Effect of NPK fertigation schedule on growth and yield of capsicum under polyhouse

Introduction

Protected cultivation of vegetables has expanded by over 300% in the Mediterranean region of Turkey during the past decade, yet fertiliser management inside polyhouses often follows the same blanket recommendations designed for open-field conditions [1] . Capsicum, a highvalue crop with a growing season of 150-180 days under cover, removes substantially more nitrogen, phosphorus, and potassium per hectare than field-grown tomato or brinjal [2] . Relying on a single basal application of granular fertiliser fails to match the crop's nutrient demand curve, which peaks during flowering and fruit-set stages [3] . Fertigation the injection of soluble fertilisers through a drip system allows growers to deliver nutrients in small, frequent doses synchronised with plant uptake [4] . Earlier trials on capsicum fertigation in southern Europe reported yield gains of 20-35% over conventional methods, but most were conducted under climate-controlled greenhouses with substrates quite different from the calcareous clay loam soils of the Antalya coastal plain [5,6] . Information on the dose-response relationship for capsicum fertigation under low-cost naturally ventilated polyhouses in southern Turkey is scarce. This research compared four fertigation doses plus a conventional soil-applied control to determine the level that maximises yield without causing nutrient toxicity or soil degradation, and to quantify the nutrient input-output balance under each treatment.

Field experimental design

The trial was laid out in a randomised block design with five treatments and four replications inside a 500 m² naturally ventilated polyhouse (galvanised iron frame, 200-µm UV-stabilised polythene cladding, 40-mesh insect net on side walls). Each plot measured 3.6 m × 2.4 m, with capsicum transplanted at 60 cm × 45 cm spacing, giving 32 plants per plot. A 0.5 m border row separated adjacent plots. Inline drip laterals (16 mm, 4 L h⁻¹ emitters at 45 cm spacing) were installed along each row. Fertigation treatments were: T1 (75% RDF), T2 (100% RDF = 200:100:100 kg N:P₂O₅:K₂O ha⁻¹), T3 (125% RDF), T4 (150% RDF), and T5 (control entire RDF applied as basal granular). Fertigation was split into 12 equal doses delivered at 10-day intervals from 20 days after transplanting [7] .

Material and Methods Material

The experiment was conducted from October 2022 to March 2023 at the polyhouse complex of the Antalya Mediterranean Agricultural University, Antalya (36.88°N, 30.70°E, 42 m elevation). Sixty-day-old seedlings of capsicum cv. Kapija (blocky type, green to red maturity) were transplanted on 10 October 2022. The soil was sandy loam (pH 6.3, organic carbon 0.48%, available N 218 kg ha⁻¹, available P₂O₅ 16.4 kg ha⁻¹, available K₂O 189 kg ha⁻¹). Water-soluble fertilisers used for fertigation were urea (46% N), mono-ammonium phosphate (12-61-0), and muriate of potash (60% K₂O), all dissolved in a 200-litre mixing tank and injected via a venturi injector [8] .

Methods

Growth observations (plant height, branch number, days to 50% flowering) were recorded at 30-day intervals on five tagged plants per plot. Yield attributes (fruit length, diameter, weight, number per plant) were measured from the third to the tenth picking. Marketable yield excluded misshapen and pest-damaged fruits. For nutrient uptake, whole-plant samples at final harvest were oven-dried at 65 °C, ground, and analysed for N (micro-Kjeldahl), P (vanadomolybdate method), and K (flame photometry) [9] . Soil samples from 0-15 cm depth were collected before planting and after the last harvest for pH, EC, and available nutrient estimation [10] . Data were analysed by ANOVA in SPSS 26; means separated by LSD at p≤0.05.

Results

Table All fertigation treatments outperformed the soil-applied control for every growth and yield parameter (Tables 123).

A clear dose-response pattern emerged up to 125% RDF, beyond which yield plateaued and soil EC rose to 0.67 dS m⁻¹ a value approaching the salt-sensitivity threshold for capsicum. Correlations between plant height, fruit weight, and yield were strong and positive (r ≥ 0.91), indicating that vigorous vegetative growth translated directly into higher productivity.

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Comprehensive interpretation

Fertigation at 125% RDF maximised both vegetative growth and fruit production without pushing soil salinity beyond safe limits. The quadratic yield response (Figure 1) confirms a diminishing return above this dose. The control treatment's poor performance underscores the inadequacy of one-time basal application for a long-duration crop like capsicum under protected conditions.

Nutrient budget

At 125% RDF, total N applied was 250 kg ha⁻¹ against a crop uptake of 187 kg ha⁻¹, giving an apparent nitrogen recovery of 74.8%. Phosphorus recovery stood at 31.4% and potassium at 68.2%. The remaining nutrients were either fixed in the soil, lost through minor leaching, or retained in crop residue. The 150% RDF treatment showed lower recovery for all three nutrients 67.1% N, 26.8% P, and 59.7% K confirming that the additional dose was not efficiently used and instead contributed to residual soil build-up [11] .

Discussion

The 22.6% yield advantage of 125% RDF over the standard dose aligns with the observation of Singh et al. [5] who recorded a 28% gain in bell pepper fertigated at 120% RDF inside fan-and-pad greenhouses in Antalya. The mechanism behind this response likely involves improved nutrient availability in the wetted root zone; because fertigation delivers dissolved nutrients directly to active roots, uptake efficiency exceeds that of broadcast granular application where nutrients must diffuse through a larger soil volume [4] . This means that a moderate excess above the conventional recommendation compensates for the inherent inefficiency of soil-mediated nutrient movement in polyhouse beds.

The yield depression at 150% RDF was accompanied by a soil EC rise to 0.67 dS m⁻¹ and a drop in soil pH to 5.97.

Capsicum is classified as moderately salt-sensitive, with yield reduction beginning around 1.0 dS m⁻¹ [12] , but the acidification and osmotic stress at the root surface may have begun to restrict water uptake even before that threshold was crossed [13] . The practical message for polyhouse

Conclusion

Capsicum cultivation under naturally ventilated polyhouses in the Antalya coastal plain demands a fertigation strategy calibrated above the conventional soil-application rate, yet restrained enough to prevent salt accumulation. Among the doses tested, 125% RDF delivered through 12 split fertigations produced the highest marketable yield (61.8 t ha⁻¹), best nutrient recovery, and acceptable post-harvest soil health. Growers are recommended to adopt this schedule with periodic soil EC monitoring. Looking ahead, multi-season trials incorporating organic amendments and micronutrients could refine the protocol further and improve long-term soil sustainability under continuous polyhouse cropping [14] .

Acknowledgement

Acknowledgements Funding sourcesFinancial support from the Antalya Mediterranean Agricultural University internal research fund is gratefully acknowledged. Institutional supportThe polyhouse complex and soil testing laboratory of the university provided the research infrastructure. Contributions not qualifying for authorshipMr. Rajesh Kumar Sahu helped with drip system maintenance and fertigation scheduling.

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