Nutrição e crescimento do eucalipto: efeito de fontes e concentrações de ácidos húmicos e de sua interação com os ácidos oxálico e cítrico

Abstract

Humic acid (HA) may increase growth and nutrition of eucalyptus. The effects normally related to the application of HA on crops are dependent on HA concentrations sources. Humic acid differs in chemical composition, molecular weight and bioactivity. Thus, its effect on plants can be optimized through techniques that allow greater bioactivity of humid fragments by it generated in the growth media. Some low molecular weight organic acids (LMWOA) may be important organic chelates to modulate effects of humic fertilization on crops. The aims of this study were to evaluate the effects of sources and concentrations of HA, combined with simultaneous application of LMWOA, on growth and nutrition of eucalyptus seedlings grown in the Hoagland´s nutritive solution. In the first experiment, a randomized block design (RBC) was used, with four replicates, in a factorial scheme 3x4+1, in which three sources of HA (HA-L1, HA-L2 and HA-Com) and four concentrations of C-HA (5, 10, 25 and 100 mg L -1 ) were combined, plus a control treatment in which plants were cultivated only in nutritive solution. In the second experiment, the following treatments were tested: HA + citric acid (CA) + oxalic acid (OA); HA+CA; HA+OA; HA; CA+OA; CA; OA and a control treatment, where the seedlings were cultivated only in Hoagland´s solution. At the end of each experiment, plants were collected and divided into root and shoot in experiment 1; and root, stem and leaf, in experiment 2 for determination of dry mass and nutrient accumulation and use efficiency. Eucalyptus growth increases over C-HA concentrations up to optimum rate of 15 mg L -1 . At the optimum concentration of C-AH, there was a greater acquisition of N, P, K, Ca, B, Cu, Zn by plants in comparison to nothumic fertilized plants. Magnitude of eucalyptus dry mass increase is HA source-dependent. With the increase of C-HA concentration, there was a reduction in Fe and Cu acquisition, possibly due to the increase of organic ligands in the growth media and the formation of organo-metallic complexes with high chemical stability. It was also observed the effect of different LMWOA in the disaggregation of HA, through the increase in the E4/E6 ratio. The combined use of HA and OA is and effective strategy top produce bioactive small humic fragments that increase plant growth and nutrient acquisition. Applying HA with LMWOA improved the acquisition of metallic micronutrients in relation to the isolated application of HA, being a strategy to minimize the decreased uptake of Fe, Cu, Zn and Mn when the HA concentration is increased. The response of eucalyptus seedlings is HA source and concentration-dependent and also rely on the synergistic effect of oxalic acid on the HA action on plants and growth media.