Soil distribution in the Mule Hole watershed was established from the observation of isolated soil profiles and toposequences, and surveys of soil electromagnetic conductivity (EM31, Geonics Ltd), lithology and vegetation (BARBIERO et AL., 2007).


Two types of soils are present in the Mule Hole watershed (Figure 6):
  • Red soils (Ferralsols and Chromic Luvisols). Ferralsols are usually 2 to 3 m thick wheras Luvisols are thinner.
  • Black soils (Vertisols and Vertic intergrades)

The major part of the watershed (80%) is covered by red soils that are about 1 or 2 m thick and can reach about 4 m at certain locations. Thin red soils (about 0.2 to 0.5 m thick) overlying loose gneiss saprolite and associated with the Shorea vegetation are mainly located on the central wash divide between the two main talwegs and in a discontinuous crescent-shaped area along the slopes.

Black soils have developed on two types of location: (i) the low-lying area, occupying the lower part of the slope and the flat valley bottoms. Black soil areas are about 2 m thick at the perimeter but can reach more than 6 m at the centre. They have developed from both gneiss and amphibolite saprolite. (ii) At higher levels black soils are about 0.2 to 0.5 m thick, except at the depressions (50 to 100 m in diameter) on the crest line where the black soils can reach 2.5 m. They are always associated with gneiss, which alternates with amphibolites.

Figure 6. Soil cover on Mule Hole watershed and distribution of erosion spots. 1–rotational slip; 2–seepage erosion at the top of black soil; 3–seepage erosion and mass movement in non-cohesive saprolite at midslope.
Figure 6. Soil cover on Mule Hole watershed and distribution of erosion spots. 1–rotational slip; 2–seepage erosion at the top of black soil; 3–seepage erosion and mass movement in non-cohesive saprolite at midslope.