3.3.3 Particle size, organic matter and nutrient availability of post-tin mining areas
Particle size in the fine earth (< 2 mm) of post-tin mining areas was dominated by sand fraction that sharply increased from 57-69% in native soils to 82-96 % in soil tailing (mostly > 90 %) (Table 5). On the other hand, the clay fraction sharply decreased from 27-38% in native soils to less than 14% in tailings and spoil (Table 5). In other words, the soil texture changed from sandy clay loam to sand. Further, organic matter (C and N) and potential P2O5 and K2O were very low status in tailings and spoil, and their content was lower than native soils.
In all cases, the layers of tailing profiles have higher pH (varying from 5.2 to 6.3) than the corresponding layer of native soil profiles (varying from 4.7 to 5.2) deriving from a similar parent material (Table 5). The higher pH of soil tailings than the native soils was associated with the lower exchangeable Al in the former owing to sandy texture, leading to very low ability to retain Al. In native soils, the high clay fraction corresponded to the high exchangeable Al and low pH (Table 6). The available P2O5 in tailing was very low (< 4 mg kg-1P2O5 according to Bray 1), suggesting the crops would experience a serious P deficiency.
Although potential K2O as extracted by 25% HCl was very low status in all tailings, there is a good trend for its higher concentration in soils derived from granite parent materials compared to tailings derived from sandstone. This is consistent with the high total K2O in granite rocks (5.2%) (Table 2). K-bearing minerals were not observed in soil tailings deriving from sandstone.
All exchangeable cations, Ca, Mg, K and Na were very low with the sum of < 0.8 cmolc kg-1 for each layer of soil tailing, and the order of decreasing content was Ca > Mg > Na > K (Table 6). The exchangeable cations in tailings were only slightly lower compared to a corresponding layer of native soil profiles. This indicates exchangeable cations are serious limiting factors for crops in both native soils and tailings. Cation exchange capacity (CEC) drastically decreased from 3.8-6.8 cmolc kg-1 in native soil profiles to 1.0 - 2.3 cmolc kg-1(mostly < 1.5 cmolc kg-1) in tailing profiles.