FIELD-SCALE BROMIDE TRANSPORT AS AFFECTED BY TILLAGE

Tillage alters soil physical properties and modifies the rates of many physical, chemical, and biological processes. The purpose of this study was to determine if subsoiling increased the depth of leaching of the nonreactive Br- anion in a Norfolk soil (fine-loamy, siliceous, thermic Typic Kandiudult) having a tillage-induced pan. Three tillage treatments, replicated three times, were selected: (i) current-year subsoiling (S88); (ii) one-year subsoiling carry-over (subsoiled in 1987: S87); and (iii) double disk tillage (DD). Mechanical impedance was measured at 35 d after tillage with a recording cone penetrometer to a depth of 60 cm on transects normal to the soybean (Glycine max [L.] Merr.) row. Cone index exceeded 3000 kPa for the DD treatment at the 20- to 45-cm depth; current-year subsoiling reduced cone index at that depth to 1300 to 2500 kPa. Six days after planting soybean, KBr was uniformly broadcast on the soil surface at the rate of 280 kg Br-/ha. Soil samples were taken after the plots received 130, 260, 545, and 1217 mm of cumulative rain and/or irrigation (28, 64, 128 and 351 d, respectively, after applying KBr). Soil samples were collected using a hydraulic probe at seven positions on duplicate transects normal to the soybean row in each plot. Bromide concentration measured with a specific-ion electrode was highly variable at all depths. For the four consecutive sampling dates, the center of Br- mass occurred at the 11-, 17-, 51-, and 90-cm depths, respectively, for Treatment DD; at the 19-, 27-, 71-, and 1002-cm depths for Treatment S88; and at the 13-, 20-, 57-, and 108-cm depths for Treatment S87. Both subsoil treatments loosened the soil and increased the depth of Br- transport. Although the depth of Br- transport was slightly deeper for the subsoiled treatments, it leached below the rooting depth for all treatments. No definitive relationship between cone index and Br- leaching depth was observed.