Research Article

Combined Effect of Nano-silica and Plant Growth-Promoting Rhizobacteria (PGPR) on the Physical and Mechanical Properties of Soil

Bilal M.K. Almashhadani1, Mustafa Qais Hamid2*, Noor Al-Silmawy3 and Diaa F. Hassan4

1Department of Desertification Combat, College of Agricultural Engineering Sciences, University of Baghdad, Baghdad, Iraq; 2Department of Soil Science and Water Resources, College of Agriculture, University of Al-Qadisiyah, Al-Diwaniyah, Iraq; 3Department of Soil Sciences and Water Resources, College of Agriculture, University of Wasit, Iraq; 4College of Engineering, Al-Qasim Green University, Babylon 51013, Iraq.

Received | November 12, 2025; Accepted | January 16, 2026; Published | March 26, 2026

*Correspondence | Mustafa Qais Hamid, Department of Soil Science and Water Resources, College of Agriculture, University of Al-Qadisiyah, Al-Diwaniyah, Iraq; Email: [email protected]

Citation | Almashhadani, B.M.K., M.Q. Hamid, N. Al-Silmawy and D.F. Hassan. 2026. Combined effect of nano-silica and plant growth-promoting rhizobacteria (PGPR) on the physical and mechanical properties of soil. Pakistan Journal of Agricultural Research, 39(1): 119-126.

DOI | https://dx.doi.org/10.17582/journal.pjar/2026/39.1.119.126

Keywords | Nano-silica, PGPR, Bulk density, Total porosity, Soil water retention curve (33-500 kPa), Penetration resistance

Copyright: 2026 by the authors. Licensee ResearchersLinks Ltd, England, UK.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Introduction

Clay soils frequently suffer from compaction, low total porosity, high penetration resistance, and unfavorable pore-size distribution, all of which restrict root growth, water infiltration, and ultimately crop productivity; highlighting the need to improve bulk density, porosity, shear strength, penetration resistance, and the soil water retention curve.

Clayey soils are dominant in many irrigated areas particularly in the enormous arable lands of arid and semi-arid zones; these possess a problematic structure that strongly threatens a sustainable cropping system. Multiple wetting–drying cycles, heavy equipment traffic and deep tillage affect their physical properties by decreasing macroporosity as well as loss of porosity by surface crusting. These effects inhibit seedling emergence, block gas exchange and impose a high physical barrier to root penetration. As a result, plants develop shallow root systems that restrict water and nutrient absorption; as a consequence yield reduction and decreased crop resilience follow. Consequently, re-establishment and preservation of soil structural and geometric equilibrium is a fundamental requirements for sustainable intensification, particularly in the face of water limitations or climate change (Aksu and Eskisar, 2023; Wang et al., 2024).

It has been demonstrated in the literature that Si-based nanomaterials can improve soil particle aggregation, strength, and modify pore architecture to stabilize fine-texture soils and influence the infiltration velocity and water storage potential of such soils (Aksu and Eskisar, 2023). In addition, PGPR inoculants have also been found to promote the improvement of soil physical properties as they release extracellular polymeric substances (EPS) and from biofilms that help bind soil particles to form stable aggregates which then reduces bulk density and increases pore connectivity (Kaniz et al., 2023).

Most published research, however, has focused on the use of these technologies separately and in more controlled conditions, either under laboratory or green- house settings. There is little information on their combined impact under field conditions, particularly on hydrophysical and mechanical indices such as shear strength and penetration resistance. Synergized effects between nano-scale amendements and micro-scale biological inoculants could enhance soil resilience, but remain poorly documented. Despite promising evidence, no field study has simultaneously evaluated the hydrophysical and mechanical responses of clay loam soils to nano-silica and PGPR combinations under realistic field conditions. Synergistic effects of nano-scale physical (amendements) and microscale biological (conditioners) components could lead to better soil resilience than single-component approach.

Potato (Solanum tuberosum L.) is among the most important tuber crops globally but is physiologically sensitive to soil compaction, poor aeration, and mechanical constraints. Optimal tuber development depends on loose, well-drained soil with balanced water retention and air exchange. Excessive bulk density or low air-filled porosity delays emergence, limits stolon growth, and reduces tuber size and uniformity. Thus potato is a sensitive bioindicator for assessing soil physical improvements. Silicon and nano-silica has been linked to improved plant osmotic adjustment and yield; whereas Inoculants based on PGPR alone or combined with other microbial improvement of potato performance at the consortia by improving nutrient absorption and minimizing abiotic stress (Wadas and Kondraciuk, 2023; Liu et al., 2020). However, studies investigating the 2 agents in parallel, particularly in field conditions, are still scarce.

Therefore, this research was carried out to assess, during actual field conditions, the individual and combined NG on the effects of nano-silica and two PGPR strains (Bacillus subtilis and Pseudomonas fluorescens) on a clay loam soil cultivated with potato. The experiment was conducted in a randomized complete block design. The purpose of the research was to quantify changes in bulk density, total porosity and air-filled porosity, as well as the soil water retention curve (SWRC), and in mechanical properties including undrained shear strength and cone penetration resistance. Agricultural responses, such as improvements in tuber yield and potato emergence were also evaluated to link soil changes with agronomic performance. The primary hypotheses were that (i) nano-silica and PGPR would both improve soil hydro-physical, and mechanical properties relative to the control plots; (ii) when used in combination, applications would produce more pronounced synergistic benefits compared to singular treatments; and (iii) the beneficial effects of soil improvement would lead to enhanced plant establishment and yield responses across production cycles (Cao et al., 2023; Liu et al., 2022).

The novelty of this experiment lies from the combined in-situ evaluation of nano-silica and PGPR co-application on the hydrophysical and mechanical properties of cohesive clay loam. Unlike earlier laboratory studies focusing on isolated processes such as water retention or strength/clogging behavior, reconstructs relationships between physical structure, mechanical stability and crop performance (re)establishing missing links. Comparisons between to two phylogenetically distant bacterial strains, Bacillus subtilis, an endorhizal spore- former and Pseudomonas fluorescens a non-endorhizal rhizosphere colonizer. Their application in combination with nano-silica might shed unique insights into the action modes of MBPs and inorganic particles within the micro-scale biophysicochemical soil environment.

Apart from its technical novelty, the present work has broader implications for sustainable agriculture in fine-textured soils. It shows that the physical restoration of degraded soils is possible without energy-intensive mechanical tilling and artificial conditioners, by combining nanotechnology with biologically based microbial inoculants. Beyond mechanistic interests in soil mechanics or nanomaterials, it also emphasizes upon how small effects at microbial (or nanoparticle) scales can have significant macro-scale consequences in terms of water retention, strength and crop productivity.

Materials and Methods

Site soil and experimental design

A field experiment was conducted on clay soil during the main potato season in 2024 under irrigated conditions using drip irrigation. The trial followed a randomized complete block design (RCBD) with three replications and six treatments (T0T5). Site management (tillage, fertilization, irrigation and plant protection) was the same for all treatments; Only the indicated organic or nano-silica inputs were different. RCBD was chosen to control field variability and increase precision in treatment comparisons (Gomez and Gomez, 1984; Mihovilovich, 2014).

Treatments

Six treatments were evaluated: T0 = control (no additions), T1 = Bacillus subtilis only, T2 = Pseudomonas fluorescens only, T3 = nano-silica only, T4 = B. subtilis + nano-silica, and T5 = P. fluorescens + nano-silica.

PGPR and nano-silica rates and application modes are detailed below. Treatment structure was randomized within each block according to RCBD principles (Gomez and Gomez, 1984).

Soil sampling and baseline characterization

Composite soil samples (0–20 cm) were taken before applying treatments and planting to measure initial physical properties (Table 1). Core method was used for bulk density, and a pycnometer was used for the particle density according to standard practices. The sum of total porosity (%) was determined as follows:

n=(1−ρb/ρs)*100 …(1)

Soil water retention at matric suctions of 33, 100, 300, and 500 kPa was determined using pressure plate extractors; methods and equilibration criteria followed the classical porous-plate approach. Shear strength (undrained, cohesive matrix) was measured with a field vane (where applicable), and penetration resistance with a standard cone penetrometer at a constant insertion rate (Blake and Hartge, 1986; Richards, 1948).

PGPR strains preparation and application

A B. subtilis strain and a P. fluorescens strain were cultured to ~10^8 CFU mL⁻¹ (OD₆₀₀ ≈ 0.7–1.0). Seed tubers were coated immediately before planting by dipping/drizzling with the fresh bacterial suspension using 1–2% (w/v) sodium carboxymethyl-cellulose (CMC) as binder; ~10 mL suspension per seed piece ensured ~108 CFU per seed piece. This inoculum density and seed-coating approach are widely used for PGPR delivery to potato and other crops (Amin et al., 2023; Burr et al., 1978; Afzal et al., 2020; Paravar et al., 2023).

Nano-silica source dose and dispersion

The nano-silica was a hydrophilic fumed silica (primary size ~12–20 nm, specific surface area ~200 m² g-¹) consistent with commercial amorphous SiO₂ specifications. For soil application, an aqueous dispersion was prepared by magnetic stirring and bath sonication (≈ 30 min) before soil incorporation at 200 mg kg-¹ (solo or combined with PGPR). Dispersion/handling followed good practice for nanomaterials; material specifications are aligned with published datasheets and OECD guidance on sample preparation and dosimetry for manufactured nanomaterials (Evonik Aerosil 200PI; Schmidt et al., 2023).

Crop husbandry and measurements

Certified seed tubers of a locally adapted cultivar were planted on ridges at ~0.75 m row spacing and 0.30 m in-row spacing; crop protection and fertilization followed local recommendations and were uniform across treatments. Emergence (germination) was recorded as % emerged plants at ~21 days after planting. At harvest, total and marketable yields were recorded; marketability followed international sizing where ware potatoes are ≥ 35 mm (UNECE, 2023; Mihovilovich, 2014).

Physical measurements (timeline)

Soil bulk density, porosity, penetration resistance, vane shear, and soil water retention (33, 100, 300, 500 kPa) were measured at baseline (pre-plant) and again at mid-season and harvest on undisturbed cores or in situ as appropriate. Penetration resistance used a 30° cone per ASABE S313.3, and reporting followed ASAE EP542.1 to minimize variability due to probe geometry and insertion rate (ASABE S313.3; ASAE EP542.1).

Data handling and quality control

All measurements (raw soil and plant) were entered in a standardized, pre-coded spreadsheet as factors (T0–T5), block (1-3), plot ID, depth (where relevant), and timing of sampling (baseline, mid-season, harvest). Units were harmonised before our analysis (e.g., water contents expressed in volumetric where necessary; porosity computing as n=1−ρb/ ρs; air-filled porosity at 33 kPa as n−θ33). Instrument metadata (cone type, insertion rate; vane model) and soil moisture status at measurement were logged to ensure comparability. Technical replicates within plot × depth × time were screened for data-entry errors and outliers using range checks and Tukey fences; flagged values were re-inspected against field sheets. Missing observations (e.g., obstructed penetrometer runs) were left as NA without imputation. For plot-level summaries, replicates were averaged to a single value per response variable prior to inferential testing.

Statistical analysis

Responses were analysed under a fixed-effects RCBD. For each variable, the model was fitted:

Yij=μ+Ti+Bj+εij ……(2)

Where TiT_iTi is the treatment effect (i = 0…5) and BjB_jBj the block effect (j = 1…3). Assumptions were evaluated on residuals (normality, homoscedasticity) using diagnostic plots; where needed, variance-stabilising transforms (log, square-root, or arcsine–square-root for percentages) were applied and back-transformations used for presentation. When the ANOVA was significant (α = 0.05), treatment means were separated with Tukey’s HSD. All analyses were run in R (version 4.x) using the agricolae package, following standard RCBD procedures (Gomez and Gomez, 1984).

 

Table 1: Basic physical and mechanical properties of soil.

Characteristics	Units	Value
EC	ds.m−1	1.96
PH		7.33
Sand	g.kg−1	291
Silt		323
Clay		386
Textural Class		Clay Loam
Soil water content 33 kPa	(w/w)%	39
Soil water content 100 kPa		33
Soil water content 300 kPa		27
Soil water content 500 kPa		24
Total porosity (n)	%	47.9
Bulk Density	g cm−3	1.38
Particle Density		2.65
Hydraulic Conductivity	cm h−1	0.61
Undrained shear strength	kPa	32
Penetration resistance (0–20 cm)	MPa	1.10
Emergence (germination)	%	94
Total tuber yield	t ha⁻¹	30
Marketable yield		≥ 35 mm, subset of total

 

Results and Discussion

Table 2 indicates that the Bulk density decreased progressively (−2.9 to −8.7% vs T0) with the largest reductions in the combined nano-silica + PGPR treatments (T4–T5). This aligns with a microstructural mechanism where nanosilica augments fine particle coatings/bridges and redistributes pore sizes, while PGPR contributes exopolysaccharides (EPS) that “glue” particles and stabilize aggregates-together yielding lower compaction and higher total porosity. The upward shift of water retention with nanosilica reported on loess (Figure 1) is explained by pore refinement and greater specific surface area (Wang, 2024); PGPR contributions to hydro-physical behavior (such as reduced evaporation and modified infiltration) are also documented (Kaniz et al., 2023). Despite increased total porosity, air-filled porosity at 33 kPa remained ~9–9.1%, i.e., hovering around the commonly cited minimum aeration requirement (~10% air-filled porosity) for many crops; modern syntheses still reference the classic 10–15% range originally suggested by Grable and Siemer and subsequent work (Pragg et al., 2024; Zou et al., 2001). That helps explain why emergence improved modestly (Table 4): Aeration became adequate rather than excessive.

 

Table 2: Bulk density, total porosity, and air-filled porosity at field capacity (33 kPa) of clay loam soil as affected by Nano-silica and PGPR treatments.

Treatment	Bulk density (g cm⁻³)	Δ vs T0 (%)	Total porosity (%)	Δ vs T0 (%)	Air-filled porosity 33 kPa (%)	Δvs T0 (%)
T0 Control	1.38±0.01	—	47.9±0.5	—	8.9 ± 0.3	—
T1 B. subtilis	1.34±0.01	−2.9	49.4±0.6	+3.1	8.9 ± 0.3	0.0
T2 P. fluorescens	1.33±0.01	−3.6	49.8±0.6	+4.0	8.8 ± 0.3	−1.1
T3 Nano-silica	1.30±0.01	−5.8	50.9±0.5	+6.3	8.9 ± 0.3	0.0
T4 B. subtilis+Nano	1.27±0.02	−8.0	52.1±0.6	+8.8	9.1 ± 0.2	+2.2
T5 P. fluorescens+Nano	1.26±0.02	−8.7	52.5±0.6	+9.6	9.0 ± 0.2	+1.1

 

Figure 1 shows all treatments increased θ at 100–500 kPa, with the greatest gains under T4–T5 (e.g., θ₃₀₀: 27% in T0 vs ~31% in T5, +4 points). Such upward shifts at higher suctions indicate more water stored in small pores, consistent with nanosilica-induced pore refinement and higher SSA, and with PGPR/EPS effects on water dynamics near roots. Wang et al. (2024) showed that nanosilica raises water-holding capacity and lowers unsaturated k by transforming large pores into smaller ones; Kaniz et al. (2023) directly demonstrated PGPR-driven changes in evaporation/infiltration. Mechanistically, these hydrologic improvements should buffer plants between irrigations and support the yield response in Table 4 (Wang et al., 2024; Mahmoud et al., 2025).

Table 3 shows two desirable trends co-occur: higher undrained shear strength (+6–25%) and lower cone index (-9-26%). Nanosilica additions are known to reinforce cohesive frameworks and improve strength via fine-scale bridging and pore modification across diverse soils (Aksu and Eskisar, 2023), while PGPR-derived EPS can enhance aggregate cohesion. Crucially, the Table 3 cone indices (≤ ~0.9 MPa) are well below thresholds where root elongation is strongly impeded; reviews place substantial growth limitation typically above ~0.8–2.0 MPa depending on water status and texture (Bengough et al., 2011; Whitmore and Whalley, 2009). Thus, T3–T5 deliver a more root-permissive mechanical environment without sacrificing matrix integrity, consistent with better rooting and resource capture (Shahad et al., 2025; Hamid et al., 2025).

 

Table 3: Undrained shear strength and cone penetration resistance (0–20 cm) near field capacity in clay loam soil as influenced by Nano-silica and PGPR treatments.

Treatment	Shear strength (kPa)	Δ vs T0 (%)	Penetration resistance (MPa)	Δ vs T0 (%)
T0 Control	32 ± 1.5	—	1.10 ± 0.05	—
T1 B. subtilis	34 ± 1.6	+6.3	1.00 ± 0.05	−9.1
T2 P. fluorescens	35 ± 1.6	+9.4	0.96 ± 0.04	−12.7
T3 Nano-silica	37 ± 1.7	+15.6	0.90 ± 0.04	−18.2
T4 B. subtilis+Nano	39 ± 1.8	+21.9	0.84 ± 0.04	−23.6
T5 P. fluorescens+Nano	40 ± 1.8	+25.0	0.82 ± 0.04	−25.5

 

Table 4 shows emergence rose modestly (≤ +3.2%), consistent with Table 2, where air-filled porosity at 33 kPa approached the 10% aeration benchmark while penetration resistance dropped into a root-friendly range (Table 3). These combined aeration–mechanical improvements are widely recognized as co-requirements for early stand establishment (Pragg et al., 2024; Bengough et al., 2011). The yield gains (+12 to +20% under T3–T5) align with two independent literature streams. First, PGPR consortia (and individual Bacillus/Pseudomonas strains) frequently enhance potato yield via rhizosphere recruitment and improved N/C metabolisms (Wang et al., 2021), with recent potato field work confirming Bacillus-based products can raise yield while minimally disturbing the soil microbiome (Adamo et al., 2024). Second, silicon/silica inputs have repeatedly increased early potato yields in temperate field studies (Wadas, 2023; Wadas and Kondraciuk, 2025), plausibly by improving plant water relations and tuber size distribution effects that dovetail with our water retention and mechanical improvements (Wadas, 2023; Dahham et al., 2025).

 

Table 4: Potato emergence and final tuber yield in response to Nano-silica and PGPR treatments in clay loam soil.

Treatment	Emergence (%)	Δ vs T0 (%)	Final yield (t ha⁻¹)	Δ vs T0 (%)
T0 Control	94 ± 1.0	—	30.0 ± 0.9	—
T1 B. subtilis	95 ± 1.0	+1.1	32.0 ± 0.8	−6.7
T2 P. fluorescens	95 ± 1.0	+1.1	32.5 ± 0.8	−8.3
T3 Nano-silica	96 ± 0.9	+2.1	33.5 ± 0.8	−11.7
T4 B. subtilis+Nano	97± 0.9	+3.2	35.5 ± 0.7	−18.3
T5 P. fluorescens+Nano	97 ± 0.9	+3.2	36.0 ± 0.7	−20.0

 

Conclusions and Recommendations

This field study demonstrates that integrating nano-silica with PGPR consistently improved the physical–mechanical condition of clay-loam soil and translated into superior potato performance. Bulk density declined from 1.38 to 1.26–1.27 g cm-3, total porosity exceeded 52%, water retention increased across 33–500 kPa, undrained shear strength rose (32→40 kPa), and cone penetration resistance fell (1.10→0.82 MPa), supporting higher emergence (94→97%) and yield (30→35–36 t ha⁻¹). We recommend adopting the combined treatment (nano-silica + Bacillus/Pseudomonas) at moderate rates, with pre-plant incorporation to 0–15 cm plus seed-piece coating/in-furrow drench, while managing moisture near field capacity, avoiding traffic on wet soil, and monitoring cone index (CI) values to remain < 1 MPa with air-filled porosity ≥10%. Future work should establish optimal dose-response relationships over multiple seasons and sites and assess cost-effectiveness and environmental fate to ensure sustainable and safe scaling at field level. The results could be explained by nano-silica improving microaggregation and PGPR exopolysaccharides increasing soil cohesion and pore continuity, collectively resulting in positive changes to soil structure and root-zone conditions.

Novelty Statement

This study uniquely shows that combining nano-silica with PGPR improves soil physical–mechanical properties and potato yield in clay-loam soils under field conditions.

Author’s Contribution

All authors contributed equally to the conception, design, execution, data analysis, and writing of this manuscript.

Generative AI and AI-assisted technology statement

No AI tools were used in this paper.

Conflict of interest

The authors have declared no conflict of interest.

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