2. Materials and methods
2.1. Characteristics of soils and olive wastes
Two types of agricultural soils, named S1 and S2 in the following, were selected among the most common in the Medi- terranean area. Their main physical and chemical properties are summarized in Table 1. The grain size analysis of the soils was performed on the fine earth obtained by sifting through a 2 mm mesh.
The texture is sandy loam for both soils. According to the FAO system of soil Classification (ISSS-ISRIC-FAO, 1994) the soil S1 is classified as Luvisols-Phaeozems, and the S2 as Phaeozems- Cambisols. Although S1 and S2 belong to the same texture class, the different percentages of gravel and sand affect their hydraulic properties such as porosity and hydraulic conductivity. In partic- ular, the soil S2 allows much more drainage than the soil S1, due to the high percentage of gravel that makes its behaviour more similar to that of a gravel soil (40% of gravel).
The OMW used in this study came from a continuous centrifu- gation system, and its main chemical and physical characteristics are reported in Table 2.
It was used either as received or after a fourfold dilution with freshwater (see Section 2.3). Olive waste compost was obtained by mixing exhausted olive pomace (91% w/w) with straw (2%) and poultry manure (7%), and its composition is reported in Table 3.