Effect of multi sourced compost on a rainfed olive tree grove in an arid land: Management practices to adapt to global climate change

In southern Tunisia, in an arid environment subjected to climate change, the olive-production sector is important for the local Economy. With low local biomass production a multi sourced compost composed of olive waste byproducts and seaweed was tested on an Arenic Calcisol. The application at the subsurface of 180 kg (0.5 m3) of compost per tree in a rainfed olive tree (Olea europaea L.) grove with declining productivity showed a significant increase in total C, N, K, P, and respiration rate in amended horizons. Surprisingly the soil volumic water content (Vw%) at the subsurface (0-20 cm) below amended (7.5% +/- 2.3) was not significantly different below non amended trees (6.9% +/- 3.3). It increased slightly under non-amended trees, from 6.6% (+/- 4.3) at a depth of 5 cm-8.1% (+/- 1.5) at 65 cm depth, while below the amended olive trees the Vw% increased from 6.7% (+/- 3.4) in the composted horizon at 5 cm depth to 13.5% (+/- 1.1) at 70 cm depth. The main positive effect of composting on soils was decreasing the water loss due to capillary rise, since the compost developed clear hydrophobic properties. Two years after applying the compost, olive tree branches were 15% longer and olives were 40% heavier on amended trees.