Historically, the use of pesticides on the Djando Plateau in Mohéli, Comoros, has led to soil contamination, especially with high pesticide doses applied over the last 20 years. As summarized in Table 1 , this practice has resulted in significant environmental consequences, including soil degradation, biodiversity loss, and a decline in agricultural productivity. Despite this, agricultural practices continued with some improvements under the supervision of international assistance, but the long-term impact of pesticide overuse has prompted the need for a more sustainable solution.

The decline in soil health and the significant reduction in agricultural yields led to the exploration of new techniques for soil rehabilitation. As shown in Table 2 , local agricultural input distribution is still centralized, with a limited number of outlets for chemical products. This situation underlines the necessity of moving towards more sustainable practices and reducing dependency on chemical inputs.

In response to the issues highlighted by the historical pesticide use, a new method was developed to address the contamination of the soils on the Djando Plateau. The primary objective was to repair the damaged soil and restore its fertility to increase agricultural yields. The method involves using halloysite-rich soils that have been impacted by high pesticide doses over the past 20 years from the Djando Plateau in Mohéli, Comoros, were carefully collected, with over 6 kilograms of soil sampled from a depth range of 0 to 40 cm. The samples were then analyzed using the method in our previous publications [13] , (Said, Hu et al. 2021). For the analysis, 2.0 g of halloysite powder was milled in a planetary ball mill (Pulverisette-7, Fritsch, Germany), equipped with two zirconia pots (45 cm³ internal volume) containing 7 zirconia balls, each 15 mm in diameter. The milling time ranged from 0 to 180 minutes, while the rotational speed was fixed at 600 rpm. The halloysite powder was mixed with SO₂, and separately, with Ca(OH)₂. To evaluate the release characteristics, a 2% citric acid solution (Sinopharm Group Co Ltd., Shanghai, Analytical reagent) was used. In each experiment, 0.02 g of the milled soil (or soil mixed with SO₂ or Ca(OH)₂ was dispersed in 100 ml of distilled water or citric acid solution, respectively. The mixture was stirred on a magnetic stirrer (524 G, Meiyingpu Shanghai, China) for 120 minutes at 500 rpm under room temperature. The pH of the solutions was measured using a pH meter (Seven Compact, Mettler Toledo, Switzerland). The suspension was then centrifuged to remove the solids, and the concentration of potassium in the supernatant was measured using Atomic Absorption Spectrometry (AA 6880, Shimadzu, Japan) to calculate the K extraction rate. X-ray diffraction (XRD) characterization of the prepared samples was performed using a MAX-RBRU-200B diffractometer (Rigaku, Japan). Fourier Transform Infrared (FT-IR) spectra were recorded over the range of 4000 - 450 cm⁻¹ using a Nicolet 6700 spectrometer (Thermo Fisher, USA), with KBr as a diluent.

The halloysite soil is distinguished by a notable chemical composition, with proportions of 33% SiO₂, 28% Al₂O₃, and 18% Fe₂O₃ listed in Table 3 , reflecting its richness in silicate and aluminosilicate minerals. These elements play a crucial role in the soil’s reactivity and its ability to adsorb pesticides. Additionally, the content of K₂O (2.5%) and CaO (1.3%) indicates that the soil contains a moderate amount of nutrients. The loss on ignition (LOI) of 19% suggests a significant presence of organic matter in the soil. The interaction between the soil’s chemical composition and the prolonged application of pesticides influences nutrient availability and the overall reactivity of the soil. Pesticides typically come into contact with the soil, where they undergo various transformations, creating a complex pattern of metabolites [14] . Proper management of these factors is crucial to prevent excessive acidification and minimize long-term contamination risks.

The XRD peaks observed for the natural halloysite sample from the Comoros correspond to the planes (001), (100), (002), (110), and (300), which are consistent with the ICDD reference 00-029-1487 [15] are shown in Figure 1 . These results align well with the reference data, although intermediate peaks may be less pronounced or slightly shift due to geological variations and extraction methods. Notably, the peaks at 2θ = 12.0˚ and 20.1˚, associated with the (001) and (100) planes, confirm an interlayer spacing of 0.73 nm for the dehydrated form and 0.44 nm for the hydrated form, similar to the halloysite reported in the literature [16] .

The analysis of soil samples in distilled water and 2% citric acid, combined with varying milling durations, provided valuable insights into potassium (K) extraction ( Figure 2 ). The data indicate that potassium extraction efficiency increases progressively with milling time, reaching maximum levels after 180 minutes in both media. In distilled water ( Figure 2(a) ), untreated halloysite soil showed limited potassium extraction, ranging from 0% to under 5%, suggesting minimal potassium solubilization. Adding SO₂ resulted in a moderate increase in extraction, reaching approximately 10%, indicating a minor role of sulfur in altering the soil’s mineral composition. In contrast, calcium hydroxide Ca(OH)₂ treatment led to a significant enhancement in potassium extraction. Starting at 15%, it reached 34% after 180 minutes, which can be attributed to the alkalizing effect of Ca(OH)₂. This pH modification promotes the dissolution of potassium-bearing minerals, a phenomenon observed by [17] .

The trends observed in citric acid ( Figure 2(b) ) followed a similar pattern to those in distilled water, but with considerably higher extraction rates. Untreated soil in citric acid showed an increase from 3.6% to 24.5% over 180 minutes, likely due to the soil’s limited buffering capacity and increasing reactivity. The addition of SO₂ in citric acid accelerated potassium extraction, rising from 3.7% to 32.5%. This can be linked to sulfur’s catalytic effect on potassium mineral dissolution, as highlighted by [18] However, the Ca(OH)₂ treatment outperformed the others, showing a rapid increase from 3.8% to 60% within 150 minutes and peaking at 70% after 180 minutes. This substantial improvement is due to the neutralizing effect of Ca(OH)₂, which reduces soil acidity and promotes potassium release, a phenomenon also noted by [19] .

Regarding the milling duration, it was observed that extending the grinding time beyond 180 minutes resulted in a decrease in potassium extraction efficiency. This phenomenon can be attributed to the formation of ultra-fine particles, which, by increasing the soil's specific surface area, promote the re-adsorption of released potassium, thereby reducing its immediate availability. This behavior is crucial for establishing a connection with field conditions, where natural cycles of weathering and dissolution influence nutrient availability over extended periods. These findings highlight the importance of soil amendments, particularly Ca(OH)₂, in enhancing potassium availability. Furthermore, the increased potassium extraction in the presence of citric acid indicates that mineral dissolution mechanisms are strongly influenced by pH variations and cation complexation. The application of these treatments thus represents a promising strategy for optimizing soil fertility, especially in regions where potassium availability is a limiting factor for agricultural productivity.

Table 4 summarizes soil properties from various samples, highlighting differences in texture, nutrient availability, and pH levels. Sandy soils (S₁ and S₅) show lower cation exchange capacity (CEC) and poorer nutrient retention, which makes them less suitable for sustained agriculture without regular fertilization. In contrast, silty (S₂, S₅) and clayey soils (S₃) exhibit higher CEC, allowing them to retain more essential nutrients like calcium (Ca) and magnesium (Mg), promoting plant growth [19] [20] (Bolan et al., 2023). Soil pH ranges from slightly acidic to neutral (4.9 - 6.1), with slightly acidic soils typically providing optimal conditions for nutrient uptake (Takahashi et al., 2001). Organic carbon is more abundant in silty and clayey soils, fostering enhanced biological activity and better water retention [21] . Silty soils display moderate acidity, while clayey soils require management due to higher acid saturation. These results indicate that silty and clayey soils offer better long-term fertility, while sandy soils require more intensive nutrient management and fertilization to maintain productivity [22] .

Table 5 shows that the soils have a low cation exchange capacity (average 2.33 cmol (+) kg⁻¹), indicating limited nutrient retention. The average pH of 5.5 suggests moderately acidic conditions, which can affect nutrient availability. Organic carbon content is relatively low (0.94%), reflecting limited organic matter. Total acidity remains low (0.192 cmol (+) kg⁻¹), but the acid saturation level (16.4%) may still impact soil health. Overall, these properties highlight the need for soil amendments to enhance fertility and structure.

Figure 3 displays the phase transformation of halloysite with milling time (0 min, 90 min, and 180 min). X-ray diffraction (XRD) analysis of soil samples subjected highlights the progressive changes in the mineral structure caused by mechanical grinding. At 0 minutes (unprocessed soil): The XRD patterns exhibit well-defined peaks corresponding to the crystallographic planes of halloysite (001, 002, 110, and 300), indicating a well-organized structure with a high degree of crystallinity. This observation aligns with previous studies describing halloysite as a tubular mineral that remains structurally stable in the absence of mechanical stress [15] . 90 minutes of grinding: A significant reduction in peak intensity is observed, suggesting partial amorphization of the mineral structure. This attenuation is directly related to particle size reduction and the introduction of crystallographic defects due to repeated mechanical impacts. Similar findings have been reported in studies on the mechanical activation of clays [23] . At 180 minutes of grinding: The XRD peaks become increasingly diffuse, indicating an advanced state of amorphization. This trend suggests extensive disruption of halloysite’s crystalline structure, making the minerals more reactive to chemical transformations and ion exchange processes. [24] have documented similar observations regarding the effects of prolonged grinding on the reactivity of clay minerals.

The progressive breakdown of the crystalline structure suggests that prolonged grinding increases the material’s surface area and chemical reactivity, Similar results have been reported in studies on the mechanical activation of phlogopite mineral [25] . This transformation is particularly relevant for agricultural and environmental applications, as it can enhance the availability of nutrients previously trapped within the mineral matrix. Unlike previous findings that involved chemical amendments, these results indicate that grinding alone is sufficient to induce significant structural modifications in the soil, potentially facilitating the release of nutrients such as potassium without the need for additional chemical treatments [13] . The increasing structural disorder may also affect soil interactions with biological agents, particularly by altering nutrient adsorption capacity and influencing microbial dynamics [26] .

In agricultural contexts, this approach could be leveraged to enhance the efficiency of natural fertilizers, particularly by promoting the release of potassium and other essential elements without excessive reliance on chemical amendments.

Figure 4 illustrates the evolution of soil pH during grinding with three treatments: unamended soil, soil mixed with SO₂, and soil mixed with Ca(OH)₂ in distilled water at different milling minutes, indicating moderate acidification. This suggests limited buffering capacity, likely due to pesticide degradation over the past 20 years [27] . In contrast, soil mixed with SO₂ shows a more pronounced pH increase, rising from 2 to 6.2 over 180 minutes, likely due to sulfur neutralizing soil acidity. This pH range aligns with optimal conditions for most plants (QDEHP, 2017) [28] . The Ca(OH)₂ treatment results in the highest pH increase, from 4.5 to 7.6, due to its strong alkalizing effect. The reason may be highly alkaline conditions, silica dissolves further as Si(OH)₄, and aluminum may form soluble complexe. The dissociation of Ca(OH)₂ releases hydroxide ions, neutralizing acidity and improving soil conditions [29] . These results suggest that Ca(OH)₂ is highly effective in raising soil pH, with SO₂ providing a more gradual pH increase. Overall, these treatments significantly alter the soil pH, which in turn affects chemical reactivity and nutrient availability [30] . Under acidic or neutral conditions, halloysite slowly dissolves in water, leading to the formation of aluminum hydroxide (Al(OH)₃) and silicic acid (SiO₂·xH₂O), as shown in Figure 4 [30] . This suggests that Equation (1) cannot adequately explain the observed phenomena.

mAl₂SiO₃(OH)₄ + nH₂O = 2 m Al(OH)₃ + 2 m SiO₂·sH₂O. (1)

In contrast, the reaction between the aluminosilicate Al₂Si₂O₅(OH)₄ and Ca(OH)₂ can lead to the formation of calcium silicate hydrates (C-S-H) and aluminate compounds, which are commonly observed in pozzolanic reactions [30] - [32] . This general reaction can be written as follows (2):

mAl₂Si₂O₃(OH)₄ + nCa(OH) = xCaO·ySiO₂·zH₂O + Al(OH)₃ (2)

This reaction is particularly relevant in cement chemistry and soil stabilization, where calcium hydroxide reacts with silicate and aluminate phases to form hydrated compounds that enhance the mechanical properties of materials [30] .

The data in Table 6 show the production of cassava, tomato, and yam in two zones (inner and outer) of rectangular plots. Cassava yields are higher in both zones (4377 kg in the inner zone and 5879 kg in the outer zone), suggesting it is less affected by pesticides or benefits more from soil treatments [33] . Tomato yields are lower in the outer zone (2233 kg) compared to the inner zone (3897 kg), likely due to the harmful effects of pesticides on soil fertility. No yam yield is recorded in the inner zone, possibly due to nutrient deficiencies from pesticide use, whereas the outer zone shows a modest yield (1384 kg), indicating yam’s sensitivity to nutrient management. Pesticides disrupt soil chemistry, leading to nutrient blockages, particularly potassium [34] . Laboratory treatments with SO₂ and Ca(OH)₂ improved potassium extraction (32.5% with SO₂, 70% with Ca(OH)₂), boosting nutrient availability. These treatments also raised the soil’s pH, enhancing mineral extraction and nutrient solubility, which led to higher yields of cassava and tomatoes in treated areas.

Figure 5 illustrates the inner and outer zone by kg/ha. The graph clearly shows that yields are generally higher in the outer zone. Cassava yields are higher in the outer zone (979.83 kg/ha) compared to the inner zone (875.4 kg/ha). In contrast, tomato yields are slightly higher in the inner zone (556.71 kg/ha) than in the outer zone (558.25 kg/ha). Finally, yam yields are only recorded for the outer zone (125.82 kg/ha), possibly due to pesticide presence in the inner zones, which reduces soil fertility [35] . SO₂ and Ca(OH)₂ treatments appear to have improved soil reactivity and nutrient availability, which may explain the higher productivity in the outer zones, particularly for crops like cassava.

Fourier transformed infrared technique was also applied to compare their difference of raw material (sample A) and the milled sample at 180 min as shown in Figure 6 . The FTIR spectra of the untreated soil sample (before grinding), Figure 6(a) show several characteristic bands as 3347 cm⁻¹: Stretching of hydroxyl groups (−OH), likely associated with adsorbed water on the mineral surface and 1626 cm⁻¹: Bending of adsorbed water (H–O–H), similar to the 1642 cm⁻¹ band in the reference extract and 1024 cm⁻¹: Si–O stretching, representing vibrations in the Si–O bonds of silicate minerals and 576 cm⁻¹ [36] , Si–O–Si bending, possibly linked to network deformation and 469 cm⁻¹: Si–O–Si bending, associated with deformation of the SiO₄ framework, consistent with the 468 cm⁻¹ band in the reference [37] . After 180 minutes of grinding ( Figure 6(b) ), the 3347 cm⁻¹ band shifts to 3456 cm⁻¹, indicating a change in hydration state. This shift suggests that grinding affects the surface sites, potentially redistributing adsorbed water or altering the coordination of hydroxyl groups. The 1024 cm⁻¹ band before grinding and the 1030 cm⁻¹/1032 cm⁻¹ bands after grinding remain relatively stable, implying that the fundamental structure of the mineral is not significantly altered by grinding [38] . The characteristic Si–O vibrations, such as Si–O stretching and Si–O–Si bending, show only slight shifts, indicating that the overall network remains stable [39] . The new band at 913 cm⁻¹ after grinding suggests a modification of the hydration state, supporting earlier observations regarding the impact of grinding on deformation of the Al–OH bonds and adsorbed water [40] . Additionally, the absorption bands at 538 cm⁻¹ and 470 cm⁻¹ are attributed to the bending vibrations of Si–O–AlVI and Si–O–Si bonds respectively [40] - [42] showing small variations, though these changes are not significant in terms of structural modification.

This study investigates the effects of chemical treatments and grinding on the reactivity and nutrient availability of halloysite-rich soils from the Djando Plateau, Moheli, Comoros. Halloysite, a clay mineral with high cation exchange capacity, is highly susceptible to pesticide contamination, which can negatively affect soil fertility. In this study, sulfur dioxide (SO₂) and calcium hydroxide Ca(OH)₂ were used as soil amendments to enhance potassium (K) extraction, modify soil pH, and alter the mineral structure. The findings show that both SO₂ and Ca(OH)₂ significantly improve potassium extraction, with Ca(OH)₂ yielding the highest potassium release (up to 70%) after 180 minutes of grinding. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses indicate that grinding disrupts the halloysite mineral structure, leading to increased surface area and enhanced reactivity. The addition of Ca(OH)₂ notably increases soil pH, neutralizing acidity and promoting nutrient availability, thus improving soil fertility. These chemical treatments improve potassium solubility and can help address nutrient deficiencies in soils impacted by pesticide use. The study further reveals that modifications to the soil structure, such as grinding and chemical treatment, can improve nutrient availability, promoting long-term agricultural sustainability. The findings highlight the importance of soil amendments like Ca(OH)₂ and SO₂ for enhancing soil fertility in regions affected by pesticide use and emphasize their potential in improving crop yields and soil health.