Using olive pruning residues to cover soil and improve fertility

The application of organic materials to land is a common practice in sustainable agriculture. The current availability of several types of pruners and choppers on the market has boosted the use of these types of residues as plant cover. Applying these types of residues increases the content of organic matter in the soil, which is very positive for the fertility of the soil and agricultural biodiversity. The latter should be taken into account in ecological olive groves where fertilisation programmes are highly limited. However, no quantitative information is available to provide farmers with a precise assessment.Experiments were conducted over a period of two agricultural years (2009/10 and 2010/11). Treatments consisted of pruning applications to fine (<8 cm in diameter) and thick (>8 cm in diameter) in the amounts indicated, I = 2.65 kg m−2 fine; II = 2.65 kg m−2 fine + 1.12 kg m−2 thick; III = 5.30 kg m−2 fine; IV = 5.30 kg m−2 fine + 2.24 kg m−2 thick; and a control of spontaneous weeds.The greatest loss of residue mass was recorded at the beginning of the sampling period. The estimated biomass loss in the first six months represented 37–50% of the total. After 704 days of decomposition, the soil maintained cover percentages of 62, 76, 74 and 88% for treatments I, II, III and IV, respectively.The various treatments applied to pruning residues have been more effective at increasing the levels of soil organic matter (SOM) than spontaneous cover. SOM values on the surface (0–5 cm) rose by 0.86, 1.04, 1.28 and 1.52% for treatments I, II, III and IV in regard to the control treatment, maintaining this improvement in fertility at a depth of 0–20 cm, where SOM increased by 0.43, 0.46, 0.84 and 0.47% for treatments I, II, III and IV, respectively, in regard to the control.Considering all the soil sampled, the largest increase in SOM in regard to the initial content of the soil was achieved by treatment III, which contained the largest amount of fine residues, with 0.63%, compared to increases of 0.33, 0.29, 0.36 and 0.10% for treatments I, II, IV and spontaneous weeds, respectively.

► Application of pruning residues between olive trees is one of the best alternatives within the group denominated inert covers.
► The initial amount of pruning residues was decisive in terms of the agronomic and environmental benefits achieved by the amendment.
► The treatment with the largest amount of residues recorded less spatial variability and greater temporal cover stability.
► The low carbon release constant measured in the different treatments confirms that it is more difficult for these types of residues to decompose.
► The good results observed in the increase of organic carbon in the soil confirm the improvement in soil fertility.