Faixas críticas de teores foliares de micronutrientes em mudas de cafeeiro (Coffeal arabica L.)

Abstract

Objetivou-se quantificar e estabelecer faixas críticas de micronutrientes em folhas de mudas de cafeeiro em experimento conduzido na Universidade Federal de Lavras, de maio de 2002 a janeiro de 2003. Utilizou-se blocos ao acaso, esquema fatorial 6 x 4, (6 níveis de adubação e 4 épocas de amostragem), com 4 repetições (9 plantas úteis na parcela). As avaliações foram realizadas com mudas de 3, 4, 5 e 6 pares de folhas verdadeiras. Os níveis de adubação utilizados foram 25, 50, 75, 100, 125 e 150% do substrato padrão. Avaliaram-se: altura de plantas, diâmetro do caule, área foliar, massa seca da raiz, caule, folhas e total e análises químicas foliares para identificação dos teores de B, Cu, Fe, Mn e Zn. Para determinação das faixas críticas encontraram-se modelos matemáticos que adequassem os níveis de adubação à performance das plantas. Quando níveis de adubação e interação de época e níveis de adubação foram significativos, realizou-se análises de regressão para cada característica e época de amostragem. O mesmo feito para teores de micronutrientes na matéria seca das folhas, interpolados com níveis de adubação. Derivou-se equações de cada característica por época, encontrando-se o ponto de máxima performance. Identificou-se faixas dos níveis de adubação, que proporcionaram acima de 90% da performance máxima substituindo-os nas equações em cada época, encontrando-se as faixas críticas (mg/kg): boro (39,74 a 39,94), cobre (6,94 a 9,29), ferro (209,01 a 213,88), manganês (33,05 a 37,21), zinco (3,68 a 4,08). A coleta de folhas deve ser no estádio de 4 pares.The objective of this work was to quantify the leaf micronutrient levels in coffee seedlings, to establish critical ranges in the experiment carried out in Federal University of Lavras, from May/2002 to January/2003. A random block design was arranged in a 6 x 4 factorial scheme (six fertilization levels and four sampling times), with four replications (nine plants were evaluated per plot). The evaluations were performed when the seedlings reached the stages of 3, 4, 5 and 6 pairs of leaves. The fertilization levels used were: 25%, 50%, 75%, 100%, 125%, and 150% with standard substrate). The following characteristics were evaluated: plant height, shoot diameter, leaf area, root dry weight, shoot and leaf dry weight s, and chemical analysis of the leaves to quantify the B, Cu, Fe, Mn, and Zn levels. To determine the critical ranges, the most adequate mathematical model to the fertilization levels in relation to plant performance was selected. For this purpose, where there was a significant effect of the fertilization level and of the interaction between time and fertilization level, regression analyses of each characteristic evaluated in each sampling time were performed. The same process was applied to the leaf micronutrient levels found in leaf dry weight, interpolated with the fertilization levels. Then the regression equations set for each characteristic in each sampling time were derived to determine the point of maximum performance. Using the equations it was possible to identify the ranges of the fertilization substrate levels that provided a maximum performance above 90% in the plants. These fertilization levels were substituted in the regression equations of the leaf micronutrient levels in each sampling time to find the critical range of micronutrients (mg/kg) : boron (39,74 to 39,94), copper (6,94 to 9,29), iron (209,01 to 213,88), manganese (33,05 to 37,21), zinc (3,68 to 4,08). The optimum stage to harvest leaves is four pairs of leaves.