Nutrient uptake dynamics and soil fertility status under foliar nutrient management in onion (Allium cepa L.) seed production in semi-arid alkaline Inceptisols

Introduction

Onion (Allium cepa L.) seed production represents a specialized agronomic enterprise fundamentally distinct from conventional bulb production, requiring successful navigation of complex phenological transitions from vegetative growth through reproductive development and seed maturation (Brewster, 2008; Kumar et al., 2018) [5,19] . As the world's second-largest vegetable crop, onion cultivation occupies approximately 3.97 million hectares globally, with India contributing substantially to international seed markets (National Horticulture Board, 2013-2014). However, onion seed productivity in semi-arid regions of India averages 700-800 kg ha⁻¹, substantially below genetic yield potential, primarily attributed to restricted nutrient bioavailability in alkaline calcareous soils (Kumar et al., 2018; Tomar et al., 2015) [19, 36] . The fundamental constraint limiting nutrient acquisition in alkaline soils derives from phosphorus chemistry and soil-mineral interactions. In calcareous soils with pH values exceeding 8.0 (characteristic of semi-arid regions), phosphorus undergoes sequential precipitation and surface complexation reactions with divalent calcium cations, forming insoluble calcium phosphate minerals including dicalcium phosphate (CaHPO₄•2H₂O), octacalcium phosphate (Ca₈H₂(PO₄)₆•5H₂O), and hydroxyapatite (Ca₅(PO₄)₃OH) (Leytem & Westermann, 2005; Nadeem et al., 2024) [21,23] . These precipitation reactions progressively render 70-95% of soil-applied phosphorus unavailable for plant uptake within 30 days of application, creating a critical nutritional bottleneck during the high-demand reproductive phase (Aimen et al., 2021; Nadeem et al., 2024) [3,23] . Onion's morphological and physiological constraints further exacerbate phosphorus acquisition difficulties. The crop possesses a shallow, sparsely-branched fibrous root system with maximum rooting density confined to the upper 30 cm of soil, fundamentally limiting soil volume exploration and acquisition of immobile nutrients such as phosphorus (Sullivan, 2001) [34] . Additionally, during the reproductive phase (bolting through seed maturation, approximately 60-120 days after planting), developing umbels function as powerful reproductive sinks, exhibiting peak nutrient demand precisely when soil phosphorus availability remains severely restricted by alkaline soil chemistry (Brewster, 2008; Kumar et al., 2018) [5,19] . Foliar nutrient application represents a mechanistically distinct delivery pathway that circumvents soil-matrix phosphorus fixation. Foliar-applied nutrients penetrate the leaf cuticle and enter the vascular symplast directly through stomatal apertures and hydrated polar pores (0.5-5 nm diameter), bypassing soil electrochemical sorption and precipitation reactions (Fernández & Eichert, 2009; Schönherr, 2006) [12,26] . This direct bypass mechanism enables immediate systemic translocation to reproductive sinks and ensures nutrient availability during phenologically-critical reproductive windows when soilacquired phosphorus remains sequestered in insoluble calcium phosphate phases. While previous investigations have established that foliar nutrient applications increase onion seed yield and quality parameters, quantitative data regarding whole-plant nutrient uptake dynamics, comparative nutrient use efficiencies across diverse foliar formulations, nutrient distribution in plant tissue, and post-harvest residual soil fertility status remain inadequately characterized. Furthermore, mechanistic understanding of how specific foliar nutrient formulations interact with soil chemistry to influence nutrient acquisition patterns remains incomplete. This knowledge gap represents a critical limitation for developing evidence-based nutrient management recommendations tailored to alkaline soil environments.

Research Hypotheses and Objectives

This investigation was predicated on the hypothesis that targeted foliar application of specific nutrient formulations during phenologically-critical reproductive stages fundamentally alters nutrient bioavailability pathways, enabling substantially increased nutrient acquisition compared to conventional soil-only fertilization approaches, while simultaneously influencing post-harvest residual soil fertility status. The specific research objectives were: (1) to quantify the effect of ten contrasting foliar nutrient management treatments on total plant uptake of N, P, and K; (2) to calculate nutrient use efficiency and agronomic efficiency metrics for each nutrient under diverse foliar formulations;

(3) to determine nutrient concentration in plant tissue at physiological maturity and assess nutrient distribution patterns; (4) to characterize post-harvest residual soil nutrient status (Olsen-extractable P, alkaline permanganateextractable N, ammonium acetate-extractable K) and relate findings to crop nutrient acquisition; and (5) to provide mechanistic evidence explaining the superior performance of phosphorus-enriched foliar formulations in alkaline soil systems.

Materials and Methods

Experimental Site Description and Soil Characterization

The field experiment was conducted at the Centre for Advanced Agricultural Science and Technology (CAAST) facility, Mahatma Phule Krishi Vidyapeeth (MPKV), Rahuri, Maharashtra, India (19°47′ N, 74°19′ E, 495 m above mean sea level) during the 2022-2023 rabi (winter) growing season. The semi-arid tropical climate at the site is characterized by bimodal rainfall distribution, with the critical kharif (monsoon) season occurring June-September, and secondary rabi season precipitation concentrated in December-January. Mean annual rainfall averaged 628 mm during the experimental year, with pronounced dry season conditions prevailing during the critical reproductive phase of onion seed production (January-March). The experimental soil was classified as a clay loam Inceptisol according to the USDA Soil Taxonomy system (Soil Survey Staff, 2014) [32] , representing 39.74% of Indian agricultural soils and characterized by limited profile development and moderate weathering intensity typical of semi-arid regions (ICAR, 2013; Supriya et al., 2019) [17,35] . The soil exhibited alkaline reaction (pH 8.20), non-saline status (EC 0.38 dS m⁻¹), and limited organic matter accumulation (organic carbon 0.54%). Available macronutrient status was characterized as: nitrogen 189.20 kg ha⁻¹ (alkaline permanganate method), phosphorus 18.30 kg ha⁻¹ (Olsen's method, 0.5 M NaHCO₃ at pH 8.5), and potassium 392.40 kg ha⁻¹ (neutral 1 N ammonium acetate).

Soil mineralogy was dominated by illite and montmorillonite clay minerals with high exchangeable calcium (Ca²⁺) and magnesium (Mg²⁺), characteristic of calcareous soils derived from parent materials containing calcium carbonate minerals. The soil contained approximately 12-15% free calcium carbonate equivalent, which represented the principal mechanistic factor predisposing phosphorus to precipitation and sorption reactions.

Experimental Design and Treatment Structure

The experiment was designed using a single-factor Randomized Block Design (RBD) with ten treatments and three replications, generating thirty experimental units (Gomez & Gomez, 1984) [15] . The RBD was selected to control spatial heterogeneity in soil fertility gradients across the experimental field, thereby reducing experimental error and increasing statistical precision for treatment effect estimation (Fisher, 1935; Cochran & Cox, 1957; Casler, 2015) [13,10,8] . Each experimental plot measured 3.0 m × 2.4 m (7.2 m²) with a net plot size of 2.4 m × 1.8 m (4.32 m²), maintaining inter-row spacing of 45 cm and intra-row plant spacing of 15 cm. The onion cultivar Phule Samarth, a short-day adapted variety bred by MPKV with documented tolerance to bolting stresses and high seed yield potential (600-800 kg ha⁻¹), was used for all experimental units (Singh et al., 2018) [29] .

Treatment Details

 T₁: Absolute Control (No basal fertilization, no foliar spray)  T₂: GRDF (120:60:60 N:P₂O₅:K₂O kg ha⁻¹ + 20 t FYM ha⁻¹) + Water spray  T₃: GRDF + Urea @ 2% (three foliar applications)  T₄: GRDF + Diammonium Phosphate (DAP) @ 2%  T₅: GRDF + Muriate of Potash (MOP) @ 2%  T₆: GRDF + Balanced NPK (19:19:19) @ 1%  T₇: GRDF + Monopotassium Phosphate (MKP, 0:52:34) @ 0.5%  T₈: GRDF + Monoammonium Phosphate (MAP, 12:61:0) @ 0.5%  T₉: GRDF + Potassium Nitrate (KNO₃, 13:0:45) @ 0.5%  T₁₀: GRDF + Sulphate of Potash (SOP, 0:0:50) @ 0.5% Basal soil application (GRDF) was uniformly applied to all treatments except T₁. Nitrogen was applied in split doses: 50% as basal application at transplanting (urea, 46% N), 25% at 30 days after planting (DAP), and 25% at 60 DAP to synchronize nitrogen availability with critical crop demand phases and minimize leaching losses. Full phosphorus and potassium doses were applied basally at transplanting. Foliar applications were conducted at three phenologically-critical growth stages: 45 DAP (vegetative phase, characterized by maximum leaf area index and rapid biomass accumulation), 60 DAP (bolting initiation, marked by scape elongation and umbel primordium differentiation), and 90 DAP (flowering initiation to early anthesis, corresponding to peak reproductive sink demand). All foliar applications were conducted during early morning hours (06:00-08:00 h) to maximize stomatal aperture and minimize evaporative losses, applying approximately 500 L ha⁻¹ spray volume using a hand-operated knapsack sprayer fitted with flat-fan nozzle to ensure uniform canopy coverage. A non-ionic surfactant (Tween-20 at 0.05% v/v) was added to all spray solutions to reduce surface tension and enhance foliar penetration through cuticular pathways (Schönherr, 2006) [26] .

Plant Sampling Protocol and Nutrient Analysis

Plant samples for nutrient analysis were collected at physiological maturity (indicated by 70% umbel browning). Five randomly selected plants per plot were harvested, separated into straw (leaves and stems combined) and reproductive organs (umbels containing mature seeds), and immediately transported to the laboratory for processing. Plant material was washed sequentially with deionized water to remove soil particles and surface contaminants, then oven-dried at 65 °C to constant weight to determine dry matter content and enable calculation of nutrient uptake on a dry matter basis. Dried plant tissue samples were ground to pass through a 60-mesh sieve, homogenized, and digested using the standard tri-acid digestion procedure (H₂SO₄:HNO₃:HClO₄, 10:4:1 v/v) (Chapman & Pratt, 1961) [9] . Total nitrogen content in the digest was determined colorimetrically using the Nessler reagent method following distillation (Bremner & Mulvaney, 1982) [4] . Phosphorus was determined colorimetrically using the vanadomolybdic acid method (Chapman & Pratt, 1961) [9] . Potassium was determined by flame photometry using a calibrated flame photometer (Hanway & Heidel, 1952) [16] . Nutrient concentrations (% dry weight basis) were multiplied by dry matter yield to calculate total nutrient uptake per hectare (kg ha⁻¹).

Soil Sampling and Nutrient Analysis Methods

Post-harvest soil samples were collected at physiological crop maturity from five randomly selected points within each plot at 0-15 cm depth (active rhizosphere zone), airdried for 48 hours at ambient temperature, ground to pass a 2 mm sieve, and stored in polyethylene containers for chemical analysis. Soil Nitrogen Analysis: Available nitrogen was determined using the alkaline permanganate method (Subbiah & Asija, 1956) [33] , wherein 5 g of soil was incubated with 0.05 M alkaline KMnO₄ solution at 37 °C for 24 hours, followed by spectrophotometric measurement of remaining permanganate absorbance. The results were expressed as kg available N ha⁻¹.

Soil Phosphorus Analysis: Available phosphorus was extracted using the Olsen method (0.5 M NaHCO₃ at pH 8.5) specifically recommended for alkaline and neutral soils (Olsen et al., 1954) [24] . Following extraction, phosphorus was determined colorimetrically using the vanadomolybdic acid yellow color method and expressed as kg available P ha⁻¹.

Soil Potassium Analysis: Available potassium was extracted using neutral 1 N ammonium acetate (Hanway & Heidel, 1952) [16] , followed by determination using flame photometry and expressed as kg available K ha⁻¹.

Calculation of Nutrient Use Efficiency Parameters

Nutrient use efficiency was calculated using multiple standardized metrics to comprehensively characterize nutrient acquisition dynamics:

Nutrient Uptake Efficiency (NUE) = Total Nutrient Uptake (kg ha -1 )

Total Nutrient Applied (kg ha -1 ) × 100

Agronomic Efficiency (AE) = Seed Yield (T₈) -Seed Yield (T₁) Total Nutrient Applied-Nutrient in Control Apparent Recovery Efficiency (ARE) = Nutrient Uptake (Treated) -Nutrient Uptake (Control) Nutrient Applied × 100

Statistical Analysis

Data were subjected to analysis of variance (ANOVA) for randomized block design using agricultural statistics software (WASP 2.0, ICAR-CCARI, Goa, India) at 𝑃 ≤ 0.05 significance level (Gomez & Gomez, 1984) [15] . Treatment means exhibiting significant F-ratios were compared using the Critical Difference (CD) test at 5% probability level following Fisher's Least Significant Difference method (Snedecor & Cochran, 1989) [31] . Homogeneity of variance was verified using Levene's test, and normality of residuals was confirmed using the Shapiro-Wilk test prior to ANOVA (Levene, 1960; Shapiro & Wilk, 1965) [20,28] . Pearson correlation analysis was conducted to quantify relationships between nutrient uptake parameters and residual soil fertility status. [30,18] . When reproductive sink strength is enhanced through phosphorus supplementation, photosynthetic source activity increases proportionally, generating enhanced carbohydrate availability for translocation to potassiumdemanding reproductive tissues.

Results and Discussion

Total Plant Nutrient Uptake and Treatment Effects

Nutrient Concentration in Plant Tissue and Tissue Diagnosis

Table 4 presents nutrient concentration in plant tissue at physiological maturity. Phosphorus concentration in plant tissue ranged from 0.34% in the absolute control (T₁) to 0.69% in the optimized foliar treatment (T₈), representing a 103% increase. This substantial enhancement reflects direct phosphorus accumulation in reproductive organs (umbels with developing seeds) responding to the increased phosphorus bioavailability from foliar application. [19,29] . Treatment T₈ achieved 0.69%, representing the upper end of the optimal range and substantially exceeding the deficiency threshold (typically ≤ 0.30% for seed production crops). Conversely, the absolute control (0.34%) remained in the marginal deficiency zone, explaining the substantially compromised seed yield (3.76 q ha⁻¹) observed in preliminary yield data. Nitrogen and potassium concentrations in plant tissue also increased substantially with improved foliar nutrient management, though the magnitude of enhancement was more moderate than for phosphorus. Nitrogen concentration increased from 1.85% (T₁) to 2.89% (T₈), and potassium increased from 1.95% (T₁) to 3.45% (T₈), reflecting that phosphorus is the more critically limiting nutrient in alkaline soils.

Post-Harvest Residual Soil Fertility Status

Table 2 presents the post-harvest residual soil nutrient status across all treatments. This analysis is fundamental to understanding the long-term sustainability implications of divergent nutrient management strategies.

Conclusion

This comprehensive investigation quantified nutrient uptake dynamics, nutrient use efficiency, and post-harvest soil fertility status under ten contrasting foliar nutrient management treatments in onion seed production on semiarid alkaline Inceptisolic soils.

Principal findings 1. Phosphorus as the primary yield-determining nutrient:

Treatment T₈ (GRDF + MAP 12:61:0 @ 0.5%, three foliar sprays) achieved total phosphorus uptake of 15.24 kg ha⁻¹, representing 82.7% increase over the absolute control, compared to 58.2% increase for nitrogen and 80.7% for potassium. Phosphorus use efficiency in T₈ (32.9%) was substantially elevated compared to T₁ (18.2%).

2. Superiority of monoammonium phosphate formulation: Among phosphorus-enriched treatments, MAP (T₈) surpassed DAP (T₄) by 13.3% in phosphorus uptake, reflecting superior molecular dissociation kinetics of MAP in the near-neutral pH environment of leaf intercellular fluids.

Mechanistic explanation through foliar penetration

pathways: Foliar-applied phosphorus bypasses soil electrochemical fixation mechanisms operative in alkaline environments, enabling direct translocation through the vascular symplast to reproductive sinks during phenologically-critical reproductive stages (60-90 days after planting) when soil-acquired phosphorus remains precipitated as insoluble calcium phosphate phases. 4. Nutrient tissue concentration enhancement: Phosphorus concentration in plant tissue increased from 0.34% (T₁) to 0.69% (T₈), representing 103% enhancement and achieving the upper end of the optimal range for seed production crops. This investigation provides mechanistic evidence that in alkaline calcareous soils characteristic of semi-arid regions, targeted foliar application of phosphorus-enriched watersoluble fertilizers (particularly monoammonium phosphate 12:61:0) applied during phenologically-critical reproductive stages (60 and 90 days after planting) fundamentally alters nutrient bioavailability pathways, enabling substantially enhanced nutrient acquisition, improved nutrient use efficiency, and maintenance of long-term soil fertility through sustainable nutrient cycling. The findings support adoption of optimized foliar nutrient management as a sustainable, economically viable strategy for enhancing productivity and nutritional quality in onion seed production systems across alkaline soil environments of the semi-arid tropics.

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