Silicate rock remineralizers sustain soil potassium availability and irrigated rice yield in subtropical agroecosystems

Technical Summary: Silicate Rock Remineralizers in Subtropical Irrigated Rice Systems

Problem Statement
Brazilian irrigated rice production, a critical component of national food security, faces significant challenges regarding potassium (K) management. The hydromorphic soils of southern Brazil, primarily Albaqualfs, possess low cation exchange capacity (CEC) and insufficient native K reserves to sustain high yields (often exceeding 8 Mg ha⁻¹). Consequently, the agricultural system relies heavily on mineral fertilization. Currently, approximately 80% of fertilizers used in Brazil are imported, with potassium chloride (KCl) being the dominant K source. This dependence creates economic vulnerability due to rising international costs and environmental concerns associated with highly soluble fertilizers, such as leaching losses. While silicate rock powders (remineralizers) offer a potential alternative by providing K alongside other nutrients and improving soil chemistry, field data validating their efficacy as a direct replacement for KCl in subtropical irrigated rice systems remains scarce.

Methodology
To address this knowledge gap, a field experiment was conducted during the 2023/24 growing season across four sites in southern Brazil (Capão do Leão, Rio Grande, and Camaquã, Rio Grande do Sul). The study utilized a randomized complete block design with four replications to evaluate six potassium fertilization treatments:

1. T1: No K fertilization (Control).
2. T2: Potassium Chloride (KCl) (Conventional management).
3. T3: Phonolite.
4. T4: Syenite.
5. T5: Nepheline syenite.
6. T6: Mica schist.

The silicate rock treatments (T3–T6) were applied at rates equivalent to three times the recommended KCl rate based on total K content, accounting for their low water solubility (<1%) compared to the 100% solubility of KCl. Application timing differed: KCl was applied at the V4 developmental stage, while silicate remineralizers were incorporated 15 days prior to sowing a winter cover crop (Persian clover) to extend soil interaction and weathering time.

Key assessments included:

• Soil Analysis: Quantification of exchangeable K (via Mehlich-1 and Ammonium Acetate) and total K (via X-ray Fluorescence) at 0–0.20 m depth. Mineralogical composition of the rocks was determined via X-ray Diffraction (XRD).
• Plant Analysis: Measurement of Nitrogen (N) and K concentrations in aboveground biomass (AGB), plant height, and chlorophyll index (SPAD) at specific phenological stages.
• Yield and Balance: Grain yield (GY) was measured at physiological maturity. A soil K balance was calculated by comparing K inputs (fertilizer) against K outputs (grain export) and changes in soil exchangeable K stocks.

Key Results

• Soil Potassium Availability: The application of silicate remineralizers significantly increased soil exchangeable K (Mehlich-1) concentrations by an average of 128% compared to KCl across the sites. While total soil K remained unaffected by treatments, the alternative sources successfully maintained or enhanced the labile K fraction.
• Plant Nutrition and Physiology: Nitrogen concentrations in biomass were unaffected by K source. However, K concentrations in AGB were significantly higher in the KCl (T2) and Mica Schist (T6) treatments (approx. 34 g kg⁻¹), representing a 55% increase over other treatments. Plant height and chlorophyll index showed site-specific responses; notably, T5 and T6 increased plant height at L1, and T6 resulted in the highest chlorophyll index at L2 and L4.
• Grain Yield: Rice grain yield was maintained at levels comparable to conventional KCl fertilization (~8.5 Mg ha⁻¹) across most sites. While performance varied by location (e.g., T5 yielded highest at L2 and L3, while T6 yielded highest at L4), the alternative sources generally did not compromise productivity relative to the control or KCl.
• Potassium Balance: The system exhibited a positive K balance (19–40 kg ha⁻¹) across all sites. The remineralizers contributed to a more favorable soil exchangeable K balance (>100% improvement over KCl in some metrics), suggesting effective accumulation of available K in the soil profile.

Significance and Claims
The authors conclude that silicate rock powders—specifically phonolite, nepheline syenite, and mica schist—constitute viable alternatives to conventional KCl fertilization in subtropical irrigated rice systems. The study demonstrates that these materials can sustain crop productivity and nutritional status while offering distinct agronomic advantages:

1. Enhanced Soil K Balance: Unlike highly soluble KCl, which is prone to leaching, the gradual release of K from silicate minerals appears to synchronize nutrient supply with crop demand, leading to greater accumulation of exchangeable K in the soil.
2. Sustainability and Resilience: Utilizing locally available rock residues reduces dependence on imported fertilizers, lowers production costs, and promotes the valorization of mining by-products.
3. System Stability: The results suggest that these remineralizers can support long-term soil fertility by acting as a reservoir for structural K, particularly in systems where redox conditions (flooding) may facilitate mineral weathering.

The paper maintains a modest tone regarding long-term implications, noting that while the single-season results are promising, the full benefits of mineral weathering and cumulative soil fertility improvements may require evaluation over successive cropping cycles. The findings highlight the potential for these sources to contribute to more resilient and sustainable agri-food systems in regions dependent on imported nutrients.