Effect of Microbial Enriched Granulated Rock Phosphate on Performance of Cluster Bean (Cyamopsis tetragonoloba L.) and Soil Phosphorus Availability

Introduction

Phosphorus (P) ranks among the most critical yet poorly managed plant macronutrients. Although P is the eleventh most abundant element in the Earth's crust, barely 0.1 per cent of soil phosphorus is readily available to plants, and the balance is locked in sparingly soluble or fixed forms (Anandham, 2007) [3] . Nearly 67 per cent of the world's cultivated land is P-deficient (Dhillon et al., 2017) [10] , and in acidic tropical and sub-tropical soils, phosphate fixation by iron and aluminium oxides such as goethite, gibbsite and kaolinite (Penn and Camberato, 2019) [24] further limits its availability, so that only 10-25 per cent of applied phosphatic fertilizer is utilised by the crop in the season of application. Global demand for phosphatic fertilizer is met chiefly from high-grade rock phosphate (HGRP, > 13.1% P), reserves of which are finite and geographically concentrated. India's own reserves of about 200 million tonnes are overwhelmingly low-grade (LGRP, < 8.73% P), forcing the country to import roughly 7.5 million tonnes of HGRP every year (Anandham, 2007) [3] . Direct field application of LGRP is agronomically inefficient because of its poor solubility, but inoculation with phosphate-solubilizing microorganisms (PSM) bacteria such as Bacillus and Pseudomonas and fungi such as Trichoderma and Aspergillus can markedly accelerate its dissolution through the production of organic acids, siderophores and phosphatase enzymes (Farrag and Bakr, 2023) [11] , offering a low-cost, sustainable route to unlock indigenous LGRP reserves. Cluster bean (Cyamopsis tetragonoloba L.), known locally as 'gorikayi', is a drought-tolerant kharif legume of considerable economic importance in the dryland tracts of Karnataka, valued for its tender pods, fodder, green manure and guar-gum industrial value. Being a legume, cluster bean fixes atmospheric nitrogen symbiotically through Rhizobium, a process itself highly Pdependent, so adequate phosphorus nutrition is central to its nodulation, growth and pod yield (Sharif et al., 2013) [27] . Despite this, biologically enriched rock phosphate has been little explored as a P source for cluster bean, and the manner in which such enrichment alters soil P fractions (saloid-, Al-, Fe-, Ca-, reductant-soluble, occluded and organic-P) and P-fixing capacity over the crop cycle remains poorly documented for the acid soils of the hilly zone of Karnataka. Against this background, the present investigation was undertaken to evaluate the effect of microbial enriched granulated rock phosphate on (i) the growth and yield performance of cluster bean, (ii) nutrient concentration and uptake by the crop, and (iii) the status and dynamics of soil phosphorus, with a view to developing a cost-effective and environmentally sustainable phosphorus nutrition strategy for cluster bean cultivation in Pdeficient acid soils.

Materials and Methods

Experimental site, soil and season

The field experiment was conducted during Rabi 2025-26 at the Main Agricultural and Horticultural Research Station (MAHRS), Iruvakki, Shivamogga, which falls under the Hilly Zone (Agro-Climatic Zone-9) of Karnataka (14° 0' N, 75° 42' E; 620 m MSL). The experimental soil was sandy clay loam in texture, strongly acidic (pH 5.80), with an electrical conductivity of 0.567 dS m -1 , organic carbon 21.6 g kg -1 , available N 175.61, available P2O5 17.83 and available K2O 321.96 kg ha⁻¹, and an initial P-fixing capacity of 89.50 mg P fixed kg⁻¹ soil.

Treatments and design

The experiment was laid out in a Randomized Complete Block Design with eight treatments replicated thrice (gross plot 5 × 3.0 m; net plot 4.5 × 2.7 m). Cluster bean variety Shigra was sown at 45 × 25 cm spacing with FYM at 10 t ha -1 (common to all treatments except T1) and a recommended dose of fertilizer (RDF) of 25:75:60 kg N:P2O5:K2O ha⁻¹. The treatment details are given in Table 1. 50% RDP through microbially enriched GRP RDP = Recommended dose of phosphorus; GRP = Granulated rock phosphate. FYM and the recommended doses of N and K were common to all treatments except T1.

Preparation of microbial enriched granulated rock phosphate

Rock phosphate (total P 9.84%, water-soluble P 0.003%, citratesoluble P 1.23%; pH 8.40) was granulated (1-5 mm) using clay and humic acid as binder, and the granules were inoculated with a consortium of phosphate-solubilizing bacteria (PSB), potassium-mobilizing bacteria (KMB) and Azotobacter at 100 ml kg⁻¹. An accompanying 8-week incubation study monitored the survival of the three microbial groups on the granulated carrier; all three commenced at 10⁸ CFU g -1 , peaked in the third week (Azotobacter ~7×10⁸, PSB ~6×10⁸ and KMB ~5×10⁸ CFU g -1 ) and thereafter declined gradually, remaining above the recommended viability threshold of 10⁷ CFU g -1 up to 7-8 weeks.

Observations and analytical methods

Plant height, number of branches, leaf area index, fresh and dry weight were recorded at 30, 60, 90 and 120 days after sowing (DAS); yield attributes (pods plant -1 , pod yield plot -1 and ha -1 ) were recorded at harvest. Nitrogen, phosphorus, potassium and zinc concentration and uptake in plant tissue were estimated at all four growth stages following standard procedures. Soil samples drawn at 30, 60, 90 and 120 DAS were analysed for pH and EC (Jackson, 1973) [17] , organic carbon (Walkley and Black, 1934) [34] , available N (Subbaiah and Asija, 1956) [31] , available P (Bray and Kurtz, 1945) [7] , available K (flame photometry), exchangeable Ca and Mg (versenate titration), available S (turbidimetry), DTPA-extractable micronutrients (Lindsay and Norvell, 1978) [21] , P-fixing capacity (equilibration with 200 ppm P followed by Bray extraction) and inorganic P fractionation (modified Chang and Jackson procedure) into saloid-P, Al-P, Fe-P, Ca-P, reductant-soluble P, occluded P, organic P and total P.

Statistical analysis

Data were analysed by analysis of variance as described by Gomez and Gomez (1984) [13] , and treatment means were compared using critical difference (CD) at P = 0.05.

Results and Discussion

Growth parameters

Plant height, branching, leaf area index (LAI) and biomass accumulation were significantly influenced by microbial enrichment of GRP at every stage of crop growth (Table 2, Fig. 1). Treatment T7 (75% RDP through enriched GRP) recorded the tallest plants (124 cm at 120 DAS), the highest branching (12 plant -1 ) and the highest LAI (4.5), and was statistically on par with T6 (100% RDP through enriched GRP: 118 cm, 11 branches, LAI 4.3) at most stages, while the absolute control (T1) remained the shortest throughout (76 cm, 7 branches, LAI 2.0 at 120 DAS). The superiority of enriched GRP treatments, and in particular the fact that T7 outperformed T5 (125% RDP through enriched GRP) despite receiving a lower P dose, reflects sustained microbial mobilisation of P from GRP through organic-acid and phosphatase activity of the PSB-KMB-Azotobacter consortium, consistent with reports of enhanced plant height and biomass in cluster bean and other legumes following PSM-enriched rock phosphate or bio-processed RP application (Yadav and [35,8,33,5] . Adequate P nutrition supports cell division, root proliferation, cytokinin-mediated lateral branching and canopy photosynthetic capacity (Grant et al., 2001; Garg and Chandel, 2010) [14,12] , explaining the parallel improvement across all growth attributes in the enriched-GRP treatments.

Yield attributes

Pod number, pod yield plot -1 and pod yield ha -1 followed the same treatment order as the growth parameters (Table 2). T7 produced the highest pod yield (62.33 q ha -1 ), statistically on par with T6 (58.00 q ha -1 ) and significantly higher than the standard RDF (T2, 41.00 q ha -1 ) and the absolute control (T1, 28.33 q ha -1 ). Notably, T7 out-yielded RDF by roughly 52 per cent while supplying only 75 per cent of the recommended P dose, underlining the extent to which microbial solubilization can substitute for chemical P input. Similar yield advantages of rock-phosphate-enrichedcompost or PSB-inoculated rock phosphate over standard fertilizer doses have been reported in mung bean (Kumari et al., 2024) [20] , chickpea (Allah Ditta et al., 2018) [1] and cluster bean (Chauhan et al., 2017) [8] , attributed to progressive P release synchronised with crop demand and sustained availability through the reproductive phase, reducing flower and pod drop (Khan et al., 2007) [18] .

Nutrient concentration and uptake

Nitrogen, phosphorus, potassium and zinc concentration in plant tissue, and their total uptake, were significantly higher under microbially enriched GRP at every growth stage (Table 3, Figs. 234). At 120 DAS, T7 removed 113.29 kg N, 12.67 kg P and 54.57 kg K ha -1 and 142 g Zn ha -1 , values statistically on par with T6 and significantly greater than RDF (T2: 93.57 kg N, 10.01 kg P, 44.85 kg K ha -1 ) and the control (T1: 88.32 kg N, 9.39 kg P, 42.63 kg K ha -1 ). Because P is a prerequisite for ATP-driven nitrogenase activity in root nodules, enhanced P supply under enriched GRP directly supported greater biological nitrogen fixation and N accumulation (Peoples et al., 2009) [25] , while P-induced improvements in root architecture increased K uptake via mass flow (Haq and Mallarino, 2000) [15] and organic-acid-mediated rhizosphere chelation mobilised zinc (Alloway, 2008) [2] , collectively explaining the parallel gains across all four nutrients.

Available soil phosphorus

Soil available P2O5 declined progressively from 30 to 120 DAS in all treatments owing to continuous crop uptake (Table 4, Fig. 5). RDF (T2) recorded the highest available P at every stage (68.65 kg ha -1 at 120 DAS), reflecting the immediate solubility of chemical superphosphate. Among the GRP treatments, however, available P was consistently graded with the enrichment and dose: T5 (125% RDP enriched) retained the most available P among GRP treatments (53.48 kg ha -1 at 120 DAS), followed by T6 and T7, all substantially higher than their unenriched counterparts (T3, T4) and the absolute control (5.10 kg ha -1 ). This confirms that microbial solubilization released P from GRP progressively through the season rather than as an immediate flush, consistent with the biphasic release pattern documented for bio-processed rock phosphate (Vassilev et al., 2001; Biswas and Narayanasamy, 2006; Meena and Biswas, 2015) [33,5,22] .

3.5 Soil phosphorus-fixing capacity P-fixing capacity was consistently highest in the absolute control (88.6-89.8 mg P fixed kg -1 across stages) and was significantly and progressively lowered by microbial enrichment, reaching its minimum under T5 (61.3 mg P fixed kg -1 at 120 DAS), followed by T6 (64.9) and T7 (68.5 mg kg -1 ), while unenriched GRP treatments (T3, T4) remained closer to the control (Table 4, Fig. 6). This reduction is attributable to organic acids (citric, gluconic, oxalic) produced by the PSB-KMB-Azotobacter consortium, which compete with phosphate ions for sorption sites on Fe and Al oxide surfaces, thereby lowering fixation (Bolan et al., 1994; Hinsinger, 2001) [6,16] , a mechanism corroborated by reports of reduced P-fixation in laterite soils following inoculation with Aspergillus tubingensis and Trichoderma viride (Pradhan and Sukla, 2006) [26] .

Soil inorganic phosphorus fractions

Microbial enrichment of GRP produced a clear redistribution of soil P among inorganic fractions (Table 5, Fig. 7). Labile fractions -saloid-P, Al-P, Ca-P and organic-P were consistently highest under T5, followed by T6 and T7, while the recalcitrant, plantunavailable fractions Fe-P, reductant-soluble P and occluded P were correspondingly lowest under the same treatments and highest in the absolute control. At 120 DAS, for instance, T5 recorded the highest Saloid-P (5.10 mg kg -1 ), Al-P (122.4 mg kg - 1 ) and Ca-P (136.4 mg kg -1 ) alongside the lowest Fe-P (96.6 mg kg -1 ), against 0.60, 40.8 and 48.2 mg kg -1 (Saloid-P, Al-P, Ca-P) and 177.4 mg kg -1 Fe-P in the control. This pattern reflects organic-acid-mediated dissolution of iron-bound phosphate complexes and re-adsorption of the released phosphate onto more labile aluminium and calcium-associated forms, i.e. an active redistribution among P pools rather than a uniform response of all fractions, as similarly reported by Chen et al. (2006) [9] , Sharma and Kapoor (2016) [29] and Singh and Tripathi (2013) [30] . Total soil P was likewise better conserved under enriched GRP (521 mg kg -1 in T5 versus 442 mg kg -1 in T1 at 120 DAS), indicating that microbial enrichment enhances P availability while sustaining the overall soil P reserve for subsequent crops. 1. Sal-P = Saloid-bound P; Al-P = Aluminium-bound P; Fe-P = Iron-bound P; Ca-P = Calcium-bound P; Red-P = Reductant-soluble P; Occ-P = Occluded P; Org-P = Organic P.

Post-harvest soil fertility status

Post harvest soil pH, EC and organic carbon were not significantly affected by treatment (pH 5.8-6.6; EC 0.116-0.176 dS m -1 ; OC 1.82-2.58%), but available N and K, exchangeable Ca and Mg, and DTPA-extractable Fe, Mn, Cu and Zn were significantly higher under enriched GRP, particularly T5 (available N 198.4 kg ha⁻¹; available K2O 354.2 kg ha -1 ; exchangeable Ca 2.74 and Mg 1.08 cmol(p⁺) kg -1 ), against the lowest values in the control (142.4 kg N ha -1 ; 291.2 kg K2O ha -1 ). This improvement in residual fertility mirrors the enhanced biological nitrogen fixation, root biomass turnover and rhizosphere organic-acid activity associated with microbial enrichment (Peoples et al., 2009; Haq and Mallarino, 2000; Alloway, 2008) [25,15,2] , and is agronomically significant for the succeeding crop in the rotation.

Microbial population dynamics during incubation

The clay-humic acid granulated carrier sustained all three inoculated groups PSB, KMB and Azotobacter above the recommended minimum viable count of 10⁷ CFU g -1 for 7-8 weeks, with Azotobacter attaining the highest peak population, followed by PSB and KMB, indicating good adaptability of these organisms to the granule matrix. The initial rise in population up to the third week reflects the favourable moisture buffering and carbon supplementing environment created by the clay-humic acid matrix, while the subsequent decline is attributable to nutrient depletion, moisture loss and intra-consortium competition, a pattern consistent with the survival kinetics reported for Burkholderia sp. on rock-phosphate-based granular carriers (Anandham et al., 2007) [4] and the beneficial role of humic acid in sustaining microbial activity on rock phosphate (Sumreen et al., 2022) [32] .

Conclusion

Microbial enrichment of granulated low-grade rock phosphate with a consortium of phosphate-solubilizing bacteria, potassiummobilizing bacteria and Azotobacter significantly improved the growth, yield, nutrient uptake and soil phosphorus status of cluster bean grown in an acidic sandy clay loam soil. Application of just 75 per cent of the recommended dose of phosphorus through enriched GRP (T7) produced pod yield and nutrient uptake statistically on par with, and in several respects superior to, 100 per cent RDP through enriched GRP (T6), and substantially exceeded the performance of the standard chemical RDF and unenriched GRP at equivalent or even higher doses. Enriched GRP treatments simultaneously reduced soil P-fixing capacity and favourably redistributed soil phosphorus toward labile, plant-available fractions while conserving the overall soil P pool. These findings establish microbially enriched granulated rock phosphate as an agronomically efficient, cost-effective and environmentally sustainable alternative to conventional chemical phosphate fertilizer, capable of reducing chemical P input by up to 25 per cent without compromising cluster bean productivity, with clear relevance for P-deficient acid soils of the hilly zone of Karnataka and similar dryland tracts.

Acknowledgement

AcknowledgementThe author gratefully acknowledges the Department of Soil Science, College of Agricultural Sciences, and the Main Agricultural and Horticultural Research Station (MAHRS), Iruvakki, Keladi Shivappa Nayaka University of Agricultural and Horticultural Sciences (KSNUAHS), Shivamogga, for providing the experimental facilities used in this study.

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