Effect of potassium solubilizing bacteria and foliar application of potassium on uptake and nitrogen use efficiency in paddy (Oryza sativa) in coastal acid soils of Karnataka

Introduction

Rice (Oryza sativa), cultivated for over 10,000 years, remains the staple food for more than half the global population, providing 21% of the per capita energy and 15% of the per capita protein worldwide. In India, rice cultivation spans 43.79 million hectares with an annual production of 118.87 million tons, forming the foundation of national food security. In Karnataka, rice is grown on 1.45 million hectares, producing 3.72 million tons annually, with productivity varying across districts due to agro-climatic diversity. However, the sustainability of rice production in coastal acid soils is threatened by nutrient deficiencies, inefficient fertilizer use, and environmental constraints, necessitating innovative solutions (Ali et al., 2007) [3] Among the macronutrients essential for crop productivity, potassium (K) plays a pivotal role in enhancing photosynthesis, enzyme activation, water regulation, and resistance to abiotic stresses. Despite its significance, potassium deficiency is prevalent in coastal Karnataka, largely due to high rainfall, which exacerbates K leaching. Potassium in soils exists primarily in unavailable forms (fixed or mineral K), with only a small fraction present in exchangeable or solution forms accessible to plants. This imbalance is further aggravated by intensive cropping systems, imbalanced fertilizer application, and the absence of indigenous potassium-bearing minerals in India, leading to heavy dependence on imported K fertilizers such as muriate of potash (MOP). To address these challenges, potassium-solubilizing bacteria (KSB) offer an eco-friendly alternative for improving K availability in soils. These rhizospheric microorganisms solubilize fixed and mineral forms of potassium through mechanisms such as organic acid production, chelation, and acidolysis. Studies have demonstrated the ability of KSB strains, including Bacillus mucilaginosus and Paenibacillus spp., to enhance K availability, improve nutrient uptake, and boost crop yields. In addition, foliar potassium application (FKA) serves as a complementary approach to soil fertilization, enabling plants to directly absorb nutrients through leaves. Foliar K applications are particularly advantageous under moisture-stress conditions or in soils with limited nutrient mobility, as they enhance nutrient use efficiency and reduce wastage. Rice production in India, particularly in Karnataka's coastal acid soils, faces compounding challenges of declining soil fertility, reduced nutrient use efficiency, and imbalanced fertilizer application. Nitrogen (N), a critical nutrient for rice, is often applied inefficiently, leading to significant losses through leaching and volatilization, further lowering its use efficiency. Integrating KSB with foliar potassium applications not only addresses potassium deficiency but also enhances nitrogen use efficiency (NUE) by optimizing nutrient availability and uptake. This integration has the potential to improve crop productivity while sustaining soil health.

Materials and Methods

Description of the experimental site

The experiment was conducted at Seed Farm, Zonal Agricultural and Horticultural Research Station, Brahmavar, Udupi district, Karnataka. The experimental site is situated between 130 25' N latitude and 740 45' E longitude with an altitude of 10 meters above mean sea level. It comes under the coastal zone (agroclimatic zone 10) of Karnataka.

Experiment details

Composite soil samples were drawn at a depth of 0 to 15 cm soil layer and a representative sample was obtained by standard procedures. The soil was analyzed for its chemical properties viz., pH, EC, organic carbon, available nitrogen, phosphorus and potassium and for biological properties viz., Bacteria, Fungi, Actinomycetes and K-solubilizers. The results of the analysis are given in Table 1. Experimental design is RCBD with eleven treatment and three replication. The cropping season is kharif with spacing 20 ×10 cm, seed rate 25 kg acre-1 and variety MO-4. Treatment details are T1: Recommended dose of nutrients (60: 30: 60 kg per ha) + 5 tonnes of FYM, T2: T1 + soil application of KSB @ 5 kg per ha, T3: T1+ 2% KCl @ 30 and 60 DAT, T4: T1 + soil application of KSB @ 5 kg per ha + 2% KCl @ 30 and 60 DAT, T5: Recommended dose of N and P2O5 + 50% K2O + soil application of KSB @ 5 kg per ha, T6: Recommended dose of N and P2O5 + 50% K2O + 2% KCl @ 30 and 60 DAT, T7: Recommended dose of N and P2O5 + 50% K2O + soil application of KSB @ 5 kg per ha +2%KCl @ 30 and 60 DAT, T8: Recommended dose of N and P2O5 + 75% K2O + KSB @ 5 kg per ha, T9: Recommended dose of N and P2O5 + 75% K2O + 2% KCl @ 30 and 60 DAT, T10: Recommended dose of N and P2O5 + 75% K2O + soil application of KSB @ 5 kg per ha + 2% KCl @ 30 and 60 DAT and T11: Absolute control. Note: FYM: Farm yard manure, KSB: Potassium solubilizing bacteria, KCl: Muriate of potash, Urea, rock phosphate and muriate of potash were used as inorganic source of N, P and K fertilizers. Five hills were selected randomly from the net plot and labelled for recording the observations in each treatment. Fisher's method of analysis of variance was applied for analysis and interpretation of data as given by Panse and Sukhatme (1967) [13] . The level of significance used in 'F' and 't' tests was P=0.05. Critical difference values were calculated wherever the 'F' test was significant.

Soil sample analysis

Soil samples were collected prior to treatment application and at harvest to assess changes in soil properties. These samples were air-dried in shade, finely powdered using a wooden mallet, sieved through a 2 mm mesh, and stored in polythene bags for subsequent analysis. Chemical properties of the soil, including pH, electrical conductivity, organic carbon, available nitrogen, phosphorus, potassium, exchangeable calcium and magnesium, available sulfur, and DTPA-extractable micronutrients (iron, manganese, zinc, and copper), were analyzed following standard protocols. The soil pH was measured using a 1:2.5 soil-to-water suspension and a digital pH meter with a combined electrode (Jackson, 1973) [9] . Electrical conductivity was determined using a conductivity bridge on a similar suspension, with results expressed as dS m⁻¹ at 25°C. Organic carbon was quantified via Walkley-Black's wet oxidation method. Available nitrogen was measured by distilling soil with potassium permanganate and sodium hydroxide, trapping the released ammonia in boric acid, and titrating against sulfuric acid (Subbiah and Asija, 1956) [18] . Available phosphorus was extracted using Bray's No.1 extractant and quantified by the chlorostannous-reduced molybdophosphoric acid blue color method at 660 nm (Bray and Kurtz, 1945) [6] . Available potassium was measured using a flame photometer after ammonium acetate extraction. Exchangeable calcium and magnesium were estimated by titration with EDTA, and available sulfur was measured turbidometrically using calcium chloride as an extractant (Jackson, 1973) [9] . DTPA-extractable micronutrients were determined using an atomic absorption spectrophotometer (Lindsay and Norwell, 1978).

Plant sample analysis

Plant samples were collected at 30, 60, and 90 days after transplanting (DAT) and at harvest to analyze nutrient concentrations and uptake. The collected samples, including grain, leaf, and straw, were initially air-dried at room temperature and then dried at 60°C in a hot air oven for 48 hours. The dried samples were powdered using a grinder with stainless steel blades and stored in polythene bags for further analysis of nitrogen (N), phosphorus (P), and potassium (K). Total nitrogen content in the plant samples was determined using the Kjeldahl method, where 0.5 g of the sample was digested with concentrated sulfuric acid and a digestion mixture containing potassium sulfate, copper sulfate, and selenium in a 100:20:1 ratio. The digested material was distilled in an alkaline medium, and the liberated ammonia was titrated against standard sulfuric acid. Nitrogen uptake (kg ha⁻¹) was calculated by multiplying nitrogen concentration with dry matter production and dividing by 100. For phosphorus and potassium analysis, 1 g of the powdered plant sample was pre-digested with concentrated nitric acid overnight. Subsequently, the sample was digested with a di-acid mixture (HNO₃: Perchloric acid in a 10:4 ratio) until a white residue was obtained. The digested material was diluted to 100 ml with distilled water for further analysis. Total phosphorus was determined using the vanadomolybdate phosphoric yellow color method in the nitric acid system, as described by Jackson (1973) [9] . Total potassium was measured using a flame photometer after diluting the di-acid digest. Uptake values for phosphorus and potassium were computed using their respective concentrations and dry matter production.

Nutrient uptake

The nutrient uptake by paddy at harvest was worked out using the following equation.

Nutrient use efficiency Recovery efficiency (RE)

Recovery efficiency of nitrogen, phosphorus and potassium was worked out for paddy and groundnut during both the years. The formula used to find out the efficiency is as follows.

Where, RE= Recovery efficiency Nuf = Amount of nutrients taken up by a crop in fertilized plot (kg ha -1 ) NuC = Amount of nutrients taken up by a crop in control plot (kg ha -1 ) Na = Amount of nutrient applied (kg ha -1 )

Results and Discussion

This study investigated the combined effect of potassium solubilizing bacteria (KSB) and foliar application of potassium chloride (KCl) on the concentration and uptake of key nutrients i.e., nitrogen (N), phosphorus (P), and potassium (K) in paddy plants at various growth stages (30, 60, 90 days after transplanting, and at harvest). The findings contribute to understanding how integrated nutrient management strategies, including microbial inoculants, enhance nutrient availability and uptake in paddy systems.

Nitrogen Concentration

• Vegetative Growth Stages: Among all treatments, the combination of the recommended dose of nitrogen (N), phosphorus (P₂O₅), 75% potassium oxide (K₂O), soil application of KSB (5 kg ha -¹), and foliar application of 2% KCl (T 10 ) resulted in the highest nitrogen concentrations of 1.291%, 1.294%, and 1.701% at 30, 60, and 90 days transplanting (DAT), respectively as given in Table 2. These results reflect enhanced nitrogen availability due to microbial activities. KSB has been widely recognized for its role in promoting nitrogen availability through nitrogen fixation, which improves nitrogen assimilation by plants (Aziz et al., 2010) [4] . Additionally, potassium is known to regulate the enzymes involved in nitrogen metabolism, which further increases nitrogen uptake (Sarwar et al., 2012) [15] . • Grain and Straw: At harvest, nitrogen concentrations in the grain and straw were highest under T10 (1.175% in grain and 0.609% in straw) (Table 2). The control treatment, without KSB and foliar KCl, recorded significantly lower nitrogen concentrations (grain: 0.996%, straw: 0.471%), suggesting that KSB and foliar KCl applications significantly enhanced nitrogen assimilation and storage in both vegetative and reproductive organs. The findings corroborate those of Surekha et al. (2003) [19] , who observed that KSB and foliar applications lead to an increase in nitrogen content in rice.

Nitrogen Uptake Grain and Straw Uptake: Nitrogen uptake was highest under T 10 , with a total of 63.87 kg ha⁻¹ in the grain and 43.11 kg ha⁻¹ in the straw. Other treatments with KSB and foliar KCl also exhibited higher nitrogen uptake compared to the control (Table 3). These results reflect the synergistic effect of microbial inoculation and potassium supplementation in improving the uptake of nitrogen by enhancing root growth, which facilitates better nutrient absorption from the soil (Dadarwal et al., 2009) [7] . In contrast, the control treatment recorded the lowest nitrogen uptake (grain: 46.67 kg ha⁻¹; straw: 27.26 kg ha⁻¹), highlighting the importance of balanced fertilization and microbial activity in nutrient mobilization.

Phosphorus Concentration

• Vegetative Growth Stages: The phosphorus concentrations were highest under T 10 (0.536%, 0.537%, and 0.521% at 30, 60, and 90 DAT, respectively). Treatments that included KSB and foliar KCl (such as T 8 and T 7 ) (Table 2)also exhibited higher phosphorus concentrations compared to the control, confirming the positive impact of KSB on phosphorus availability. KSB not only solubilizes insoluble phosphorus but also aids in enhancing microbial mineralization of organic phosphorus (Badr et al., 2006) [5] . Moreover, foliar potassium application has been shown to help in the mobilization of phosphorus from soil reserves, as potassium ions play a key role in enhancing the activity of phosphorus-mobilizing microorganisms (Meena et al., 2014) [11] . • Grain and Straw: At harvest, phosphorus concentrations in grain (0.370%) and straw (0.328%) were highest in T 10 , indicating better nutrient translocation and assimilation. The control treatment, with no KSB or foliar application, exhibited significantly lower phosphorus content (grain: 0.218%, straw: 0.173%) (Table 2). This underscores the critical role of KSB in phosphorus solubilization, as noted in similar studies by Kuotsu et al. (2016) [10] , which showed that microbial inoculants improve phosphorus uptake by reducing the fixation of phosphorus in soils.

Phosphorus Uptake

Grain and Straw Uptake: The uptake of phosphorus in both grain and straw was significantly higher in the T 10 treatment, with 20.10 kg ha⁻¹ in the grain and 26.18 kg ha⁻¹ in the straw (Table 3). Other treatments also exhibited elevated phosphorus uptake, indicating the beneficial role of microbial inoculation and foliar potassium in improving phosphorus uptake from both organic and inorganic pools in the soil. The control treatment showed the lowest phosphorus uptake (grain: 12.92 kg ha⁻¹, straw: 17.92 kg ha⁻¹), indicating the necessity of both microbial activity and foliar nutrients for improved phosphorus utilization.

Studies have confirmed that microbial inoculants, such as KSB, enhance phosphorus uptake by increasing phosphorus solubilization in the rhizosphere (Badr et al., 2006) [5] .

Potassium Concentration

• Vegetative Growth Stages: Potassium concentrations were highest under T 10 (1.113%, 1.120%, and 1.523% at 30, 60, and 90 DAT, respectively). The positive impact of KSB and foliar KCl on potassium concentration was evident across growth stages (Table 2). Potassium is essential for enzyme activation and photosynthesis, both of which are crucial for plant growth. The availability of potassium was significantly enhanced in treatments involving KSB, as KSB is known to release potassium from insoluble mineral sources through the production of organic acids (Sheng & He, 2006) [17] . • Grain and Straw: At harvest, potassium content was highest in grain (1.064%) and straw (1.187%) under T 10 , indicating efficient nutrient translocation (Table 2). Other treatments with KSB and foliar KCl also exhibited significant potassium content, emphasizing the role of KSB in potassium solubilization and availability. The control treatment showed the lowest potassium content, underscoring the importance of microbial intervention in enhancing potassium uptake in paddy.

Potassium Uptake

Grain and Straw Uptake: Potassium uptake was highest under T 10 , with 57.31 kg ha⁻¹ in the grain and 84.04 kg ha⁻¹ in the straw. These values were significantly higher than those in the control treatment (grain: 41.26 kg ha⁻¹, straw: 58.25 kg ha⁻¹), demonstrating the critical role of KSB and foliar KCl in enhancing potassium uptake (Table 3). Potassium solubilizing bacteria aid in the mobilization of potassium from nonexchangeable forms in the soil, thus improving its uptake by the roots (Meena et al., 2014) [12] . This result aligns with findings by Kuotsu et al. (2016) [10] , who showed that KSB treatment led to improved potassium uptake in rice.

Key Findings and Implications 1. Synergistic Effects of KSB and Foliar KCl:

The integration of KSB with foliar KCl significantly enhanced the concentration and uptake of all three key nutrientsnitrogen, phosphorus, and potassium-across the growth stages of paddy. This synergistic effect was evident in higher nutrient content in both grain and straw at harvest. The positive influence of KSB on nutrient availability is well-documented, as KSB not only enhances potassium availability through solubilization but also influences nitrogen and phosphorus dynamics in the soil (Meena et al., 2014) [11] . The addition of foliar potassium application ensures efficient nutrient translocation, promoting better growth and yield.

Optimizing Potassium Fertilization:

The 75% K₂O dose combined with KSB and foliar KCl yielded the highest nutrient uptake, indicating that reduced potassium application can still achieve optimal nutrient use efficiency when combined with biofertilizers. This finding suggests that farmers can achieve high yields with lower chemical fertilizer inputs, promoting sustainable agriculture practices (Sarwar et al., 2012) [15] . The study emphasizes the potential for optimizing potassium fertilization, particularly in regions with limited access to chemical fertilizers.

Microbial Mechanisms in Nutrient Cycling:

The results highlight the critical role of microbial inoculants in enhancing nutrient solubilization and availability in paddy cultivation. The improved nutrient uptake is likely a result of KSB-induced solubilization of insoluble phosphorus and potassium as well as their role in nitrogen fixation (Aziz et al., 2010) [4] . Microbial activity also improves soil health by stimulating nutrient cycling, which ultimately leads to better nutrient utilization and crop growth.

Enhanced Nitrogen Use Efficiency:

The study shows that the combination of KSB and foliar KCl improves nitrogen uptake and assimilation. Potassium is essential for nitrogen metabolism as it regulates the activity of nitrate reductase and other enzymes involved in nitrogen assimilation. This synergistic interaction suggests that optimizing potassium nutrition could improve nitrogen use efficiency, reducing the need for excessive nitrogen fertilization (Aziz et al., 2010) [4] .

Nitrogen Use Efficiency of Paddy as Influenced by KSB and Foliar Application of 2% KCl

The study indicate that the integration of KSB with foliar potassium application led to significant improvements in both total nitrogen uptake and NUE across treatments, with synergistic effects observed in the combined use of biofertilizers and potassium supplementation.

Total Nitrogen Uptake

The total nitrogen uptake, including both grain and straw nitrogen, showed substantial variation across treatments. The highest nitrogen uptake (106.97 kg ha⁻¹) was observed in treatment T10, which combined 75% potassium oxide (K₂O) with soil-applied KSB and foliar sprays of 2% KCl at 30 and 60 days after transplanting (DAT) (Table 4). This treatment demonstrated the effectiveness of combining KSB with potassium supplementation in enhancing nitrogen uptake. The second-highest nitrogen uptake was recorded in T8 (104.78 kg ha⁻¹), where KSB was applied with 75% K₂O but without the foliar potassium spray. The absence of the foliar application slightly reduced nitrogen uptake, indicating the critical role of foliar potassium in improving nutrient translocation. Conversely, the absolute control (T11), which lacked any nutrient input, exhibited the lowest nitrogen uptake at 24.55 kg ha⁻¹, highlighting the importance of nutrient supplementation for optimizing nitrogen assimilation in paddy crops. Grain nitrogen uptake, a key factor in total nitrogen uptake, was highest in T10 (63.87 kg ha⁻¹), further supporting the beneficial effects of combining KSB and foliar potassium (Adam et al., 2011) [1] Nitrogen Use Efficiency (NUE) NUE values reflected the efficiency with which nitrogen was converted into plant biomass. The highest NUE (178.28) was observed in T10, where the combination of KSB, potassium supplementation, and foliar potassium application enhanced both nitrogen uptake and utilization (Table 4). This indicates that the integrated approach significantly improved nitrogen use efficiency. Treatment T8, which lacked the foliar potassium spray but included KSB and 75% K₂O, exhibited a high NUE of 174.63, emphasizing the role of KSB in enhancing nutrient use even in the absence of foliar potassium. On the other hand, T11, which received no nutrient inputs, showed the lowest NUE (40.92), underscoring the inefficiency of nitrogen use when fertilizers or biofertilizers are not applied. Interestingly, treatments with reduced potassium inputs, such as T7 (161.68) and T5 (157.43), also achieved high NUE values. This highlights that KSB can improve NUE even with lower potassium doses, offering a more cost-effective and sustainable alternative to full potassium fertilization (Adenawoola et al., 2005) [2] Enhanced Nutrient Solubilization by KSB The significant improvements in nitrogen uptake and NUE can be attributed to several mechanisms facilitated by KSB. KSB are known to solubilize unavailable potassium from soil minerals, increasing the potassium pool in the rhizosphere. Potassium is crucial for plant growth, as it regulates enzyme activation, photosynthesis, and nutrient translocation. By improving potassium availability, KSB indirectly support better nitrogen uptake, as potassium enhances the activity of nitrate reductase and other enzymes involved in nitrogen assimilation (Meena et al., 2014) [12] .

Microbial Activity and Nutrient Cycling

KSB also contribute to enhanced microbial activity in the soil, which is essential for efficient nutrient cycling. The increased microbial population facilitates the breakdown of organic matter, releasing nitrogen and other nutrients that are subsequently available to plants. This microbial activity not only supports better nitrogen uptake but also improves soil fertility by enhancing the mineralization of nitrogen and other essential nutrients (Sheng et al., 2010) [16] . Therefore, KSB help maintain a healthy soil ecosystem that supports efficient nutrient utilization and cycling.

Foliar Potassium Application

Foliar application of 2% KCl at critical growth stages (30 and 60 DAT) further optimized nutrient availability. Potassium applied in this manner reduces nutrient leaching and ensures that the plant can effectively absorb and utilize nitrogen during key phases of growth. Potassium is involved in the synthesis of amino acids, proteins, and chlorophyll, and plays a vital role in nitrogen metabolism. The foliar application of potassium enhances the efficiency of nitrogen assimilation and translocation, ensuring that the plant utilizes the available nitrogen to its fullest potential (Parmar & Sindhu, 2013) [14] .

Sustainable Input Management

The results also suggest that reduced potassium inputs can still result in high NUE when KSB is used. Treatments like T7, which involved reduced potassium application, achieved high NUE values, demonstrating that KSB can enhance nutrient efficiency even with lower potassium doses. This is a promising finding for sustainable agricultural practices, as it indicates that KSB can help farmers reduce their reliance on chemical fertilizers while still achieving optimal crop performance. The integration of biofertilizers like KSB with reduced chemical inputs offers a cost-effective and environmentally friendly alternative to conventional farming practices (Ali et al., 2005)

Conclusion

The integration of potassium solubilizing bacteria (KSB) with foliar potassium chloride (KCl) significantly enhances nutrient dynamics and uptake efficiency in paddy cultivation. This synergistic approach improves the concentration and uptake of essential nutrients like nitrogen, phosphorus, and potassium across various growth stages, leading to better plant growth and higher rice yields. By solubilizing unavailable potassium and improving nutrient availability in the rhizosphere, KSB facilitates nutrient cycling and uptake. The addition of foliar potassium ensures efficient nutrient absorption during critical growth periods, minimizing nutrient losses and addressing deficiencies effectively. This combination not only enhances soil biological activity and fertility but also supports sustainable farming practices by reducing reliance on chemical fertilizers. As a cost-effective and eco-friendly solution, the integration of microbial inoculants like KSB with foliar nutrient application promotes higher productivity while maintaining long-term soil health. It represents a practical and sustainable strategy for improving paddy cultivation in nutrient-depleted soils.

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