Genesis, morphology, and classification of soils in two ecological zones in Swaziland.

摘要

The objectives of this study are: (1) to use Soil Taxonomy to classify some representative soils of Swaziland; (2) to examine the relationship between the soil-forming processes and landscape position; (3) and to compare soil-forming processes in two different ecological zones of Swaziland.;In order to undertake this work, representative pedons were described and sampled in the field according to USDA criteria. The pedons were located along two topographic transects in two of the four ecological zones in Swaziland, one in the Highveld (Sigangeni transect) and another in the Middleveld (Lozitha transect). Soil samples from the pedons were physically, chemically, mineralogically, and morphologically characterized.;The soils of the study areas are predominantly clayey and red and characterized by low pH, high iron oxide contents, low CEC's and low ECEC's. The mineralogy is dominated by kaolinite, iron oxides, and gibbsite, however, mica is evident in the Lozitha-A pedon. Other minerals occurring in small amounts are hydroxyl interlayered vermiculite, anatase, quartz, and feldspar. Using Soil Taxonomy, all soils in the Middleveld (Lozitha study site) classify as Oxisols, whereas Alfisols and Oxisols occur in the Highveld (Sigangeni study site).;In studying soil genesis, total phosphorus, CBD-extractable iron oxide from whole soil, CBD-extractable iron oxide from the clay fraction, kaolinite, and gibbsite were used. Each transect shows an increase in soil weathering stage downslope, characterized by an increase in iron oxides downslope through each entire transect and an increase in gibbsite and a decrease in kaolinite through the backslope positions (mid-transect). The change in the distribution of the latter parameters at mid-transect could be attributed to differences in surface age or geomorphologic processes, at least in the Lozitha transect, and possibly in both transects. Total phosphorus contents are not at minimal levels as expected in such highly weathered soils. It seems that phosphorus is retained in these soils by the abundance of iron oxides present, but only minimal portions will be available for plant use. The study also shows that the soils have reached a more advanced weathering stage in the Highveld than in the Middleveld.