Optimizing polyhalite (POLY-4) use in the maize-wheat system: A comparative case study from upper and Trans Indo-Gangetic plains of India

Abstract

Increasing complexity in crop nutrient requirement in intensive crop production systems needs alternate multi-nutrient sources. Polyhalite (POLY-4) which contains 14% K₂O along withcalcium (17% CaO), magnesium (6% MgO), and sulfur (19% S) can be a possible recourse in this regard. In maize-wheat systems, it was evaluated for productivity, profitability, nutrient usage, and nutrient use efficiency under Indo-Gangetic plain (IGP) zones for consecutive two years (2018-19 and 2019-20). The results revealed that 150% K through POLY-4 produced the maximum maize grain yield under the Trans Indo-Gangetic plains (TGP). The maize grain yield increased by 20.8% and 26.2% under 100% K (POLY-4) and 150% K (POLY-4) over No–K, respectively. But statistically, 100% K (POLY-4) stands similar with both 150% K (POLY-4) and 150% K through muriate of potash (MOP) and equivalent. The trends were noticed under upper Indo-Gangetic plain zones (UGP) also. Similarly, the maximum wheat grain yield (6.12 and 6.29 t/ha under TGP and UGP, respectively) was obtained under 150% K (POLY-4), and remained statistically at par with 100% K (POLY-4), but significantly higher than 150% K (MOP). Under both agro-ecologies i.e. TGP and UGP, the highest system productivity was obtained with recommended N, P, and 150% K application through POLY-4. The added return over NPK remained highest with 150% K (POLY-4) for both maize and wheat. However, higher partial factor productivity for N and S, agronomic, physiological, and translocation efficiencies were noticed under 150% K (POLY-4), and remained at par with 100% K (POLY-4). Increased system yield, added returns, partial factor productivity, agronomic, physiological, and translocation efficiencies under 100% K through POLY-4 (along with recommended N and P) proved its effectiveness as multi-nutrient source for the maize-wheat system under TGP and UGP.

1. Introduction

Maize (Zea mays L.) - wheat (Triticum aestivum L.) system is the 3rd most important agricultural production system in the Indo-Gangetic plains (IGP) after rice (Oryza sativa L.)-wheat and cotton (Gossypium hirsutum L.)-wheat systems, occupying an area of about 1.13 million hectares [1]. IGP is considered as the food bowl of the entire Indian sub-continent (Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka). The increasing population pressure in the Indian sub-continent has compelled us to intensify the food production system to meet the food and fodder requirements of humans as well as livestock adequately. Cropping system intensification with enhanced system productivity leads to massive nutrient mining from the soils. Inadequate nutrient supply in intensive production systems and the inclusion of high-yielding crop varieties (HYVs) with poor agronomy are some of the reasons for the renewed threats to yield sustainability. To attain a sustainable higher productivity, balanced fertilization, site-specific precision nutrient management, customized fertilizers, etc., are some of the best nutrient management approaches to be adopted. There is an unrest need for some effective multi-nutrient sources, which not only have high nutrient content, but also ensure synchronous nutrient release as per crop need. Therefore, best management practices (BMPs) in nutrient management are centered around the right rate, right timing, right source, and right placement (4Rs) for better crop response and enhanced nutrient use efficiency. Ironically, in the Indian sub-content, N fertilizer is being used in the largest quantity for raising the crops, whereas, potassium (K) is being removed in the largest quantity by the crops. Therefore, the importance of K in Indian agriculture for balanced fertilization and achieving higher farm and crop efficiency has been widely felt , especially when21% of Indian agricultural soils have been categorized as low, 51% as medium, and while only 28% soils are high in available K status [2,3].

Low K intakes from Indian soils has also become a serious threat to human health. India requires 2.0 million tonnes (MT) ofK fertilizers every year. The main source of potassium fertilizer in the country is murate of potash (KCl). As an alternative to conventional K fertilizers, POLY-4 is a naturally occurring polyhalite having 14% K₂O [3]. Along with potassium, polyhalite also contains calcium (17% CaO), magnesium (6% MgO), and sulfur (19% S). It occurs with 90% content as ore and the remaining 10% is anhydrite, magnesite, kieserite, hexahydrite, szaibelyite, gypsum, halite, mica, and syngenite. Besides four major nutrients, POLY-4 also contains eight main micronutrients, including boron, copper, iron, manganese, molybdenum, selenium, strontium, and zinc. POLY-4 as a polyhalite is mined from the earth, along the north-eastern coast of the United Kingdom [4], and has lower environmental impacts than other fertilizers [5]. It is a raw-processed but not a refined product, therefore involves lesser environmental footprint. In the present time fertilizers have become one of the main sources of greenhouse gas (GHGs) emissions among all the sources in agriculture. In this background, the newer nutrient sources with minimal environmental footprint are of great interest. The multi-nutrient nature and lesser losses cause higher nutrient use efficiency along with better crop growth and productivity and low cost of production. In spite of being a multi-nutrient source, POLY 4 usage among farmers remains a challenge due to its availability, ease in its application, and overall effectiveness in enhancing crop yield and economics.

Polyhalite releases nutrients more slowly than traditional fertilizers [6], which may contribute to higher fertilizer use efficiency [7]. The transport and leaching of different minerals present in POLY-4 are relatively less. The efficacy of polyhalite has been examined for many field crops viz. maize [8]; cabbage (Brassica oleracea var. capitata) [9]; mustard (Brassica spp.); sorghum (Sorghum bicolor, L.) [10], potato (Solanum tuberosum) [4]; tomato (Solanum lycopersicum L.) [11,12] in different parts of the world. However the efficacy of POLY-4 as a K source has not been evaluated for intensive agro-regions of IGPs. Also, the relative performance of POLY-4 over conventional K fertilizers has not been assessed for maize and wheat crops in IGP so far. Thus, in order to explain the scientific application of POLY-4 in TGP and UGP, the response, uptake, and efficiency studies were carried out in maize and wheat. The hypothesis was to evaluate the performance of POLY-4, assess its efficacy over MOP, and optimize the POLY-4 application rates for maize and wheat under two diverse eco-regions, i.e. TGP and UGP.

2. Materials and Methods

2.1. Site description

The field experimentations on the assessment of the efficacy of POLY-4 (Polyhalite) in the maize-wheat system under Indo-Gangetic Plains (IGP) for enhanced yield, K, and S use efficiency were conducted at two locations. The on-station trial was carried out at the research farm of ICAR-Indian Agricultural Research Institute located at Latitude 28°38′23"N, Longitude:77°09′27"E., Altitude:228.61 m above mean sea level. Geographically, the on-station site comes under Trans Gangetic Plain (TGP) zone. The soil at TGP was sandy loam, comprising of ∼ 67, 18 and 15% sand, silt and clay, respectively. Taxonomically, the experimental soil belongs to Inceptisol (Typic and Haplustept) of Gangetic alluvial origin, slightly sodic in reaction (pH (1: 2.5: soil: water) 7.9), non-saline (electrical conductivity (1:2.5 soil: water) 0.42 dSm⁻¹), very deep (>2.0 m) and are well-drained. Regarding the fertility level, the experimental soils were low, medium, and medium category of oxidizable soil organic carbon (Walkley–Black C 4.4–4.7 g/kg), plant available P (16 kg/ha) and plant available K (155 kg/ha), respectively. The details about soil properties are being given in Table 1. While another experiment with same set of treatments was conducted on-farm, under Upper Gangetic Plain (UGP) zones in Indian at Bulandshahr district in the North India state of Uttar Pradesh. It is located (Fig. 1) between 28.4° and 28.0° north latitudes and between 77.0° and 78.0 east longitude. The soils at UGP are loamy sand (60, 22, 18% sand, silt and clay, respectively). Bulandshahr is located in the UGP, has a semi-arid subtropical climate with, dry hot summers and cold winters. The average maximum temperature is 45°C and the average minimum temperature is 4.5°C. The soils of the experimental sites are sandy loam of Gangetic alluvial origin, very deep (>2 m), well-drained, and low in organic C. The maize-wheat system has been widely practiced in TGP as well as in UGP. Hence, the experiments were conducted in maize-wheat system for two consecutive years (2018-19 and 2019-20 respectively) at both locations.

Table 1.

Soil characteristics in experimental site at two locations.

Parameter		TGP	UGP
Soil type (Texture)		Loamy sand	Sandy loam
Soil organic carbon (g/kg)		4.4	4.7
pH		7.91	8.01
EC (dS/m)		0.36	0.33
BD (Mg/m3) (0–15 cm)		1.49	1.40
Available N (kg/ha)	0–15 cm	284	291
	15–30 cm	226	237
Available P (kg/ha)	0–15 cm	21.5	22.2
	15–30 cm	18.5	19.4
Available K (kg/ha)	0–15 cm	243	267
	15–30 cm	211	216
Available S (mg/kg)	0–15 cm	10.6	10.5
	15–30 cm	9.3	9.2

Fig. 1.

Exact location of experimental sites in IGP.

2.2. Experimental design and treatments details

The experiments were laid out in a completely randomized block design with a total of eight treatments replicated thrice at on-station (TGP), while un-replicated traits were conducted at five locations at farmers field (UGP) with the same set of treatments in maize-wheat system. The field experiments in maize and wheat crops in system mode were carried out, with 08 treatments in completely randomized block design (CRBD), replicated thrice. The 08 treatments of multi-nutrient source, POLY-4 (polyhalite) were tested. as, T1-Recommneded (rec.) NP and S (no-K), T2- Recommended NP and K (no-S), T3- Recommended NP and 50% of recommended K through POLY-4, T4- Rec. NP and 100% of rec. K through POLY-4, T5-Recommended N, P and 150% of rec. K through POLY-4, T6- Rec. NP and 50% of rec. K through MOP + S equal to 50% K supply in T₃ through Bentonite, T7- Rec. NP and 100% of rec. K through MOP + S equal to T₄ through Bentonite, T8-Rec. NP and 150% of rec. K through MOP + S equal to T₅ through Bentonite for efficacy in crop yield, nutrient usage, acquisitions etc. The T1 and T2 were used as control for K and S, respectively, while treatments T3-T5 were comprised of POLY-4 based on recommended K and in treatments T6-T8, a similar amount of K was applied through MOP, and sulfur was compensated through bentonite application corresponding to supply of S through POLY-4 in treatments T3-T5. POLY 4 is the trademark name of all polyhalite from Sirius Minerals. Polyhalite is a naturally occurring, evaporated mineral formed from the dried-up bed of an ancient sea or ocean. Chemically, it is a hydrated potassium, calcium, and magnesium sulphate salt with the chemical formula K₂SO₄.MgSO₄.2CaSO₄.2H₂O. The recommended doses of N, P, K and S were 150, 33.5, 62.5, 30 and 120, 26.2, 50, and 30 kg/ha for maize and wheat crops, respectively in Indo Gangetic Plain (IGP) zones of Northern India [55]. The S source of POLY 4 was evaluated against Bentonite S, based on S supply from POLY-4 for maize and wheat crops under TGP and UGP locations. The details of nutrients added under different treatments have been shown in Table 2.

Table 2.

Nutrients applied in maize and wheat crops (Based on elemental content of P and K).

Treatments	Maize(kg/ha)				Wheat (kg/ha)
	N	P	K	S	N	P	K	S
No–K	150	33.5	0.0	30.0	120.0	26.2	0.0	30.0
No–S	150	33.5	62.5	0.0	120.0	26.2	50.0	0.0
50% K (POLY-4)	150	33.5	31.3	50.9	120.0	26.2	25.0	40.7
100% K (POLY-4)	150	33.5	62.5	101.8	120.0	26.2	50.0	81.4
150% K (POLY-4)	150	33.5	93.8	152.7	120.0	26.2	75.0	122.1
50% K (MOP) + S1	150	33.5	31.3	50.9	120.0	26.2	25.0	40.7
100% K(MOP) + S2	150	33.5	62.5	101.8	120.0	26.2	50.0	81.4
150% K (MOP) + S3	150	33.5	93.8	152.7	120.0	26.2	75.0	122.1

Note: N: nitrogen, P: phosphorus, K: potassium, S: sulfur, MOP: murate of potash, rec.: recommended. The recommended dose of N, P, K and S for maize and wheat remains 150:75:75:30 and 120:60:60:30, respectively; S₁: S equal to T3 through Bentonite, S₂: S equal to T4 through Bentonite, S₃: S equal to T5 through Bentonite.

2.3. Polyhalite application and crop management

The standard field operations were followed for the preparation of soil for both crops. Maize was sown on 2nd July across both locations and was harvested during October 15–16 in both crop seasons. After the harvest of maize, the sowing of wheat was done on 10th November during both the years. The duration of wheat variety (HD 2967) used was 145 days, while PMH-1, a maize hybrid of 110 days duration was sown. The irrigation scheduling was done on the basis of critical physiological stages in both crops. The weeds were controlled with pre and post-emergence use of selective herbicides in maize and wheat. The plant protection against weeds, insects, pests and diseases was done with strict adherence of standard plant protection protocols. Polyhalite (POLY-4) was used on the basis of the supply of potash to the crops and accordingly, the doses of other nutrients were adjusted. POLY-4 is a multi-nutrient source as it contains 14% K₂O, 14% S, 6% MgO and 17% CaO. The soils in the experimental site were deficient in available K and S, along with N and P. The whole amount of POLY-4 was applied as a basal dose at the time of last ploughing before sowing. The doses of N were split in both the crops for ensuring higher nitrogen use efficacy. In maize, recommended dose of atrazine (750–800 g ai/ha) as pre-emergence followed by either of the post-emergence herbicide of Tembotrione @ 34.4% SC (150 g ai/acre) at 20–30 days after swing for effective weed control. In maize, for control of fall armyworms and other insects, timely and uniform sowing over a larger area, seed treatment with cyantraniliprole 19.8% + Thiamethoxam 19.8% FS @ 6 ml/kg of seed offers protection for 15–20 days of crop growth, application of the recommended insecticides for FAW, viz., Thiamethoxam 12.6 % + Lambda-cyhalothrin 9.5% ZC (50ml/acre) @ 0.25 ml/L; Spinetoram 11.7% SC (100ml/acre) @ 0.5 ml/L; Emamectin benzoate5% SG (80g/acre) @ 0.4 g/L was done. For control of leaf blight in maize, two sprays of Mancozeb (2-3 gm/litre) were done at 15 days interval immediately after disease appearance. In wheat, for cost-effective broad-spectrum weed control, sulfosulfuron 0.025 kg/ha and a mixture of clodinofop + metsulfuron (4g) were used. . The mixture of isoproturon 1.0 + 2,4-D @ 0.5 kg/ha and clodinofop 0.06 fb 2,4-D 0.50 kg/ha have also been recommended. The IPM module of the seed treatment with T. viride (@4 g/kg seed) + carboxin (75WP @1.25 g/kg seed) or tebuconazole (@ 1.0 g/kg seed) for the control of loose smut, followed by broadcast of insecticide-treated soil (with chloropyriphos @ 3L/ha) at 15 DAS for termites. Rust disease management in wheat was done through spraying the crop with propiconazole (Tilt 25 EC @ 0.1%). The details of nutrient applied in different treatments has been given in Table 2.

2.4. Soil and plant analysis

The soil analysis for available N, P, K S, and micronutrients was done from the representative soil samples. The soil samples were collected from each plot from three points and appropriately sorted and made into one truly representative sample of the specific plot, from a soil depth of 0-15-cm at on-station (Trans Gangetic Plains) and Upper Gangetic Plains zone (on-farm), before start of the field experiment in 2018–19. Soil samples were also collected from each plot after wheat harvest at 0-15-15 cmcm soil depth during 2019. The collected soil samples were composited and a sub-sample was pounded using a ceramic pestle and mortar and passed through a 100-mm sieve. Alkaline KMNO₄ method [13], extractable P (0.5 M NaHCO₃, pH 8.5 extraction) [14], exchangeable K (1 M NH₄OAc, pH 7.0 extraction) [15], non-exchangeable K (1 M HNO₃ extraction) [15], extractable S (0.15% CaCl₂ extraction) [16], were used to analyze these finely-ground and pulverized soil samples. The pipette method was used for particle size analysis for initial soil samples. The initial soil properties for each location are reported in Table 2. The plant samples from vegetative and grains were ground in a stainless steel Wiley mill and the representative sub-samples were dried at 70 °C, and then wet digested with concentrated H₂SO₄ for determination of total N, or digested with concentrated HNO₃ and HClO₄ (mixed in 4:1 ratio) for determination of total P, K, S, and Zn. The N content was determined by the Kjeldahl method using an auto-analyzer, and P was determined by vanadomolybdate yellow color method [17]. Total K content was determined by flame photometry. S was determined turbidimetrically [18 ]using an ultraviolet–visible (UV–VIS) spectrophotometer.

2.5. Nutrient use efficiencies (NUE), acquisitions and economics

Nutrient use efficiency was estimated for NPK and S as partial factor productivity (PFP_N,P,K,S) (equation I), agronomic efficiency (equation II) and recovery efficiency (equation III). The nutrient source was compared with POLY-4 on the basis of various NUE measurements and calculations [19,20].

$$\begin{array}{cc}\begin{array}{c}\text{Partial}\text{Factor}\text{Productivity}(\text{PFP})=\\ (\text{kg}\text{grain}/\text{kg}\text{nutrient})\end{array}& \mathrm{Y}/\mathrm{F}\end{array}$$ I

$$\begin{array}{cc}\begin{array}{c}\text{Agronomic}\text{Efficiency}(\text{AE})=\\ (\text{Kg}\text{grain}/\text{kg}\text{nutrient})\end{array}& (\mathrm{Y}-\mathrm{Y}0)/\mathrm{F}\end{array}$$ II

$$\begin{array}{cc}\text{Recovery}\text{Efficiency}(\text{RE})\%=& (\mathrm{U}-\mathrm{U}0)/\mathrm{F}\end{array}$$ III

Here, Y is the yield of the harvested portion of crop with nutrient application; Y0 = yield without nutrient application; F = amount of nutrient applied; UH = nutrient content of harvested portion of the crop; U = total nutrient uptake in aboveground crop biomass with nutrient applied; U0 = nutrient uptake in aboveground crop biomass with no nutrient applied; N = nutrient in percentage.

The economic analysis of the various treatments was performed to assess the economic viability of various combinations. The economics was estimated based on actual expenditure incurred in field experimentation and in imposing the specific treatments. The common cost of cultivation of maize and wheat was as per the recommendation of the Directorate of Economics and Statistics, Ministry of Agriculture and Farmers Welfare. However, the added cost of specific treatments was calculated based on prevailing market prices of those nutrient sources and the dose of the nutrient sources. The gross returns from wheat and maize grain was estimated by multiplying grain yield with the minimum support price of a particular crop, as declared by the Commission of Cost and Prices (CACP). The cost of Polyhalite was estimated as ₹ 14/kg product, based on all the expanses to produce Polyhalite in a commercial product. The added returns were estimated as.

Added gross return: Price of produce x quantity of additional produce due to specific treatment

Added return = A₁-A₀ IV

Where, A₁ denotes added gross return due to specific treatment over, A₀ means the cost of specific added treatments in the study.

2.6. Statistical analysis and computations

Data were analyzed to determine the effects of fertilizer treatment on the productivity of maize and wheat, nutrient usages, efficiency, nutrient harvest index, soil properties, and changes in soil nutrients. The homogeneity of variance for all variables was first tested to ensure that residual errors have identical variances across all treatment groups [21]. The normality of the residuals was tested using the Shapiro–Wilk statistic obtained from the NORMAL option in the PROC UNIVARIATE statement, and data were transformed as required for analysis. Among the dependent variables, maize and wheat grain yield, maize equivalent yield, nutrient uptake (N and S), nutrient acquisition in the form of various indices, microbial biomass carbon in soil, and dehydrogenase activity in soil were analyzed using a model with a pooled error variance. Comparisons among treatment means were determined using the Duncan Multiple Test Range (DMRT) at α = 0.05. The mention of significance refers to the 5 % level of probability (α = 0.05) unless otherwise specified.

3. Results

3.1. Agronomic productivity

The highest maize grain yield under TGP was obtained with the application of 150% K through POLY-4 (Table 2). In TGP, the maize grain yield obtained under 100% K (POLY-4) and 150% K (MOP) + S₁ remained statistically at par. A 20.8% and 26.2% increase in maize grain yield was obtained with 100% K (POLY-4) and 150% K (POLY-4) over No–K. However, this increase with 100% and 150% K (MOP) was 15.5 and 18.9% only over no-K. Likewise, a 12.9% and 14.9% increase in maize grain yield was obtained with 100% and 150% K through (POLY-4) over no-S. This increase remained as 5.1 and 8.2% with 100% and 150% K (MOP) over no-S. The 100% K (POLY-4) stands similar with both 150% K (POLY-4) and 150% K (MOP) and equivalent S. Under UGP, the highest maize yield was also obtained under 150% K (POLY-4), which remained at par with 150% K (MOP) and statistically superior over 100% K application either through POLY-4 or MOP indicating a higher demand for K in these areas. The comparable results of 100% K either through POLY-4 or MOP highlight that using POLY-4 as a K source may omit the requirement of additional S application for maize. A 3.5% increase in maize grain yield with 100% K (POLY-4) over N, P and K (No–S) emphasizes the advantage of S applications in the systems. This increase in maize grain yield was recorded as high as 23.9% with 150% K (POLY-4). A 17.8% and 19.7% increase in the maize grain yield was obtained with 100% and 150% K (MOP), respectively over no K. Also, a 3.6 and 7.5% increase in the maize grain yield with 100 and 150% K (POLY-4), respectively over no-S emphasizes the importance of balanced nutrition (Table 3).

Table 3.

Grain yield of maize, wheat and system productivity under different nutrient management.

Treatments	Maize grain yield (t/ha)		Wheat grain yield (t/ha)		System productivity (t/ha)
	TGP	UGP	TGP	UGP	TGP	UGP
No–K	6.19e	6.28e	5.12e	5.00e	11.77e	11.72e
No–S	6.80d	7.24cd	5.53cd	5.40d	12.81d	13.12cd
50% K (POLY-4)	6.91d	7.02d	5.62d	5.49cd	12.87cd	12.99d
100% K (POLY-4)	7.48ab	7.50b	5.95ab	5.88b	13.96ab	13.90b
150% K (POLY-4)	7.81a	7.78a	6.12a	6.29a	14.48a	14.63a
50% K (MOP) + S1	6.64de	6.89d	5.30de	5.35d	12.35de	12.71d
100% (MOP) + S2	7.15bcd	7.40bc	5.71bc	5.75bc	13.36bc	13.65bc
150% K (MOP) + S3	7.36abc	7.52ab	5.86b	5.97b	13.73b	14.02b

Note: S₁: S equal to T3 through Bentonite, S₂: S equal to T4 through Bentonite, S₃: S equal to T5 through Bentonite; TGP = trans-Gangetic Plains, UGP = upper-Gangetic Plains.

Values followed by different letters as superscripts within a column are significant at p < 0.05.

Similarly, the highest wheat grain yield in TGP was recorded with 150% K (POLY-4) which remained at par with 100% K (POLY-4). A 16.2% and 19.5% increase in the grain yield was recorded with 100% and 150% K (POLY-4), respectively over no K. This increase was, however 11.5 and 14.5%, respectively with 100 and 150% K (MOP) over no-K. Likewise, a 7.6 and 19.5% increase in wheat yield with 100 and 150% K (POLY-4) was recorded over no-S and this increase was only 3.3 and 6.1% over no-S with 100 and 150% K (MOP). In UGP, the highest wheat yield was obtained with 150 % K (POLY-4), followed by 150 %K (MOP). But, the wheat yield obtained with 150% K (MOP) remained statistically similar with 100% K either through POLY-4 or MOP, highlighting the similar response of 100% K (POLY-4) and 150% K (MOP). An additional 50% K (MOP) can be thus saved if the recommended K is applied through POLY-4. Interestingly, 100% K (MOP) also remains similar to 50% K (POLY-4). The wheat yield increased by 17.6 and 25.8% with 100 and 150% K (POLY-4), respectively over no-K. Not only K, but S application, through 100 and 150% K (POLY-4) also increased wheat yield by 8.9 and 16.5% respectively, over no-S. In both TGP and UGP, the wheat grain yield obtained under 100% K (POLY-4) and 150% K (MOP) remained statistically at par which implies that 100% K (POLY-4) remained similar to 150% K (MOP). In both TGP and UGP, the highest system productivity was obtained with full N, P and 150% K application through POLY-4 (T5). The T7 treatment where 100% recommended K was applied through MOP produced statistically at par maize and wheat yields and system productivity invariably in both regions with T8 where an additional 50% K was applied.

3.2. Economic returns

In TGP, the highest added cost over NPK remained at 150% K (MOP), which was 13.1% higher over 150 % K (POLY-4) in maize (Table 4). The added cost was 73.7 % higher with 150 % K over 100 % K (POLY-4). In wheat, a 12.2 % higher added cost was incurred with 150 % K (MOP) over 150 % K (POLY-4). The system-added cost was 73.8 and 70.7% higher with 150% K through POLY-4 and MOP over 100% K from both sources. Under TGP, the added return over NPK remained highest with 150% K (POLY-4) for both maize and wheat. In maize, a respective increase of 49.9% and 57.9% was recorded in the added returns with 150% K over 100% K (POLY-4) and 150% K over 100% K (MOP). The added return over NPK with 150% K (POLY-4) and 150% K (MOP) was 44.5 and 66.7% over 100% K through POLY-4 and MOP, respectively. In TGP, with an added cost of 73.7% with 150% K (POLY-4) over 100% K (POLY-4) in both maize and wheat, a 48.8 and 37.2% increase in the added return was recorded with maize and wheat, respectively. Under UGP, with a similar added cost as in TGP, the added returns in both maize and wheat remained negative with no-K. Unlike TGP, the higher added returns were recorded with the application of POLY-4 over MOP. A 125.9 and 68.2% increase in the added cost was recorded with 150% K (POLY-4) over 100% K (MOP) in maize and wheat, respectively. This increase was however, 19.1 and 56.1% only with 150% K (MOP over 100% K (MOP) in maize and wheat, respectively.

Table 4.

Economics (₹/ha) terms of added cost and added return over RDF (NPK) of maize, and wheat under different nutrient management.

Treatments	Maize				Wheat				System
	TGP		UGP		TGP		UGP		TGP		UGP
	Added cost over NPK	Added return over NPK	Added cost over NPK	Added return over NPK	Added cost over NPK	Added return over NPK	Added cost over NPK	Added return over NPK	Added cost over NPK	Added return over NPK	Added cost over NPK	Added return over NPK
No–K	−650	−10643	−650	−1769	−200	−9970	−200	−8759	−850	−20613	−850	−10528
No–S	0	0	0	0	0	0	0	0	0	0	0	0
50% K (POLY-4)	1238	2160	1238	−121	1009	−1486	1009	3268	2247	674	2247	3148
100% K (POLY-4)	4714	12304	4714	990	3818	9293	3818	12816	8532	21597	8532	13806
150% K (POLY-4)	8202	18452	8202	2236	6627	12753	6627	21559	14829	31205	14829	23795
50% K (MOP) + S1	1591	−2018	1591	34	1279	−7506	1279	−1227	2870	−9524	2870	−1192
100% (MOP) + S2	5429	6698	5429	1346	4357	4309	4357	10023	9786	11008	9786	11370
150% K (MOP) + S3	9271	10578	9271	1603	7435	7765	7435	15650	16706	18343	16706	17253

Values followed by different letters as superscripts within a column are significant at p < 0.05.

3.3. Partial factor productivity of nutrients

Under TGP, in maize, the highest PFP-N was recorded with 150% K (POLY-4), which remained at par with 100% K (POLY-4) (Table 5). The PFP-N with 100% K (POLY-4) remained at par with 100 and 150% K (MOP). An increase of 25.9 and 14.8% in the PFP-N was recorded with 150% K (POLY-4) over no-K and no-S, respectively. This increase was, however, 18.6 and 8.2%, respectively with 150% K (MOP) over no-K and no-S. Under TGP in wheat, the maximum PFP-N was recorded with 150% K (POLY-4). The PFP-N with 100% K (POLY-4) remained at par with 100 and 150% K (MOP). A 19.4% and 10.9% increase in the PFP-N was recorded with 150% K (POLY-4) over no-K and no-S, respectively. This increase was, however, 14.2 and 6.1% with 150% (MOP) over no-K and no-S, respectively. In UGP, however, the maximum PFP-N was recorded with 150% K (POLY-4) which was significantly superior over other treatments. Like TGP, the PFP-N in wheat under 100% K (POLY-4) remained at par with both 100 and 150% K (MOP). Under both TGP and UGP, the PFP-P in maize remained highest with 150% K (POLY-4). It remained statistically superior over all other treatments in TGP, however, under UGP it remained at par with 150% K (MOP). The on-par PFP-P with 100% K (POLY-4) and 150% K (MOP) under both TGP and UGP has been recorded.

Table 5.

Partial factor productivity (kg grain/kg nutrients) of nitrogen, phosphorus, potassium and sulfur of maize and wheat as influenced by various nutrient management options.

Treatments	PFP-N				PFP-P				PFP-K				PFP-S
	Maize		Wheat		Maize		Wheat		Maize		Wheat		Maize		Wheat
	TGP	UGP	TGP	UGP	TGP	UGP	TGP	UGP	TGP	UGP	TGP	UGP	TGP	UGP	TGP	UGP
No–K	41.3e	41.8e	42.7e	41.7e	184.8e	187.3e	195.5e	190.9e	–	–	–	–	206.33a	209.2a	170.8a	166.8a
No–S	45.3cd	48.3cd	46.0c	45.0d	202.8cd	216.1cd	210.9c	206.1d	108.7d	115.8b	110.5d	108.0C	–	–	–	–
50% K (POLY-4)	46.1cd	46.8d	45.6cd	45.7cd	206.4cd	209.6cd	208.9cd	209.3cd	220.9a	224.3a	218.9a	219.4a	135.8b	137.9b	134.4b	134.7b
100% K (POLY-4)	49.8ab	50.0ab	49.6ab	49.0b	223.3ab	223.8ab	227.2ab	224.5b	119.7C	120.0b	119.0c	117.6b	73.5C	73.7C	73.1d	72.2c
150% K (POLY-4)	52.0a	51.9a	51.0a	52.4a	233.2a	232.2a	233.0a	240.1a	83.3e	82.9c	81.6e	83.9d	51.2d	51.0d	50.1e	51.5d
50% K (MOP) + S1	44.3d	45.9d	43.7de	44.5d	198.2cd	205.7d	200.0de	203.9d	212.4b	220.1a	209.6b	213.8a	130.5b	135.4b	128.7c	131.3b
100% (MOP) + S2	47.6bc	49.3bc	47.6bc	47.9bc	213.3bc	220.7bc	218.0bc	219.4bc	114.3cd	118.3b	114.0cd	115.0bc	70.2C	72.6c	70.2d	70.6c
150% K (MOP) + S3	49.0b	50.2ab	48.8b	49.7b	219.7b	224.5ab	223.0b	227.8b	78.5e	80.2c	78.1f	79.56d	48.2d	49.2d	48.0e	48.8d

Note: S₁: S equal to T3 through Bentonite, S₂: S equal to T4 through Bentonite, S₃: S equal to T5 through Bentonite.

Values followed by different letters as superscripts within a column are significant at p < 0.05.

For succeeding, wheat crop, under TGP the PFP-N with 150 and 100% K (POLY-4) remained statistically similar, significantly superior over 100 and 150% K (MOP). A 19.2 and 10.5% increase in the PFP-P has been recorded with 150% K (POLY-4) over no-K and no-S, respectively. This increase was, however, 14.1 and 5.7% only with 150% K (MOP) over no-K and no-S, respectively. However, in UGP, a 25.7 and 16.4% increase in the PFP-P was recorded with 150% K (POLY-4) over no-K and no-S, respectively, however, this increase was only 19.3 and 10.5% with 150% K (MOP). Under TGP, in maize, the highest PFP-K was recorded with 50% K (POLY-4), followed by 50% K (MOP). The PFP-K with 100% K either through POLY-4 or MOP remained statistically similar. The minimum PFP-K was however recorded with 150% K (POLY-4). Under UGP, in maize, a similar PFP-K was recorded with 100% K either through POLY-4 or MOP, which remained similar with no-S also. In wheat under TGP, the highest PFP-K was recorded with 50% K (POLY-4), followed by 50% K (MOP). However, in UGP, a statistically similar PFP-K was recorded with 50% K either through POLY-4 or MOP. Under TGP in wheat, although, a statistically similar PFP-K was obtained with 100% K either through POLY-4 or MOP, but under 150% K application, a higher PFP-K was obtained with POLY-4 over MOP. Under TGP, in maize, the PFP-S was recorded highest with no-K. The respective PFP-S under 50, 100, and 150% K remained statistically similar either through POLY-4 or MOP. But, under TGP, in wheat, the PFP-S under 50% K (POLY-4) remained superior over 50% K (MOP). However, under 100 and 150% K application, the PFP-S remained similar with both POLY-4 and MOP. The response of PFP-S under UGP in wheat remained similar to that of maize.

3.4. Recovery efficiency

The amplitude of recovery efficiency of K (RE_K) in maize was higher under TGP over UGP (Fig. 2A). In TGP, the RE_K ranged from 45.1 to 78.8% and the RE_K remained highest with 50% K (POLY-4), followed by 50% K (MOP). The RE_K gradually reduced with higher K doses and a 16.6 and 25.5% reduction was recorded with 100 and 150% K (POLY-4) over 50% K (POLY-4). This reduction was however, 18.5 and 30.2%, respectively in their corresponding doses of K through MOP. Under UGP also, the highest RE_K was recorded with 50% K (POLY-4). However, the reduction with 100 and 150% K (POLY-4) was 7.3 and 10.3%, respectively over 50 % K (POLY-4), and the reduction in corresponding K application with MOP was 7.9 and 9.6% only. The RE_S under TGP ranged between 18.7 and 38.8%. Under TGP, the maximum RE_S was recorded with no-K which was 38.8% over no-S. The RE_S was reduced by 8.2 and 12.7% with 100 and 150% K (POLY-4) over 50% K (POLY-4), respectively. However, the reduction with 100 and 150% MOP was 7.4 and 2.6%, respectively over 50% K (MOP). Under UGP, the RE_S was recorded higher over TGP and ranged from 19.7 to 43.8 in various treatments. The maximum RE_S (43.8%) was recorded with no-K and RE_S declined with increasing K application from 50% to 150% K either through POLY-4 or MOP. A 12.6 and 11.3% reduction in RE_S was recorded with 100 and 150% K (POLY-4), which was 1.2 and 11.3% less with 100 and 150% (MOP) over 50% K (MOP).

Fig. 2.

Recovery efficiency of maize and wheat (A & B) and agronomic efficiency of maize and wheat (C & D)

Note: T1-Rec. NP and S (no-K), T2-Rec. NP and 50% of rec. K through POLY-4, T3-Rec. NP and 100% of rec. K through POLY-4,T4- Recommended N, P and 150% of rec. K through POLY-4, T5-Rec. NP and 50% of rec. K through MOP + S equal to T3 through Bentonite,T6-Rec. NP and 100% of rec. K through MOP + S equal to T4 through Bentonite, T7-Rec. NP and 150% of rec. K through MOP + S equal to T5 through Bentonite.

Under TGP, in wheat, the RE_K ranged between 32.9 and 70.6% (Fig. 2B). The maximum RE_K was recorded with 50% K (POLY-4), followed by 100 and 150% K (POLY-4). The RE_K obtained with 50, 100 and 150% K (POLY-4) was recorded 37.6, 11.5 and 5.3% higher over 50, 100 and 150% K (MOP), respectively. Under both, TGP and UGP, the RE_K declined with increasing K doses by POLY-4, however, with MOP the RE_K increased with 100% K over 50% K and declined thereafter. In UGP, the highest RE_K was recorded with 50% K (POLY-4) and the respective decline in the RE_K was 28.6, 4.4 and 5.7% respectively. The RE_S in both TGP and UGP remained highest with no-K. A 20.6 and 27.7% increase in RE_S was recorded with no-K over no-S. Under both TGP and UGP, the RE_S declined with increasing S dose either through POLY-4 or MOP. In TGP, the RE_S in TGP ranged between 10.8 and 20.6% and a relatively higher RE_S was recorded under UGP. The higher RE_S under UGP indicates a higher response with S application.

3.5. Agronomic efficiency

The agronomic efficiency of K (AE_K) in maize varied between 12.4 and 23.8 kg grain/kg K applied. Under TGP, AE_K was recorded highest with 50% K (POLY-4) and declined thereafter with 100 and 150% K (POLY-4) (Fig. 2C). The AE_K declined by 2.47 and 5.84 kg/kg K applied with 100 and 150% K (POLY-4), respectively over 50% K (POLY-4). But with MOP as K source, the highest AE_K was recorded with 100% K (MOP). This indicates that POLY-4 as K source gave the highest response to applied K even at 50% of the recommended dose. In UGP, the AE_K again remained highest with 50% K (POLY4) and declined thereafter with higher K doses. The AE_K with MOP under UGP remained higher over TGP. Also, the AE_K declined by 1.75 and 6.39 kg/K applied with 100 and 150% K (MOP). In wheat, the AE_S increased with increasing K application, through either source, but the response of K application through Poly-4 was higher over MOP (Fig. 2D). Under TGP, 100% K (POLY-4) resulted in higher AES over 50% K (POLY-4), the response, however, declined with 150% K (POLY-4). The AE_S with POLY-4 as a K source remained higher over MOP, both at 100 and 150% K supply.

4. Discussion

The higher maize yield under TGP with the application of POLY-4 affirms the synergies between the four essential nutrients, viz. potassium, sulfur, magnesium and calcium. The prolonged nutrient availability and the synergistic interaction among all essential nutrients improved the agronomic performance of the crop under both TGP and UGP [6,7]. The higher yield response with K over S application with either source envisages the upcoming K deficiencies in the intensively cultivated areas. The synergies in S and N interactions were also reported in wheat seed yield in a sustainable manner [19,20]. POLY-4 as a slow-release polyhalite increases the availability over readily soluble forms of K and therefore might have contributed to yield enhancement (25, 28). The available Ca from polyhalite maintains plant membrane stability, cell integrity, cell division, and elongation [22] and regulates several signal transduction pathways and activation that promote growth [23]. Likewise, magnesium (Mg) also supports plant photosynthesis and glucose partitioning [24]. The increased plant height and leaf area due to this allows plant to capture more sunlight with higher canopy coverage. Sulfur is also essential for improving the effectiveness of nitrogen fertilizers [25]. K is a crucial element for metabolic and physiological processes, including improved photosynthesis, protein synthesis, starch production, and solute translocation. It also reduces the adverse effects of water stress and improves root growth due to efficient turgor maintenance, thereby increasing water and nutrient use efficiencies [26]. POLY-4 reduces the competition for the absorption of K⁺ by plant roots and improves K uptake [27]. MOP as K source increases chloride content in the soils and reduces sulphate content, also, K⁺ in MOP gets fixed more strongly to clay particles, than does K released from polyhalite, due to the competition between monovalent (K⁺) and divalent (Ca²⁺, Mg²⁺) cations. The lower yield with MOP in the absence of S is due to competition between chloride ion for root absorption. Thus, the optimum K release pattern along with Ca, Mg and S rich compound POLY-4 also reduces chloride ions uptake by the plants. Therefore, a balanced nutrient supply for plants with POLY-4 remains easy over MOP as K source [28,29]. The long-term impact of using POLY-4 on soil health and its advantages for sustainable production, especially under limited K and Ca supplying soils have been reported by many researchers [26,[28], [29], [30], [31]].

Though POLY-4 was used as K equivalent basis and compared with MOP, although, POLY 4 ensures additionally four essential macro-elements to make available to the crops. This helped in the elimination of separate application of different nutrient sources and hence improved the economics [5,6,8]. Potassium application through POLY-4 improves the activity of different enzymes; maintains optimum cell pH; influence photosynthesis and transport sugar within plant system [2,8]. It also improves the nutrients and water uptake and ultimately improves crop quality. The synchronization in nutrient demand and supply under polyhalites lead to better economics compared to other nutrient sources [30]. The root growth, nutrient-supplying capacity, and crop quality is influenced by carbohydrate metabolism and the role of POLY-4 in cell growth and development in plants. The crop yield and quality improve with POLY-4 due to readily available S in a balanced form and its synergy with other elements prompts nitrite and sulfate reduction in plants. Sulfur limitation can severely impact seed yield and quality [7,10]. A large number of studies [3,31,32] have shown the impact of customized fertilizers on seed yield by minimizing overall fertilizer inputs. Therefore, the rising fertilizers costs, increasing environmental impacts in synthesis of different nutrient sources separately can be avoided and this also highlighted comparative better economics from POLY-4 in different crops and locations.

Several works have supported the strong interactions between S and N metabolism [19,25,33,34] leading to enhanced productivity and nutrient acquisition. The limited S application can significantly decrease NUE [35] and vice-versa [36]. When either of these elements remains low or in excess it can lead to reduced seed yield, growth, and NUE [35,37]. Polyhalite use in maize confirmed that the seed tends to accumulate S and N and suggests that S and N were sequestered where POLY-4 was applied. The positive effects of S and N supplies on plant growth, dry matter allocation, and the development of higher sink capacity have been reported [38,39]. An increase in S fertilization in non-limiting K conditions leads to improvement in the seed yield and, consequently NUE [40]. The low K and S availability also promotes N remobilization and recycling [39]. Therefore, adjustment of S and K application with N and P fertilizers may lead to higher seed yield and agronomic performance while increasing sustainability [40,41,43]. POLY-4 makes S available for the succeeding crop also and thus, the available S was recorded higher after wheat. Besides four major nutrients, POLY-4 contains eight main micro-nutrients, viz. boron, copper, iron, manganese, molybdenum, selenium, strontium, and zinc [6,8,41]. These micronutrients have very crucial roles in plant growth and development. Therefore, the slow, steady, and prolonged supply of these major and micro-nutrients as per the demand of the crop through POLY-4 enhanced the agronomic efficiency of N and S and also RE [31,42]. It is both physically and chemically compatible with urea, DAP, rock phosphate, ammonium nitrate, and MOP as dry blends. The assessment of polyhalite and its benefits in terms of increased NUE have been demonstrated by various researchers [[41], [42], [43], [44], [45], [46], [47]]). The physical characteristics of POLY-4 among polyhalite as a commercial fertilizer has shown several agricultural benefits to crops [28,48]. The improvement in yield and NUE of potatoes, beets [41], barley and maize [49] with the use of polyhalite have been reported.

The twin advantage of ideal solubility and saturation levels makes POLY-4 suitable for soil application [[41], [42], [43]]. Once POLY-4's nutrients are in the soil solution they are immediately available for plants and are as effective as other sulphate-based fertilizers. The macro-nutrients in the polyhalite were available in the soil solution for plant uptake for a long time, after application and alleviated nutrient losses [41,48]. POLY-4 simultaneously releases all the macro-nutrients into the soil solution and is therefore, supportive of rapid plant growth. The results of the studies show that potassium, sulfur, magnesium, and calcium interact with the soil exchange complex and disperse in the soil solution efficiently by making the nutrients continually available for plant uptake. Polyhalite behaves like sulphate of potash and has lower salt index than MOP [28,51,52]. It also does not raise soil salinity and has no negative effect on soil pH or soil conductivity [30]. The site-specific nutrient management through POLY-4 has helped to achieve the maximum fertilizer use efficiency of applied nutrients in a cost-effective manner [11]. POLY-4 carries macro and micro-nutrient forms to meet out crop's nutritional needs as an organic source, specific to its site, soil, and crop stage [8,9,12]. This assumption was based on the slower solubility of polyhalite than the sulfate salts of K, Ca, and Mg [6,10,50,53]. Cereal yield increased significantly due to slow-release fertilizer sources by enhanced biomass accumulation and yield [10,29,54].

The higher available K and S contents in the top soil layer (0–15 cm) at the end of the experiment, further support the notion that polyhalite represents a more sustainable fertilizer with long-term residual effects. The potential advantage of using polyhalite is the simultaneous release of the four major nutrients through the dissolution of the mineral. However, it was assumed that ion (Ca⁺², Mg⁺², K⁺, and SO4⁻²) transport in the soil was controlled by their adsorptive affinity to soil particle surfaces [55,56]. The application of polyhalite would be most relevant in soils where the availability of these nutrients is low: in sandy soils, in highly leached soils, or in areas where crops are irrigated by water with low content of these nutrients or are rain-fed. Polyhalite plus rain led to increased crop yield due to augmented Ca uptake in sandy soil [[51], [52], [53]]. In both soils, polyhalite behaved as a prolonged availability fertilizer with more nutrients retained in the topsoil layer and not leached below the root zone.

5. Conclusion

Study conclude that polyhalite shows potential as a commercial fertilizer to supply K, Ca, Mg, and S nutrients, especially under conditions of dryland agriculture where occasionally leaching by rainfall occurs. The study was planned to compare the efficacy of polyhalites vis-à-vis MOP as K sources. Though the doses of polyhalite were fixed on the basis of the recommended K level, but the multi-nutrient nature of polyhalites were found effective over the use of high analysis MOP. POLY-4 was found to be an efficient multi-nutrient fertilizer for supplying K, Ca, Mg, and S. To meet the plant-required ratios of different elements, 100% K through POLY-4 was found optimum under both TGP and UGP for the maize-wheat system. The increased system productivity of the maize-wheat system was recorded with the use of POLY-4 due to better response compared to other potassium sources. The cost of fertilizer use was also reduced substantially due to use of POLY-4 and this in turn increased the added returns in both maize and wheat crops. The added cost of POLY-4 remained <20.0%, while it was >50.0% in the case of MOP with the equivalent amount of potassium in both maize and wheat showingbetter prospects of POLY-4 as a source of K. The multi-nutrient nature of POLY-4 product might also be a strong reason for better response to the crops compared to MOP. The pH-neutral behaviour of POLY-4 further validates it as a valuable crop nutrient product under semi-arid environments. Avoiding individual fertilizer inputs for various nutrients while maintaining, rather improving seed yield, quality and nutrient use efficiency through the use of POLY-4 makes it a potential nutrient source in the coming time. However, the availability of POLY 4 will remain a challenge and its location and crop-specific response needs to studied for its advocation as a complete K nutrient source for soils.

Funding statement

The research was a part of collaborative research project with Sirius Minerals UK (Code 179-24).

Data availability statement

All data required to substantiate the conclusions have been either provided in the article or additional data can be made available by the corresponding author upon request.

Additional information

No additional information provided.

CRediT authorship contribution statement

Vinod Kumar Singh: Conceptualization, Data curation, Formal analysis, Funding acquisition, Methodology, Supervision. Kapila Shekhawat: Data curation, Formal analysis, Investigation, Methodology, Validation. Rajiv Kumar Singh: Data curation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing. Subhash Babu: Formal analysis, Methodology, Writing – original draft, Writing – review & editing. Pravin Kumar Updhyay: Data curation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Pradeep Kumar Rai: Formal analysis, Writing – original draft, Writing – review & editing. Abhinav Kumar: Formal analysis, Investigation, Methodology, Writing – review & editing. Neeraj Kumar Awasthi: Funding acquisition, Methodology, Resources, Writing – review & editing. Sanjay Singh Rathore: Conceptualization, Data curation, Formal analysis, Funding acquisition, Supervision, Writing – original draft, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article.