Reduced Chemical Fertilizer Combined with Microbial Inoculants: Implications for Soil Fertility and Profitability in Mediterranean Vegetable Production

A three-year field experiment (2021–2023) in southeast Spain evaluated whether reduced mineral fertilization, with or without plant-growth-promoting microorganisms, could maintain crop productivity and modify selected soil indicators in a Mediterranean vegetable rotation. Four treatments were compared: conventional fertilization (T1), reduced fertilization (T2; −30% or −50%), reduced fertilization plus bacterial inoculants (T3), and reduced fertilization plus bacterial–fungal inoculants (T4). Crop yields were not significantly affected by fertilization strategy. Potato yields ranged from 55,661 to 60,741 kg ha−1, those of broccoli from 14,928 to 16,797 kg ha−1, and those of melon from 30,815 to 33,423 kg ha−1. Inoculated treatments were associated with some quality responses, including higher potato tuber firmness in T4 (16.0 vs. 13.2 kg cm−2 in T1), whereas melon soluble solids tended to be slightly lower. Soil analyses showed changes in some nutrient-related indicators, including a 217% increase in NH4+ in T4 and a 0.75% decrease in pH in T3. Reduced fertilization lowered production costs by about 9%. Under the conditions of this field trial, reduced fertilization maintained yield and gross margin relative to conventional fertilization, and inoculated treatments under reduced fertilization showed differences in selected soil indicators.