Effects of Different Fertilizer Regimes and Irrigation Water Qualities on Soil Fertility under Lettuce Production

Purpose: This study aimed to evaluate the effect of organic compost as a substitute for synthetic nitrogen (N) and the use of wastewater irrigation on lettuce cultivation by assessing changes in key soil fertility parameters. Methods: A greenhouse experiment was conducted at the Soil Science Department of the School of Agricultural Sciences in Botucatu, Brazil (22°50′S, 48°22′W), on a medium-textured Rhodic Hapludox. The experimental design was completely randomized in a split-plot arrangement, with six N fertilization treatments (organic and synthetic) combined with either wastewater or freshwater irrigation, and five replicates. The N treatments were as follows: T1 – no N; T2 – 0.54 g N applied as urea, split at 7, 14, and 28 days after transplanting; T3 – 0.27 g N from organic compost and 0.27 g N from urea, split at 7, 14, and 28 days after transplanting; T4 – 0.54 g N from organic compost; T5 – 0.81 g N from organic compost; T6 – 1.08 g N from organic compost. Results: The highest compost rate (1.08 g N, T6) significantly improved soil fertility by increasing pH, cation exchange capacity, base saturation, and the availability of macro- and micronutrients, under both freshwater and wastewater irrigation. These treatments also enhanced soil organic matter and dissolved organic carbon contents, which were positively correlated with nutrient availability. Conclusion: The T6 treatment also produced the highest green biomass yield of lettuce compared to the exclusive use of urea, highlighting organic compost as a sustainable alternative for N fertilization. Highlights · Organic compost improved soil fertility, nutrient availability, and lettuce yield. · Wastewater irrigation enhanced nutrient recycling and crop productivity. · Combined organic and chemical fertilization reduced soil acidity and Al³⁺ toxicity. · Long-term compost uses increased soil organic carbon and nutrient correlations. · Organic compost offers a sustainable alternative to synthetic nitrogen fertilizers.