Abstract
Relations of green matter yield to nutrient availability in a dystrophic Oxisol (Goias, 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 Barreirao 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.
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