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. 2025 Sep 30;15:33895. doi: 10.1038/s41598-025-08132-w

Optimizing root physiology and soil nutrient balance in rice–wheat sequence using polyhalite for enhanced profitability in the Indo-Gangetic plains of India

Vipin Kumar 1, Kapila Shekhawat 1,✉, Sanjay Singh Rathore 1, P K Upadhyay 1, Rajiv Kumar Singh 1, Ananya Gairola 1, Diksha Sharma 1, Aastika Pandey 2, Sougata Roy 4, G D Sanketh 3, Khushboo Devi 1, Subhash Babu 1
PMCID: PMC12484773  PMID: 41028084

Abstract

Excessive application of high-analysis fertilizers disrupts ecosystem balance and raise environmental concerns. Potassium (K) and sulphur (S) are crucial nutrients for plants optimum growth and development. Potassium uptake exceeds nitrogen (N) by almost 1.5. Its deficiency diminishes yield (~ 20%) and compromise stress resilience. Potassium application through MOP observes low recovery, poor nutrient equilibrium, suboptimal monetary returns, and environmental threats. In contrast, polyhalite, a natural multi-nutrient mineral containing K, magnesium (Mg), calcium (Ca), and S, presents promising alternative for mitigating nutrient imbalances, enhancing soil health, and higher farm profitability. A field experiment at ICAR-Indian Agricultural Research Institute, New Delhi, was conducted during rabi 2021–22 to evaluate K sources viz. polyhalite, bentonite, and MOP in optimizing plant growth and nutrient uptake. The grain N content under 100% K (polyhalite) was 4.36, and 1.89% higher over 100% K (MOP), and 100% K (MOP)+ bentonite-S equivalent, respectively. Potassium content in grain and straw was 30.3% and 10.38% higher, respectively, over K supply with sole MOP. The optimal K dose (polyhalite) registered 23% higher root length density, 2.64% higher root volume, and 1.28-folds higher root diameter over 100% (MOP). Polyhalite as a K source offered 1.83 and ~ 2-folds higher agronomic efficiency over 100% K (MOP) and 75% K (MOP), respectively. The greater actual net gain in soil K was observed and recorded with 100% K (polyhalite; 20.1 kg ha−1), followed by 100% K (MOP)+ S-equivalent to T8 (19.1 kg ha−1). A net gain of 2 kg ha−1 in Soil S was registered under polyhalite over MOP application. Across the treatments, 100% K (polyhalite) revealed 1.35 and 1.22-times higher net returns over conventional (100% K-MOP) and blend combination (50:50; MOP+ polyhalite), respectively.

Keywords: Polyhalite, Potassium, Balanced nutrition, Profitability, Root dynamics

Subject terms: Ecology, Environmental sciences

Introduction

The widespread application of various synthetic fertilizers has garnered considerable attention within agrarian economies; however, since the 1980s, their detrimental effects have posed severe challenges to both ecological systems and human health1–3. Unsustainable nutrient management strategies under intensive agro-ecosystems have precipitated yield plateaus and a pronounced negative nutrient equilibrium, exacerbating depletion of soils’ ecosystem services and several nutrient deficiencies4. Soil nutrient imbalance, cooperated by nutrient mining identified as the primary driver of poor crop productivity and lower crop response to fertilizer5. A net negative balance due to annual loss of ~ 10 Mt of primary nutrients (NPK) comprises a loss of 12% phosphorus (P), 19% nitrogen (N), and a staggering 69% potassium (K), further intensifying soil fertility constraints6. In the hierarchy of nutrient roles and soil deficiencies, K assumes a critical position following N in its essential contribution to plant metabolic functionality7. Contrary to widespread belief, Indian soils, once deemed abundant in K-minerals, have been conventionally viewed as requiring minimal replenishment. As a result, potassium fertilizer has typically been applied at only 35% of the amount removed by crops, which may lead to nutrient depletion over time8,9. However, not all the soils of India are abundant in K-bearing minerals, and mere presence of soil K does not ensure that crops will be able to access required quantities of K at the optimal stage10–12. These factors combine to deplete the readily available reserves of soil potassium, creating an imbalance, often termed K-mining, i.e., 3.3 Mt per annum13,14. In IGP’s rice–wheat sequence (RWS), K uptake surpasses nitrogen utilization by a factor of 1.515. Therefore, the inadequacy of soil K culminates in a substantial diminution of economic yield (~ 20%) and quality, particularly within intensive cereal-based cropping sequences prevalent in semi-arid regions of various parts of India12. Escalating temperature levels and rising adverse effects of terminal heat stress during the post-anthesis period in wheat further intensify this challenge16. This situation perturbs the balance between reactive oxygen species (ROS) and the defense system of the plant, resulting in oxidative damage to cellular organelles. Heat stress, when observed during the grain filling stage, expedites senescence of the flag leaf by 25%, severely impairing photosynthetic efficiency, stomatal conductance, and chlorophyll fluorescence (Fv/Fm) in wheat17–20. K’s role is paramount in promoting vascular tissue lignification, thereby reducing lodging susceptibility and alleviating vulnerability to field stresses (biotic and abiotic). The optimal supply of K in root and leaf tissues activates defense mechanisms against elevated reactive oxygen species (ROS) levels, protecting against membrane damage and supporting smoother physiological functioning21,22. Soil K is inherently immobile, migrating gradually via diffusion, and is partitioned into soluble, exchangeable, non-exchangeable, and mineral reservoirs. These pools critically regulate the equilibrium between soil K availability and plant uptake. Further, the additional soil K supply is essential in order to maintain the soil nutrient balance and long-term functionality of the soil’s ecosystem23. Maintaining balanced fertility for plant nutrition is imperative, as nutrient interactions can be either synergistic or antagonistic in nature24–26. The judicious application of inorganic nutrients in appropriate proportions exerts minimal adverse effects on soil health and the environment. However, high-analysis potassium fertilizers, such as muriate of potash (MOP), supply only a single nutrient, resulting in diminished nutrient use efficiency and increased losses within the rhizosphere. Consequently, this imbalance hinders availability and uptake of essential macro- and micronutrients, thus compromising systems’ resilience, soil fertility, and environmental sustainability27,28. To sustain uptake-replenishment synchronization for optimal plant-soil health and improved fertilizer use efficiency with cost minimization, a comprehensive custom-made fertilizer strategy has become an indispensable necessity in modern agronomic approaches. Amidst the growing challenges of ecosystem instability and nutrient imbalance in RWS, the integration of polyhalite (K2SO4·MgSO4·2CaSO4·2H2O)—a naturally occurring multi-nutrient carrier enriched 14% K2O, 6% MgO, 17% CaO, and 19% S—emerges as a strategic approach to mitigate these pressing concerns. Its well-balanced nutrient profile rectifies deficiencies with precision while optimizing nutrient use efficiency, fostering an integrated approach toward food-soil-ecosystems’ security29,30. This strategy mitigates nutrient deficiencies and promotes long-term agroecological stability and resilience. Compared to conventional fertilizers, polyhalite is characterized by a relatively reduced environmental footprint as it generates only 7% CO2 compared to SOP and 15% CO2 relative to MOP31. It is a salt of neutral Ph, solubility 27 g L−1 at 25 °C and < 2% chloride32. A massive stock of evaporite mineral has been found at a depth of 1,200 m beneath surface of Earth within North Sea, situated along northeastern coastline of the UK (United Kingdom)33,34. Unlike traditional materials, it is imbued with a protracted nutrient release mechanism, a distinctive attribute that holds the potential to enhance nutrient utilization efficiency within rhizosphere35,36. Several studies have evaluated polyhalite efficacy across diverse range of crops (such as, maize, sorghum, kiwifruit, potato, mustard, etc.)34,37–39. However, absence of robust comparative analysis examining its utilization efficiency and soil nutrient dynamics against conventional fertilizers (MOP) in pivotal cropping sequences like the RWS of Indo-Gangetic plains in India, remains evident. In India, numerous researchers have elucidated the advantageous effect of polyhalite on soybean, potato, maize and sugarcane26,36,40,41. However, empirical investigations on its impact on major cereal staples, namely rice and wheat in semi-arid regions remain conspicuously inadequate. There exists a critical need to assess potential of polyhalite as a tailored nutrient source for maintaining nutrient balance, particularly potassium (K) and sulfur (S), within wheat-based agroecosystems. On the other hand, feasibility of substituting MOP with polyhalite as an alternative K source in cereal-based production scenarios warrants rigorous exploration to optimize yield, agronomic efficiency, and positive economics for sustainable as well as cost-effective production. We postulated that the utilization of polyhalite as a multi-nutrient supplier would exhibit comparable efficacy to conventional K-based fertilizers in augmenting both plant uptake and soil balance, while enhancing profitability, particularly under scenarios of semi-arid regions of the country. Thus, in order to investigate the effect of different doses of polyhalite, bentonite, and MOP on crop growth, nutrient utilization, soil nutrient balance and economic returns in wheat under rice–wheat sequence, field experiment at ICAR-Indian Agricultural Research Institute, New Delhi, was planned and conducted during rabi 2021–22. The outcomes from this study could promote balanced fertilization and ensure food security with a greener economy.

Materials and methods

Experimental site

A field experiment at ICAR-Indian Agricultural Research Institute, New Delhi, India, was planned and conducted during rabi 2021–22 (Fig. 1; Map was created using QGIS 3.34 Prizren; Download QGIS). The farm is positioned at 28°38′23″ N latitude, and 77°09′27″ E longitude, at 228.61 m above mean sea level (MSL). The site of the experiment lies in the western part of the Indo-Gangetic Plains (IGP), receiving an average annual rainfall of 714 mm, out of which 75% falls during the July to October (monsoon season). The experimental site is under a semi-arid type of climate and displays sandy loam soil. The physico-chemical properties of the experimental soil before treatment application are presented in Table 1,

Fig. 1.

Fig. 1

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Map of experimental site (Map was created using QGIS 3.34 Prizren) (Download QGIS).

Table 1.

Physico-chemical properties of experimental soil (0–15 cm depth).

S. No Soil parameter Value
1 Sand 64%
2 Silt 17%
3 Clay 19%
4 Typic Haplustert
5 Soil pH 7.2 (Neutral)
6 Organic carbon 0.41% (Moderate)
7 Available soil nitrogen (oxidizable by KMnO4) 130.01 kg ha−1
8 Available soil phosphorus (0.5 M NaHCO3-extracted) 11.03 kg ha−1
9 Available soil potassium (neutral 1 N NH4OAc-extracted) 230 kg ha−1
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Experimental design and details

The field experiment design completely randomized block, total treatments of 12 combinations having 3 replications for each: T1: No-K, no-S, T2: Recommended S (30 kg ha−1) through bentonite, no-K; T3:50% K through MOP; T4:75% K through MOP; T5:100% K through MOP; T6:50% K through polyhalite; T7:75% K through polyhalite; T8:100% K through polyhalite; T9:50% K through MOP + S–equal toT6 (33.9 kg ha−1) via bentonite; T10:75% K through MOP + S–equal to T7 (50.9 kg ha−1) via bentonite; T11:100% K through MOP + S–equal toT8 (67.9 kg ha−1) via bentonite; and T12:50% K through MOP + 50% through polyhalite. The recommended dose of fertilizer applied was 150 kg ha−1 N, 60 kg ha−1 P2O5, and 50 kg ha−1 K2O, respectively. After being leveled by a tractor-mounted bund maker, the experiment was laid out at 8.0 m × 3.50 m gross plot size (i.e., 28 m2).

Crop management

The experimental field was prepared by ploughing twice using disc harrows and a cultivator, followed by planking. The outstanding variety of wheat HD-2967 was sown on 20th November (seed rate: 100 kg ha−1) by tractor-operated seed drill with a spacing of 22.5 × 10 cm. For proper irrigation, the bunds were manually trimmed with a spade. A channel between the plots was built with adequate provisions for irrigation. The nitrogen and phosphorus recommended dose was applied using urea and DAP fertilizers, while the potassium requirement was fulfilled using multi-nutrient carrier polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) and conventional fertilizer muriate of potash (MOP) or a combination of both.

Weeds were controlled using a post-emergence herbicide, Total @ 32 g a.i. ha−1 [Sulfosulfuron 75% (1-(4,6-dimethoxypyrimidin-2-yl)-3-(2-ethylsulfonylimidazo[1,2-a] pyridin-3-yl) sulfonylurea) + Metsulfuron-methyl 5% WG (methyl 2-(4-methoxy-6-methyl−1,3,5-triazin-2-ylcarba Moylsulfamoyl) benzoate)] at 32 DAS.

Root studies

The plants in each replication were de-topped from a one-square-meter field. At 60 DAS, up to a depth of 15 cm, soil core samples were obtained utilizing a root sampling pipe. Soil samples were passed through a 1 mm nylon mesh sieve and gently washed under running water to remove impurities. The cleaned roots were analyzed in WinRHIZO Root Scanner for the estimation of root parameters such as root length, surface area, volume, and diameter etc. Data obtained were used to assess root growth dynamics under different treatments.

Agronomic indices

Agronomic efficiency

Agronomic efficiency determines the yield increment due to the application of unit fertilizer material. It was calculated based on the difference in economic yield obtained from the fertilized plot and the control plot by using the following formula:

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Recovery efficiency

Recovery efficiency or apparent nutrient recovery denotes the percentage of the applied nutrient taken up and accumulated by the crop. It was calculated based on the difference in nutrient uptake obtained for fertilized crops and the control plot by applying this formula:

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Partial factor productivity for N, P, K, S

The calculation of Partial factor productivity was done by dividing grain yield (kg ha−1) by the amount of nutrient (N, P, K, S) applied and expressed as kg kg−1.

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Plant analysis

After harvesting, the collection of grain and straw samples was done and oven-dried at 65°C for 24 h to obtain a constant weight. Afterward, samples were ground and passed through a 40-mesh sieve. From each plot, plant samples were collected for estimation of N, P, K, and S. The grain and straw N were estimated via the micro-Kjeldahl method42. The phosphorus, potassium, and sulphur content in grain as well as straw were estimated using the mentioned analysis procedure42, through spectrophotometer and flame emission photometry.

Statistical analysis

A completely randomized block design was used for analysing individual parameters, and evaluated using the standard analysis of variance methodology (ANOVA). The critical differences at a 5% level of significance (P = 0.05) were estimated to find differences between treatment means, and the significance of treatment means was evaluated using the F-test43.

Results

Nutrient concentration in grain and straw

A significant difference in nutrient concentration in plant material (grain and straw) has been observed under various treatment applications (Table 2). In the present study, the N content in grain and N content straw was recorded highest with the applying 100% K dose through polyhalite (2.15%; 0.52%), and it was found to be statistically at par with T11 (2.11%; 0.52%), T12 (2.08%; 0.51%). The lowest N concentration was observed with T1 and T2, where K application was absent. The soil application of 100% K (polyhalite) revealed 4.36 and 1.89% higher grain N content over 100% K (MOP), and 100% K (MOP) + S-equivalent to T8, respectively. A similar trend was registered with P concentration in grain and P concentration in straw. However, K concentration in grain was higher under 100% K dose through polyhalite, 30.3% and 186.66% higher over 100% K (MOP) and control, respectively, while statistically at par with T11. The straw K content was observed maximum under T8 (0.25%) and 10.38% greater over the K dose through sole MOP. Regarding S concentration, T8 (0.28%; 0.25%) outshone across all the treatments, showing a 33.33% and 25% increase in grain and straw, respectively, over 100% K (MOP) application. The mean concentration of N, P, K, and S in grain was 1.96, 0.28, 0.31, and 0.24%, while in straw, 0.48, 0.24, 2.25, and 0.23%, respectively.

Table 2.

Effect of polyhalite on nutrient concentration (%) of grain and straw in wheat.

Treatment N concentration (%) P concentration (%) K concentration (%) S concentration (%)
Grain Straw Grain Straw Grain Straw Grain Straw
T1 No-K, no-S 1.78 0.42 0.2 0.18 0.15 1.79 0.21 0.19
T2 Rec. S $, no-K 1.79 0.44 0.21 0.2 0.19 1.91 0.27 0.25
T3 50% K MOP 1.90 0.47 0.25 0.21 0.23 2.04 0.22 0.20
T4 75% K MOP 1.93 0.48 0.27 0.22 0.27 2.18 0.23 0.22
T5 100% K MOP 2.06 0.51 0.33 0.24 0.33 2.31 0.21 0.20
T6 50% K polyhalite 1.94 0.48 0.27 0.23 0.29 2.20 0.24 0.23
T7 75% K polyhalite 1.95 0.49 0.29 0.28 0.38 2.43 0.26 0.24
T8 100% K polyhalite 2.15 0.52 0.35 0.3 0.43 2.55 0.28 0.25
T9 50% K MOP + S*-T6 1.90 0.47 0.26 0.22 0.27 2.18 0.24 0.23
T10 75% K MOP + S*-T7 1.94 0.49 0.28 0.23 0.37 2.43 0.25 0.24
T11 100% K MOP + S*-T8 2.11 0.52 0.34 0.29 0.42 2.54 0.27 0.25
T12 50% K MOP + 50% K polyhalite 2.08 0.51 0.34 0.28 0.38 2.42 0.23 0.22
SEm ±  0.035 0.01 0.01 0.01 0.011 0.04 0.009 0.009
LSD (P = 0.05) 0.103 0.02 0.03 0.03 0.03 0.11 0.03 0.03
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N: Nitrogen; P: Phosphorus; K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment; SEm ± : Standard Error of the Mean; LSD: Least Significant Difference.

Root dynamics

Root length density (RLD) functions as a critical parameter for evaluating the growth potential of plants under varying soil conditions, as it reflects the plant’s capacity to effectively explore its forage environment for optimal nutrient uptake. Moreover, it offers valuable insights into soil structure and health, as a higher root length density often signifies more favourable soil conditions for robust root development. The present study revealed that the root length density of wheat was influenced by polyhalite and other K sources application significantly (Table 3; Fig. 2). Among all treatments, the maximum RLD was observed with a 100% K dose through polyhalite (3578.5 cm cm−3), which was 23% and 10% higher than 100% K (MOP) and the blend composition (T12), respectively, while remaining statistically on par with T11. The mean RLD was measured at 2638.1 cm cm⁻3, with values ranging impressively from 1804.02 cm cm⁻3 to 3578.5 cm cm⁻3, showcasing notable variability across treatments.

Table 3.

Effect of polyhalite on root parameters at the flowering stage in wheat.

Treatments Root length density
(cm cm−3)		 Surface area
(cm−2 cm−3)		 Average root
diameter (mm)		 Root volume
(cm3)
T1 No-K, no-S 1804.2 375.9 0.51 11.1
T2 Rec. S $, no-K 1926.6 376.4 0.55 11.6
T3 50% K MOP 2039.2 387.7 0.58 12.7
T4 75% K MOP 2163.3 412.8 0.68 13.2
T5 100% K MOP 2908.6 524.4 0.76 15.1
T6 50% K polyhalite 2587.1 400.0 0.67 14.1
T7 75% K polyhalite 3193.1 600.7 0.82 14.7
T8 100% K polyhalite 3578.5 765.5 0.98 15.5
T9 50% K MOP + S*-T6 2334.7 435.8 0.63 13.2
T10 75% K MOP + S*-T7 2548.3 567.0 0.74 14.6
T11 100% K MOP + S*-T8 3318.1 735.9 0.94 15.4
T12 50% K MOP + 50% K polyhalite 3254.9 576.9 0.90 15.4
SEm ±  87.2 16.1 0.02 0.5
LSD (P = 0.05) 263.4 48.4 0.08 1.4
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K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment; SEm ± : Standard Error of the Mean; LSD: Least Significant Difference.

Fig. 2.

Fig. 2

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Scanned images of roots analyzed using WinRHIZO. (a) 100% K polyhalite; (b)75% K polyhalite; (c) 50% K polyhalite; (d)100% K MOP; (e) 75% K MOP; (f) 50% K MOP; (g) Blend dose of 50%K of each MOP and polyhalite; (h) Control: No-K, No-S; (i) Rec. S-bentonite, No-K.

Root surface area analysis reveals root-soil interactions, aiding in optimizing root architecture and enhancing crop productivity. The varying doses and sources of potassium in the study exhibited statistically significant effects on root surface area (Table 2; Fig. 2). The average root surface area was recorded as 513.2 cm2 cm⁻3, with a range spanning from 375.87 cm2 cm⁻3 to 765.53 cm2 cm⁻3. The maximum value was observed with 100% K (polyhalite), while the least value was observed under control (No-K, No-S; 375.87 cm−2 cm−3). The application of S-equivalent to 100% K (polyhalite) with 100% K (MOP) registered 40.33% more surface area over 100% K (MOP). However, K application through polyhalite revealed 1.5-times higher surface area over K with MOP.

Root diameter, key to estimating root biomass, is vital for understanding carbon sequestration, as finer roots, with rapid turnover, significantly impact carbon cycling in ecosystems. A significant variation in average root diameter was observed across the different potassium sources (Table 2; Fig. 2). The average root diameter ranged between 0.51–0.98 mm with a mean value of 0.71 mm. The application of 100% K (polyhalite) revealed 1.28 and ~ 2-times progression in the value of average root diameter over 100% K (MOP) and control treatment, respectively. The mean root diameter increased by 7.89% and 20.58% under 75% K (polyhalite) compared to 100% K (MOP) and 75% K (MOP), respectively. However, the minimum average root diameter was registered under control followed by No-K, Recommended S (bentonite).

Root volume represents the three-dimensional space occupied by a plant’s root system, including fine and coarse roots, and serves as a crucial metric for assessing root size and biomass (cm3). The root volume of wheat exhibited significant variation in response to the application of different K doses and sources (Table 2; Fig. 2). Across the treatments, the maximum root volume was observed under 100% K (polyhalite), which was 2.64% higher over 100% K (MOP) while at par with T5, T6, T7, T10, T11, and T12. However, 75% K (polyhalite) revealed 11.36% greater root volume over 75% K application (MOP). The root volume under various treatments ranged between 11.06 and 15.50 cm3 with a mean value of 13.9 cm3.

Nutrients efficiency indices

Agronomic efficiency (AE) stands as a pivotal metric in modern agriculture, encapsulating resource optimization and sustainability by measuring the effectiveness of applied inputs in enhancing crop production. The agronomic efficiency of K (AEk) was evaluated using a control treatment (T2; Table 3). The highest AEk was recorded under 100% K (polyhalite) (53.8 kg yield kg K−1 applied), followed by T7. The application of 75% K (polyhalite) revealed remarkable 1.83 and ~ 2-fold higher AEk over 100% K (MOP) and 75% K (MOP), respectively. However, the blend combination (MOP + polyhalite) registered ~ 32% greater efficiency over the complete dose through MOP. The AEk varied within a range of 16.8 to 53.8 kg yield kg of K−1 applied in response of various K doses and sources. The mean AEk was observed to be 36.9 kg yield kg of K−1 applied across treatments. Across the different treatments, AEs was registered highest under 100% K (polyhalite); T8 followed by T11 (Table 4). The lowest AEs was measured with Recommended S, no-K (− 44.8 kg yield kg nutrient−1 applied) followed by T9 (− 15.1 kg yield kg nutrient−1 applied). The utilization of 75% K (MOP) registered 22% higher AEs over T10, whereas application of 50% K (polyhalite) resulted in 18.5% more AEs over T9.

Table 4.

Effect of polyhalite on the agronomic efficiency of potassium and sulphur.

Treatments Agronomic efficiency
(kg yield kg nutrient−1 applied)
K S MOP vs MOP + S MOP vs Polyhalite
T1 No-K, no-S – – – –
T2 Rec. S $, no-K – − 44.8 – –
T3 50% K MOP 16.8 – – –
T4 75% K MOP 23.5 – – –
T5 100% K MOP 26.9 – – –
T6 50% K polyhalite 37.1 − 12.3 – 15.1
T7 75% K polyhalite 49.3 10 – 19.1
T8 100% K polyhalite 53.8 19.8 – 19.8
T9 50% K MOP + S*-T6 33.2 − 15.1 12.1 –
T10 75% K MOP + S*-T7 46.9 8.2 17.2 –
T11 100% K MOP + S*-T8 45.6 13.8 13.9 –
T12 50% K MOP + 50% K polyhalite 35.5 12.8 −  –
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K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment.

A comparative analysis of MOP and MOP + S revealed that the highest AEs was recorded under 75% K (MOP) + S-equivalent to T7 (control: T7), followed by 100% K (MOP) + S equivalent toT8 (control: T8). The minimum AEs was registered under 50% K (MOP) + S-equivalent to T6 (Control: T6). Similarly, a comparison between MOP and polyhalite showed that the maximum AEs was noticed with the application of 100% K (polyhalite; Control: T5) followed by 75% K (polyhalite; Control: T7; Table 4).

Partial Factor Productivity (PFP) is a key metric used to assess the efficiency of a single input—such as fertilizer, water, or labour—in generating crop yield. It provides a simple ratio of total crop output to the amount of a specific input used. The partial factor productivity of N, P, K, and S varied significantly under various K doses and sources (Table 5). The PFPN was found to be highest under 100% K through polyhalite (39.1 kg grain kg N−1 applied) followed by 100% K through MOP + S-equivalent to T8 (36.41 kg grain kg N−1 applied). A similar trend was observed with PFPP under different treatments. In reference to K, the highest PFPK was registered under 50% K (polyhalite; 164.3 kg grain kg K−1 applied) application followed by 50% K (MOP) + S-equivalent to T6 (160.4 kg grain kg K−1 applied). The least value of PFPK was observed with the application of 100% K (MOP; 90.5 kg grain kg K−1 applied). The partial factor productivity of sulphur (PFPS) was estimated highest under blend doses (50% MOP + 50% polyhalite; 146.3 kg grain kg S−1 applied).

Table 5.

Effect of polyhalite on partial factor productivity of nitrogen, phosphorus, potassium, and sulphur (kg grain kg nutrient⁻1 applied) in wheat.

Treatments Partial factor productivity
(kg grain kg nutrient−1 applied)
N P K S
T1 No-K, no-S 18.4 46.0 – –
T2 Rec. S $, no-K 21.2 53.0 – 106
T3 50% K MOP 24.0 60.0 144.0 –
T4 75% K MOP 27.1 67.7 108.3 –
T5 100% K MOP 30.2 75.4 90.5 –
T6 50% K polyhalite 27.4 68.4 164.3 121.2
T7 75% K polyhalite 33.5 83.8 134.1 98.8
T8 100% K polyhalite 39.1 97.8 117.4 86.5
T9 50% K MOP + S*-T6 26.7 66.8 160.4 118.3
T10 75% K MOP + S*-T7 32.9 82.3 131.7 97.1
T11 100% K MOP + S*-T8 36.4 91.0 109.2 80.4
T12 50% K MOP + 50% K polyhalite 33.0 82.6 99.1 146.3
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N: Nitrogen; P: Phosphorus; K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment.

Recovery efficiency (RE) quantifies plant’s nutrient absorbing and utilizing capacity, thereby reflecting nutrient use efficiency. This metric is crucial for assessing fertilizer effectiveness in enhancing crop yield and mitigating nutrient losses in environment. The REk was found highest under 100% K (polyhalite; 2.53 kg uptake kg K−1 applied) followed by 75% K (MOP) + S-T7 (2.51 kg uptake kg K−1 applied). The minimum REk was obtained with polyhalite @50% K (Table 6). The recovery efficiency of S was recorded maximum with 100% K (polyhalite) and minimum under T2 (Rec. S, no-K).

Table 6.

Effect of polyhalite on recovery efficiency (kg uptake kg nutrient−1 applied) of potassium, and sulphur.

Treatments Recovery efficiency
K S
T1 No-K, no-S – –
T2 Rec. S$, no-K – -0.09
T3 50% K MOP 0.50 –
T4 75% K MOP 0.96 –
T5 100% K MOP 1.49 –
T6 50% K polyhalite 1.58 − 0.01
T7 75% K polyhalite 2.61 0.15
T8 100% K polyhalite 2.53 0.19
T9 50% K MOP + S*-T6 1.37 − 0.02
T10 75% K MOP + S*-T7 2.51 0.12
T11 100% K MOP + S*-T8 2.19 0.14
T12 50% K MOP + 50% K polyhalite 1.88 0.12
Open in a new tab

K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment.

Financial returns

The integration of financial budgeting within the evaluation process, stakeholders such as farmers and decision-makers are empowered to make judicious choices about implementing technologies or practices that not only enhance productivity but also contribute to greater economic stability and profitability.

The cost of cultivation ranges between 435.8 to 505.9 USD ha−1 under different doses of K and S application with polyhalite and MOP (Table 7). The maximum cost was incurred in with 100% K MOP + S-equivalent to T8 (505.9 USD ha−1), whereas least cost of cultivation under no-K, no-S (control). Across the treatments, the maximum gross return was registered under 100%K (polyhalite; 1661.6 USD ha−1) followed by100% K through MOP + S-equivalent to T8 (1545.5 USD ha−1), while control treatment (T1) revealed minimum gross returns 805.7 USD ha−1). However, 100% K through polyhalite results 1.35 and 1.22-times higher net returns over 100% K usage of conventional (MOP) and blend combination (MOP + polyhalite), respectively. Polyhalite application @75% K resulted in statistically at par values with 75% K through MOP + S-equivalent toT7, and 50% K (MOP) + 50% K (polyhalite).

Table 7.

Effect of polyhalite on the economics of wheat.

Treatments COC Gross returns Net returns Net B:C
(USD ha−1)
T1 No-K, no-S 435.8 805.7 370.0 0.85
T2 Rec. S$, no-K 462.8 920.7 457.9 0.99
T3 50% K MOP 443.5 1026.5 582.9 1.31
T4 75% K MOP 447.4 1157.6 710.2 1.59
T5 100% K MOP 451.2 1309.6 858.4 1.90
T6 50% K polyhalite 469.0 1170.9 702.0 1.50
T7 75% K polyhalite 485.6 1445.1 959.6 1.98
T8 100% K polyhalite 502.0 1661.6 1159.6 2.31
T9 50% K MOP + S*-T6 469.8 1143.3 673.6 1.43
T10 75% K MOP + S*-T7 486.7 1419.2 932.5 1.92
T11 100% K MOP + S*-T8 505.9 1545.5 1039.8 2.06
T12 50% K MOP + 50% K polyhalite 476.6 1423.9 947.3 1.99
SEm ±  42.4 23.0 –
LSD (P = 0.05) 127.0 68.9 –
Open in a new tab

K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment; COC: Cost of cultivation; B:C: Benefit Cost Ratio; USD: United States Dollar; SEm ± : Standard Error of the Mean; LSD: Least Significant Difference.

In this study, net benefit–cost ratio ranged between 0.85–2.32 under various K and S treatments (Table 7). The maximum net B:C was found under 100% potassium dose (polyhalite; 2.32) followed by 100% K (MOP) + S-equivalent to T8 (2.06), whereas minimum was obtained with no-K, no-S (0.85).

Soil nutrient balance

Soil nutrient balance used to assess the nutrient input to agricultural soil and associated losses of nutrients to the environments with them. The soil nutrient balance can be measured with actual net gain or loss and apparent gain or loss of nutrients (kg ha−1). Soil K balance is a critical measure in agriculture and soil management, as it reflects the equilibrium between K inputs and outputs. Maintaining an optimal K balance is essential for sustaining soil fertility, crop productivity, and environmental health. In present study, soil K balance was significantly affected by the treatments involving addition of polyhalite and conventional fertilizer (Table 8). The peak actual net loss of soil K was observed in treatment T2 (− 5.5 kg ha−1), followed by T1 (− 4.7 kg ha−1). Conversely, the greatest actual net gain in soil K was recorded with 100% K supplied through polyhalite (20.1 kg ha−1), closely followed by the treatment entails 100% K (MOP) + S-equivalent to T8 (19.1 kg ha−1). However, the maximum apparent gain of soil K was achieved with 100% K (polyhalite; 193.8 kg ha−1), while the least gain was registered under the no-K, no-S treatment (76.7 kg ha−1).

Table 8.

Effect of polyhalite on soil potassium balance after wheat.

Treatment A B C = (A + B) D X = (C−D) E Y = (E−A) Z = (E−X)
T1 No-K, no-S 291.4 0.0 291.4 81.4 209.9 286.6 − 4.7 76.7
T2 Rec. S$, no-K 295.6 0.0 295.6 97.2 198.5 290.1 − 5.5 91.6
T3 50% K MOP 306.4 25.0 331.4 109.7 221.7 312.2 5.8 90.5
T4 75% K MOP 320.2 37.5 357.7 133.2 224.5 330.1 9.9 105.6
T5 100% K MOP 331.2 50.0 381.2 171.6 209.6 346.4 15.1 136.8
T6 50% K polyhalite 320.9 25.0 345.9 136.7 209.2 334.2 13.3 125.0
T7 75% K polyhalite 339.3 37.5 376.8 195.1 181.7 355.2 15.9 173.5
T8 100% K polyhalite 352.9 50.0 402.9 223.7 179.2 373.0 20.1 193.8
T9 50% K MOP + S*-T6 318.3 25.0 343.3 131.5 211.8 326.3 8.0 114.5
T10 75% K MOP + S*-T7 333.8 37.5 371.3 191.1 180.2 352.5 18.7 172.3
T11 100% K MOP + S*-T8 349.5 50.0 399.5 206.9 192.7 368.7 19.1 176.0
T12 50% K MOP + 50% K polyhalite 336.4 50.0 386.4 191.3 195.1 349.8 13.4 154.7
Open in a new tab

K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment; A-initial available nutrient; B-nutrient added; C-total nutrient; D-crop uptake; X-expected nutrient balance; E-Available nutrient after harvest of crop; Y-Actual gain/loss; Z-Apparent gain/loss.

In context of soil sulphur balance, actual net loss was reported maximum under 50% K dose (polyhalite) and 50% K (MOP; − 1.07 kg ha−1) followed by control one (− 0.93 kg ha−1). Furthermore, the maximum net gain was obtained with 100% K dose (polyhalite; 1.24 kg ha−1) followed by 100% K (MOP) + S -equivalent to T8 (1.20 kg ha−1). The highest apparent gain was registered under 100% K (MOP) while the maximum apparent loss under 100% K (MOP) + S-equivalent to T8 (Table 9).

Table 9.

Effect of polyhalite on soil sulphur balance after wheat.

Treatment A B C = (A + B) D X = (C-D) E Y = (E-A) Z = (E-X)
T1 No-K, no-S 9.13 0.00 9.13 14.00 − 4.87 8.20 − 0.93 13.07
T2 Rec. S$, no-K 14.98 30.00 44.98 20.51 24.47 16.28 1.30 − 8.19
T3 50% K MOP 10.13 0.00 10.13 17.86 − 7.73 9.32 − 0.81 17.05
T4 75% K MOP 10.72 0.00 10.72 21.67 − 10.95 9.84 − 0.88 20.79
T5 100% K MOP 10.81 0.00 10.81 23.07 − 12.26 10.05 − 0.76 22.31
T6 50% K polyhalite 12.65 33.92 46.57 22.90 23.67 13.42 0.77 − 10.25
T7 75% K polyhalite 14.10 50.89 64.99 30.45 34.54 15.17 1.07 − 19.37
T8 100% K polyhalite 15.06 67.85 82.91 35.89 47.02 16.30 1.24 − 30.72
T9 50% K MOP + S*-T6 12.40 33.92 46.32 22.36 23.96 13.20 0.80 − 10.76
T10 75% K MOP + S*-T7 13.73 50.89 64.62 29.41 35.21 14.78 1.05 − 20.43
T11 100% K MOP + S*-T8 15.04 67.85 82.89 32.84 50.05 16.24 1.20 − 33.81
T12 50% K MOP + 50% K polyhalite 11.36 33.92 45.28 27.09 18.19 10.29 − 1.07 − 7.90
Open in a new tab

K: Potassium; S: Sulphur; T: Treatment; $Recommended S through bentonite: 30 kg ha−1; MOP: Muriate of potash; *S through bentonite equal to respective treatment; A-initial available nutrient; B-nutrient added; C-total nutrient; D-crop uptake; X-expected nutrient balance; E-Available nutrient after harvest of crop; Y-Actual gain/loss; Z-Apparent gain/loss.

Discussion

Nutrient concentration

The absorption and accumulation of nutrients within plant tissues are predominantly depends upon the facile accessibility of nutrients. The bioavailable forms of the nutrients are synergistically bolstered by the efficient root system and the metabolic processes in plants44. The findings of the study unveiled that maximum N concentration in grain as well as straw was observed under the treatment involving exclusive application of K through polyhalite45. Polyhalite application led to a notable improvement in grain N content over MOP alone and MOP + Bentonite-S, underscoring its integrated nutrient supply advantage. A balanced supply of readily available S through polyhalite application significantly enhances N uptake in plants, while also fostering synergistic interactions with other essential nutrients. Sulphur augments N assimilation, it facilitates crucial metabolic processes (NO2− and SO4− reduction), remains fundamental to the amino acid biosynthesis and for the efficient nutrient translocation within the plant tissues46. Several researchers have highlighted a strong relationship between N and S metabolism, leading to greater nutrient acquisition. The uptake of N and P improved due to their synergistic relationship with K. The synergistic behaviour of K enhances P uptake by promoting robust root biomass and length, whereas K deficiency restricts rhizosphere growth, limiting P utilization and below-ground mobilization47,48. The enhanced K concentration in grain under polyhalite-based fertilization suggests its superior efficacy over MOP in meeting crop potassium requirements. Similarly, the elevated straw K under T8 and consistent S enrichment in both grain and straw indicate the advantage of polyhalite in supplying secondary nutrients, particularly S, in a more balanced and bioavailable form. Potassium regulates plant ion transport, including functionality of K+ channels and transporters (Shaker, TPK, TPC, KT/HAK/KUP, and HKT families), ensuring improved structural integrity and root functionality for efficient P uptake. The synchronization in demand and supply with balanced K and S fertilization ensures adequate energy (ATP synthesis) for the active transport of nutrients, while also promoting the production of coenzymes and proteins that further support energy production, thereby accelerating nutrient uptake processes. Polyhalite mitigates competition for K+ absorption by roots49, thereby enhancing K+ uptake, whereas K released from MOP shows higher adsorptive affinity with clay particles due to monovalent (K+) and divalent cations (Ca++, Mg++, etc.) competition30,50,51. Additionally, chloride toxicity associated with MOP impedes S uptake, further limiting nutrient assimilation52.

Change in root physiology

Sustainability in agro-ecosystem relies on the efficient acquisition and utilization of soil resources. Roots serves as a structural anchor and function as a primary organ for the uptake of water and nutrient, determining plants’ accessibility to the available resources, resilience to biotic and abiotic environmental stresses, and overall contribution to crop yield. The root morphology, encompassing root depth and branching, is highly plastic in nature, adapting to soil water, soil strength, and nutrient availability changes. The root parameters indicate ability of the plant to absorb nutrients in adverse conditions, and the cation exchange capacity of roots.

The outcomes of the study revealed the superior root length density under polyhalite application that underscores its favorable influence on below-ground growth, likely due to its balanced nutrient composition over conventional MOP or blended treatments. The K supply through polyhalite ensures sufficient K+ in the cytoplasm, maintaining pH balance and acting as an osmotically active agent to generate turgor pressure in elongation zone of root. Simultaneously, the presence of magnesium ions (Mg++) stimulates root hair density, thereby promoting enhanced nutrient absorption. Similar results were obtained by other researchers45,53.

The addition of sulphur alongside MOP enhanced root surface area, while polyhalite application further amplified it, showing a 1.5-fold increase over MOP alone, suggesting a synergistic benefit from its multi-nutrient composition. Sufficient S boosts nutrient uptake (N, K, P) essential for root development, acting as a redox regulator and precursor for compounds vital to root growth. Plant starvation with S disrupts N and S metabolism, leading to reduced fine root length, surface area, and biomass, ultimately impairing root activity and nutrient absorption54. The abundant Ca++ in polyhalite actively contributes to cell differentiation at the root tip and root elongation by regulating root morphogenesis through phytochrome mediation and stress signaling (auxin) in the primary root55–57.

The application of 100% K dose through polyhalite increased average root diameter by 28% over MOP and doubled it over the control. At 75% K dose, polyhalite also resulted in a greater root diameter over both 100% and 75% K dose applied through MOP. This could be due to sustained nutrient release through polyhalite, which builds up a favorable soil environment for the root growth and improves soil physical properties29,58,59.

Efficiency indices

Polyhalite at 100% K resulted in the highest agronomic efficiency (AE) and partial factor productivity (PFPNPK), outperforming other treatments including T7. At 75% K, polyhalite was more efficient over both optimal and sub-optimal doses of MOP. The blend dose of MOP and polyhalite also revealed greater efficiency over 100% K (MOP). Polyhalite, being a slow-release fertilizer, boosts crop performance by providing sustained nutrient availability. Its calcium content stabilizes membranes, supports cell division and elongation, and regulates signal transduction, while magnesium enhances photosynthesis and glucose partitioning, contributing to overall plant growth and productivity. The K supply, improves photosynthesis, protein synthesis, and starch production while mitigating water stress and promoting root growth for better water and nutrient use efficiency. The integration of S and K with N and P fertilizers enhances agronomic performance and crop-soil sustainability. Moreover, Ca++ modulates phosphorylation mechanisms that play a direct role in governing the uptake and equilibrium of essential plant nutrients60. Beside macro-nutrients (K, Ca, Mg, S), it carries eight micro-nutrients crucial for plant development. The slow-release pattern aligns with crop demand, boosting N and S efficiency while improving resource utilization. Compatible with common fertilizers, polyhalite has been widely validated for enhancing nutrient use efficiency and driving superior agricultural outcomes46.

Economics

The cost of cultivation is a cornerstone of economic decision-making in agriculture. It provides essential insights for improving productivity, optimizing resources, and ensuring both economic and environmental sustainability in farming systems. The maximum cost was incurred in treatment with 100% K MOP + S (equivalent to T8) due to the extra cost of bentonite equal to T8, followed by a 100% K dose through polyhalite. The reason for higher COC with polyhalite is that it does not come under the subsidized items under the Govt. of India so far, whereas the lowest cost of cultivation under no-K, no-S treatment is due to the absence of cost of MOP and bentonite fertilizer.

Polyhalite application consistently yielded higher net returns compared to other treatments, with the 100% K dose (polyhalite) outperforming the 100% K MOP + S (T8) treatment. At 75% K, polyhalite produced higher net returns comparable to those from MOP + S (T7) and the 50:50 K blend. The higher net returns with polyhalite (T8) registered due to the higher economic yield with efficient resource utilization and compatibility to blend physically or chemically with other fertilizers (urea and DAP). This eliminates the separate application of inputs repeatedly hence enhance profitability31,37,61. Polyhalite application also resulted in higher nutrient utilization efficiency and the synchronization over other nutrient sources leads to excellent economic budget29. Moreover, polyhalite plays a significant role in sustainable agriculture as it emits ~ 74 lb CO2 ton−1 fertilizer which is 9.45,5.40, 13.51, 20.72, 21.62 times lower carbon foot print over MOP, SSP, MAP, SOP, and DAP, respectively, making it an eco-friendly alternative. This can potentially reduce carbon emission aligns with global climate goals, lowering GHGs contributions from agricultural activities.

The highest net B:C ratio was achieved with 100% K (polyhalite), followed by the 100% K MOP + S treatment (T8). Economically, its slow-release nature optimizes nutrient efficiency, reducing the frequency and quantity of applications required, thereby cutting operational costs for farmers. Furthermore, its multi-nutrient composition enhances crop yield and quality, increasing profitability while promoting sustainable farming practices26,30,62. By combining environmental benefits with cost-effectiveness, polyhalite stands as a valuable tool for achieving greener and more economically viable agriculture.

Soil nutrient balance

Soil K balance was influenced significantly under treatments of polyhalite application (Table 7) and the maximum actual net loss of soil K was observed in treatment T2, followed by T1 as K was not applied in these treatments. Conversely, the highest actual net gain in soil K was observed under polyhalite and MOP + bentonite-S (T8). The increase in the soil available K is due to the longer-term supply of K through polyhalite because of the slow release, and efficient nutrient delivery system46. However, the maximum apparent gain of soil K was achieved with 100% K (polyhalite), while the least gain was registered under the no-K, no-S treatment. This build-up in the soil might be due to increased availability of K through mineralization with polyhalite application and the addition of K through decomposition of root biomass. The application of sub-optimal doses of S under T12 revealed actual net loss of available S in soil. Across the polyhalite doses, highest net gain was registered with 100% K. Compared to other sources, polyhalite supplies higher S to the soil and remains available for long-term in soil and maintain soils’ ecosystem services properly. The apparent gain was highest under 100% K (MOP), while the apparent loss was maximum with 100% K MOP + S (equivalent to T8). This gain was maximum with S applied through bentonite equal to S-T8 as polyhalite and bentonite contain 19% S and 90% elemental S, respectively. However, sulphur use efficiency is low through polyhalite, ranging between 12 and 15%. Therefore, it leads to soil S fixation and may be available in the next crop.

Conclusion

The study proved that the multi-nutrient carrier polyhalite exhibits remarkable potential to seamlessly deliver K, Ca, Mg, and S in readily available forms, aligning with the plant’s nutritional demands at critical growth stages. Among the different K doses and sources, 100% K (polyhalite) revealed outshined performance in terms of nutrient acquisition, improved root functionality, and soil nutrient balance. The application of 100% K (polyhalite) demonstrated a significant advantage in soil nutrient enrichment, resulting in a net gain of 5 kg ha−1 in available K and 2 kg ha−1 in available S compared to 100% K (MOP). These findings underscore the efficacy of polyhalite as a superior nutrient source for improving soil K and S reserves, contributing to sustainable soil fertility management. Moreover, the multi-nutrient nature of polyhalite, fostering a balanced nutrient supply and synchrony among various nutrient and ion interactions, ultimately enhancing crop responses.

Author contributions

VK, KS: Experimentation, data recording, analysis, writing manuscript; SB, SSR: Supervision, Resources, Software, reviewing and editing; PKU, RKS: Data recording, visualization, software; AG, AP & SR: Review and editing, Resources; SGD & KD: Review and editing; DS: Final editing of manuscript (Revised) All author reviewed the manuscript and agreed for submission.

Data availability

Data is provided within the manuscript.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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