Potassium is one of the most essential macronutrients required by plants in large quantities, playing a critical role in crop growth, development and stress resilience. Despite its importance, potassium deficiency in agricultural soils is becoming increasingly prevalent due to intensive cropping systems, imbalanced fertilization and excessive removal of crop residues (Tisdale et al., 1985). Over time, intensive cultivation practices have reduced the soil’s ability to release non-exchangeable K, necessitating alternative management strategies to maintain soil fertility (Lalitha and Dhakshinamoorthy, 2014). This deficiency leads to poor root development, reduced lodging resistance, lower yields and increased susceptibility to diseases and environmental stresses (Srinivasarao et al., 2003). In rice (Oryza sativa L.), potassium is crucial for maintaining stomatal function, enzyme activity and osmotic balance, particularly under drought and salinity stress (De Datta and Mikkelsen, 1985). Rice in hilly and shifting cultivation systems of Northeast India shows clear yield and growth responses when K availability is improved via agronomic interventions and balanced fertilization. Improved Jhum practices and full recommended nutrient management increased yields markedly and raised soil available K, while site experiments in hill ecosystems reported higher yield components and NPK uptake under recommended fertilization practices (Kumar et al., 2015; Layek et al., 2023 and Vijaykumar et al., 2021). Studies reported that increased K concentration and uptake in grain and straw with higher K rates, corresponding to greater biomass accumulation and grain yield (Vijaykumar et al., 2021). Local experimental evidence from northeastern plains and multi-site trials elsewhere indicates beneficial responses across a broad K range and split/top-dress timings often improve efficiency; however, exact hill-specific numeric optima are not established. Trials showed positive responses up to 80 kg K ha^-1 with split applications improving biomass and yields and multi-site analyses suggested moderate rates (≈93-135 kg ha^-1) as reasonable in other rice agroecosystems, with K120 often giving good yield versus no-K controls (Singh and Prasad, 2020).  Soil surveys in Assam reported 1 N NH₄OAc-extractable K averaging 48-63 mg kg^-1, classed as low, with a majority of plant samples showing moderate to extreme K deficiency (Dutta and Zaman, 2013). In Manipur, the State Agriculture Department recommends applying 30 kg ha^-1 of K for rice cultivation, while the ICAR Research Complex for the North Eastern Hill (NEH) Region, Manipur Centre suggests a K application rate of 40 kg ha^-1 for lentil crops. However, continuous rice cultivation with inadequate K replenishment has led to negative K balances in many rice-growing regions, diminishing soil K reserves and buffering capacity. Similarly, in lentil (Lens culinaris), an important protein-rich legume, potassium plays a vital role in nodulation, grain quality and protein synthesis (Islam et al., 2018). Despite its nutritional and soil-enriching benefits through biological nitrogen fixation, lentil production remains low in regions like Manipur due to insufficient potassium fertilization. Given these challenges, this study aims to evaluate the impact of potassium on growth, yield and nutrient uptake of rice and lentil under rice-lentil cropping system. The findings will contribute to better fertilization strategies, helping farmers achieve higher yields while maintaining soil health in intensive cropping systems.

The experiment was conducted in Lourembam, Thoubal District under Central Agricultural University, Imphal, Manipur, India during Kharif and Rabi season of 2022–2023 and 2023-2024. The experimental site was located at 24.644573^oN latitude and 94.074931^oE longitude and at an elevation of 769 m above mean sea level. Soil is clay in texture and acidic (pH 5.42), organic carbon (1.58%), CEC of 16.30 cmol (p+) kg^-1, low in available nitrogen (225.79 kg ha^-1), medium in available phosphorus (24.83 kg ha^-1) and available potassium (212.48 kg ha^-1). The experiment was laid out in randomized block design (RBD) with 7 K levels and 3 replications.  The K levels of the experiment during Kharif rice (var. RC Maniphou-13) were  0, 10, 20, 30, 40, 50 and 60 kg K₂O ha^-1. During Rabi season, no fertilizers were applied in lentil crop (var. IPL-316). For all the treatments, recommended dose of nitrogen and phosphorus were applied @ 60 and 40 kg ha^-1 respectively. Nitrogen, phosphorus and potassium were applied in the form of urea, single super phosphate (SSP) and muriate of potash (MOP), respectively. At harvest, grain and straw for rice and seed and stover yields for lentil were recorded. Grain and straw samples for rice and seed and stover samples for lentil crop for each treatment were used for determination of nitrogen content by modified Kjeldahl’s method, phosphorus content - Di-acid digestion and yellow colour development method and potassium content- Flame photometric method (Jackson, 1973). The data collected from the field experiment were subjected to analysis of variance (ANOVA) using the procedure described by Gomez and Gomez (1984) for a randomized block design (RBD). Treatment means were compared using the least significant difference (LSD) test at a 5% probability level (p≤0.05). Standard errors of mean (SEm ±) and critical differences (CD) were calculated for significant effects. All statistical analyses were performed in R software (version 4.3.1) using the packages agricolae for ANOVA and multiple comparison tests, stats for basic models and dplyr for data handling.

Yield and yield attributes of rice
 
The increasing potassium application significantly increased the number of effective tillers per sq. m of rice at all growth stages, with the greatest differences observed between K level 0 kg K₂O ha^-1 (254) and higher potassium levels (20, 30, 40, 50, 60) kg K₂O ha^-1. In the case of K level 40 kg K₂O ha^-1 (410), significant differences were found when compared to 20, 30, 50 and 60 K levels, with p-values <0.05 as given in Table 1. Potassium application has been linked to a significant rise in the number of effective tillers in rice plants. Mirza et al. (2010) reported that increase in number of tillers in rice plants was due to influence of different fertilizer combinations. This may also be probably due to increased availability and quick accessibility of nutrients. Similar findings were reported by Siavoshi et al. (2011) and Gebreslassie (2016).

   

Data in Table 1 revealed that K level of 40 kg K₂O ha^-1 (190) showed significantly higher and increase in grain number compared to 50 kg K₂O ha^-1 (172) and 60 kg K₂O ha^-1 (168). This suggests that the highest potassium levels (50 and 60 kg K₂O ha^-1) did not result in the highest number of filled grains per panicle compared to the 40 kg K₂O ha^-1. In an experiment, the application of K not only enhanced potential photosynthetic activity but also reduced sodium (Na) and magnesium (Mg) concentrations. This adjustment improved the ratios of K/Na, K/Mg and K/Ca, which are critical for effective grain filling (Bohra and Doerffling, 1993).
       
Among the different  levels of K, maximum test weight (28.52 g) was observed under 40 kg K₂O ha^-1 followed by 30 kg K₂O ha^-1 (28.20 g), 50 kg K₂O ha^-1 (27.84 g) and 20 kg K₂O ha^-1 (27.71 g). Potassium application shown to substantially boost the yield of various rice varieties, with optimal rates typically between 40 and 60 kg K₂O ha^-1. This level of application has been associated with improved grain weight and a reduction in pest infestations (Sarwar, 2012).
       
K level of 0 kg K₂O ha^-1 recorded significantly lower yield (4020 kg ha^-1) than most other  K levels. The highest grain yield was observed under 40 kg K₂O ha^-1 (5620 kg ha^-1), which was significantly superior to all other treatments (p<0.001).  Overall, the potassium levels could be ranked in descending order of effectiveness as 40>30>50>60>20 >10 ≈0 kg K₂O ha^-1. Increase in yield of rice might be due to prolonged availability of K in soil, significant decrease in number of chaffy grains, increased tillering and concentration of K in straw and grain. Similar findings was also reported by Ravichandran and Sriramachandrasekharan (2011). In this study, 40 kg K₂O ha^-1 (6720 kg ha^-1) and 30 kg K₂O ha^-1 (6500 kg ha^-1) observed the most effective K levels for increasing straw yield of rice, with consistent performance across both years. Studies have demonstrated that potassium application rates between 40 and 80 kg ha^-1 can led to significant increases in straw yield. This is in conformity with the findings reported by  Khan et al. (2006).
 
Yield and yield attributes of lentil
 
Potassium application significantly influenced lentil growth and yield parameters under residual fertility conditions in the rice-lentil cropping system.
       
Data presented in Table 2 showed that the number of branches per plant was highest in K level 50 kg K₂O ha^-1  (21), followed by 40 kg K₂O ha^-1 (20), which was at par with K level  60 kg K₂O ha^-1 (20), all significantly exceeding 0 kg K₂O ha^-1 (13). In a study, Singh et al. (2011) reported that secondary branches per plants were increasing with increasing the level of nutrient. Pods per plant of lentil crop recorded the highest values in K level 50 kg K₂O ha^-1  (99) and 60 kg K₂O ha^-1 (90), significantly surpassing 0 kg K₂O ha^-1 (46). This improvement might be due to the fact that potassium acts as catalytic agent in activating a number of enzymes and synthesis of peptide bonds (Sahay et al., 2013). Similar findings were also reported by Srinivasarao et al. (2003).The highest test weight (28.40 g) was observed in K level 50 kg K₂O ha^-1, significantly greater than the control 0 kg K₂O ha^-1 (24.07 g), indicating better grain filling due to improved K availability. Application of potassium at higher levels resulted in higher seed weight which may be probably due to its involvement in translocation of photosynthates and its ability to develop well- developed bold seeds. These findings are in close agreement with those of Ali et al. (2007).


       
The analysis of results shown in Table 2 indicated that the seed yield of lentil was observed highest in K level 50 kg K₂O ha^-1 (762 kg ha^-1) across all comparisons, while 0 kg K₂O ha^-1 (544 kg ha^-1) the lowest. This may also be due to the excess K applied in K levels 50 kg K₂O ha^-1 and 60 kg K₂O ha^-1 in rice which remained in the soil, making more potassium available for the lentil crop leading to more nutrient uptake and growth. Similar results was also reported by Fratini and  Ruiz (2001). Stover yield was maximized in K level 50 kg K₂O ha^-1 (1219 kg ha^-1), followed by 60 kg K₂O ha^-1 (1108 kg ha^-1). This might be due to the cumulative effect of yield attributing characters and enhanced photosynthetic efficiency and greater diversion of assimilates towards reproductive organs. Similar findings were also reported by Farjam et al. (2014).
 
Nutrient content and uptake in rice
 
The N content in grain and straw of rice was statistically significant which ranges from 0.93% to 1.13% and 0.32% to 0.63%. The phosphorus content due to different K levels in grain of rice varied from 0.26% to 0.42% and in straw from 0.08% to 0.24%. Potassium content in rice grain was non significantly affected by different levels of K but was statistically significant in straw which ranges from 2.29% to 3.38% as observed in Table 3. It was observed that potassium content in rice straw was higher than that of grains in all the K levels. Krishnappa et al. (1990) reported that K application increased K content in rice. In terms of nutrient ratios, potassium application raises the concentration of K in rice straw while simultaneously decreasing levels of sodium (Na) and magnesium (Mg). This adjustment improves the ratios of K/Na, K/Mg and K/Ca ratios (Bohra and Doerffling, 1993).


       
Data in Fig (1 to 6) showed that K level 40 Kg K₂O ha^-1 recorded the highest uptake of N, P and K in grain and straw of rice with other level comparisons. This might be due to the fact that nutrients are absorbed by plants proportionately as the available nutrients pool in soil solution increases. This was very close with the findings reported by Singh et al. (2005) and Rayar (1990). In a study, Frederick et al. (2025) reported that a significant increase in yield, yield components and NPK uptake of rice was noted with increase in potassium levels from 0 to 30 and 60 kg ha^-1.



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Nutrient content and uptake in lentil
 
The percentage of N, P and K concentration in lentil seed and stover was recorded maximum in K level 50 kg K₂O ha^-1 and 60 kg K₂O ha^-1, although they did not differ significantly from each other and the lowest in 0 kg K₂O ha^-1 and 10 kg K₂O ha^-1 (Table 4). Similarly, 50 kg K₂O ha^-1 and 60 kg K₂O ha^-1  both observed the highly effective nutrient uptake of N, P and K compared to lower K levels, but there was no yield  advantage to increasing the potassium level beyond 50 kg K₂O ha^-1 for maximizing uptake in seed and stover as shown in Table 5. This may be due to the involvement of potassium nutrient to increase the crop growth with increase in utilization and translocation of other essential nutrient especially N to plant and synergy between N and K in soil system resulting in boosting crop yield (Guo and Zhu, 2004; Bruns and Ebelhar, 2006).




 
Limitations and future scope
 
The present study focused primarily on the agronomic and nutrient response of rice- lentil cropping system to potassium fertilization under acid soil conditions. Economic analysis of treatments was not conducted, which restricts direct recommendations on the profitability of different potassium levels. In addition, the experiment was carried out over two years at a single location; therefore, the findings should be validated under diverse agro-ecological conditions and longer-term trials before drawing generalized recommendations. Future investigations should integrate both agronomic and economic analyses to identify the economically optimal potassium dose for rice-lentil systems. Multi-location and multi-season trials across varied soil types and altitudes in the North-Eastern region would help refine recommendations and capture site-specific responses. Long-term studies are also needed to evaluate the residual and cumulative effects of potassium on soil fertility, nutrient balances and system sustainability. In addition, incorporating advanced approaches such as soil test-based fertilizer recommendations, nutrient budgeting and precision agriculture tools could further improve potassium management strategies in acid soils.

The present study clearly demonstrates that potassium (K) plays a pivotal role in enhancing crop productivity and soil health in a rice-lentil cropping system under acid soil conditions. Application of potassium significantly improved plant growth, yield attributes and final yields of rice with K level 40 kg K₂O ha^-1 and lentil with 50 kg K₂O ha^-1 consistently producing the highest grain and biomass yields. Notably, rice grain yield increased from 4020 kg ha^-1 under no K application (0 kg K₂O ha^-1) to 5620 kg ha^-1 under K level (40 kg K₂O ha^-1), while lentil seed yield rose from 544 kg ha^-1 to 762 kg ha^-1, highlighting the substantial yield benefits of optimal potassium nutrition. These findings underscore the importance of balanced fertilization practices and soil-specific nutrient management for sustainable agriculture. However, further research is needed to refine potassium recommendations under varied soil textures, altitudes and climatic conditions.

The authors declare that there are no conflicts of interest within them.