Atributos de qualidade do solo em resposta à aplicação de gesso agrícola e compactação

Abstract

No till system (NTS) has countless agronomic and environmental benefits. However, problems related to the accumulation of nutrients and soil compaction on surface layers has been recurrent and are constituted as limitations, which restrict the management under NTS. Phosphogypsum has been used to enrich soil chemical fertility on both surface and subsurface layers, as a source of sulfur and calcium, also having, in the literature, records of effects on soil physical properties. Particularly in relation to the sulfur, the implantation of more restrictive legislation have reduced both the emissions and the inputs, by chemical deposition, of this nutrient to the soil. The aims of the study were: (i) to evaluate, in a no-till area, the effect of phosphogypsum rates and traffic on soil chemical and physical attributes, as well on some yield components of common bean (Phaseolus vulgaris) crop; (ii) to evaluate, using a chronological dataset with 13 years of soil chemical tests, the evolution of soil S levels in the northwest Ohio (USA) region. To accomplish the objective (i), an experiment was setup in a very clayey Typic Hapludox, using a split-splot design with four replicates and plots of 102.4 m2. In November of 2009 PG rates (0, 3, 6, 9 and 12 Mg ha-1) were broadcasted on soil surface. In November of 2012, the traffic treatment was added by 15 overlapped passages using a tractor, creating the positions of traffic (tractor wheel), between wheel's track and no traffic. The common bean sowing was done in February of 2013. Soil samplings for chemical and physical attributes determination were done, in the 0.0-0.1 and 0.1-0.2 m layers, at six (2013) and fourteen (14) months after the traffic application. Soil aggregate samples from the traffic area were also analyzed in the scanning electronic microscope with an attached X-ray energy-dispersive spectrometer. The aim (ii) was evaluated by a dataset composed of 8,428 soil tests, collected between 2002-2014, from the 0.0-0.19 m layer, on 143 farm fields located in the northwest Ohio region, at Allen, Defiance, Paulding and Williams counties. The S from samples was extracted by Mehlich III and determined by the turbidimetric method. The results were organized according to the years and grouped into one database for analyses. As results for the objective (i) it was found that phosphogypsum reduced the potential acidity and Al3+ levels, as well as increased pH values and Ca2+ levels in the 0.0-0.2 m layer; SO42- contents were increased only in the 0.1-0.2 m layer. Phosphogypsum has decreased traffic impacts on aggregate tensile strength and soil penetration resistance at 0.0-0.1 m layer, also promoting bigger C counts in the wheel traffic position aggregates. The association between machinery traffic and phosphogypsum has increased soil water-dispersible clay, as well as reduced soil total porosity and macroporosity at 0.0-0.1 m layer in 2013. Machinery traffic has increased soil bulk density and tensile strength, reducing yield, aerial dry mass, weight of thousand seeds and number of pods per plant of common bean crop. As a result of objective (ii) it was found the existence of a linear decreasing trend of S concentrations in the soils of the northwest Ohio-USA. These data also indicated that S soil test values were often below critical concentrations needed to support high crop yields. The findings of objective (i) were: phosphogypsum was effective to promote improvements on soil chemical and physical fertilities, attenuating the traffic effects on aggregate tensile strength. The conclusions of objective (ii) were: current fertilization practices and wet deposition of S (acid rain) have not been efficient to maintain adequate plant-available S concentrations in soil, being essential to maintain and/or increase soil test S concentrations to assure crop productivity.