Abstract
The study aims to map areas sensitive to erosion by water and rainfall erosivity after addition of organic matter (OM) in highly unstable soils. A soil association map was created using digital soil mapping methodology. Soil samples from six soil associations were incubated and analysed for several soil erodibility measures and inferred to the soil association map. Soil stabilization against soil erosion by use of OM was evaluated for 30 weeks under two simulated rainstorms, intermittent rainstorms (IR) and single rainstorm (SR). Rainfall erosivity (R-factor) was calculated from theduration of a rainstorm and the total amount of rainfall received under rainfall simulations. Erodibility factor (K-factor) was estimated using the soil OM content and texture. Largest area (40%) was covered by shallow soils and K-factor range of 0.0693-0.0778 t.ha.hha-1MJ-1mm-1. Largest (60.2%) area had a structural stability index of 0.8 and 42.7% of the area was covered by a dispersion ratio value range of 0.65-0.70. The area size with erosion rates of > 15 t/ha/yr was drastically reduced from 1 to 8 weeks after OM application thereafter gradually increased under both IR and SR. Soil erosion rates of < 5 t-1 ha-1 yr-1 and > 15 t-1 ha-1 yr-1 were most and least observed respectively under both storms. R-factor was higher under IR than SR and the smallest areas with soil erosion rates of > 15 t-1 ha-1 yr-1 contributed most to the lost soil. Organic matter confers soil resistance to erosion up to a certain period before losing its effectiveness. The study provided first assessment of erosion dynamics, basis for identifying conservation priorities which may be applicable in similar areas.
 Keywords: Erosivity, planning, rainstorm, soil conservation, soil degradation
Highlights
Accelerated soil erosion is a serious global problem that threatens the sustainability of agriculture and infrastructural development (Parwada and Van Tol, 2016)
Soil Maps: The soil association map (Figure 2) achieved a point accuracy of 65% and a Kappa value of 0.57, indicating a moderate agreement with reality
These values are slightly lower than comparable digital soil mapping maps created in southern Africa, such as the 73% point accuracy van Zijl et al (2014) obtained for 4 001 ha in the Kruger National Park, and the 80% point accuracy obtained by Van Zijl et al (2014) for 10 970 ha near Gurue, Mozambique
Summary
Accelerated soil erosion is a serious global problem that threatens the sustainability of agriculture and infrastructural development (Parwada and Van Tol, 2016). When one understands how the different mechanisms operate, it is clear to see that different mitigation strategies must exist for the different mechanisms of gully formation (Van Tol et al, 2014) In this regards, it is advisable to closely monitor the soil erosion process and rates of soil loss even after soil amelioration. In many parts of the world, soil erosion risk mapping is used for identifying high erosion areas where resources of soil water conservation programs can be effectively concentrated (Van Tol et al, 2014).Often, a quantitative assessment is needed to infer the extent and magnitude of soil erosion problems so that effective management strategies can be compiled. There are few inventory maps drawn using the soil erosion indices and the response of the soil to erosion after amelioration indicating erosion sensitive areas
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