Soybean crops cultivated with dacite rock by-product: A proof of a cleaner technology to soil remineralization

Abstract The concept of remineralizing soils with the application of rock in the form of bran or powder is not new, but the term remineralizer is new. It was defined by Brazilian Law No. 12,890 / 2013, which included it in Law No. 6894 / 1980 (Law of Fertilizers). In 2016 the Ministry of Agriculture, Livestock and Supply (MAPA) published normative instructions dealing with the parameters of geochemical quality and the research of applicability and agronomic efficiency necessary to register the remineralizing product. This legal and normative process follows the increase in studies of characterization and application of dacite rock by-product as a sustainable environmental and economic alternative to soil fertilization in several regions of Brazil and also worldwide, since this legislation is the first to address the use of rock meal as soil fertilizers. The study brings the results of research on material from silicate rock (acidic volcanic rock - dacite) mined in an aggregate production quarry in Nova Prata, RS, Brazil. The methodology used in the study were: i) the geochemical characterization of samples of comminuted material, covering chemical, mineralogical and leaching analysis; and ii) statistical research of feasibility and agronomic efficiency application in soybean crop (Glycine max (L)). The dacite rock by-product had a Sum of Bases content (CaO + MgO + K2O) of approximately 9%, within a tolerance of 10% to less. K2O values were above 1% and free silica content well below the maximum limit of 25%. The analyzes of toxic elements content were below that established by NI No 5/2016 which are: As The laboratory tests of chemical leaching in a 24 h period showed the availability of fertilizing elements even under conditions of minimal ionic strength (deionized water). The elements with the highest concentration in solution were Si, Ca, P, Mg and K. The high availability of Si stands out, which characterizes the silicate remineralizer. The agronomical test unit consisted of 4 different areas, which one divided into plots 4.0 m wide and 21.0 m long, totaling 84 m2 per area, and 336 m2 in the study (84 × 4). The experimental design used was a randomized block with 7 treatments and each plot was subdivided into 3 subplots, corresponding to the application times, counted in days before sowing (DBS), for: 50 DBS, 20 DBS and 0 DBS From the study it is observed that the product remineralizer having different elements according to their prescribed limit and quality which could be used as substitute of limestone or others. The experiment conducted for the 50 days before sowing (50 DBS) showed a statistically significant difference between treatments for the parameters 3 grains per pod, 1000-grain weight (g) and productivity (kg ha-1). The application carried out 20 days before sowing (20 DBS) showed a statistically significant difference between treatments for the parameters number of pods per plant, 3 grains per pod and 1 grain per pod, grains per pod, 1000-grain weight (g) and productivity (kg ha-1). For the application carried out on the day of sowing (0 DBS), a statistically significant difference was observed between treatments for the parameters number of pods per plant, grains per pod, 1000-grain weight (g) and productivity (kg ha-1) When compared to the main productivity numbers of soybean in Brazil, which were 3.379 kg, the suggested treatment increased the production about 18% for 0 DBS and 16% for 20 DBS. At 20 DBS the difference between the treatments in the dose of 3 ton ha-1 and the Control was 1023 kg ha-1 and the difference with the standard treatment Limestone in the dose of 3 ton ha-1 was 831 kg ha-1. In the application at the time of sowing (0 DBS) the difference between the treatments in the dose of 3 ton ha-1 and the Control was 1153 kg ha-1 and with the standard treatment Limestone in the dose of 3 ton ha-1 was 875 kg ha-1. In conclusion, the best time of application found was 0 days before sowing at the rate of 3 ton ha-1 followed by 20 DBS. However, the application of fertilizers in the same day of sowing, for commercial crops is not recommended by the governmental agencies, due to the fact that all staff and machineries are used to sow the soybean, making very difficult to apply the dacite rock by-product in the same day, so the final recommendation of this study was to apply the soil fertilizer as close as possible to 0 DBS, not exceeding 20 DBS. The results of the study recommend that the dacite rock by-product can be used as soil fertilizer. The by-product studied here has potential to be an environmental solution to soil fertilization problem because it does not involve chemical processing and can be used in its natural form and also can solve the problem of by-products deposited outside the mines, increasing the consumption of highly soluble fertilizers.