RELATIONS OF Brachiaria brizantha TO GREEN MATTER YIELD AND NUTRIENT AVAILABILITY RATES IN SOILS MANAGED UNDER THE "BARREIRÃO" SYSTEM

Abstract

Relations of green matter yield to nutrient availability in a dystrophic Oxisol (Goiás, Brazil) were assessed in a pasture area derived from an original savannah opening. Pasture was established by applying 1.5 t ha^-1 of calcitic lime and direct sowing of Brachiaria decumbens. After ten years, this pasture was replanted using the Barreirão system. In a single operation, rice and Brachiaria brizantha were sown together and fertilizers were applied at depths of 3-5 cm and 8-10 cm, respectively. Areas where the system had been ineffect for one to six years were monitored, using a native forest area and another with 17-year old traditional pasture as reference. The data obtained allow to conclude the following: i) a critical level of 19.5g OM kg^-1 of soil was estimated by relating green matter yield to organic matter contents in the soils, but this is not a reliable index for available nitrogen, because of the dynamic character of soil organic matter; ii) critical levels for nutrients in soils were determined for: exchangeable calcium + magnesium ? 4.5 cmolc dm^-3; exchangeable potassium ? 123 mg dm^-3; available zinc ? 1.7 mg dm^-3; available copper ? 2.2 mg dm^-3; iii) no critical level for available phosphorus has been established, because the applied rates were insuffi-cient to sustain productivity through the years.

KEY-WORDS: Cerrado; macronutrient; micronutrient; tropical pastures.