Anion movement in a soil under pasture

Abstract

Laboratory and field studies of anion movement in soils were made. In the laboratory the movement of a solution containing chloride and phosphate through saturated field cores (143 mm in diameter and 170 mm long) was studied. Both the anions appeared in the effluent almost immediately after application to the soil surface. Dye studies showed the movement was highly preferential, occurring mainly through biogenic macropores such as root and worm channels. Unsaturated solute flow was also studied in the laboratory. When the matric potential in these field cores was reduced from zero to -2.0 J/kg (-20 mb), the volumetric water content decreased from 0.56 to 0.53, while the hydraulic conductivity decreased two orders of magnitude. The flow of surface-applied bromide and dye in the slightly unsaturated soil was much less preferential than in the saturated soil, a finding that agrees in general terms with [he predictions of theory previously published. In the field, the flow of a surface-applied chloride and phosphate solution to a mole drain (at 0.4 m depth) was also highly preferential. In one experiment the concentration of both chloride and phosphate in the mole effluent exceeded 0.8 of that in the applied solution within 5 min of its application to the soil surface, when only 20 mm of solution had infiltrated. Dye studies again showed root and worm channels, sometimes in association with incipient fracture planes, were the preferential pathways. This indicates the importance of biological factors in maintaining hydraulic conductivity and mole drain performance.