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Olive grove soils in semi-arid Mediterranean areas are very poor in organic matter and are exposed to progressive degradation processes. Therefore, additions of the olive mill by-products as organic amendments can improve soil quality and hence mitigate the negative environmental and agronomic limitations of these soils. A field study was conducted to assess the short-term effects of two oil mill by-products [olive leaves and alperujo (a two-phase olive mill waste)] and determine soil organic carbon (SOC), total nitrogen (TN), the C:N ratio, and their stratification across the soil profile, in an area under Mediterranean conditions.

The experimental design consisted on three large plots each spanning a 100 m x 100 m area covered with unirrigated olive trees of the Picual variety less than 30 years old under conventional tillage (CT) in the province of Jaen (southern Spain). One of the plots (CT) was used as control plot and the other two were treated with alperujo (CTa) and olive leaves (CTol), respectively. Soil properties were assessed three years after the by-products were applied. Supplying the soil with the by-products increased C and N stocks. Thus, the SOC stock was 75.4 Mg ha(-1) in CT, 91.5 Mg ha(-1) in CTa and 136.3 Mg ha(-1) in CTol; and the TN stock 12.1, 13.9 and 16.1 Mg ha(-1) in CT, CTa and CTol, respectively. SOC, TN and the C:N ratio in the surface horizon were higher in the treated plots (viz. 18.5 g Ckg(-1), 1.4 g N kg(-1) and 13.2:1 in CTa, and 84.1 g C kg(-1), 5.7 g N kg(-1) and 14.7:1 in CTol) than in the control plot (8.9 g C kg(-1), 0.9 g N kg(-1) and 9.8:1, respectively). In addition, application of the by-products improved soil quality (the stratification ratio, SR, for SOC was greater than 2).

Oil mill by-products such as olive leaves and alperujo are thus potentially useful as soil amendments since they are effective sources of organic matter and nitrogen, improve soil quality and alleviate the environmental and agronomic limitations of Mediterranean agricultural soils, even those under using CT. In addition, the by-products can be recycled on site. (C) 2012 Elsevier B.V. All rights reserved.