Evaluation of Organic Amendments and Biofertilizers in Combination with Varying Mineral Fertilizer Levels on Soil Fertility and LUPIN (Lupinus albus L.) Productivity in Sandy Soil

Integrating organic amendments and biofertilizers with mineral fertilizers sustainably improves sandy soil fertility and crop productivity This study evaluated the effects of organic amendments and biofertilizers, applied alone or combined with varying rates of mineral nitrogen, on soil fertility and the productivity of lupin (Lupinus albus L.) cultivated in sandy soil. Two field experiments were conducted at the Ismailia Agricultural Research Station (30°35′41.901″N, 32°16′45.843″E) during the 2023/2024 and 2024/2025 winter seasons using a split-plot design with three replications. The main plots were assigned to control and two organic amendment treatments (biochar and compost) in addition to biofertilization with R. leguminosarum, whereas the sub-plots included four levels of mineral nitrogen fertilizer (0, 20, 30, and 40 kg N fed⁻¹). Statistical analyses were performed to validate the results and ensure their reliability. Results showed that the combined application of compost or biochar with mineral nitrogen significantly increased soil nutrient availability. Compost with 40 kg N fed⁻¹ enhanced available soil nitrogen and phosphorus by 32.49% and 22.66%, respectively, compared with compost alone. Similarly, biofertilizer with 30 kg N fed⁻¹ increased soil available nitrogen and phosphorus by 24.0% and 20.20%, respectively, compared with biofertilizer alone. These improvements in soil chemical properties were reflected in plant performance. Biofertilizer combined with 30 kg N fed⁻¹ resulted in the highest values of plant height (120.77 cm), branches per plant (12.6), pods per plant (58.7) and seed yield (1.44 t fed⁻¹), representing increase of 25.64% in plant height and 68.95% in the number of branches compared with mineral N alone. Seed nutrient concentrations were also increased under integrated treatments; biofertilizer with 30 kg N fed⁻¹ increased seed Fe and Zn concentrations by 26.55% and 46.57%, respectively, compared with biofertilizers alone. Overall, combining organic amendments and biofertilizer with moderate mineral nitrogen rates improves soil fertility, lupin growth and seed nutrients, offering a sustainable strategy for sandy soils.