Effect of Some Organic Fertilizers on Yield and Quality of Olive Oil in Semiarid Conditions

Abstract

This two-year field experiment evaluated the effects of different organic fertilizers on olive oil quality of the Gemlik cultivar grown under semiarid conditions in Mardin, Türkiye. Treatments of olive pomace (P1: 4, P2: 8, P3: 12 kg tree-1), leonardite (L1: 1.5, L2: 3.0, L3: 4.5 kg tree-1), and vermicompost (SG1: 3, SG2: 6, SG3: 9 kg tree-1) were applied in a randomized complete block design with three replicates. Oils were analyzed for free acidity (FA), peroxide value, and phenolic profile using high-pressure liquid chromatography. FA ranged from 0.27% to 0.42% (as oleic acid), remaining well below the International Olive Council extra virgin limit; the highest mean value occurred in L2, whereas lower means were found in P2, P3, L3, and SG2. Mean total phenolic content was 307 mg L-1, and several treatments (P2, L1, SG3, P3) exceeded this average. Two-way analysis of variance revealed significant effects of treatment, year, and their interaction on multiple parameters, indicating the influence of both fertilization and climatic variability. The findings highlight that quality assessment based solely on FA may overlook treatment-related differences, suggesting that phenolic and oxidative indices should also be considered. As the study was limited to a single cultivar, site, and two growing seasons, further research across diverse genotypes and environments is required before broader policy implications are drawn.