The experiment was conducted in a controlled plastic greenhouse located at Al-Azhar University, Cairo, Egypt (30° 2′ 44″ N, 31° 15′ 44″ E), over the 2022–2023 growing season. The site is situated in a hot desert climate (BWh, Köppen classification), characterized by long, hot summers and mild winters. Average annual temperatures range from 13.5 °C in January to 35.2 °C in July, with relative humidity fluctuating between 50% and 60%, and low annual precipitation (20–25 mm), mostly concentrated in the winter months. Sweet pepper (Capsicum annuum L.), hybrid F1 cultivar 702, was selected for this study due to its suitability for greenhouse cultivation. Seeds were sown in 1:1 peat moss–vermiculite trays and grown in a nursery for 40 days before transplanting on November 5, 2022. Each pot contained a single seedling. In winter, greenhouses were covered with 120-micron white polyethylene plastic, while in summer, shading nets were used to moderate temperature and light.

The experimental workflow was designed to evaluate the integrated effects of compost and nano-fertilizer application on sweet pepper growth under greenhouse conditions. The process started with mechanical compost preparation and soil amendment, followed by transplanting uniform seedlings into pots and applying the experimental treatments according to a factorial arrangement. Plants were subsequently maintained under standard greenhouse management, during which physiological, biochemical, and yield-related traits were monitored. At harvest, data were collected and subjected to statistical and multivariate analyses to assess treatment effects and relationships among measured traits. The outline of the experimental work is shown in Figure 1.

Sweet pepper vegetative growth and average fruit weight (Figure 3) improved significantly with compost amendment, especially when combined with 2 cm³ L^-1 nano-fertilizer. The highest fruit mass occurred in sandy soils with 20–25% compost. These gains are attributed to enhanced soil moisture retention, greater nutrient availability, and foliar nano-calcium and nano-nitrogen supply, which may stimulate cell division and expansion. The combined root and foliar nutrient delivery created an optimal nutrient environment, consistent with EL-Mogy et al. (2024), who reported similar benefits from integrating organic and nano-based amendments.

The number of fruits per plant (Figure 4) was highest in sandy soil with 20% compost plus 2 cm³ L^-1 nano-fertilizer. This likely reflects improved flowering, pollination, and fruit set due to enhanced nutrient status. Boron in the nano-fertilizer may have supported pollen viability and hormone regulation, while compost-stimulated soil microbiota could have improved nutrient mobilization. These results align with those of Esringü et al. (2011), who reported increased fruit set and yield in strawberries and tomatoes following boron application.

In contrast, fruit diameter (Figure 5) showed no statistically significant differences among compost levels alone, but slight improvements were observed when nano-fertilizer was applied. This suggests that while compost improved general growth conditions, the foliar-applied nutrients played a more critical role in stimulating cell elongation and fruit expansion. The modest increase could be associated with calcium’s role in maintaining cell wall integrity and osmotic regulation (Hepler and Winship, 2010). Though the diameter differences were not dramatic, even marginal improvements in marketable fruit size can have significant economic implications.

The total yield per plant, as depicted in Figure 6, reflects the cumulative effect of improved vegetative growth, fruit set, and fruit size. The highest yield was achieved with the application of 20% compost in sandy soil and 2 cm³ L^-1 nano-fertilizer. This outcome confirms the agronomic effectiveness of the combined strategy, particularly in soils with limited resources. The compost’s capacity to improve soil water retention and root environment, along with the nano-fertilizer’s efficiency in nutrient delivery, synergistically enhanced the crop’s productivity. This finding aligns with that of EL-Mogy et al. (2024), who found that integrating compost and biochar enhanced yield components in greenhouse peppers.

Regarding biochemical traits, total chlorophyll content (SPAD index, Figure 7) increased significantly with the addition of compost and nano-fertilizer, especially at a concentration of 2 cm³ L^-1. The highest SPAD values were observed in sandy soils with 20% compost, indicating enhanced photosynthetic activity and nitrogen assimilation through nano-nitrogen supply, as well as improved soil fertility. These findings align with those of Pôrto et al. (2014), who validated SPAD as an indicator of nitrogen status in vegetables, and with Amirahmadi et al. (2024), who reported yield gains driven by compost and reduced ecosystem impacts in wheat systems.

Fruit ascorbic acid content, shown in Figure 8, was significantly improved by the integrated application of compost and nano-fertilizer. The highest ascorbic acid levels were recorded in plants grown in sandy soil with 15% compost and 2 cm³ L^-1 nano-fertilizer, followed closely by those grown in clay soil with 25% compost under the same foliar treatment. These increases can be attributed to enhanced metabolic activity and stress resilience, as compost improves antioxidant enzyme activity and nano-fertilizers boost micronutrient uptake. Ascorbic acid is a key quality parameter that enhances the nutritional and commercial value of sweet pepper. This result aligns with Dinesh et al. (2015) and Youssef et al. (2017), who highlighted the role of micronutrients in stimulating the biosynthesis of secondary metabolites in vegetables.

Similarly, Figure 9 shows that the total soluble solids (TSS) content was significantly higher in fruits from plants grown with 15% compost and treated with 2 cm³ L^-1 nano-fertilizer. This effect was particularly pronounced in sandy soil. The elevated TSS levels suggest improved carbohydrate accumulation, likely driven by enhanced photosynthetic efficiency and nutrient translocation under optimal nutritional conditions. These findings suggest that integrated nutrient management not only enhances yield but also improves taste and shelf life, two key attributes for market competitiveness.

To better understand the relationships among the measured variables, a correlation matrix was generated (Figure 10), revealing strong positive associations between chlorophyll content, fruit weight, fruit number, ascorbic acid, and TSS. This suggests that treatments that enhance one trait tend to improve others, reflecting systemic physiological improvement rather than isolated responses. These correlations support the hypothesis that nutrient efficiency and improved soil-plant interactions have a comprehensive impact on sweet pepper performance.

The heatmap cluster analysis (Figure 11) visually reinforces the superior performance of specific treatments. T24 (20% compost in sandy soil + 2 cm³ L^-1 nano-fertilizer) emerged as the top-performing treatment across most traits, followed by T23 and T27. In contrast, control treatments (T1, T6), which received no compost or nano-fertilizer, performed poorly across all parameters. These visual insights provide strong evidence of the benefit of integrating organic and nano-nutrient strategies, especially in sandy soils where baseline fertility is low.

Lastly, PCA (Figure 12) captured over 79% of total variability in the first two components, confirming the robustness of the dataset. Treatments were clearly divided into two distinct clusters, with high-performance treatments grouped in Cluster 2 (positive PC1), characterized by superior fruit weight, number, ascorbic acid, and chlorophyll content. T24 stood out as the most optimal treatment, highlighting its potential for adoption as a best-practice nutrient management strategy in greenhouse systems. Collectively, these results underscore the significance of integrating engineered compost and nano-fertilizer technologies for sustainable sweet pepper production. The findings confirm that compost produced via mechanical turning not only meets agronomic standards but also provides a sustainable and cost-effective organic amendment when combined with precision foliar nutrition. This approach enhances yield, fruit quality, and nutrient efficiency, offering a scalable and environmentally conscious solution for intensive greenhouse agriculture.

Application of mechanically produced compost markedly improved sweet-pepper growth, chlorophyll content, and yield parameters. These findings agree with previous studies reporting that organic compost enhances soil physicochemical properties, increases cation-exchange capacity, and stimulates microbial activity responsible for nutrient mineralization. The engineered compost produced by the turning machine of Sayed et al. (2022) maintained an optimal C:N ratio and aeration level, accelerating microbial decomposition and releasing available N, P, and K to the rhizosphere. Enhanced microbial biomass and enzyme activity likely promoted ammonification and nitrification processes, improving N assimilation and root growth. Consequently, plants grown in compost-amended sandy soils exhibited higher chlorophyll and SPAD values, reflecting improved nitrogen uptake and chloroplast development (Arancon et al., 2003; Jamir et al., 2017; Azim et al., 2018). These results confirm that compost not only serves as a nutrient source but also acts as a biostimulant through its effect on microbial-driven nutrient cycling and root–microbe interactions.

Foliar application of nano-fertilizer further enhanced physiological performance by improving nutrient penetration and translocation within leaf tissues. Nano-particles possess a high surface-to-volume ratio, allowing efficient diffusion through stomata and cuticular pathways. In this study, nano-Ca–Mg–B fertilization increased chlorophyll concentration, ascorbic-acid content, and total soluble solids, indicating improved photosynthetic efficiency and carbohydrate metabolism. Calcium plays a key role in stabilizing thylakoid membranes and activating photosystem II, whereas boron regulates carbohydrate transport and reproductive development. Hence, the observed increase in fruit quality and antioxidant levels can be attributed to enhanced ion homeostasis and redox balance within chloroplasts. Similar results were reported by Avila-Quezada et al. (2022) and Esringü et al. (2011), who demonstrated that nano-nutrient sprays elevated photosynthetic pigments and sugar accumulation through efficient foliar uptake.

The integrated application of compost and nano-fertilizer produced the most pronounced improvements in physiological and biochemical attributes. This synergy arises from complementary nutrient pathways: compost enhances soil nutrient reservoirs and root uptake, while nano-fertilizer ensures rapid foliar nutrient supply and metabolic activation. The concurrent stimulation of root and shoot nutrition likely established a balanced C:N ratio and improved carbon fixation efficiency. Enhanced chlorophyll content under the integrated treatment signifies higher photosynthetic capacity, while elevated ascorbic acid and TSS reflect stronger antioxidant metabolism and sugar transport. These results align with Mukanov and Gulin (2024) and Channab et al. (2024), confirming that combining organic and nano inputs can synchronize macro- and micro-nutrient dynamics to achieve superior plant metabolic performance.

The response magnitude was greater in sandy than in clay soils, consistent with earlier findings that compost markedly improves water-holding capacity and nutrient retention in light-textured soils. In sandy soil, compost mitigated leaching losses and created a micro-environment favourable for microbial proliferation and root extension, enhancing nutrient accessibility and photosynthetic stability. Clay soils, in contrast, already possess high nutrient-holding capacity; thus, the relative improvement from compost–nano integration was less pronounced. This differential response highlights the importance of soil texture in regulating nutrient availability and physiological outcomes.

The combined use of engineered compost and nano-fertilizer established a multi-level enhancement in nutrient uptake, photosynthetic metabolism, and antioxidant defence. Mechanistically, compost improved rhizosphere nutrient availability and microbial activity, while nano-nutrients intensified leaf metabolic activity. Their synergy fostered efficient nutrient partitioning between roots and shoots, reinforcing photosynthetic machinery and promoting metabolic homeostasis. This integrative mechanism explains the substantial improvement in both yield and fruit biochemical quality observed in greenhouse sweet pepper. Such physiological insights underline the potential of compost–nano integration as a sustainable strategy for optimizing nutrient efficiency in controlled-environment agriculture.