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Article

Assessments on the Potential Use of Rhyolite Filler as a Soil Remineralizer in Agroecological Practices in the Fourth Colony of Italian Immigration, Rio Grande do Sul, Brazil

by
João Pedro Foletto
1 and
Augusto Nobre
2,*
1
Departamento de Geociências, Centro de Ciências Naturais e Exatas, Universidade Federal de Santa Maria, Santa Maria 97105-900, Brazil
2
Instituto de Geociências, Universidade de Brasília, Brasília 70910-900, Brazil
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(19), 8955; https://doi.org/10.3390/su17198955
Submission received: 22 July 2025 / Revised: 2 October 2025 / Accepted: 8 October 2025 / Published: 9 October 2025
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Abstract

This study examined the application of rhyolite filler in agroecological farming systems in the Fourth Colony of Italian Immigration (Quarta Colônia de Imigração Italiana), Rio Grande do Sul, Brazil. The aim was to explore sustainable alternatives to synthetic fertilizers in line with organic agriculture principles. The region’s designation as the Quarta Colônia UNESCO Global Geopark reinforces the relevance of this initiative. The research involved petrographic characterization, incubation experiments, and chemical analyses of rhyolite samples. Incubation tests with pigeon pea and elephant grass assessed combinations of rock filler and poultry litter. The results showed that rhyolite filler supported plant growth, especially with poultry litter, which supplies nitrogen and microorganisms that accelerate mineral weathering. Petrographic observations indicated that elephant grass promoted stronger mineral alteration, likely due to its dense fibrous roots and substrate interaction. Chemical analyses confirmed the rhyolite’s quartz content and trace elements remain within safety limits defined by Brazilian legislation on soil remineralizers. The K2O content and the base sum (CaO, MgO, and K2O) also complied with minimum legal requirements. Beyond mineralogical and chemical aspects, the study emphasized the economic feasibility of using locally sourced rock fillers, offering lower costs than synthetic fertilizers and supporting sustainable, resilient agroecological systems.

1. Introduction

Amid technological advancements driven by Industry 4.0 and 5.0, the increasing risk of natural resource scarcity, and growing demands for more circular economies, agriculture faces progressively greater challenges related to soil degradation and the need for more sustainable production methods, particularly in the context of climate change [1,2,3,4,5]. Regarding small-scale farming, the pursuit of sustainable and innovative methods is imperative to ensure long-term productivity and food quality. In this context, Brazil stands out for its vast geodiversity, which grants the country a strategic position in the global mineral landscape by hosting a wide range of rocks suitable for use both as soil amendments and in advanced mineral applications [6,7,8].
Remineralization involves converting specific rocks naturally enriched with macro- and micronutrients into soil remineralizers capable of restoring degraded soil and reducing dependence on external synthetic chemical fertilizers [9]. According to the study by Burbano et al. [10], the use of remineralizers derived from basaltic rocks, whether combined with organic sources or not, proved effective for soil fertilization. Furthermore, the study demonstrated that rock powders have a long-term residual effect when compared to soluble fertilizers. This aspect is particularly relevant when considering the constant need to maintain soil fertility [11].
In this context, shaped by a blend of agroecological practices and traditional knowledge, regulations such as Brazilian Law No. 10831/2003 [12] have emerged to promote the replacement of synthetic inputs with cultural, biological, and mechanical methods, including remineralization techniques. This approach not only offers income-generating alternatives and supports the permanence of families in rural areas but also reduces production costs and enhances food quality [13,14]. According to Law No. 6894/1980—Article 3 of the Ministry of Agriculture and Livestock (MAPA), fertilizers are mineral or organic products, natural or synthetic, that supply one or more nutrients to plants [15]. Conventional fertilizers are highly water-soluble and contain elevated concentrations of nutrients immediately available to plants. However, it is important to note that, except for nitrogen-based fertilizers, most fertilizers are derived from chemically processed rocks [16,17]. Normative Instruction (NI) 05/2016 [18] plays a key role in regulating soil remineralizers in Brazil by establishing specific criteria to ensure their safety and effectiveness. These include essential compound parameters that define standards for macro- and micronutrients necessary for plant growth. In order to be certified, a remineralizer must meet the following minimum requirements: (I) physically, it must be in filler, powder, or ground form; (II) the sum of bases (CaO, MgO, K2O) must be at least 9% by weight; (III) K2O content must be ≥1%; (IV) quartz must not exceed 25% in filler weight; and (V) maximum limits for potentially toxic elements (PTEs) must be respected. It is important to highlight that soil remineralizers are regulated by different normative instructions [15,19], reflecting the specific regulatory approach and complexity attributed to remineralizers within Brazilian agricultural legislation.
The crop species selected for the eight-month experiment—elephant grass (Pennisetum purpureum) and red pigeon pea (Cajanus cajan)—are aligned with sustainable practices aimed at improving soil fertility and promoting resilient, balanced agricultural systems [20,21]. Red pigeon pea is recognized for its nitrogen-fixing ability, typical of legumes [22], and serves as a cover crop, green manure, windbreak, and food source for both humans and livestock [23]. Notably, it is considered a Non-Conventional Food Plant (NCFP), underscoring its nutritional potential and contribution to dietary diversification. Elephant grass stands out as one of the most important forage crops, widely cultivated in tropical and subtropical regions due to its high productivity and superior forage quality [24]. Although agroecological research on its benefits for producers and sustainable dairy farming is still at an early stage in Brazil [25,26], its continued widespread use by the target farmers of this study highlights its practical value and acceptance despite the research gaps. The choice of these crops is further supported by the prominence of family and agroecological farming in Brazil, which accounts for 76.8% of the 5,073,324 agricultural and aquaculture establishments, totaling 3,897,408 units. However, family farming occupies only 23.0% of the total area dedicated to agricultural activities, revealing the typically smaller scale of these operations [27]. For many of these family farmers, access to agricultural inputs poses a significant challenge, as most fertilizers available on the market are imported, increasing production costs. According to data from the National Fertilizer Diffusion Association [26], Brazil imported 32,872,543 tons of intermediate and complex nitrogen-phosphorus-potassium (NPK) fertilizers in 2023. This highlights the agricultural sector’s dependency on external inputs and its vulnerability regarding their domestic availability. In this way, this work is aligned with the Sustainable Development Goals (SDGs) of the UN 2030 Agenda, contributing to the reduction of hunger and the promotion of sustainable agriculture (SDG No. 2), as well as to responsible consumption and production (SDG No. 12).
The present study aims to evaluate the use and initial mineralogical transformations of filler material derived from a felsic volcanic rock (rhyolite)—a rock type less studied for remineralization compared to mafic rocks, like basalts [16]—applied to crops commonly cultivated by agroecological farmers in the Fourth Colony of Italian Immigration region in Brazil. To achieve this, the study addresses the following objectives:
  • Assess whether the chemical and mineralogical composition of the rhyolite filler, sourced from a quarry in Itaara, Rio Grande do Sul, near the Fourth Colony, complies with the criteria established by NI No. 05/2016 [18];
  • Conduct an eight-month incubation experiment using crops grown by agroecological farmers (red pigeon pea and elephant grass);
  • Analyze the weathering processes, nutrient leaching, and nutrient release from the rock throughout the incubation period;
  • Evaluate the market feasibility of this potential remineralizer as a viable tool for supporting small-scale agroecological producers.
The practice of remineralization is emerging as a promising alternative for family farmers and agroecological producers. The use of crushed rocks as a nutrient source can offer a cost-effective option. Local production of these inputs, particularly when rock sources are located near cultivation areas, has the potential to reduce overall production costs by lowering transportation expenses, thereby generating positive economic impacts for farmers.

2. Materials and Methods

2.1. Geological Context of the Study Area

The Serra Geral Group, located within the Paraná Sedimentary Basin, covers an area of approximately 917,000 km2. Its total volume has been estimated at over 600,000 km3, with at least 450,000 km3 attributed to extrusive rocks and 112,000 km3 to sills. The broader Paraná-Etendeka Continental Flood Basalt Province has a minimum estimated volume of 1,700,000 km3 [5,27,28]. The Serra Geral Group predominantly consists of tholeiitic rocks, such as basalts and andesites, which account for more than 90% of its total volume. However, significant amounts of felsic rocks—rhyolites, rhyodacites, dacites, and quartz latites—are also present, representing approximately 2.5% of the total volcanic rock volume in the Paraná Magmatic Province. These felsic rocks are mainly concentrated near the continental margin and can reach thicknesses exceeding 400 m in the depocenter of the Paraná Basin [29,30]. In some areas, the volcanic sequence can attain a total thickness of up to 1700 m. The felsic rocks of the Serra Geral Group are classified into two chemical types: the Chapecó type (high Ti and P) and the Palmas type (low Ti and P) [31,32]. The study area is located geomorphologically on the edge of the Southern Plateau of Rio Grande do Sul, within the Palmas Formation [28]. This formation comprises primarily rhyodacites (including undifferentiated rhyolites and dacites) and quartz latites (Figure 1). It lies at the contact zone between the volcanic rocks of the Paraná Basin and the sedimentary rocks, where many of the region’s family-owned and agroecological farms are concentrated.
Volcanic rocks, particularly those of basic and acidic composition, are regarded as suitable for agricultural applications, in accordance with the principles of rock remineralization. This is especially pertinent in southern Brazil, where a significant portion of the territory is composed of rocks from the Serra Geral Group, characterized by gently undulating plateaus [33]. The utilization of rock powders, particularly those derived as byproducts from basic and ultrabasic rocks mining activities, to restore soil fertility through remineralization has been identified as a promising strategy. Such materials (are typically already partially disaggregated and contain appreciable concentrations of phosphorus and calcium oxides, enhancing their agronomic potential [34]. Moreover, the application of this technique has demonstrated the potential to mitigate environmental impacts in areas such as the Ametista do Sul Mining District [35]. The district’s proximity to a major agricultural region in Rio Grande do Sul further underscores its potential as a supplier of agricultural inputs [36,37].
Considering the limited scientific knowledge regarding the remineralization potential of felsic volcanic rocks and recognizing that the municipality of Itaara hosts quarries producing crushed stone from rhyolites for the construction industry, it is relevant to investigate the potential of the fine dust generated as a byproduct of these operations. This material may represent a low-cost, natural alternative for the gradual release of nutrients into the soil, with the aim of enhancing long-term productivity, particularly for organic and agroecological farming systems [9].

2.2. Agroecological Initiatives in the Fourth Colony of Italian Immigration

The target audience of this study comprises farmers with organic certification, specifically those participating in the Ana Primavesi Organic Market in the Fourth Colony region. This market is held weekly in Santa Maria, the most populous and economically developed municipality in the central Rio Grande do Sul state where Fourth Colony is located. Agroecology is defined as a body of systematized knowledge grounded in both technical and traditional practices, including those of Indigenous peoples and small-scale farmers. It is a discipline that revives agronomic principles predating the Green Revolution, treating agricultural fields as ecosystems characterized by predator-prey interactions, competition, commensalism, and ecological succession. Its primary objective is to understand the relationship between human societies—including their cultures, habits, and traditions—and the dynamics and interactions within and between biotic and abiotic components of the environment [38].
This group of producers represents a specific market niche distinguished by sustainable production practices. The market space functions not only as a site for commercial transactions but also as a reference point for agroecological initiatives, encouraging community engagement in supporting local agriculture. This marketing model is particularly effective for consumers seeking a diverse range of organic foods adapted to seasonal availability, as it offers subscription options wherein consumers pay a monthly fee to receive a weekly basket of fresh products. The Ana Primavesi Organic Market is the only market within the urban area of Santa Maria—and among the municipalities of the Fourth Colony of Italian Immigration—dedicated exclusively to the direct sale of certified organic products, playing a crucial role in promoting sustainable agricultural practices [39].

2.3. Study Area

The geographic locations relevant to this study are depicted in Figure 2, including the Ana Primavesi Organic Market in the municipality of Santa Maria—where organic products from small-scale producers in the Fouth Colony are commercialized—the rhyolite Conpasul company quarry in Itaara, which serves as the source of the rock filler utilized in the incubation experiments, and the municipalities within the Fourth Colony of Italian Immigration, where agroecological farming systems are implemented.
The quarry from which the rhyolite samples were obtained is geologically situated within the Serra Geral Group, specifically in an area characterized by felsic volcanic flows. As illustrated in Figure 3, the quarry face exhibits a moderately fractured structure, incipient weathering features, and a shallow soil profile. These attributes render the rock suitable to produce crushed stone, with fine rock powders generated as a byproduct. The mine has been in operation since 2000. The rhyolite is isotropic, gray in coloration, and possesses a fine-grained groundmass, which precludes macroscopic identification of its mineralogical composition. Additionally, the presence of small pockets of phenocrysts imparts a glomeroporphyritic texture, while vesicles and amygdales are absent. The characterization and confirmation of the rhyolite composition, along with its chemical potential for nutrient release, were integral components of the present investigation.
Since the rock massif exhibits several fracture sets, which prevent the extraction of large rock blocks, the quarry’s main product is crushed stone for the regional construction industry. The rock massif is blasted according to production demand, and the material is directed to two sets of crushers: a primary crusher, which breaks the blasted blocks into smaller particles, and a secondary crusher, which produces aggregates of different grain sizes. The aggregate size is controlled by the duration of rock comminution in the secondary crusher and can be adjusted depending on the particle size of interest. Rock powders (particle size < 2 mm) are inevitable by-products of the crushing processes, constituting residues of quarry operations. The opportunity to screen this residue to separate the filler fraction (<75 μm) and use it to produce remineralizers represents not only a new commercial product option for the quarry but also a contribution to reducing solid waste generation in its operations.

2.4. Analytical Methods

Morphological analyses of the rock filler, as well as chemical tests of the rhyolite samples, were conducted using Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Compliance with Brazilian quality standards was assessed based on Normative Instruction No. 05/2016 [18], particularly regarding chemical suitability. The analyses compared quartz content, concentrations of potentially toxic elements (PTEs) such as Cd, As, and Pb, the sum of basic oxides (CaO, MgO, K2O), and the total K2O content. SEM experiments were performed using a JSM 6360 microscope equipped with a QUEST EDS detector (ThermoSystem, Hillsboro, OR, USA). The analyses were carried out at the Scanning Electron Microscopy Laboratory (UFSM). Images were obtained in topographic mode, which provides morphological contrast based on variations in secondary electron emission from the sample surfaces. Analytical conditions included an accelerating voltage of 20 kV and a beam current of 20 nA, following the procedure described by Ceccato et al. [40]. For ICP-OES analyses, a Spectro Cirus instrument (SPECTRO Analytical Instruments) was used, enabling simultaneous spectrum acquisition within the 125–770 nm wavelength range. The powdered samples were digested in hot nitric acid using microwave-assisted digestion in a Synthos 3000 system (Anton Paar, Graz, Austria).
The incubation experiments were designed following the methods of Medeiros et al. [41] and Cardozo et al. [36]. In the employed method, plant precursors in the form of seedlings are used, as they possess greater resilience and sufficient nutritional reserves to initiate the weathering process. In contrast, seeds, due to various factors, might fail to germinate or fail to produce relevant weathering results within the timeframe of the experiment. Plant development was monitored by measuring plant height (from the base of the stem to the apex) and stem circumference (measured near the base of the plants, 2 cm above the substrate level). Two plant species—pigeon pea (Cajanus cajan) and elephant grass (Pennisetum purpureum)—were cultivated in identical 3 L pots. Thick, impermeable plastic liners were used to line the pots to prevent the leaching of secondary minerals and dissolved materials. All cultivation setups were performed in duplicate to ensure the survival of plant specimens at the end of the experiment for comparative analysis. In addition, control pots were prepared: one containing only the rock filler and another containing the rock filler combined with poultry litter, both without plants, to validate whether observed transformations in the substrate were attributable to plant activity. A detailed description of the experimental variables used in the incubation study is provided in Table 1. The incubation period for the plant species was 245 days (just over 8 months).
Soil was deliberately excluded as a variable in the experiment, as the study focuses on the accelerated weathering of rocks rather than conventional agronomic assessments. The inclusion of soil could obscure the specific processes associated with the rock filler, complicating the interpretation of the resulting data. Poultry litter was the only external material added, alongside the rock filler and the incubated plants. Poultry litter refers to the material accumulating on the floors of poultry houses or facilities used for chicken meat or egg production. This substrate forms naturally due to the movement of the birds, the accumulation of manure, feathers, and other physiological components, as well as the introduction of bedding materials such as wood shavings or rice husks, and the natural decomposition of organic matter. Poultry litter plays a crucial role in enhancing nitrogen availability and retention in rock powders, which are traditionally deficient in this essential nutrient. Furthermore, it acts as a weathering accelerator, enabling the incubation experiments to yield measurable results within the designated timeframe [7,9,41,42,43,44,45].
The incubation experiment was conducted in a covered environment designed to shield the pots from precipitation, thereby allowing precise control over the quantity of water applied to each vessel. Additionally, the orientation of the site ensured that the plants received direct sunlight during the morning hours and throughout the afternoon. The pots were arranged in organized groups with a spacing of 2 to 3 cm between them, totaling ten pots. Considering the dimensions of the pots and the spacing adopted, the total area occupied by the experiment was approximately 1 m2, facilitating efficient management.
The pot samples were identified using tags containing their respective IDs. Cultivation commenced at the end of the summer season, during which maximum temperatures ranged between 30 °C and 35 °C. During the autumn months, temperatures were milder, while in winter, minimum temperatures ranged from 5 °C to 6 °C. Each pot contained approximately 1.8 kg of rock filler, and the pots receiving poultry litter were supplemented with 15 g of this material.
Since the pots were subjected to different experimental conditions requiring distinct irrigation volumes, it was established that water should be added weekly to maintain constant visual substrate moisture and ensure chemical weathering pathways throughout the experiment.

3. Results

3.1. Rock Filler Characterization

To determine the rock type and ensure the use of a felsic-composition material, total rock chemical analysis was conducted using ICP-OES to identify the mineral components, which were subsequently processed through the Minetosh Online platform for geological data (https://www.minetoshsoft.com/geology/index.html, accessed on 30 September 2025). The normative composition of the rock indicates that plagioclase ((Na,Ca)(Si,Al)AlSi2O8), accounting for 38.8%, is the main mineral phase, serving as a significant source of sodium and calcium. Orthoclase (KAlSi3O8), comprising 28.2%, also plays an important role, substantially contributing to the rock’s alkalinity by providing potassium, an essential element for plant development. Mafic minerals such as hypersthene ((Mg,Fe)SiO3) and diopside (CaMgSi2O6) occur in lower proportions, playing a more limited yet relevant role in supplying magnesium and iron. Figure 4 shows the compositional distribution of the mineralogy of pure filler (sample F6). The results show that sample F6 contains a quartz (SiO2) content of 20.9%. This value complies with the current regulatory standards for soil remineralizers [18], which set a maximum allowable quartz content of 25% in weight. The moderate quartz concentration is consistent with the characteristics of rhyolite, an igneous rock of predominantly acidic composition. It is important to note that the application of rock powders with high quartz content may lead to desertification processes [46,47]. This phenomenon occurs when the soil loses its capacity to support plant life due to nutrient depletion, compaction, and poor water retention—factors that can be exacerbated by the excessive presence of inert minerals [47,48]. Desertification reduces vegetation cover, intensifies erosion processes, and accelerates soil degradation, resulting in unproductive areas that directly affect ecosystems and the sustainability of agricultural practices [49,50].
The Streckeisen Diagram [51] (Figure 5), used for the classification of igneous rocks, places the studied rock within the rhyolite field. This classification is supported by the normalization of the rock’s composition based on the proportions of alkali feldspars, plagioclase, and quartz. These components highlight the acidic nature of the rock, associated with a moderate degree of alkalinity. As a complementary approach to the Streckeisen diagram, the Total Alkali–Silica (TAS) Diagram [52], shown in Figure 6, was employed to verify the rock classification. This diagram is based on the contents of Na2O + K2O (alkalinity) vs. SiO2 (silica content). The plotted point falls within the “R” field—Rhyolite—confirming that the rock contains more than 70% silica in its total composition.

3.2. Analysis of Potentially Toxic Elements

The assessment of the safety and environmental compliance of soil remineralizers is essential for their safe application in agricultural settings. For this purpose, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was employed to verify whether the concentrations of potentially toxic elements (PTEs) in the rock comply with the limits established by current regulations. It is worth noting that, due to the low concentrations of these metals, ICP-OES is the only technique with sufficient sensitivity to resolve the samples at the µg/g level. The results of this analysis are presented in Table 2, enabling an evaluation of the material’s suitability in terms of safety parameters. According to the specifications of Normative Instruction No. 05/2016 [18], the criteria related to PTEs were met. The concentrations of cadmium (Cd), arsenic (As), and lead (Pb) were below the safety thresholds defined by the regulation, confirming the sample’s compliance with regulatory requirements. Specifically, cadmium was detected at concentrations below 1 µg/g, arsenic below 2 µg/g, and lead below 2 µg/g—values that are below the detection limits of the equipment and substantially lower than the regulatory limits of 10, 15, and 200 µg/g, respectively.

3.3. Morphology of Filler Mineral Grains

In Figure 7A, the presence of agglomerates formed by filler-sized particles can be observed, resulting from electrostatic attraction among low-mass particles. In Figure 7B, the sample clearly exhibits mineralogical heterogeneity, with a range of particle sizes predominantly within the 1–20 µm scale, yet still consistent with the filler classification. Additionally, numerous crystals display well-defined edges (Figure 7B), indicating that cleavage and fracture were the dominant processes during sample micronization, with minimal evidence of surface weathering.
Figure 8 addresses the grain morphology of sample F1 after the incubation experiment period. In Figure 8A, the distribution of mineral agglomerates formed by electrostatic attraction is observed, along with isolated, larger quartz grains. The agglomerates exhibit irregular morphologies, with rough surfaces and well-defined edges, typical of fragments resulting from milling processes, but without evidence of chemical destabilization. In Figure 8B, the slightly more rounded shape of the crystals clearly indicates the interaction between the remineralizer, microorganisms, and pigeon pea (Cajanus cajan) roots. The morphology suggests a more pronounced weathering process compared to the filler sample (F6), implying the action of chemical agents over the course of the experiment, which led to the comminution of mineral edges and their alteration—potentially releasing ions and promoting ion exchange with plant roots. This result confirms one of the key assumptions underlying remineralizer technology, as suggested by several authors [53,54,55,56], indicating that the solubility of nutrients derived from rock powders increases over time through interaction with the soil. This process can be further enhanced by the introduction of cover crops or legumes, such as pigeon pea (Cajanus cajan) or elephant grass (Pennisetum purpureum), which promote chemical and biological transformations within the soil–plant system.
Figure 9 shows the crystal morphology of sample F3 after the incubation experiment. A rounded morphology is observed in some mineral grains, with smoothed contours in the more unstable minerals, whose edges appear rounded, suggesting the action of intense chemical weathering. The presence of microorganisms (highlighted with red arrows) can also be seen, which are key agents in the accelerated weathering of rocks.

3.4. Plant Development in the Incubation Experiment

All plants showed some degree of development during the incubation period. However, those grown in substrates containing rock filler combined with poultry litter exhibited clearly superior performance. For pigeon peas, significant differences were recorded in terms of plant height, number of leaves, and sub-woody stem circumference. In experiment F2 (Figure 10A), the plant reached a height of 21 cm, with a stem circumference of 0.8 cm. In experiment F1 (Figure 10B), the plant reached 40 cm in height and a stem circumference of 1.7 cm, indicating more vigorous development in this treatment. Figure 11 presents the results for elephant grass. In Figure 11A (F4), the plant reached 61 cm in height, with a culm circumference of 3 cm. In experiment F3 (Figure 11B), the plant reached 141 cm in height and a culm circumference of 3.5 cm, demonstrating significantly greater vegetative development.

3.5. Analysis of the Nutritional Adequacy of the Filler

Since the material was previously processed to achieve filler-grade granulometry, the rock particles meet the particle size requirements established by IN 05/2016 [18]. Consequently, it is necessary to assess their nutritional suitability for use as a remineralization substance. The total chemical composition of the original rock filler (sample F6) is presented in Table 3. The most abundant chemical element in the sample is oxygen (O), which is also the most abundant element in the Earth’s crust and is present in all minerals identified in the rock. Silicon (Si) follows, consistent with the predominance of silicate minerals. Subsequently, aluminum (Al), potassium (K), iron (Fe), sodium (Na), calcium (Ca), magnesium (Mg), and manganese (Mn) are identified as the main major and minor elements in the sample. As discussed in Section 3.3 and Section 3.4, the most significant plant development—particularly in terms of vigor, foliage, and stem support—along with the successful detection of fragmented filler grains under microscopy, led to the selection of sample F1 for a comparative chemical analysis with the original rock (sample F6), aiming to assess the nutrient release potential of the chemical elements present. The analysis results are presented in Table 4, where oxygen and silicon remain the most abundant chemical elements in the sample. These elements are fundamental constituents of quartz, the most chemically stable mineral in the rock’s mineral assemblage. They are followed, in descending order, by Al, K, Na, Ca, Fe, N and Mg. Based on the chemical data, it is important to note meaningful differences between the samples. Most notably, nitrogen (N) was detected in the sample containing poultry litter, accounting for 1.56% of the normalized mass, whereas this element was absent in the pure rock filler sample. The presence of nitrogen indicates the contribution of poultry litter, which is rich in nitrogen-containing organic compounds. Iron (Fe), a chemical element commonly found in mafic minerals, showed a significant proportional decrease in sample F3, supporting the model that weathering of mafic minerals (such as hypersthene and diopside in the studied rhyolite assemblage) is more intense under surface conditions. Manganese (Mn), previously present in low concentrations and associated with mafic minerals, was no longer detected in the analysis. The remaining chemical elements maintained similar proportions, suggesting that the mineral composition of the rock remains largely unchanged, confirming one of the underlying assumptions of remineralizer technology: its nutrient release occurs slowly but is sustained over an extended period.
In relation to the nutritional standards established by [16], both the original rock (sample F6) and the sample after the incubation experiment (F1) meet the regulatory requirements, with K2O contents of 7.54% and 9.20% by weight, respectively. The base sum (K2O + CaO + MgO) also exceeded 9% by weight in both cases, with values of 10.51% and 12.76%, respectively. The oxide proportions are presented in Table 5. These results also support the hypothesis that mafic minerals weather at a proportionally faster rate, as the content of K2O—associated exclusively with orthoclase—increased following the incubation experiment. CaO displays a mixed behavior within the rock’s mineral assemblage, as it is a constituent of both plagioclase (a felsic mineral) and mafic minerals such as diopside. MgO, on the other hand, is geochemically associated with mafic minerals (diopside and hypersthene) and was the only analyzed oxide to show a decrease from the beginning to the end of the experiments, further reinforcing the preferential weathering of mafic minerals observed in experimental conditions.

4. Discussion

The results of the incubation experiment offer valuable insights into plant development under different substrate compositions. All plants survived the tested treatments, which is significant evidence of the substrates’ basic capacity to support life. Even when the rock filler was used alone or combined with poultry litter, plant viability was maintained throughout the experiment, indicating that they were able to extract at least minimal nutrients from the substrates, despite differing growth patterns. The addition of poultry litter enhanced plant development, likely due to its high nitrogen content—a key macronutrient for plant growth—as well as the presence of microorganisms that may accelerate mineral weathering and contribute to nitrogen fixation. Plant performance also varied according to species-specific root systems. Pigeon pea (Cajanus cajan), with its deep taproot system, could explore water and nutrients from deeper layers, while elephant grass (Pennisetum purpureum), with its dense fibrous roots, occupied surface-level substrate space. These root system characteristics, combined with the spatial constraints of the experimental setup, played a crucial role in plant development. Notably, both species are traditionally used in agroecological systems of the Fourth Colony region.
The ICP analysis confirmed low levels of potentially toxic elements, indicating that the tested material is environmentally safe for use as a soil remineralizer. Although F1 and F3 showed detectable Pb levels, they remain significantly below the maximum permitted limits. Since sample F6 (rock filler without additives or plants) did not show detectable Pb levels, it is possible that the values observed in F1 and F3 originated from poultry litter, even though there is no concrete evidence to support this hypothesis. This reduces the risk of soil contamination and potential impacts on human and ecosystem health. Other nutritional quality parameters required by Brazilian legislation [18] were adequately met. Both K2O content and the base sum (K2O + CaO + MgO) exceeded the legal thresholds, and the moderate quartz content remained within acceptable limits, ensuring compliance with current regulations.
As this study had an initial exploratory focus on the material’s potential, the limitations of the method include the absence of agronomic tests with soil, the low statistical representativeness and reproducibility of the data, and the relatively short duration of the incubation experiments, which restricts more detailed observation and interpretation of weathering and ion release processes. Nevertheless, for the purpose of specifically testing the rock filler under restricted variables, the study provided sufficient conditions to validate the rock as a potential soil remineralizer.

5. Conclusions

Although the incubation period of 540 days is relatively short to demonstrate more advanced weathering stages and nutrient release, the results support the long-term nature of soil remineralization processes. This is particularly relevant for agroecological farmers who, due to either regulatory restrictions or financial limitations, cannot access synthetic fertilizers. Therefore, rock filler remineralizers represent a sustainable alternative that should be encouraged, especially among smallholder farmers in the Fourth Colony of Italian Immigration, southern Brazil. Further studies evaluating other rock types and initiating the formal registration process of this rock filler as an official soil remineralizer are possible. Alternatively, this filler can be blended with other rock powders to create a more effective soil remineralizer, targeting specific plant nutrients. This would help make a sustainable, locally sourced product available to family farmers in support of ecologically responsible agricultural practices.

Author Contributions

Conceptualization, J.P.F. and A.N.; methodology, J.P.F. and A.N.; validation, J.P.F. and A.N.; formal analysis, J.P.F. and A.N.; investigation, J.P.F. and A.N.; resources, J.P.F. and A.N.; data curation, J.P.F. and A.N.; writing—original draft preparation, J.P.F. and A.N.; writing—review and editing, J.P.F. and A.N.; visualization, J.P.F. and A.N.; supervision, J.P.F. and A.N.; project administration, A.N.; funding acquisition, A.N. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The entire database used in this work is available upon request from the authors.

Acknowledgments

The authors thank the Sedimentology Laboratory of the Department of Geosciences at the Universidade Federal Santa Maria (UFSM) for their assistance in sample preparation for the incubation experiments. They also thank the Department of Chemistry at UFSM for providing access to the equipment used in the chemical analyses presented in this study and the Rapid Prototyping, Additive Manufacturing, Innovation, and Services Laboratory (LAB3D+) of the UFSM Technology Center for enabling microscopy analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Regional geological context and geodiversity of Itaara and the municipalities that make up the Fourth Colony of Italian Immigration.
Figure 1. Regional geological context and geodiversity of Itaara and the municipalities that make up the Fourth Colony of Italian Immigration.
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Figure 2. Location map of the study area, showing the rhyolite Conpasul quarry in Itaara, the municipalities of the Quarta Colônia of Italian Immigration, and the Ana Primavesi Organic Market in Santa Maria, Rio Grande do Sul State, southern Brazil.
Figure 2. Location map of the study area, showing the rhyolite Conpasul quarry in Itaara, the municipalities of the Quarta Colônia of Italian Immigration, and the Ana Primavesi Organic Market in Santa Maria, Rio Grande do Sul State, southern Brazil.
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Figure 3. Rhyolite quarry in Itaara, illustrating the shallow soil profile and fractured rock. Color variations are due to heterogeneous exposure to sunlight and moisture.
Figure 3. Rhyolite quarry in Itaara, illustrating the shallow soil profile and fractured rock. Color variations are due to heterogeneous exposure to sunlight and moisture.
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Figure 4. Normative composition of mineralogy by weight of rock filler.
Figure 4. Normative composition of mineralogy by weight of rock filler.
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Figure 5. Rock classification in the Streckeisen Diagram. Field 2 corresponds to rhyolite. The remaining fields correspond to the following rocks: 1—alkali feldspar rhyolite; 3 and 3a—dacite; 4—quartz alkali feldspar trachyte; 5—quartz trachyte; 6—quartz latite; 7 (a, b, c, and d)—basaltic andesite; 7 (e and f)—trachybasaltic andesite; 8—alkali feldspar trachyte; 9—trachyte; 10—latite; 11—foid-bearing alkali feldspar trachyte; 12—foid-bearing trachyte; 13—foid-bearing latite; 14—phonolite; 15—tephritic phonolite; 16—phonolitic basanite or phonolitic tephrite; 17—basanite or tephrite; 18—phonolitic foidite; 19—tephritic foidite; 20—foidite.
Figure 5. Rock classification in the Streckeisen Diagram. Field 2 corresponds to rhyolite. The remaining fields correspond to the following rocks: 1—alkali feldspar rhyolite; 3 and 3a—dacite; 4—quartz alkali feldspar trachyte; 5—quartz trachyte; 6—quartz latite; 7 (a, b, c, and d)—basaltic andesite; 7 (e and f)—trachybasaltic andesite; 8—alkali feldspar trachyte; 9—trachyte; 10—latite; 11—foid-bearing alkali feldspar trachyte; 12—foid-bearing trachyte; 13—foid-bearing latite; 14—phonolite; 15—tephritic phonolite; 16—phonolitic basanite or phonolitic tephrite; 17—basanite or tephrite; 18—phonolitic foidite; 19—tephritic foidite; 20—foidite.
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Figure 6. Rock Classification in TAS Diagram. R—Rhyolite. Thus, the rock was properly correlated with the regional geological context shown in Figure 1. The other fields of the diagram are represented as follows: F—foidite; Ph—phonolite; U1—tephrite–basanite; U2—phono-tephrite; U3—tephri-phonolite; S1—trachybasalt; S2—basaltic trachyandesite; S3—trachyandesite; T—trachyte; Pc—picrobasalt; B—basalt; O1—basaltic andesite; O2—andesite; O3—dacite.
Figure 6. Rock Classification in TAS Diagram. R—Rhyolite. Thus, the rock was properly correlated with the regional geological context shown in Figure 1. The other fields of the diagram are represented as follows: F—foidite; Ph—phonolite; U1—tephrite–basanite; U2—phono-tephrite; U3—tephri-phonolite; S1—trachybasalt; S2—basaltic trachyandesite; S3—trachyandesite; T—trachyte; Pc—picrobasalt; B—basalt; O1—basaltic andesite; O2—andesite; O3—dacite.
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Figure 7. SEM images of mineral morphology in sample F6 (untreated rock filler) at magnifications of (A) Red arrows indicate examples of particle agglomerates of the filler. (B) Red arrows indicate examples of minerals with well-defined edges.
Figure 7. SEM images of mineral morphology in sample F6 (untreated rock filler) at magnifications of (A) Red arrows indicate examples of particle agglomerates of the filler. (B) Red arrows indicate examples of minerals with well-defined edges.
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Figure 8. SEM images of mineral morphology. (A) sample F1 (rock filler + poultry litter + pigeon pea). Red arrows indicate examples of particle agglomerates of the filler. (B) Red arrows indicate examples of minerals with rounded edges, evidencing a morphological wear process at the microscopic scale.
Figure 8. SEM images of mineral morphology. (A) sample F1 (rock filler + poultry litter + pigeon pea). Red arrows indicate examples of particle agglomerates of the filler. (B) Red arrows indicate examples of minerals with rounded edges, evidencing a morphological wear process at the microscopic scale.
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Figure 9. SEM image of mineral morphology in sample F3 (rock filler + poultry litter + elephant grass), at 500× magnification, showing the presence of microorganisms (red arrows).
Figure 9. SEM image of mineral morphology in sample F3 (rock filler + poultry litter + elephant grass), at 500× magnification, showing the presence of microorganisms (red arrows).
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Figure 10. Comparison of pigeon pea seedlings: (A) grown with rock filler (F2); (B) grown with rock filler and poultry litter (F1).
Figure 10. Comparison of pigeon pea seedlings: (A) grown with rock filler (F2); (B) grown with rock filler and poultry litter (F1).
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Figure 11. Comparison of elephant grass seedlings: (A) grown with rock filler (F4); (B) grown with rock filler and poultry litter (F3).
Figure 11. Comparison of elephant grass seedlings: (A) grown with rock filler (F4); (B) grown with rock filler and poultry litter (F3).
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Table 1. Experimental variables of the plant incubation trials.
Table 1. Experimental variables of the plant incubation trials.
Test IDNumber of PotsSubstrate CompositionPlant Culture
F12Rock filler + poultry litterPigeon pea
F22Rock fillerPigeon pea
F32Rock filler + poultry litterElephant grass
F42Rock fillerElephant grass
F51Rock filler + poultry litterNo plant
F61Rock fillerNo plant
Table 2. Comparison of sample content with legal limits for Potentially Toxic Elements (PTEs).
Table 2. Comparison of sample content with legal limits for Potentially Toxic Elements (PTEs).
ElementLIMIT (ppm)Sample F6 (µg/g)Sample F3 (µg/g)Sample F1 (µg/g)Conformity
Cadmium (Cd)10<1<1<1Approved
Arsenic (As)15<2<2<2Approved
Lead (Pb)200<23.92.5Approved
Table 3. Normalized chemical composition (wt%) of sample F6.
Table 3. Normalized chemical composition (wt%) of sample F6.
ElementAtomic Number (Z)Normalized Mass (%)
O858.66
Si1425.83
Al135.83
K193.13
Fe262.29
Na112.18
Ca201.45
Mg120.56
Mn250.07
Sum100.00
Table 4. Normalized chemical composition (wt%) of sample F1.
Table 4. Normalized chemical composition (wt%) of sample F1.
ElementAtomic Number (Z)Normalized Mass (%)
O853.76
Si1426.14
Al137.82
K193.82
Na112.81
Ca201.90
Fe261.68
N71.56
Mg120.54
Sum100.00
Table 5. Estimated contents of basic oxides (K2O, CaO, MgO) and base sum (wt%) from normalized mass in the analyzed samples.
Table 5. Estimated contents of basic oxides (K2O, CaO, MgO) and base sum (wt%) from normalized mass in the analyzed samples.
OxideSample F6Sample F1
K2O (%)7.549.20
CaO (%)2.032.66
MgO (%)0.950.90
Base sum (%)10.5112.76
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Foletto, J.P.; Nobre, A. Assessments on the Potential Use of Rhyolite Filler as a Soil Remineralizer in Agroecological Practices in the Fourth Colony of Italian Immigration, Rio Grande do Sul, Brazil. Sustainability 2025, 17, 8955. https://doi.org/10.3390/su17198955

AMA Style

Foletto JP, Nobre A. Assessments on the Potential Use of Rhyolite Filler as a Soil Remineralizer in Agroecological Practices in the Fourth Colony of Italian Immigration, Rio Grande do Sul, Brazil. Sustainability. 2025; 17(19):8955. https://doi.org/10.3390/su17198955

Chicago/Turabian Style

Foletto, João Pedro, and Augusto Nobre. 2025. "Assessments on the Potential Use of Rhyolite Filler as a Soil Remineralizer in Agroecological Practices in the Fourth Colony of Italian Immigration, Rio Grande do Sul, Brazil" Sustainability 17, no. 19: 8955. https://doi.org/10.3390/su17198955

APA Style

Foletto, J. P., & Nobre, A. (2025). Assessments on the Potential Use of Rhyolite Filler as a Soil Remineralizer in Agroecological Practices in the Fourth Colony of Italian Immigration, Rio Grande do Sul, Brazil. Sustainability, 17(19), 8955. https://doi.org/10.3390/su17198955

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