Influence of zeolite and fertilizer dosages on macronutrient status of soil under incubation

Introduction

Nutrient management is one of the most critical determinants of soil fertility and crop productivity in tropical agricultural systems. Continuous cultivation with imbalanced fertilizer use has led to progressive nutrient depletion, while applied nitrogen, phosphorus and potassium are subject to substantial losses through leaching, volatilization and fixation, particularly in coarse-textured, acidic soils (Baligar et al., 1986) [1] . Fertilizer-use efficiency under tropical conditions is generally below 50 per cent for nitrogen, less than 10 per cent for phosphorus and around 40 per cent for potassium (Baligar et al., 1986) [1] and about half of the world's agricultural soils are reported to be deficient in phosphorus (Lynch, 2011) [2] . Zeolites are naturally occurring crystalline hydrated aluminosilicate minerals of alkali and alkaline-earth metals, built up of a three-dimensional framework of AlO 4 and SiO 4 tetrahedra that generates a system of channels, cavities and pores of molecular dimensions (Ming et al., 1989; Ramesh et al., 2014) [3,4] . Isomorphous substitution of aluminium for silicon in the framework imparts a permanent negative charge with a marked preference for monovalent cations, conferring on zeolites a very high cation exchange capacity (CEC), high water-holding capacity in the free channels and high adsorption capacity (Vaughan, 1978; Mumpton, 1999) [6,5] . These properties enable zeolites to function as carriers and controlled-release dispensers of plant nutrients, thereby improving nutrient-use efficiency and soil fertility (Sangeetha et al., 2016; Jakkula et al., 2018) [8,7] . The agronomic relevance of these mechanisms is amplified in Alfisols, the soil order to which the sandy loam of the present study belongs, since their inherently low native cation exchange capacity and pronounced susceptibility to nutrient leaching under intense tropical rainfall make them especially reliant on amendments capable of expanding the exchange complex. Working specifically in semi-arid Alfisols, Ramesh et al. (2024b) [9] demonstrated that locally sourced mordenite zeolite, characterized by a cation exchange capacity several-fold higher than that of the unamended soil, markedly improved the water-and nutrient-use efficiency of NH 4 attributing the response to the combined action of high specific surface area, internal void structure and moisture-retentive porosity of the mineral. The selectivity of the zeolite framework for particular cations is not uniform but follows a defined exchange series governed by ionic radius, hydration energy and charge density, so that monovalent ions of intermediate hydrated radius such as NH 4 + and K + are held preferentially over more strongly hydrated or more weakly charged species (Wang et al., 2010) [10] . This selective, reversible exchange behaviour, rather than irreversible fixation, is what permits zeolite to function simultaneously as a nutrient reservoir and a slow-release dispenser, distinguishing its mode of action from that of conventional soil conditioners such as farmyard manure or lime. Laboratory incubation studies provide a controlled means of characterizing the nutrient-release behaviour of soil amendments independent of the confounding influences of weather, irrigation and crop uptake that operate under field conditions. Zeolite has been reported to significantly enhance available N, P and K over a 90-days incubation in tropical Ultisols (Ramesh et al., 2024a) [11] , to reduce ammonia volatilization and improve retention of exchangeable ammonium in sandy loam soil (Rabai et al., 2012) [12] , to raise soil pH and favours moderately labile, slow-release phosphorus fractions in acid soils when combined with phosphatic fertilizers (Hasbullah et al., 2020) [13] and to increase the adsorption of NH 4 + and K + while lowering their desorption, in addition to improving soil pH buffering capacity (Palanivell et al., 2021) [14] . However, information on the combined influence of graded zeolite rates with varying fertilizer dosages on the release pattern of all three major nutrients in the acid sandy loam soils typical of the Southern Transition Zone of Karnataka is limited. The present investigation was therefore undertaken with the objective of studying the influence of zeolite and fertilizer dosages on the macronutrient (nitrogen, phosphorus and potassium) status of soil under incubation, so as to identify the rate of zeolite application that most effectively conserves and sustains nutrient availability prior to its evaluation under field conditions.

Material and methods

Experimental site

The incubation study was conducted at the College of Agriculture, Navile campus, Keladi Shivappa Nayaka University of Agricultural and Horticultural Sciences (KSNUAHS), Shivamogga (13° 58' to 14°1

' N latitude and 75° 34' to 75° 42' E longitude; altitude 650 m above mean sea level), which falls under the Southern Transition Zone (Zone 7) of Karnataka, during 2026.

Experimental design and treatments

The incubation study was laid out in a Completely Randomized Design (CRD) with eleven treatments replicated thrice, using 1 kg of processed soil per treatment maintained at field capacity moisture throughout the incubation period. The treatments imposed, on the basis of the recommended dose of fertilizer (RDF: 25:50:25 kg N:P 2 O 5 :K 2 O ha -1 ) applied through urea, single super phosphate and muriate of potash, were as follows: T 1 -Absolute control; T 2 -100% RDF; T 3 -75% RDF; T 4 -100% RDF + zeolite @ 50 kg ha -1 ; T 5 -100% RDF + zeolite @ 75 kg ha -1 ; T 6 -100% RDF + zeolite @ 100 kg ha -1 ; T 7 -100% RDF + zeolite @ 150 kg ha -1 ; T 8 -75% RDF + zeolite @ 50 kg ha -1 ; T 9 -75% RDF + zeolite @ 75 kg ha -1 ; T 10 -75% RDF + zeolite @ 100 kg ha -1 ; T 11 -75% RDF + zeolite @ 150 kg ha -1 .

Initial soil and zeolite characterization

Bulk surface soil was collected from the Southern Transitional Zone of Karnataka (Agro-Climatic Zone VII), air-dried under shade, powdered and sieved through a 2 mm mesh. The processed soil was sandy loam in texture with an acidic reaction (pH 5.80), electrical conductivity of 0.31 dS m -1 and organic carbon content of 4.02 g kg -1 . The initial available N, P 2 O 5 and K 2 O status of the soil was 323.80, 37.03 and 305.02 kg ha -1 , respectively and the cation exchange capacity was 15.20 cmol (p + ) kg -1 (Table 1). The zeolite used was procured from Global Earths Chemicals, Vijayapura, Karnataka. Characterization was performed after acid digestion (HNO 3 -HClO 4 , supplemented with HF for silicon estimation), which revealed an alkaline reaction (pH 7.68), electrical conductivity of 0.08 dS m -1 , organic carbon of 4.84 g kg -1 , a very high cation exchange capacity of 120 cmol (p + ) kg -1 and Ca, Mg, Al 2 O 3 and SiO 2 contents of 5.20, 1.20, 13.00 and 60.00 per cent, respectively (Table 2).

Results and discussion

Available nitrogen status of soil under incubation

Available nitrogen did not differ significantly among treatments at 5 and 10 DAI. At 5 DAI, the numerically highest available N (347.70 kg ha -1 ) was recorded in T 7 (100% RDF + 150 kg ha -1 zeolite) and the lowest (322.27 kg ha -1

) in T 1 . At 10 DAI, the numerically highest value (359.20 kg ha -1 ) was recorded in T 2 (100% RDF) and the lowest (329.53 kg ha -1 ) in T 8 . From 15 DAI onward, treatment differences became significant. At 15 DAI, T 7 recorded the highest available N (355.63 kg ha -1 ), on par with T 6 (354.97 kg ha -1 ) and T 10 (352.10 kg ha -1 ); the lowest (314.93 kg ha -1 ) was recorded in T 1 . At 30 DAI, T 7 remained highest (346.67 kg ha -1 ), on par with T 6 (342.37 kg ha -1 ) and T 11 (342.23 kg ha -1 ), while T 1 recorded the lowest (314.57 kg ha -1 ). At 60 DAI, T 6 recorded the highest available N (341.47 kg ha -1 ), on par with T 7 (336.80 kg ha -1 ) and T 11 (336.23 kg ha -1 ), whereas T 1 remained lowest (313.30 kg ha -1 ). Notably, every zeolite-amended treatment maintained available N above the initial soil status (323.80 kg ha -1 ) throughout the 60-day incubation, while T 1 remained below this baseline at every interval (Table 3). The failure of treatments to differ significantly in available nitrogen up to 10 DAI, followed by significant divergence from 15 DAI onward, is itself the most instructive feature of the incubation study and constitutes direct evidence of slow-release behaviour. In the first ten days, the mineral nitrogen released from urea hydrolysis was comparable across all fertilized treatments; as time advanced, the unamended treatments lost nitrogen through volatilization and nitrification-driven mobility, whereas the zeolite-amended treatments retained it. The mechanism underlying this retention is the selective adsorption of ammonium ions within the tetrahedral aluminosilicate framework of the mineral. Zeolites are crystalline hydrated aluminosilicates whose three-dimensional network of channels, cavities and pores accommodates exchangeable cations (Ming et al., 1989) [3] and the isomorphous substitution of aluminium for silicon generates a permanent negative charge with a marked preference for monovalent cations such as NH 4 + and K + (Vaughan, 1978) [6] . Ammonium adsorption by clinoptilolite increases with soil pH in the range 4 to 7 and with the initial ammonium concentration of the medium (Kithome et al., 1999) [15] . Clinoptilolite added to a sandy loam soil has been reported to reduce ammonia loss appreciably by favouring retention of ammonium over its conversion to ammonia gas, so that zeolite-amended soils held higher available nitrogen at the end of incubation than urea applied alone (Rabai et al., 2012) [12] , a finding closely mirrored by the present results. Amendment of light-textured soils with clinoptilolite has likewise been shown to curtail nitrogen losses through both ammonia volatilization and nitrate leaching by enhancing ammonium retention and microbial immobilization (He et al., 1999) [16] and a 90day incubation in tropical acid soils demonstrated that zeolite significantly increased available N, P and K, offering a route to reduced dependence on chemical fertilizers (Ramesh et al., 2024a) [11] . A point of considerable practical relevance emerges on comparing the incubation values with the initial available nitrogen status of the soil (323.80 kg ha -1 ). T 3 (75% RDF), which received a reduced fertilizer dose without any amendment, held only 318.73 kg ha -1 at 60 DAI, whereas T 11 (75% RDF + 150 kg ha -1 zeolite) at the identical reduced fertilizer dose held 336.23 kg ha -1 , statistically on par with the best treatment. This demonstrates that the amendment compensated, in terms of retained nitrogen, for a twenty-five per cent reduction in fertilizer dose, consistent with the finding that zeolite addition along with half the recommended nitrogen and potassium reduced fertilizer cost without compromising crop performance (Zhaohui et al., 2013) [17] .

Available phosphorus status of soil under incubation

Unlike nitrogen, available phosphorus differed significantly among treatments from 5 DAI itself and at every subsequent interval (Table 4). At 5 DAI, T 6 (100% RDF + 100 kg ha -1 zeolite) recorded the highest available P (57.83 kg ha -1 ), on par with T 7 (57.27 kg ha -1 ) and T 5 (56.12 kg ha -1 ); the lowest (36.39 kg ha -1 ) was recorded in T 1 . At 10 DAI, T 6 remained highest (64.70 kg ha -1 ), on par with T 7 (64.65 kg ha -1 ) and T 4 (64.47 kg ha -1 ); T 1 recorded the lowest (34.33 kg ha -1 ). At 15 DAI, T 6 recorded 73.28 kg ha -1 , on par with T 7 (71.15 kg ha -1

); T 1 recorded the lowest (34.05 kg ha -1 ). Available phosphorus rose progressively up to 30 DAI in all nutrient-applied treatments, the highest value (81.94 kg ha -1 ) being recorded in T 7 , on par with T 6 (81.60 kg ha -1 ) and T 11 (78.18 kg ha -1 ); the lowest (32.57 kg ha -1 ) remained in T 1 . Thereafter, available P declined marginally by 60 DAI in all treatments, with T 6 recording the highest value (77.35 kg ha -1 ), on par with T 7 (77.33 kg ha -1 ) and T 11 (74.83 kg ha -1 ), while T 1 continued to decline to its lowest value (31.52 kg ha -1 ). In contrast to the nutrient-applied treatments, T 1 showed a continuous decline throughout the incubation period, from 36.39 kg ha -1 at 5 DAI to 31.52 kg ha -1 at 60 DAI. Available phosphorus rose progressively up to 30 DAI in all nutrientapplied treatments and declined thereafter, reflecting the dissolution of applied single super phosphate together with mineralization processes up to 30 DAI, followed by the onset of fixation by iron and aluminium oxides characteristic of acid soils. The important observation is that the extent of this decline was much smaller in zeoliteamended treatments; between 30 and 60 DAI, T 6 lost only 4.25 kg ha -1 of available phosphorus, whereas T 2 (100% RDF, without amendment) lost 7.78 kg ha -1 . Three mechanisms explain this protective effect. First, clinoptilolite amendment of acid soils raises soil reaction and lowers exchangeable aluminium and titratable acidity and although immediate phosphorus availability is not always altered, zeolite favours accumulation of moderately labile aluminium-, iron-and calcium-bound phosphorus fractions that behave as slow-release reserves rather than irreversibly fixed forms (Hasbullah et al., 2020) [13] . Second, the exchange-induced dissolution mechanism operating in zeolite-phosphate systems releases phosphorus gradually in response to plant demand rather than in a single flush (Handreck, 1997) [18] and clinoptilolite has been shown to lower the initial rate of phosphorus release while raising the average rate of dissolution over the whole period (He et al., 1999) [16] . Third, ammonium-and potassium-saturated clinoptilolite increases phosphorus solubility while si-multaneously supplying NH 4 + and K + to the soil solution, so that the amendment performs a dual nutritional function (Hua et al., 2006) [19] . The magnitude of the response observed here, with available phosphorus in T 6 at 60 DAI being nearly two and a half times that in T 1 , is consistent with the general finding that zeolites raise phosphorus availability from phosphatic sources through their ion-exchange properties (Jakkula et al., 2018) [7] .

Available potassium status of soil under incubation

Available potassium followed a pattern similar to that of available nitrogen, with treatments remaining statistically at par at 5 and 10 DAI and differing significantly from 15 DAI onward (Table 5). At 5 DAI, the numerically highest available K (324.43 kg ha -1 ) was recorded in T 7 and the lowest (301.83 kg ha -1 ) in T 8 . At 10 DAI, the highest value (327.40 kg ha -1 ) was recorded in T 5 and the lowest (305.63 kg ha -1 ) in T 9 . At 15 DAI, T 7 recorded the highest available K (317.20 kg ha -1 ), on par with T 6 (315.50 kg ha -1 ) and T 11 (313.47 kg ha -1 ); the lowest (287.47 kg ha -1 ) was recorded in T 1 . At 30 DAI, T 6 recorded the highest value (310.50 kg ha -1 ), on par with T 7 (308.23 kg ha -1 ) and T 11 (307.30 kg ha -1 ); T 1 recorded the lowest (284.73 kg ha -1 ). At 60 DAI, T 6 recorded the highest available K (303.10 kg ha -1 ), on par with T 7 (297.03 kg ha -1 ) and T 5 (296.97 kg ha -1

); T 1 remained the lowest (276.93 kg ha -1 ). Taken together, the incubation results show that all zeolite-amended treatments maintained available N, P and K above the initial soil status throughout the 60-day period, whereas the absolute control (T 1 ) declined continuously below the baseline for all three nutrients, confirming the nutrient conserving effect of zeolite in this acid sandy loam soil. Available potassium behaved much like available nitrogen, treatments remaining statistically indistinguishable at 5 and 10 DAI and differing significantly from 15 DAI onward. Potassium is lost from light-textured soils primarily through leaching and through fixation in the interlayer positions of clay minerals, its use efficiency being restricted to about 40 to 70 per cent of the applied quantity (Baligar et al., 1986) [1] . The strong preference of the zeolite framework for monovalent cations (Vaughan, 1978) [6] means that K + released from muriate of potash is held on the exchange complex of the amendment instead of moving down the profile. Direct evidence for this comes from sorption desorption studies in acid soils amended with graded rates of clinoptilolite, in which zeolite markedly increased adsorption of both NH 4 + and K + and, importantly, coupled higher potassium adsorption with lower potassium desorption, indicating that the mineral locks up the element against leaching while retaining it as a plant-accessible reservoir; the same study also reported improved pH buffering capacity, critical to macronutrient availability in weathered acid soils (Palanivell et al., 2021) [14] . Enhancement of available soil potassium and its subsequent uptake following zeolite application has also been documented (Kavoosi, 2007) [20] , as has an increase in the concentration of calcium, sodium and potassium in zeolite-treated soils (Ghazavi, 2015) [21] . The three properties of zeolite of greatest agricultural interest, namely high cation exchange capacity, high waterholding capacity in the free channels and high adsorption capacity (Mumpton, 1999) [5] , thus operated together in this incubation study to conserve all three major nutrients. These sorption-desorption patterns are consistent with controlled kinetic studies on clinoptilolite, which show that potassium desorption from zeolite-amended systems proceeds in two distinct phases, an initial rapid release lasting less than 200 hours followed by a much slower phase extending to several weeks, with the desorption rate accelerating in proportion to the calcium and ammonium concentration of the surrounding solution because these divalent and monovalent competitors progressively displace K + from exchange sites (Rezaei et al., 2009) [22] . Field-scale confirmation of this behaviour comes from a two-year trial in which incorporation of zeolite together with graded potassium fertilizer increased the cumulative potassium release rate constant by over 13 per cent relative to the unamended control, the effect being attributed to the mineral's capacity to intercept K + before it could be fixed in the interlayer positions of clay minerals or lost to leaching (Karim et al., 2022) [23] . The two-phase release pattern reported in these kinetic studies mirrors the trajectory observed in the present investigation, wherein available potassium in zeolite-amended treatments rose sharply during the first fortnight and thereafter declined only gradually, in contrast to the more precipitous decline recorded in treatments receiving fertilizer without the amendment.

Conclusion

The incubation study clearly demonstrated the slow and sustained nutrient-release behaviour of natural zeolite when combined with recommended fertilizer dosages in an acidic sandy loam soil. Available nitrogen and potassium did not differ significantly among treatments up to 10 DAI but diverged significantly from 15 DAI onward, while available phosphorus differed significantly from as early as 5 DAI and peaked at 30 DAI before declining marginally, with the decline being appreciably smaller in zeolite-amended soils. Treatments receiving 100 per cent RDF combined with 100 or 150 kg ha -1 zeolite (T 6 and T 7 ) consistently recorded the highest and statistically comparable levels of available N, P and K throughout incubation, while treatment T 11 (75% RDF + 150 kg ha -1 zeolite) demonstrated that a twenty-five per cent reduction in fertilizer dose could be compensated by zeolite application. All zeolite-amended treatments closed the incubation period with available N, P and K above the initial soil status, whereas the absolute control declined continuously below baseline. It may therefore be concluded that the application of 100 kg ha -1 zeolite along with the recommended dose of fertilizer is the most effective combination for conserving and sustaining the macronutrient status of acid sandy loam soils under incubation, providing a sound basis for its evaluation under field conditions.

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