Fig. 1 Results of acid leaching experiment for samples from different depths in the profile (Table 2); a) oxalate extraction; b) pyrophosphate extraction; Al_ox and Al_p:oxalate and pyrophosphate extractable Al in untreated samples; Al^R_ox and Al^R_p:oxalate and pyrophosphate extractable Al in solid residues after successive acid leaching; Al_L:total Al removed by acid leaching; Al_L + Al^R_ox and Al_L + Al^R_p:sum of total leached Al and extractable Al in solid residues