-20 cm. One day after application of the chemi cals the plots were irrigated with a sprinkling apparatus. The cumulat ive amounts of infiltration until the time of sampling were 30, 60, an d 90 mm within 1, 2, and 3 days, respectively. A trench was excavated, and soil cores were taken horizontally from a 1 X 1 m profile in a re gular 0.1 X 0.1 m grid. The loamy and the sandy soil showed completely different transport patterns. In the loamy soil the bulk mass of herb icides remained in the top layer; however, considerable amounts of her bicides were transported below the root zone. A few percent for triasu lfuron and atrazine and <1% for terbuthylazine were detected below 0.5 m depth after 90-mm cumulative infiltration. Traces of all herbicides were found down to 0.9 m. The depth distributions for anions and all herbicides were similar. These results show that the herbicides were o nly partly adsorbed by the soil matrix. A fraction of these chemicals was transported with or without minor adsorption along cracks or fissu res. In the sandy soil, chemical movement was confined to the top 0.4 m, and the penetration depth of the herbicides was consistent with the ir mobility characteristics: triasulfuron showed greatest mobility, at razine was moderately mobile, and terbuthylazine was the least mobile of all three.