*2.1. Soils*

Eight composite top soil samples (0–20 cm) representing a range of different physicochemical properties were collected from arable lands of Peloponnese (Greece) and used in this study. The main criterion for the selection of sampling sites was the soil pH. Four of the soils were acid and four alkaline. The samples were transferred in sterile sampling bags to the laboratory, air-dried, crushed, passed through a 2-mm sieve and finally stored again in sterile sampling bags. Particle size distribution was determined by the hydrometer method [22], while pH and EC were measured in a 1:1 (w/v) soil/water ratio [23]. Total carbonates content (CaCO3) was calculated by measuring the evolved CO2 following HCl dissolution [24]. The Loeppert and Suarez [25] ammonium oxalate method was used in order to determine active carbonate fraction. Available phosphorous (*p*) was obtained by using the Olsen method [26] and organic carbon (OC) content was determined by the Walkley-Black's protocol [27]. Amorphous and free Fe, Mn and Al oxide contents were calculated by the ammonium oxalate buffer methods [28] and by the sodium–bicarbonate–dithionate (CBD) [29], respectively. Total Se was extracted by aqua regia [30].
