Chemical forms in soil and availability of manganese and zinc to soybean in soil under different tillage systems

Abstract

Chemical forms of Mn and Zn in the soil and its availability to soybeans grown in a Latosol for 12 years under different tillage systems were studied. The effects of these systems on Mn complexation by soil humic acid (HA) were also evaluated. The field experiment was carried out in a randomized block (three blocks) split-plot experimental design. The main treatments consisted of four tillage systems and the secondary, of three soil sampling depths (0.00–0.05 m, 0.05–0.10 m and 0.10–0.20 m). The tillage systems were no tillage (NT), conventional tillage (CT), minimum tillage (MT) and NT with scarification, every three years. The results presented refers to the last 2 years of the experiment, 2000 and 2001. In average numbers, the Zn content of the 0.00–0.05 m layer was lower in soil under CT (2 mg kg−1), compared to the other tillage systems (NT and NT with scarification = 2.7 and MT = 2.8 mg kg−1). In general, Zn percentages in exchangeable, organic, oxides and residual forms were not modified by the tillage systems. In turn, the Mn content of the soil surface layer under the NT was higher than in the soil under CT. Mn contents extracted with DTPA were 11 and 5.2 mg kg−1 in the soil under NT and CT, respectively. The Mn contents extracted with Mehlich I, Mehlich III and HCl in the soil under CT were 20.8, 8.7 and 16.8 mg kg−1, respectively. In the soil under NT, the contents extracted with the same extractors were 27.9, 15 and 29.2 mg kg−1. In the surface layer, higher Mn percentages in the organic fraction were found in the soil under NT (19.2%) and NT with scarification (15.9%) than in soil under CT (11.8%). The Mn content of this layer was also positively correlated with the soil organic matter, but none of the extractors used (DTPA-TEA pH 7.3, HCl 0.1 mol L−1, Mehlich I and III) properly assessed the levels of Mn and Zn available to plants, suggesting the need for further studies. It was not observed presence of Mn in the HA samples by electron paramagnetic resonance (EPR). Since the digestion by nitro-perchloric acid of the HA samples showed the presence of Mn in these samples, it strengthens the hypothesis that Mn was bound to the HA in the form of stable complexes (covalent bonds), since this form of Mn cannot be detected by EPR. During soybean cultivation, Mn concentrations in the leaves did not vary with the tillage methods and were near or below the range considered adequate for the crop. Furthermore, visual symptoms of Mn deficiency in the leaves were observed in the initial stages of plant development, regardless of treatment. This suggests the need for new correlation and calibration studies for micronutrients in Brazil, especially for soils cultivated under the NT.