Efeitos de corretivos da acidez do solo associados ao gesso agricola sobre os atributos físicos e químicos do solo

Abstract

Soil acidity is an important factor that restricts agricultural production in the world. Acid soils have low pH values, high exchangeable aluminum (Al3 +) content and low availability of exchangeable calcium (Ca2+), magnesium (Mg2+) and potassium (K+). To correct the problems caused by the soil acidity, several materials can be used, with emphasis on carbonates and silicates of Ca and Mg. These products present distinctions as to their chemical composition and physical characteristics, which has repercussions on their reactivity on the soil. However, with the advent of no-till (NT), acidity correctives are applied to the soil surface, which restricts the correction of acidity in subsurface. In this context, the phosphogypsum is applied in association with acidity correctives. Although the phosphogypsum does not act on the active acidity of the soil, in subsurface it acts reducing the Al3+ contents and increasing the Ca2+ contents, which improves the root environment. The chemical management of the soil, through the application of correctives of acidity and of agricultural gypsum can alter the soil structure and the thermodynamic processes that occur in it. In this context, the general aim of this study was to evaluate the effects of soil acidity correctives [ground rock limestone (GRL), steel slag (SS) and calcined limestone (CL)], associated or not with soil physical and chemical attributes and crop productivity in NT. For this purpose, an experiment was carried out on a sandy-clay Typic Hapludox, at Ponta Grossa-PR. The treatments were the three soil acid correctives: GRL (3.8 Mg ha-1), SS (4.2 Mg ha-1) and CL (2.7 Mg ha-1), plus one control treatments (without acidity correction), associated or not with phosphogypsum (2.4 Mg ha-1). The corrective doses were calculated to raise the base saturation of the 0-0.20 m layer to 70%, based on their carbonate equivalents. The dose of phosphogypsum was calculated based on the soil clay content. Correctives and phosphogypsum were applied to the soil surface in August 2015. The crop rotation was corn (2015/16) – wheat (2016) – soybean (2016/17) – black oat (2017) – beans (2017/18). After the corn and wheat were harvested, at 7 and 15 months after the treatments, undisturbed and disturbed soil samples were collected. Undisturbed soil samples were collected in (i) monoliths, in the 0–0.05; 0.05–0.10 and 0.10–0.20 m layers, for evaluation of aggregation and aggregates stability; and in (ii) volumetric rings, in the 0–0.05; 0.05–0.10; 0.10–0.20; 0.20–0.40 and 0.40–0.60 m layers, to evaluate the soil bulk density (BD), total porosity (TP), macroporosity (Ma), microporosity (Mi) and soil water retention. The deformed samples were collected in the same collection layers of the volumetric rings to determine the soil acidity components (pH, H+Al and Al3+), exchangeable basic cations (Ca2+, Mg2+ and K+) and labile (POxOC) and total (OC) carbon contents. The yield crops was evaluated and used to determine the agronomic efficiency of the correctives and the phosphogypsum. Both at 7 and 15 months after application of treatments, the basic cations contents were more affected than the acidity components. As a rule, the application of phosphogypsum promoted intense Mg2+ and K+ leaching in the more superficial layers of the soil, which resulted in increased concentration of nutrients in depth. Referring to the components of acidity, Al3+ was the most altered component by the application of treatments, especially at 15 months after application. One result to be highlighted is that SS with phosphogypsum increased Al3+ contents in depth. The POcOC and OC contents were not affected by the treatments. The changes in the soil chemical attributes had repercussions on the physical attributes. The weight mean diameter of the aggregates and the proportions of large macroaggregates were more influenced at 15 months 11 after application, in relation to the 7 months, demonstrating the importance of the reaction time of the materials in the soil on their aggregation. BD, TP and Ma were altered by treatments in the layer of 0.40–0.60 m, while Mi was affected in the most superficial layers of the soil (0–0.20 m). The soil water retention was sensitive to changes in soil structure, especially when the correctives were applied associated with the phosphogypsum. In this way, it can be concluded that soil ion dynamics by the acidity correction and phosphogypsum application influences the physical attributes of the soil, both in surface and in depth. The increase in the Ca2+ and Mg2+ contents and the Al3+ reduction improve the soil structure, reducing its BD and increasing its TP; and, the substitution of Al3+ for Mg2+ in the soil exchange complex, due to the application of SS and phosphogypsum influences soil water retention. The higher concentration of Mg2+ favors retention. The effects of correcting soil acidity, regardless of the corrective, and the phosphogypsum on crop yield, occur independently. Also, crops respond differently to acidity correction and phosphogypsum application. However, the calcined limestone was the corrective with higher agronomic efficiency, both when applied alone and when associated with agricultural gypsum.