Nutrient concentrations in soil solution of some Brazilian Oxisols under conventional and no-tillage systems in the early part of the rainy season

Abstract

In the South American savanna, no-tillage (NT) is implemented to improve the sustainability of cropping systems. At the beginning of the rainy season, however, more plant nutrients may be leached under NT than under conventional tillage (CT) because of more pronounced organic matter mineralisation and increased pore continuity. To test this hypothesis, we analysed the chemical composition of the soil solution under conventional (CT) and no-tillage (NT) soybean (Glycine max (L.) Merr.) fields in the Brazilian savanna (Cerrados).

We collected the soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m depth and the precipitation in 1- to 3-day intervals from 28 October to 23 December 1998 on plots under CT and NT, replicated 3 times. We determined pH, electrical conductivity (EC), Ca, K, Mg, Na, NO₃^– , NH₄⁺, Cl^–, and total organic carbon (TOC) concentrations in soil solution and precipitation.

Soil solution pH under NT was 0.3–0.8 units lower than under CT and was inversely related to TOC concentrations (average under NT, 1.02; CT, 0.70 mmol C/L) at all depths. Average Cl^– , Ca, and Mg concentrations at 0.15–0.3 m depth were significantly higher under CT (1.09, 1.1, and 0.25 mmol/L) than under NT (0.50, 0.83, and 0.17 mmol/L), respectively. No difference was observed in average Na (0.09 mmol/L) and NO₃^– concentrations (2.2 mmol/L) between CT and NT. At 0.8–2 m, average NO₃^– (0.30 mmol/L), Cl^– (0.18 mmol/L), Ca (0.19 mmol/L), Mg (0.05 mmol/L), and Na (0.04 mmol/L) concentrations under CT were significantly lower than under NT (NO₃^– , 0.38; Cl^– , 0.40; Ca, 0.23; Mg, 0.09; Na, 0.06 mmol/L). In the monitored period, the Cl^– which had accumulated during the dry season and which was applied with KCl fertiliser on 29 October reached a depth of 0.3 m under CT and of 1.2m under NT.

The results suggest higher mineralisation rates and faster leaching in the NT than in the CT systems, being the combined result of different plowing practices and different cropping sequences.