Control of soil acidity in no-tillage system for soybean production

The common practice for liming no-tillage consists of applying dolomite to soil surface without incorporation into the soil by plowing. The effectiveness of surface application of lime to oxisols under no-tillage, particularly with regard to subsoil acidity, is uncertain. A field experiment was conducted from 1986 to 1991 in the State of Parana, Brazil, to evaluate the extent of downward movement of dolomite and phosphogypsum in an acid soil profile under a no-tillage system, and the effect on soybean yields. The experimental treatments were: control (no amendments), surface dolomite, incorporated dolomite (0-20 cm), surface phosphogypsum, surface 1/2 dolomite (half of the lime requirement) plus surface 1/2 phosphogypsum (half of the phosphogypsum requirement), and surface 1/4 dolomite (one-quarter of the lime requirement applied per year over 4 years). The following results were obtained: a significant reduction in Al and increase in Ca and Mg because of increased pH were measured about 40 cm deep when dolomite is surface applied; the effects of surface dolomite on the acid soil chemistry were essentially the same as those produced by incorporated dolomite; surface phosphogypsum had no effect on pH, increased Ca and decreased Mg; combining phosphogypsum with dolomite improved the soil fertility levels; soybean yields increased with surface and incorporated dolomite, phosphogypsum plus dolomite, and surface 1/4 dolomite annually; and soybean yields decreased with single application of phosphogypsum. It was concluded that for a no-tillage system, surface applied dolomite was effective in improving the fertility of an acid soil below the point of placement, and increased soybean grain yields.