GESSO AGRÍCOLA NO CULTIVO DE CITROS E PLANTAS DE COBERTURA DO SOLO

Abstract

The citriculture has considerable economic and social importance in Brazil, and the density of citrus orchards is directly related to the rootstock used. Cover plants associated with citrus orchards can result in several benefits to the cropping system, improving the physical, chemical and biological fertility of the soil. The objectives of this study were i) to evaluate the effects of gypsum on soil fertility, growth, nutritional status and nutrient export rate in Satsuma mandarin (Citrus unshiu Marc.) and cover crops in-between the rows, under field conditions without use of high solubility fertilizers, and ii) to examine the effects of lime and gypsum on growth and nutrition of trifoliate orange [Poncirus trifoliata (L.) Raf.] rootstock, under greenhouse conditions. The field experiment was alloted in a completely randomized design, with eight replications, composed of two gypsum treatments, 0 (control) and 6 Mg ha-1, applied in total area, without incorporation. Setaria grass (Setaria anceps Stapf) and black oat (Avena strigosa Schreb.) were cultivated in-between the rows as cover plants. In greenhouse experiment, trifoliate orange was grown in columns: the upper ring, 15 cm high, and the lower ring, 20 cm high. The experimental design was a randomized block design, with four replications, in a factorial scheme (2×4)+1, being two liming treatments without and with applications of calcium and magnesium carbonates, carried out only in the soil of the upper ring, four doses of gypsum 0,0 2,5, 5,0 and 10,0 g kg-1, carried out only in the soil of the upper ring, and an additional treatment (with calcium and magnesium carbonate applications, carried out in the soil of both upper and lower ring). In field experiment, after 12 months, the results showed that gypsum reduced Al3+ level and increased Ca2+ and S-SO42- levels in subsurface and surface layers of the soil. The other chemical components of the soil (pH, pH, Al, P, K, Mg and micronutrients) are not affected by the gypsum. The gypsum did not affect the growth of Satsuma mandarin, but increased the S concentration in the leaves, due to the higher availability of this nutrient in the soil. However, the growth rate of tangerine seedlings was low. In-between the rows, gypsum did not affect the biomass production of setaria grass and black oat, but lower N extraction by setarian grass and higher Ca and S extractions by black oat were obtained with gypsum. However, setaria grass exhibited higher efficiency in recycling nutrients from chemically poor soils than black oat, due to higher dry matter yield and total nutrient extraction per unit area. Thus, both species can alternatively be used as cover crops in areas of organic tangerine yield, whereas there are restrictions on the use of industrialized fertilizers under organic agriculture. In greenhouse experiment, the results showed that lime and gypsum applications did not affect the shoot growth of the trifoliate orange seedlings. However, liming increased root growth in superficial soil layer. There was a higher chlorophyll concentration at 55 days after transplanting with gypsum and liming. Lime increased Ca concontration and reduced Mn concentration in roots of both superficial and subsuperficial layers of the soil, and there were also increases in Mg and S concentrations in roots of the superficial soil layer. Gypsum without liming resulted in higher N, K, and Mn concentrations and lower Mg concentration in roots of the subsurface soil layer. Ca and S concentrations in root of the superficial soil layer were higher with gypsum. In trifoliata orange shoot, the concentrations of K, Ca, S and Cu were higher with lime addition, and concentrations of Ca and P were higher and lower, respectively with gypsum application.