Comparative evaluation of indigenous glauconitic Nanopotassium and Muriate of potash on soil potassium availability: An incubation study

Introduction

Potassium (K) is a primary macronutrient essential for maintaining soil fertility and ensuring sustainable agricultural productivity. Within the soil ecosystem, K exists in four distinct pools: soil solution K, exchangeable K, interlayer K, and mineral or structural K. While structural K constitutes 90-98% of total soil potassium, the soil solution K the only form immediately available for plant uptake represents merely 0.1-0.2%. Continuous agricultural intensification combined with lack of K fertilization has led to excessive mining of native soil K reserves, resulting in a gradual decline in soil potassium fertility. Furthermore, the Indian fertilizer market is almost entirely dependent on imported soluble potassium salts, such as muriate of Potash (MOP), importing approximately 4 million tonnes annually due to a lack of high-grade domestic K reserves (Ministry of Chemicals and Fertilizers-GOI, 2022) [1] . Glauconite as an indigenous alternative considering the global demand and supply uncertainties, exploring locally available alternate resources is imperative. India possesses significant deposits of glauconite across various sedimentary basins, it was estimated that India has 3,149 million tonnes of glauconite reserves (Ministry of Mines-GOI, 2024) [2] . Glauconite is a naturally occurring, dioctahedral, potassium and iron-rich micaceous clay mineral with the generalized chemical formula: of (K, Na, Ca) (Fe 3+ , Fe 2+ , Al, Mg) 2 (Al, Si) 4 O 10 (OH) 2 . nH 2 O Containing approximately 4-8% K 2 O (Odin and Matter, 2003) [3] . Glauconite represents a massive indigenous mineral resource (Chand and Swami, 2000) [4] . However, its historically poor structural organization and slow-release nature in its native granular form have limited its widespread adoption compared to highly soluble commercial K fertilizers (Annapurna et al., 2017) [5] . Nanotechnological intervention might help in effective usage of glauconite and improve its nutrient release efficiency, nanotechnology offers a significant technical advantage. By manipulating materials at the atomic and molecular scale (typically 1-100 nm), nanomaterials exhibit drastically increased surface areas and enhanced physicochemical reactivity. Transforming bulk glauconite into glauconitic nanopotassium can fundamentally alter its dissolution kinetics, allowing for a controlled, highly efficient release of potassium that minimizes leaching losses and sustains the available soil K pool over time. However, the precise temporal availability of potassium within the soil matrix as influenced by glauconite nanopotassium are not yet fully documented. Therefore, an incubation study was conducted to evaluate the potassium availability patterns of glauconitic nanopotassium in comparison with commercial source MOP, therefore assessing its contribution towards a sustainable alternative to conventional K fertilizers.

Material and Methods

Soil collection and processing

The soil used for the laboratory incubation study was collected from the top 30 cm layer Upon collection, the soil was manually processed and passed through a 2-mm standard sieve to ensure uniform particle size prior to use in the incubation experiment. Prior to the imposition of treatments, the initial physicochemical properties of the revealed that soil was sandy loam in texture, neutral in reaction with a pH of 6.9, a low EC of 0.17 dS m -1 , a low OC of 0.27%. Low levels of available N (85.7 mg kg -1 ) and low available P 2 O 5 (17.0 mg kg -1 ), whereas the available potassium K 2 O (ammonium acetate extractable) was categorized as medium at 83.0 mg kg -1 .

Experimental design and incubation setup

The 90 days incubation experiment was aimed to compare conventional muriate of potash (MOP) with glauconite nano potassium and different combinations of these two fertilizers, therefore we have procured MOP (58% K 2 O) from local market sources and Glauconite nanopotassium was synthesized and procured from nanotechnology laboratory, RARS-ANGRAU, Tirupati (fig. 1), the estimated K 2 O content of glauconite nanopotassium was 6.95% using four acid digestion (Chao and Sanzolone 1992) [6] . And the size of the nano particle was around 89 nm (TEM image as presented in fig. 2). The dosage of fertilizer was calculated as to supply 50 kg K 2 O ha -1 to the 250 g of the processed soil, 100% MOP require 9.6 mg of MOP and 100% glaconite NanoK require 79.7 mg, the treatments were designed in such a way that 10% replacement at every treatment interval along with an absolute control. The experiment was laid out in a Completely Randomized Design (CRD) comprising 12 treatments with three replications (total of 36 experimental units). For each unit, the processed soil was taken into a 500 ml capacity plastic container equipped with a basal drainage hole to allow for the leaching of excess water. staggered mixing protocol was employed. thoroughly mixed with the respective treatment dose using a glass rod. This moisture level was continuously monitored and maintained throughout the 90-day incubation period. The containers were incubated in a controlled greenhouse environment, with ambient temperatures maintained between 27 °C and 30 °C and relative humidity ranging from 65% to 75%. Samples were collected and analysed for available potassium at scheduled intervals (0, 15, 30, 45,60, 90 days) over the 90-day duration.

Results and Discussion

The 90-day incubation experiment revealed that conventional MOP provided a rapid increase in potassium availability during the initial stages, whereas treatments containing higher proportion of glaconite based nano potassium maintained greater potassium availability during the later stages of incubation. The data presented in table 1. and fig. 3. Meanwhile, the integrated treatments began to show the gradual contribution of the nano-source. For example, T 8 (4.8 mg MOP + 39.85 mg glauconite nanoK) increased steadily to reach 94.81 mg kg -1 , while the pure nano-source T 3 showed a modest but steady rise to 87.21 mg kg -1 . This controlled release mechanism is highly advantageous, as it mitigates the risk of rapid fixation while initiating a steady replenishment of the soil solution pool.

The results are conclusive with Bharti (2022) [7] . The 30-day interval marked a critical transitional phase in the potassium release pattern. While T 2 remained high, it exhibited its first notable decline from the day-15 peak, dropping to 99.94 mg kg -1 . Similar declines were noted in other high-MOP treatments, with T 11 dropping to 95.86 mg kg -1 and T 12 to 96.23 mg kg -1 . This reduction indicates the gradual depletion of the readily available potassium pool and the onset of fixation into non-exchangeable forms. In stark contrast, treatments containing higher proportions of glauconitic nanopotassium continued a steady upward trajectory. Treatment T 10 peaked in this window at 101.93 mg kg -1 , while T 9 increased to 97.97 mg kg -1 , and T 5 rose to 92.76 mg kg -1 . This progressive increase highlights that the glauconite nanoparticles, vastly enhanced by their mechanically reduced size and increased surface area, were actively weathering and continuously replenishing the soil solution with K⁺ ions, buffering the decline seen in purely soluble sources. Similar results were reported by Rakesh et al.

(2020) [8] . At 45 days, the influence of conventional MOP continued to wane as the sustained release from the nano-fertilizer became increasingly prominent. The highest available K 2 O content firmly shifted away from the 100% MOP treatment to the integrated applications, with T 10 reaching its absolute peak at 102.13 mg kg -1 and T 9 maintaining a high availability of 97.35 mg kg -1 . Meanwhile, T 2 declined further to 98.65 mg kg -1 . This stage illustrates the distinct agronomic advantage of partial substitution. Treatments successfully combined the immediate availability of a reduced MOP dose with the sustained, steadystate dissolution of the nanopotassium. As the initial MOP flush was fixed or depleted, the massive specific surface area and elevated reactive sites of the nano-glauconite ensured a continuous mineral weathering process, maintaining optimum potassium availability exactly when conventional sources began to falter, aligning with Bhattacharyya and Poonia (1996) [9] . The 60-day sampling reinforced the shifting dominance toward the integrated and nano-rich treatments. Treatment T 9 recorded the highest availability at 101.08 mg kg -1 , while T 6 (2.88 mg MOP + 55.79 mg glauconite nanoK) climbed steadily to 96.24 mg kg -1 . Conversely, the purely conventional T 2 suffered a dramatic drop to 88.71 mg kg -1 , representing a massive depletion from its earlier peaks. The accelerated decline of MOP treatments compared to the steady climb of the nanopotassium treatments clearly demonstrates that the high surface energy of the nano-sized particles facilitates a prolonged, steady equilibrium between the fixed, exchangeable, and solution pools of soil potassium. By 75 days of incubation, the limitations of highly soluble, conventional fertilizers under prolonged conditions became glaringly evident. Treatment T 2 recorded only 86.33 mg kg -1 , nearly returning to its Day 0 baseline. In contrast, the heavily nano-reliant treatments surged to the forefront. Treatment T 5 secured the highest available K 2 O at 97.82 mg kg -1 . Notably, other nano-heavy treatments remained highly elevated: T 6 reached 96.60 mg kg -1 , and the pure-nano T 3 climbed to 94.26 mg kg -1 . The severe decline in MOP-dominated treatments can be directly attributed to the irreversible fixation of unprotected potassium into clay lattices over time. The glauconitic nanopotassium, however, acts as a protective mineral reservoir; its structural potassium is slowly liberated only as the nanoparticle weathers, effectively bypassing the rapid fixation pathway and maintaining a higher net availability in the soil solution.

At the end of incubation (90-days), the sustained release capability of glauconitic nanopotassium was unequivocally established. The purely nano-based Treatment T 3 (79.7 mg glauconite nanoK) recorded the highest available K 2 O at an impressive 98.78 mg kg -1 , which was nearly 19% higher than the control (82.89 mg kg -1 ). It remained statistically at par with a suite of highly nano-enriched treatments, including T 4 (96.80 mg kg -1 ), T 5 (96.24 mg kg -1 ), and T 6 (95.83 mg kg -1 ). These treatments significantly outperformed the conventional T 2 , which had completely degraded down to 85.64 mg kg -1 , a value lower than its day 0 starting point. The 90-day trajectory confirms that mechanically reducing glauconite to the nanoscale fundamentally alters its dissolution profile. By exponentially increasing the specific surface area, the nanoparticles permit a slow, continuous release of K⁺ that perfectly mirrors the concept of a "smart" slow-release fertilizer. This minimizes wasteful fixation and ensures a steady nutrient supply for extended durations, making partial or full substitution of MOP with glauconitic nanopotassium a highly efficient and sustainable soil management strategy. These results are in strong correlation with Praveen (2018) [10] . ). The study highlights the agronomic superiority of an integrated approach. Treatments substituting 50% to 70% of MOP with glauconitic nanopotassium proved highly effective, balancing the immediate K release from MOP with the steady, long-term mineral dissolution of the nano-source. Ultimately, the substitution of imported, highly soluble K salts with indigenous glauconitic nanopotassium presents a highly viable and ecologically sustainable soil management strategy.

References

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