Influence of Compost and Biological Fertilization with Reducing the Rates of Mineral Fertilizers on Vegetative Growth, Nutritional Status, Yield and Fruit Quality of ‘Anna’ Apples

A field trial was conducted on eight-year-old ‘Anna’ apple (Malus domestica) trees from 2021 to 2023 in northern Egypt. The objective of this study was to determine the effects of replacing mineral fertilizer with compost and microorganism applications. Treatments were prepared using combinations of three mineral fertilizer NPK (nitrogen (N), phosphorus (P) and potassium (K)) levels (75% recommended NPK rate, 50% and 25% recommended rate), with two compost levels (with/without) and two bacteria/fungi biological blend (PGPM) levels (with/without). This design resulted in 12 treatments in addition to a control treatment consisting of the full NPK recommended rate (100% NPK). Leaf nutrient concentrations, vegetative growth, fruit set percentage, fruit drop percentage, yield and fruit quality were measured in 2022 and 2023. Our results indicated that vegetative growth parameters were significantly influenced by the fertilizer treatments in both seasons. The application of 75% NPK + compost + PGPM or 50% NPK + compost + PGPM significantly increased shoot length, shoot diameter, leaf area and leaf-specific weight compared with the control (100% NPK). The greatest values of leaf nutrients and production and quality parameters were obtained with treatments 75% NPK + compost + PGPM or 50% NPK + compost + PGPM. Applying 75% NPK + compost + PGPM or 50%NPK + compost + PGPM increased total soluble solids and anthocyanin concentrations but reduced fruit nitrate and nitrite levels compared with the control (100% NPK). This study shows that it is possible to reduce mineral fertilizer application by 25–50% while improving the yield if compost and microbial inoculants are applied.