Calagem e uso de gesso agrícola na produção de arroz irrigado

Abstract

In the State of Santa Catarina (SC), irrigated rice is traditionally grown in the pre-germinated system. However, in recent years, with the advent of Clearfield® technology, the minimum cropping system has been gaining ground, reaching close to 30% of the cultivated area in this system. This has generated demands for information regarding the correction of soil acidity because these areas remain drained until the V3-V4 development stage of rice plants. In this period, does not occur the phenomenon is known as "self-liming", so that soil acidity can be corrected in the early stages of plant development, with the lime application to maintain the productive potential of the crop. Another demand for information is related to the use of phosphogypsum in the irrigated rice crop. This product has been studied in several rainfed crops, but few studies have demonstrated the effects of its application on rice cultivation, especially in flooded areas. This study aimed to evaluate soil chemical changes and the response of irrigated rice in a minimum crop/dry soil system as a function of the application of dolomitic lime and phosphogypsum. The experiments were carried out in two locations in the southern region of the State of Santa Catarina. Experiment 1 was carried out in Santa Rosa do Sul in the 2013/14, 2014/15 and 2015/16 agricultural years, in a soil with low saturation by Al3+, and experiment 2 was carried out in Praia Grande in 2017/18, in a soil with high saturation by Al3+. A randomized complete block design with three replications was used in both experiments. Three rates of dolomitic lime were applied in the plots, as well as a control treatment with no lime, corresponding to 0, 0.5, 1.0 and 1.5 times the liming requirement estimated by the buffer SMP method to raise the pH in water at 5.5 (0, 2, 4, and 6 t ha-1 in experiment 1 and 0, 2.4, 4.8 e 7.2 t ha-1 in experiment 2), and in the subplots, three rates of phosphogypsum (2, 4, and 6 t ha-1) were applied, in addition to a control treatment without phosphogypsum. The rice was sown in a minimum cropping system, with the exception of the 2015/2016 harvest in experiment 1, which was managed in the pre-germinated system. The effects of liming and phosphogypsum on the soil chemical attributes were similar in the two experiments, presenting similar responses to the studies carried out on upland soils without irrigation. Liming was efficient in correcting the soil acidity in the incorporation layer, increasing the pH in CaCl2 and the exchangeable Ca2+ and Mg2+ contents, and reducing the exchangeable Al3+ content. In experiment 1, the lime rates practically did not influence the concentrations of nutrients in the rice leaf. In experiment 2, there was an increase in Mg concentration and a decrease in the Ca and S contents in the rice leaf with the increment of lime rate; liming also increased Ca exportation by grains and Mg uptake by plants, and decreased the uptake of K and S by plants and the export of S by grains. The phosphogypsum rates, in both experiments, increased the Ca2+ and S-SO42- levels in the soil profile (0-60 cm) already in the first crop, whose effects lasted until the 33 months after the application, and decreased the Al3+ levels in the soil after 33 months of application. In experiment 1, the phosphogypsum rates increased the concentrations of P, K, Ca and S in the leaf experimento 2, as doses de gesso aumentaram a concentração de S na folha bandeira do arroz, a extração de P e S pelas plantas, as concentrações de P, K, Ca e S nos grãos, bem como a exportação de K, Ca, Mg e S pelos grãos. Os componentes de rendimento, analisados somente no experimento 2, não foram influenciados significativamente pela calagem. Apesar de os benefícios na correção da acidez do solo proporcionados pela calagem, não houve resposta positiva na produtividade do arroz com as doses de calcário aplicadas nos dois experimentos e, ainda, no primeiro cultivo do experimento 1, as doses de calcário deprimiram a produtividade de arroz. Esses resultados revelaram que a calagem foi desnecessária na cultura de arroz irrigado com o objetivo de aumentar a produtividade de híbridos com alto potencial produtivo. As doses de gesso aumentaram o número de panículas por m2, o peso de 1000 grãos, o número de grãos por panícula, o número de grãos cheios por panícula e o peso da panícula. A produtividade do arroz irrigado foi aumentada com doses de gesso de forma consistente, nos dois experimentos. Concluiu-se que existe grande potencial de uso do gesso agrícola na cultura do arroz irrigado em sistema de cultivo mínimo visando aumentar a eficiência da utilização dos nutrientes do solo e a produtividade de grãos de híbridos com alto potencial produtivo.