Soil science
S.H. Mousavi Dizkouhi; M. Barani Motlagh; E. Dordipour; E. Malekzadeh; F. Sadeghzadeh; M. Ghasem Nejhad
Abstract
Introduction
The olive tree (Olea europaea L.) is one of the most significant and ancient cultivated plants in the Mediterranean region, prized for its edible fruit and high-quality oil. However, the increasing scale of olive oil production has led to the accumulation of large quantities of solid waste, ...
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Introduction
The olive tree (Olea europaea L.) is one of the most significant and ancient cultivated plants in the Mediterranean region, prized for its edible fruit and high-quality oil. However, the increasing scale of olive oil production has led to the accumulation of large quantities of solid waste, particularly Olive Solid Pomace (OSP), which poses considerable environmental challenges due to its high organic load and phytotoxic components. Sustainable management and valorization of this waste are crucial for reducing environmental risks and improving the circular economy in agricultural systems. This study aimed to evaluate the effects of compost and vermicompost derived from OSP, both in enriched and non-enriched forms, on the yield of olive fruit and the quality characteristics of the resulting olive oil. To this end, a field experiment was conducted during the 2018 growing season in a traditional olive orchard located in Rudbar, Gilan Province, Iran. The experiment was carried out using a randomized complete block design (RCBD) with 12 treatments and three replications using the ‘Arbequina’ cultivar, a well-known olive variety cultivated for its high oil content and quality.
Materials and Methods
Compost and vermicompost were first produced from olive solid pomace. After analyzing their basic physicochemical properties, several treatments were biologically enriched using plant growth-promoting rhizobacteria (PGPR), including Bacillus megaterium (phosphorus-solubilizing), Azotobacter chroococcum (nitrogen-fixing), and Thiobacillus thioparus (sulfur-oxidizing). Additional treatments were chemically enriched by incorporating 1 kg each of urea (as a nitrogen source), triple superphosphate (as a phosphorus source), and elemental sulfur at a rate of 1% by weight. The experimental treatments included: raw olive pomace, unenriched compost, chemically enriched compost, biologically enriched compost, unenriched vermicompost, chemically enriched vermicompost, biologically enriched vermicompost, a full NPK fertilizer treatment, a manure-only treatment (10 kg of animal manure), and a no-fertilizer control. All olive waste-based amendments were applied at 3% w/w. NPK fertilizers included urea (750 g in three split applications), triple superphosphate (250 g), and potassium sulfate (750 g). Micronutrients such as magnesium sulfate, manganese, iron, zinc, copper, boric acid, and elemental sulfur were applied based on soil test recommendations. Uniform horticultural practices, including surface drip irrigation, weed control, pest management, and other cultural operations, were applied across all plots. Post-treatment, soil samples were collected at depths of 0–30, 30–60, and 60–90 cm to measure pH, EC, organic carbon, and available phosphorus. Foliar sprays were prepared with 1,000 ml solutions of urea and potassium sulfate (10 g/L), zinc sulfate (3 g/L), and boric acid (5 g/L) and applied twice at sunset using a handheld sprayer. Fruit yield, oil content, and selected oil quality parameters were then assessed.
Results and Discussion
Application of biologically enriched vermicompost significantly improved olive yield and oil quality. Trees receiving this treatment produced 50.33 kg of fruit per tree—an increase of 93.58% compared to the control. Similarly, the highest oil yield (11.14 kg per tree) was recorded in the biologically enriched vermicompost treatment. The lowest peroxide value (1.06 meq O₂ kg-1 oil) was also observed in this treatment, representing an 88.27% reduction compared to the control. Organic fertilizers positively influenced the oil percentage of the fruit, with biologically enriched compost yielding the highest oil content (57.7%), which was 132.2% higher than the control. The extinction coefficients K270 and K232, indicators of oil oxidation, were reduced by 96.2% and 78.5%, respectively, in the biologically enriched vermicompost treatment. Furthermore, this treatment resulted in the lowest free fatty acid content, 94.7% lower than the control. Leaf phosphorus content was also significantly enhanced, reaching 0.33% in the biological vermicompost treatment, a 230% increase over the control. These findings underscore the beneficial role of organic fertilizers, particularly biologically enriched vermicompost, in improving soil fertility, nutrient availability, and plant performance. The high phosphorus content in the compost and vermicompost, combined with microbial activity, played a pivotal role in enhancing both yield and oil quality. The application of PGPRs proved particularly effective, as they not only facilitated nutrient cycling but also contributed to improved physiological responses in olive trees.
Conclusion
Overall, the results suggest that olive trees fertilized with biologically enriched organic amendments derived from olive pomace benefit from improved oil quality and fruit yield. The presence of adequate phosphorus and beneficial bacteria played a pivotal role in enhancing plant nutrition and oil characteristics. Therefore, the use of PGPR in the enrichment of composted organic materials can be an effective and sustainable strategy to improve the productivity and quality of olive oil. Among the treatments, biologically enriched vermicompost emerged as the most effective and is recommended for further field application. Future research should explore other organic amendments and their long-term effects on olive orchards.
