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Global Research journal of Natural Science

& Technology (GRJNST)

Volume: 04 Issue3 (2026), 2098

ISSN P: 2790-7643 ISSN E: 2790-7651

www.grjnst.net

https://doi.org/10.53762/grjnst.04.03.19

Magnesium Fertilization Improves Grain Yield, Net Photosynthetic

Rate, and Thousand Grain Weight of Spring Hybrid Maize (Zea mays

L.) under Alkaline-Calcareous Soils of Southern Punjab

Received: 12 March 2026. Accepted: 03 April 2026. Published: 05 May 2026

Muhammad Javid Qamar

Soil Fertility (Field),

Bahawalpur, Punjab, Pakistan

https://orcid.org/0009-0005-7271-0807

Muhammad Rashid Farooq

Soil Fertility (Field),

Bahawalpur, Punjab, Pakistan

https://orcid.org/0009-0005-3121-7397

Sehrish Jameel

Soil and Water Testing Laboratory,

Bahawalnagar, Punjab, Pakistan

https://orcid.org/0009-0003-2983-9336

Muhammad Bilal

Soil and Water Testing Laboratory,

Dera Ghazi Khan, Punjab, Pakistan

https://orcid.org/0009-0003-6512-9446

Abdul Ghaffar Khan

Soil Fertility Research Institute,

Punjab, Lahore, Pakistan

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Commons Attribution 4.0 International (CC BY 4.0) license, which permits unrestricted use and distribution

G. 2098

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https://orcid.org/0009-0003-5070-5519

Muhammad Khalid

Soil and Water Testing Laboratory,

Bhakkar, Punjab, Pakistan

https://orcid.org/my-orcid?orcid=0009-0009-6297-7144

Abdul Sattar

Soil and Water Testing Laboratory,

Vehari, Punjab, Pakistan

https://orcid.org/0009-0004-9693-4357

Muhammad Asif

Soil and Water Testing Laboratory,

Khanewal, Punjab, Pakistan

https://orcid.org/0009-0008-9139-7870

Hafiz Muhammad Rafiq (Corresponding Author)

Soil and Water Testing Laboratory for Research,

Bahawalpur, Punjab, Pakistan

rafiqqais@gmail.com

https://orcid.org/0000-0001-6711-2072

Saeed-ur-Rehman

Soil and Water Testing Laboratory for Research,

Bahawalpur, Punjab, Pakistan

https://orcid.org/0009-0007-4437-0854

Ali Rizwan

Soil and Water Testing Laboratory,

Bahawalnagar, Punjab, Pakistan

https://orcid.org/0000-0002-8248-8016

Alamgir Alvi

Soil Salinity Research Institute,

Pindi Bhattian, Punjab, Pakistan

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G. 2098

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Abstract

Maize (Zea mays L.) productivity in southern Punjab is increasingly limited by

latent secondary-nutrient deficiencies on the region's alkaline-calcareous soils,

where free CaCO₃, low organic matter, and Ca²⁺-dominated cation exchange sites

jointly constrain the plant availability of magnesium. Despite magnesium's central

role as the chlorophyll metal, an essential Rubisco cofactor, and the principal

cation for sucrose phloem loading, calibrated soil-applied Mg dose–response data

for spring-planted hybrid maize on Pakistani alkaline soils are limited. A two-

location field trial was therefore conducted during the Rabi 2024–25 season at

Hasilpur and Chishtian (District Bahawalnagar) to evaluate the response of spring-

planted DK6317 hybrid maize to five graded levels of magnesium sulphate

heptahydrate (MgSO₄·7H₂O), at 0, 20, 40, 60, and 80 kg MgSO₄·7H₂O ha⁻¹ and

applied as a broadcast basal alongside a uniform NPK (225–150–125 kg ha⁻¹)

plus Zn–B supply. The experiment was laid out as a randomized complete block

design with three replications at each site and analyzed as a two-factor combined

ANOVA. Treatment effects were highly significant (p < 0.01) for grain yield, net

photosynthetic rate (Pn), and thousand-grain weight (TGW). Pooled mean grain

yield rose progressively from 7,624.1 kg ha⁻¹ at the control to 7,964.8 kg ha⁻¹ at

the highest Mg rate (+4.5%), while Pn increased significantly from 24.67 to 29.90

mmol CO₂ m⁻² s⁻¹ (+21.2%) and TGW increased from 281.1 to 328.1 g

(+16.7%) over the same Mg gradient. The disproportionately large gain in TGW,

set alongside the parallel rise in Pn, established a coherent mechanistic chain in

which magnesium supply enhanced photosynthetic capacity through the late grain-

filling window and translated into greater photo-assimilate partitioning to the

developing kernel. All three response curves were still rising at the highest tested

rate. On this evidence, the inclusion of 80 kg MgSO₄·7H₂O ha⁻¹ as a basal soil-

broadcast input is recommended as a regional Mg fertilization practice for spring

hybrid maize across the canal-irrigated districts of southern Punjab.

Keywords: Hybrid maize, magnesium fertilization, MgSO₄·7H₂O, net

photosynthetic rate, thousand grain weight.

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1. Introduction

Maize (Zea mays L.) is the third most important cereal crop of the world after wheat and

rice, providing food, feed, fodder, and industrial raw material across a wide range of agro-

ecological zones (FAOSTAT, 2023). Global maize production has grown rapidly over the

past two decades on the strength of hybrid technology, improved agronomy, and rising

demand from the livestock and starch industries, reaching roughly 1.2 billion tonnes from

about 200 million hectares (FAO, 2023). In Pakistan, maize has emerged as the third major

cereal after wheat and rice and the fastest-growing cereal crop in terms of both acreage and

per-hectare yield, currently cultivated on approximately 1.44 million hectares with an

annual production of around 8.24 million tonnes (GoP, 2025). The expansion has been

driven largely by the rapid uptake of single-cross hybrids, the growth of the poultry and

dairy feed industries, and the establishment of a Rabi-season maize crop in the irrigated

districts of Punjab, where spring-planted hybrids now contribute a substantial share of

national output (Ali et al., 2020). Despite these gains, the realized on-farm yield remains

well below the genetic potential of available hybrids, and the persistent yield gap has been

attributed to imbalanced fertilization, sub-optimal plant population, and the increasing

prevalence of micro- and secondary-nutrient deficiencies on the country's predominantly

alkaline-calcareous soils (Gen et al., 2021; Bibi et al., 2024). The soils of southern Punjab,

where much of the country's Rabi maize is now grown, are characteristically alkaline,

calcareous, and low in organic matter, with pH values typically ranging from 7.8 to 8.5 and

substantial free CaCO₃ contents (Waheed et al., 2025). Under these conditions, the plant

availability of several essential cations, including magnesium, is constrained by competition

with Ca²⁺ on exchange sites, by the alkaline reaction that favours Mg fixation in carbonate

complexes, and by the relatively low Mg content of the alluvial parent material (Gransee

and Führs, 2013). The Rabi maize crop adds physiological constraint: with sowing in early

February and harvest in late May, grain filling coincides with rapidly rising temperatures

and high evaporative demand during late April and May, when canopy photosynthesis

becomes the rate-limiting step for kernel weight accumulation (Lobell et al., 2013).

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Any nutritional constraint on photosynthetic capacity during this window, therefore,

translates directly into a penalty on thousand-grain weight and grain yield. Although

nitrogen, phosphorus, and potassium responses in maize have been extensively documented

in Pakistan, the role of magnesium, and particularly its interaction with location-specific

soil conditions and end-of-season heat stress, has received comparatively little attention.

Magnesium occupies a uniquely central position in plant metabolism. It is the central atom

of the chlorophyll molecule, a structural and functional cofactor of more than three

hundred enzymes (including Rubisco, the rate-limiting carboxylase of C₃ and C₄

photosynthesis), an essential activator of ATP-dependent reactions, and the principal

counter-cation for phloem loading of sucrose and amino acids (Cakmak and Yazici, 2010;

Gransee and Führs, 2013). Under Mg-sufficient conditions, leaf chlorophyll content,

photosystem II efficiency, and net photosynthetic rate are all enhanced, with downstream

consequences for assimilate partitioning to reproductive sinks (Tränkner et al., 2018). In

maize specifically, adequate Mg supply has been shown to increase leaf chlorophyll

concentration, stabilize photosynthetic performance under high-light and heat stress,

improve phloem loading of sucrose from source leaves to developing kernels, and

consequently raise thousand-grain weight and grain yield (Jezek et al., 2015; Hauer-Jákli

and Tränkner, 2019). Mg deficiency, by contrast, first manifests as interveinal chlorosis on

older leaves, reduces photosynthetic rate even before visible symptoms develop, and reduces

the duration and effective rate of grain filling, a particularly damaging combination for

spring-planted maize maturing under thermally stressful conditions (Cakmak and Yazici,

2010). Despite this physiological centrality, calibrated soil-applied Mg dose–response data

for hybrid maize on alkaline-calcareous soils of Pakistan are sparse, and current provincial

fertilizer recommendations for maize do not routinely include magnesium as a maintenance

input.

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Where regional studies on Mg fertilization in maize have been reported, they have typically

been confined to a single site or a single Mg rate, making it difficult to identify an

agronomically optimal dose or to establish whether the response is consistent across soils

with differing fertility status. A multi-location, dose–response evaluation of Mg fertilization

in hybrid maize under the alkaline-calcareous, semi-arid conditions of southern Punjab is

therefore warranted. The present study was designed (i) to evaluate the response of spring-

planted hybrid maize (cv. DK6317) to four graded levels of soil-broadcast magnesium

sulphate heptahydrate (MgSO₄·7H₂O), and applied over a uniform NPK basal dose, at

two contrasting sites in District Bahawalnagar during the Rabi 2024–25 crop season; (ii) to

quantify the response in terms of three complementary variables, grain yield, net

photosynthetic rate, and thousand grain weight, that together capture both the yield and the

underlying physiological mechanism; and (iii) to test whether the Mg response interacts

significantly with location, thereby clarifying whether a single regional recommendation is

justified or whether site-specific calibration is required.

2. Materials and Methods

2.1 Experimental sites

The field experiment was conducted during the Rabi 2024–25 cropping season at two

collaborating sites in District Bahawalnagar, southern Punjab, Pakistan, one at Hasilpur and

one at Chishtian, under the framework of the Soil Fertility (Field) Bahawalpur research

project SF-111 ("Evaluation of the effect of magnesium sulphate on spring hybrid maize").

Both sites lie within the canal-irrigated, semi-arid tract of the Indus Basin and are

characterized by hot summers, mild winters, low to moderate annual rainfall, and alluvial,

calcareous soils developed on Indus Basin sediments. Prior to land preparation, composite

soil samples were collected from each site at two depths (0–15 cm and 15–30 cm)

following standard auger sampling procedures (Ryan et al., 2001). Samples were air-dried,

ground, sieved through a 2 mm screen, and analyzed for electrical conductivity of the

saturation extract (ECₑ) by conductivity meter, pH (1:1 soil-to-water suspension) by glass

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electrode, organic matter by the Walkley–Black wet oxidation method, available

phosphorus by the AB-DTPA extraction procedure, and exchangeable potassium by

ammonium acetate extraction followed by flame photometry (Ryan et al., 2001; Estefan et

al., 2013). The soil analytical data for the two experimental sites are presented in Table 1.

Both sites carried alkaline (pH 8.16–8.40), moderately saline (ECₑ 2.70–3.19 dS m⁻¹) soils

with low organic matter content (0.55–0.79%). Available phosphorus was generally low to

medium across both sites (4.38–10.9 ppm), while available potassium ranged from 96 to

130 ppm. These soil characteristics, alkaline pH, free CaCO₃, moderate salinity, and low

organic matter, represent conditions widely recognized to limit the plant availability of soil-

bound magnesium through Ca²⁺/Mg²⁺ exchange-site competition and carbonate fixation

(Gransee and Führs, 2013), and were therefore expected to amplify the agronomic value of

Mg supplementation.

Table 1. Soil physico-chemical properties of the two experimental sites at two sampling

depths.

Soil parameter

pH

OM (%)

Available P (ppm)

Available K (ppm)

EC (dS m⁻¹)

Hasilpur

Depth 0–15 cm

Depth 15–30 cm

Chishtian

2.73

3.19

8.24

8.16

0.79

0.55

6.98

4.38

130

104

Depth 0–15 cm

Depth 15–30 cm

2.90

2.70

8.40

8.30

0.71

0.77

10.90

9.10

125

96

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2.2 Plant material

A commercially available single-cross maize hybrid, DK6317 (Dekalb, Bayer Crop Science),

was used at both locations. DK6317 is a medium-maturing single-cross hybrid widely

adopted for spring planting in the canal-irrigated districts of Punjab and was selected for its

consistent performance under the prevailing Rabi-season conditions of southern Punjab.

Disease-free, certified seed was procured from authorized hybrid seed distributors and used

for sowing at both sites.

2.3 Treatments and experimental design

The experiment evaluated five treatments comprising a graded series of magnesium

fertilization rates applied over a uniform NPK basal dose. The Mg treatments, expressed on

a MgO-equivalent basis, were: T1 = Control (NPK only, 0 kg MgSO4 ha⁻¹); T2 = NPK

+ 20 kg MgSO4 ha⁻¹; T3 = NPK + 40 kg MgSO4 ha⁻¹; T4 = NPK + 60 kg MgSO4

ha⁻¹; and T5 = NPK + 80 kg MgSO4 ha⁻¹. Magnesium was supplied as magnesium

sulphate heptahydrate (MgSO₄·7H₂O; Epsom salt), which contains approximately 16.26%

MgSO4 by mass; the corresponding actual fertilizer rates applied in the field were

approximately 0, 123, 246, 369, and 492 kg MgSO₄·7H₂O ha⁻¹ for T1 through T5,

respectively. The uniform basal NPK dose comprised 225 kg N ha⁻¹, 150 kg P₂O₅ ha⁻¹,

and 125 kg K₂O ha⁻¹, applied as urea, di-ammonium phosphate (DAP), and sulphate of

potash (SOP), respectively. In addition, all experimental plots, including the unfertilized

Mg control, received a uniform basal application of zinc at 5 kg ha⁻¹ (as zinc sulphate) and

boron at 1.0 kg ha⁻¹ (as borax) to ensure that the response to graded Mg fertilization could

be evaluated independently of any underlying micronutrient limitation other than Mg itself.

Treatment compositions are summarized in Table 2.

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The experiment was laid out as a randomized complete block design (RCBD) at each site,

with three replications, and the data from the two sites were subsequently combined and

analyzed as a two-factor combined ANOVA with locations and Mg fertilization levels as

the two factors. The same randomization protocol was followed at both sites. Individual

plot size was 250 m², equivalent to one-fortieth of a hectare, which was sufficient to

accommodate a representative net plot area for harvesting after exclusion of border rows.

Table 2. Treatment compositions used to study the response of spring hybrid maize to

graded levels of magnesium fertilization.

Treatment

Treatments

N

MgO

P₂O₅

K₂O

(kg

(kg ha⁻¹)

ha⁻¹)

Control (NPK only) T1_Control

225

150

125

0

NPK + 20 kg

T2_MgO_2

0

20

MgSO₄

NPK + 40 kg

T3_MgO_4

0

225

150

125

40

MgSO₄

T4_MgO_60

T5_MgO_80

225

150

125

60

80

NPK + 60 kg MgSO₄

NPK + 80 kg MgSO₄

2.4 Crop management

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Maize was sown on 4 February 2025 at both locations and harvested on 29 May 2025,

giving a crop cycle of approximately 114 days. At sowing, the full doses of P₂O₅, K₂O,

MgSO₄·7H₂O, ZnSO₄, and borax, together with one-third of the total nitrogen, were

broadcast uniformly over the prepared seedbed and incorporated into the surface soil

through a final shallow tillage operation immediately before sowing. The remaining

nitrogen was applied in two further equal splits: one-third at the knee-height stage

(approximately 35–45 days after sowing) and the final one-third at the three-feet-height

stage, applied as a top-dressing approximately 15–20 days after the second N application.

Seed was placed in opened furrows at the recommended depth, and a uniform plant

population consistent with standard production recommendations for spring hybrid maize

in Punjab was maintained at both sites through manual thinning at the early vegetative

stage. Irrigation was applied through the canal supply system: the first irrigation was given

immediately after sowing to ensure uniform germination, and subsequent irrigations were

applied at critical growth stages including knee-height, tasseling, silking, and grain filling,

with frequency adjusted to crop demand, evaporative demand, and rainfall. Weeds were

controlled by a combination of pre-emergence herbicide application and one manual hoeing

during the early vegetative phase. Plant protection measures against stem borer (Chilo

partellus) and fall armyworm (Spodoptera frugiperda) were applied uniformly across all

plots whenever scouting indicated treatment thresholds had been reached. All other

agronomic operations were carried out according to the standard production

recommendations for spring-planted hybrid maize in Punjab.

2.5 Data recording

Three response variables were recorded: grain yield, net photosynthetic rate, and thousand

grain weight.

1. Grain yield (kg ha⁻¹). At physiological maturity, all cobs were harvested from the net

plot area, leaving border rows excluded to minimize edge effects. Harvested cobs were sun-

dried, threshed, and the grain was cleaned, weighed, and adjusted to a standard moisture

basis of 14% using a calibrated digital grain moisture meter. Plot-level grain yield was

extrapolated to a per-hectare basis (kg ha⁻¹) at 14% moisture content.

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2. Net photosynthetic rate (Pn, mmol CO₂ m⁻² s⁻¹). Net photosynthetic rate was measured

on fully expanded ear leaves at the silking stage using a portable infrared gas analyzer-based

photosynthesis system (CI-340 Handheld Photosynthesis System, CID Bio-Science, USA).

Measurements were taken on randomly selected plants per plot between 09:00 and 11:00

hrs local time on cloud-free days, under ambient photosynthetic photon flux density, CO₂

concentration, and leaf temperature. The plant-level readings per plot were averaged to give

a single plot-level Pn value used in subsequent analyses.

3. Thousand grain weight (TGW, g). Following threshing and moisture adjustment, three

random sub-samples of 1,000 grains were drawn from the cleaned grain of each plot using a

seed counter and weighed on an electronic precision balance (resolution 0.01 g). The three

sub-sample weights were averaged to give a single plot-level TGW value at 14% moisture

basis.

2.6 Statistical analysis

The data for grain yield, net photosynthetic rate, and thousand grain weight were each

subjected to two-factor combined analysis of variance (ANOVA) appropriate for a multi-

location randomized complete block design, with locations and treatments treated as fixed

effects and replications nested within locations. Where the location × treatment interaction

was significant, simple effect means were computed for each treatment within each location;

where it was non-significant, treatment means were pooled across locations and separated

using Fisher's protected least significant difference (LSD) test at the 5% probability level

(Steel et al., 1997). The significance of location, treatment, and location × treatment

interaction effects was tested at p ≤ 0.05 and p ≤ 0.01 levels. The yield, Pn, and TGW

response of each Mg treatment over the unfertilized control was computed for each location

as the absolute and percentage difference from T1. Statistical analyses were performed using

Statistix 8.1, and graphical presentations were prepared using R/RStudio and Microsoft

Excel.

3. Results and Discussion

3.1 Soil status of the experimental sites

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The soil analytical data presented in Table 1 confirmed that both experimental sites carried

alkaline, calcareous, low-organic-matter soils typical of the southern Punjab cropping plain,

with surface pH ranging from 8.16 to 8.40 across the two locations and both sampling

depths. Electrical conductivity values (2.70–3.19 dS m⁻¹) indicated moderate salinity at

both sites, with the Hasilpur subsoil registering the highest EC (3.19 dS m⁻¹). Organic

matter content was uniformly low (0.55–0.79%), and available phosphorus was generally

inadequate at the Hasilpur subsoil (4.38 ppm) while ranging from low-medium to medium

at the other sampling positions (6.98–10.9 ppm). Available potassium ranged from 96 to

130 ppm across the four soil samples. Collectively, these soil conditions, alkaline pH, free

CaCO₃, moderate salinity, and low organic matter, represent the broader fertility

constraints under which the maize crop was raised and were expected to amplify the

agronomic value of magnesium supplementation.

The combination of high pH, calcareous mineralogy, and moderate salinity is widely

recognized as constraining Mg availability through three interacting mechanisms: Ca²⁺

dominance of cation exchange sites under high CaCO₃ loading; precipitation of Mg in

carbonate complexes at alkaline pH; and Na⁺ competition with Mg²⁺ for root uptake under

elevated soil salinity (Gransee and Führs, 2013; Tränkner et al., 2018). The site-to-site

contrast in EC and available P further provided a useful natural gradient against which the

consistency of the Mg response across locations could be evaluated.

3.2 Analysis of variance

The combined analysis of variance across the two locations revealed significant treatment

effects on all three response variables (Table 3). For grain yield, the location main effect

was non-significant, while the treatment effect was highly significant (p < 0.01) and the

location × treatment interaction was also significant (p < 0.01). For net photosynthetic

rate, both the location main effect and the treatment effect were highly significant (p <

0.01), while their interaction was non-significant. For thousand grain weight, only the

treatment effect was significant (p < 0.01), with both the location main effect and the

interaction term remaining non-significant.

Table 3. Combined analysis of variance (mean squares) for grain yield and associated traits

of spring hybrid maize across two locations and five magnesium fertilization levels.

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Grain yield

(MS)

Source of variation

DF

Pn (MS)

TGW (MS)

Replications (A)

Locations (B)

Error A × B

2

1

22,986

41,813 ⁿˢ

34,726

4.67

43.20 **

0.22

734.23

3.33 ⁿˢ

2

570.03

**

Treatments (C)

B × C interaction

Error A × B × C

Total

4

149,747

22.71

1,284.05^**

28.92 ⁿˢ

501.01

**

4

23,479

2.92 ⁿˢ

16

29

8,990

1.92

Note: * = significant at 5% probability level; ** = significant at 1% probability level; ⁿˢ =

non-significant. Grain yield in kg ha⁻¹; Pn in mmol CO₂ m⁻² s⁻¹; TGW in g.

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These ANOVA patterns convey three useful pieces of information for downstream

interpretation. First, the highly significant treatment effects across all three variables

confirm that the graded Mg fertilization rates produced statistically discernible responses

over and above the uniform NPK basal dose. Second, the contrasting pattern of the location

main effect, non-significant for grain yield and TGW but highly significant for Pn,

indicates that the two sites achieved broadly similar absolute yield ceilings but differed

measurably in their photosynthetic machinery, with Hasilpur registering consistently higher

Pn values than Chishtian across all five Mg levels. Third, the significant location ×

treatment interaction for grain yield, set against non-significant interactions for the two

upstream physiological variables, suggests that the yield-level response of Mg differed

slightly between the two sites despite a broadly consistent underlying physiological

response.

3.3 Grain yield response to magnesium fertilization

Across both locations, soil-applied magnesium sulphate heptahydrate produced a

progressive improvement in grain yield as the Mg rate increased from 0 to 80 kg MgSO4

ha⁻¹, with the highest yield recorded at the highest tested dose (Figure 1, Table 4). The

unfertilized NPK-only control (T1) produced a pooled mean grain yield of 7,624.1 kg

ha⁻¹, while application of 20, 40, 60, and 80 kg MgSO4 ha⁻¹ produced pooled mean yields

of 7,606.1, 7,757.7, 7,886.0, and 7,964.8 kg ha⁻¹, respectively. The yield response over the

unfertilized control therefore rose from a marginal −0.2% at T2 to +1.8% at T3, +3.4%

at T4, and +4.5% at T5, with the highest grain yield gain of +340.7 kg ha⁻¹ over the

NPK-only control recorded at the 80 kg MgSO4 ha⁻¹ rate. The small yield depression

observed at T2 in the pooled mean reflects a site-specific dip at Hasilpur, where T2

produced a yield 76.8 kg ha⁻¹ below the control, while at Chishtian T2 produced a small

positive gain of 40.7 kg ha⁻¹; this divergence is the principal driver of the significant

location × treatment interaction.

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Figure 1 | Performance of maize crop at five different magnesium fertilization levels at two

different locations

The progressive and monotonic shape of the yield response curve, with the peak observed at

the highest tested rate and no evidence of a plateau or decline, is physiologically consistent

with established understanding of Mg nutrition in maize. Hybrid maize is a high-yielding

C₄ crop with intense reproductive sink strength and substantial Mg uptake during the

grain-filling phase, and the demand for Mg at this stage routinely exceeds the supply that

alkaline-calcareous soils can deliver from native pools (Hauer-Jákli and Tränkner, 2019).

The continuing yield response at 80 kg MgSO4 ha⁻¹ suggests that the sufficiency threshold

for Mg on these soils, under spring planting and the prevailing heat stress regime of the late

grain-filling window, has not yet been reached within the rates tested. The absolute yield

gain of +340.7 kg ha⁻¹ from T1 to T5 represents a measurable agronomic response on top

of an already optimized NPK schedule and confirms that magnesium is a latent yield-

limiting factor for spring-planted hybrid maize on these soils.

Table 4. Site-wise mean grain yield, net photosynthetic rate, and thousand grain weight of

spring hybrid maize under five graded levels of magnesium fertilization at Hasilpur and

Chishtian.

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Grain Yield

Chishtian

Net Photosynthetic rate

Thousand Grain weight

Treatmen

t

Hasilpur

7633.3

7556.6

7789.5

7979.5

8066.7

Pooled

7624.1

7606.1

7757.7

7886.0

7964.8

Hasilpur

Chishtian

24.10

25.70

26.37

25.77

28.03

Pooled

Hasilpur

270.4

286.5

301.7

315.2

321.5

Chishtian

T1 (0)

T2 (20)

T3 (40)

T4 (60)

T5 (80)

7614.8

7655.6

7725.9

7792.6

7863.0

25.23

26.63

28.67

29.67

31.77

24.67

26.17

27.52

27.72

29.90

291.7

308.7

309.0

318.0

334.7

3.4 Net photosynthetic rate response to magnesium fertilization

Net photosynthetic rate showed a strong, monotonically rising response to magnesium

fertilization at both locations (Figure 2). Pooled across the two sites, net photosynthetic

rate increased progressively from 24.67 mmol CO₂ m⁻² s⁻¹ at the unfertilized control (T1)

to 29.90 mmol CO₂ m⁻² s⁻¹ at the highest Mg rate (T5), corresponding to an absolute

increase of +5.23 mmol m⁻² s⁻¹ and a relative gain of +21.2% over the control. The site-

wise responses were directionally identical: at Hasilpur, Pn rose from 25.23 to 31.77 mmol

m⁻² s⁻¹ (a gain of +6.53 mmol m⁻² s⁻¹ or +25.9% over control), and at Chishtian from

24.10 to 28.03 mmol m⁻² s⁻¹ (a gain of +3.93 mmol m⁻² s⁻¹ or +16.3% over control).

The non-significant location × treatment interaction (Table 3) confirms that the broad

shape of the Pn response was consistent across the two sites, although the absolute

magnitude was greater at Hasilpur, in line with the significant location main effect.

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Figure 2 | Net Photosynthetic rate of maize crop at five different magnesium fertilization

levels at both Hasilpur and Chistian locations and difference from control

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The strong photosynthetic response to Mg fertilization is mechanistically the central

finding of the present study and is fully consistent with magnesium's biochemical role in C₄

photosynthesis. Magnesium is the central atom of the chlorophyll molecule, an essential

structural cofactor of Rubisco and PEP carboxylase, and an obligatory activator of more

than three hundred enzymes, including those of the Calvin–Benson cycle and the C₄ shuttle

(Cakmak and Yazici, 2010; Tränkner et al., 2018). Under Mg-deficient conditions, leaf

chlorophyll concentration declines, photosystem II efficiency falls, and net photosynthetic

rate is depressed even before visible interveinal chlorosis develops on older leaves (Jezek et

al., 2015; Hauer-Jákli and Tränkner, 2019). Restoration of Mg supply through soil-applied

fertilization has been shown to elevate leaf Mg concentration, restore chlorophyll content,

and increase the maximum rate of carboxylation, with corresponding improvements in

measured Pn in maize and other C₄ cereals (Jezek et al., 2015). The +21.2% pooled

increase in Pn recorded here at the highest Mg rate falls within the upper range of values

reported in the recent meta-analysis of Hauer-Jákli and Tränkner (2019), which found

mean photosynthetic gains of 15–25% in Mg-supplemented field crops under conditions

of moderate to severe baseline Mg limitation.

3.5 Thousand-grain weight response to magnesium fertilization

Thousand grain weight showed the strongest relative response of any variable measured in

the present study, rising progressively at both locations across the full Mg dose series

(Figure 3). Pooled across the two sites, TGW rose from 281.1 g at the unfertilized control

to 328.1 g at the highest Mg rate, corresponding to an absolute increase of +47.0 g and a

relative gain of +16.7% over the control. At Hasilpur, TGW increased from 270.4 g (T1)

to 321.5 g (T5), a gain of +51.1 g (+18.9%), while at Chishtian TGW rose from 291.7 g

to 334.7 g, a gain of +43.0 g (+14.7%). The TGW response was monotonically positive

across the dose series at both sites, with no evidence of a plateau within the range tested,

and the non-significant location × treatment interaction (Table 3) confirms that the broad

TGW response was conserved across both locations.

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Figure 3 | Thousand Grain Weight of maize crop at five different magnesium fertilization

levels at both Hasilpur and Chistian locations and difference from control

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The magnitude of the TGW response, substantially larger in relative terms than the grain

yield response, is mechanistically informative and represents arguably the most interpretable

finding of the present study. Because grain yield in maize is the product of plant population,

kernels per plant, and individual kernel weight, the disproportionately large gain in TGW

indicates that magnesium fertilization improved the present crop primarily through

enhanced grain filling rather than through increased kernel set or stand density (Tollenaar

and Lee, 2002). This is the classical signature of improved late-season source capacity,

where supply of photosynthate to the developing kernel during the grain-filling window

becomes the rate-limiting step for individual kernel growth (Borrás et al., 2004). The

TGW response observed here is therefore the direct, downstream agronomic expression of

the upstream photosynthetic response documented in section 3.4: Mg supplementation

elevated leaf Pn through the silking and early grain-filling phases, and the additional photo-

assimilate produced was preferentially partitioned to the kernel, increasing individual grain

weight. This source–sink chain is the established mechanism by which Mg fertilization

improves cereal grain yield on Mg-limiting soils (Cakmak and Yazici, 2010; Hauer-Jákli

and Tränkner, 2019).

3.6 Location × treatment interaction and the consistency of the Mg response

The significant location × treatment interaction for grain yield (Table 3, p < 0.01), set

against non-significant interactions for Pn and TGW, requires careful interpretation. The

site-wise yield responses (Figures 4 and 5) show that both Hasilpur and Chishtian

responded positively to Mg fertilization across the full dose series, with the highest yield at

each site recorded at T5 (8,066.7 kg ha⁻¹ at Hasilpur; 7,863.0 kg ha⁻¹ at Chishtian) and

the absolute yield gain over the control ranging from +241.1 kg ha⁻¹ at Hasilpur (+3.2%)

to +248.2 kg ha⁻¹ at Chishtian (+3.3%). The principal source of the interaction was the

divergent response at T2 (20 kg MgSO4 ha⁻¹): at Hasilpur, T2 produced a small yield

depression of −76.8 kg ha⁻¹ (−1.0%) relative to control, while at Chishtian T2 produced a

small positive gain of +40.7 kg ha⁻¹ (+0.5%). The two response curves converged at T3

onwards, with both sites showing parallel yield gains at the 40, 60, and 80 kg MgSO4 ha⁻¹

rates.

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The small T2 dip at Hasilpur is most plausibly attributed to the higher baseline soil salinity

at that site (ECₑ 3.19 dS m⁻¹ in the subsoil, the highest value recorded in either trial

location), where the sulphate ion supplied with the modest Mg dose at T2 may have

transiently elevated rhizosphere ionic strength without delivering enough Mg to offset the

osmotic effect on early-season root activity. Once the Mg dose was raised to 40 kg MgSO4

ha⁻¹ and above, the agronomic benefit of Mg supply outweighed any transient ionic effect,

and the yield response at Hasilpur converged with Chishtian. Importantly, the broad

pattern of monotonic improvement up to the highest tested rate was preserved at both

locations, and the formal statistical significance of the interaction notwithstanding, the

practical extension message, that 80 kg MgSO4 ha⁻¹ produces the highest grain yield

response at both sites, is unaffected.

Figure 2 | Yield performance of maize crop at five different magnesium fertilization levels

at Hasilpur location and yield difference from control

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Figure 3 | Yield performance of maize crop at five different magnesium fertilization levels

at Chistian location and yield difference from control

The non-significant location × treatment interactions for both Pn and TGW further

reinforce that the underlying physiological response of the crop to Mg fertilization was

conserved across the two sites; only the yield-level expression differed marginally at the

lowest Mg rate.

3.7 Mechanistic synthesis and the recommended Mg rate

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Taken together, the present results establish a coherent and physiologically interpretable

mechanistic chain: soil-applied magnesium fertilization at 80 kg MgSO4 ha⁻¹ raised leaf

magnesium status; elevated leaf chlorophyll content and Rubisco activation, as expressed in

a +21.2% increase in net photosynthetic rate over the unfertilized control; enhanced the

supply of photo-assimilate to developing kernels through the late grain-filling window;

produced a +16.7% gain in thousand grain weight; and translated into a +4.5% gain in

grain yield on a pooled-mean basis. It is worth emphasizing that the observed gains were

obtained over an NPK basal already supplemented with Zn and B at agronomic rates,

indicating that the response is attributable specifically to relief of the Mg constraint rather

than to a generalized correction of micronutrient deficiency. This source-driven yield

response is the established physiological signature of improved Mg nutrition in cereals

(Cakmak and Yazici, 2010; Tränkner et al., 2018; Hauer-Jákli and Tränkner, 2019) and is

particularly relevant for spring-planted Rabi maize in southern Punjab, where the grain-

filling phase falls in late April and May under rising temperatures and high evaporative

demand, precisely the conditions under which Mg-mediated stabilization of photosynthetic

machinery is most agronomically valuable (Lobell et al., 2013; Tränkner et al., 2018).

On the strength of the largest grain yield response, the largest Pn gain, and the largest

TGW gain over the unfertilized NPK-only control, the T5 treatment (80 kg MgSO4 ha⁻¹,

is identified as the recommended Mg rate for spring-planted hybrid maize on the alkaline-

calcareous soils of District Bahawalnagar and the wider southern Punjab cropping plain.

The fact that the grain yield, Pn, and TGW response curves were all still rising at the

highest tested rate, with no evidence of a plateau, indicates that the true sufficiency

threshold for soil-applied Mg under these conditions may lie at or beyond 80 kg MgSO4

ha⁻¹, and that future work should evaluate even higher Mg rates (e.g., 100–120 kg MgSO4

ha⁻¹) to determine the upper limit of the agronomic response. For current extension

purposes, however, the inclusion of 80 kg MgSO4 ha⁻¹ as a basal soil-broadcast input

alongside the standard NPK schedule emerges as both the agronomically and

physiologically defensible recommendation supported by the present data.

4. Conclusion

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This multi-location field study demonstrated that soil-applied magnesium sulphate

heptahydrate, broadcast at sowing alongside a uniform NPK basal dose, improved the

productivity of spring-planted hybrid maize (cv. DK6317) under the alkaline-calcareous,

canal-irrigated conditions of southern Punjab during Rabi 2024–25. Across the 0–80 kg

MgO ha⁻¹ series, pooled grain yield rose from 7,624.1 to 7,964.8 kg ha⁻¹ (+4.5%), net

photosynthetic rate from 24.67 to 29.90 mmol CO₂ m⁻² s⁻¹ (+21.2%), and thousand

grain weight from 281.1 to 328.1 g (+16.7%), with the disproportionate gain in grain

weight alongside the parallel rise in photosynthesis indicating that enhanced Mg supply

sustained late-season photosynthesis and improved photo-assimilate partitioning to the

kernel. The response was positive at both sites, peaking at the 80 kg MgSO₄ ha⁻¹ rate;

accordingly, 80 kg MgSO₄ ha⁻¹ as a basal soil-broadcast input, complementing existing N,

P, and K recommendations, is recommended for spring-planted hybrid maize across canal-

irrigated southern Punjab, while future work should test higher rates (100–120 kg MgSO₄

ha⁻¹), Kharif maize, wider soil gradients, and grain quality to develop a fully integrated

regional Mg recommendation.

Author Contributions

MJQ; Conceptualization and Execution, Supervision, Planning, Writing – original draft;

MRF, SJ; Experimental layout, Field execution and crop management at Hasilpur and

Chishtian sites; MBL, AGK; Treatment application, Field supervision; MK, AS; Data

analysis, Statistical computation, Data visualization; MA, HMR; Soil sampling, Soil

chemical analysis, Soil fertility characterization; SUR, AR, GM; Grain yield, Pn, and TGW

data recording, Data compilation; HMR, MJQ, AGK; Writing – review & editing.

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