Spatial distribution of bowal and differences in physicochemical characteristics between bowal and woodland areas in Benin

analyses on soil samples Within each of the six phytogeographical districts where bowé were identified, nine homogeneous topsoil sample pairs were taken on bowé and woodland plots. Woodland soil was sampled as adjacent material. Seventeen woodland samples were considered for the entire study as some of the bowé sites were adjacent to the same woodland sample. We used a soil auger to a depth of 10 cm because bowé is hard like rock with few topsoil. Out of the nine sample pairs; we selected randomly one composite topsoil sample composed of the nine homogenous ones from bowal and woodland of each phytogeographical district for further analysis. The composite soil samples were dried in the laboratory at 105°C, then crushed and sieved through screen of 2 mm. The particle size was measured by the pipette method (Loveland and Whalley, 1991). The electrical conductivity (EC) and the pH-value were measured according to the methods of Rhoades (1982). Soil organic matter (OM) was determined by the K2Cr2O7–H2SO4 oxidation method of Walkey and Black (Nelson and Sommers, 1982). The total nitrogen was extracted by using the Kjeldahl System 1026 distilling unit. Potassium was determined after extraction of ammonium acetate-extractable cations using atomic absorption spectrometry (the centrifuge procedure described by Thomas, 1982). The extractable phosphorus was analyzed using a Bray-2 extract as described by Bray and Kurtz (1945).