MACROPORE EFFECTS IN CONVENTIONAL TILLAGE AND NO-TILLAGE SOILS

In an earlier study, it was shown that short-term (< 1 h) infiltration rates in the conventionally-tilled treatment (CT) of a 10-year-old ti llage study near Griffin, Georgia, were reduced compared to the no-til l treatment (NT), because of the presence of a surface crust. Our obje ctive was to determine whether macropores were present in the NT treat ment of this study and, if so, what effect macropores might have on th e infiltration rate in NT and on chloride movement. We measured steady -state infiltration rates in the field with 98 cm (38.6 in) diameter r ings. Macropore distributions were measured on intact soil columns 30 cm (11.8 in) in diameter and 40 cm (15.7 in) in length using ultramari ne blue dye. We also used columns to determine the effect of a small s imulated rain of 1 cm (0.4 in) immediately after chemical application on chloride (Cl) leaching in a larger rain of 16 cm (6.3 in) one week later. Macropores were evident near the surface in the NT treatment an d constituted 15% of the cross-sectional area based on image-analysis of the dye stain. Long-term (> 2 h) steady-state ring infiltration rat es were not reduced in NT plots compared to CT, in spite of a shallow hardpan in NT; this was attributed to macropores that penetrated the h ardpan. A 1-cm rain immediately after chemical application reduced lea chate concentrations of Cl by about half in both the NT and CT treatme nts. With the 1-cm initial rain, chloride had an opportunity to diffus e into stagnant regions of the matrix and as a result was excluded fro m flow in macropores and interaggregate space. Macropores will probabl y cause the greatest leaching losses when a chemical is applied to the surface of NT and a large rain occurs soon after application.