*3.2.1.3 Exploiting soil depth*

Soil depth was assessed to see the storage medium for the year–round rainfall that could cause landslides as well as to understand the rooting depth required for the crops to be grown in the area. Field survey of soil depth identified three levels (0–50, 50–100 and 100–200+ cm) for each soil types within different slope categories of the study area using augering method. The pedological prospections were conducted

*Erosion Control Success Stories and Challenges in the Context of Sustainable Landscape… DOI: http://dx.doi.org/10.5772/intechopen.96267*

**Figure 4.**

*Maps of slope gradient, soil depth, erosion risk and dominant soil types in Gishwati area. Source: GWLM project.*

on 52 depth tests on the dominant slope class of 16–40%, 29 tests on 40–60% slope class, 34 tests on 6–16% slope class, 27 depth tests on 0–6% slope class, and 14 depth tests on >60% slope category (**Figure 4**). Based on the soil database of Rwanda, field pedological prospection resulted in 156 soil depth tests for dominant soil types such as *Andosols*, *phaeozems*, *Acrisols*, *Cambisols*, *Lixisols* and *Leptosols* [18].

Results showed that most soils are very deep and well developed. More than 80% of conducted auger tests ranged between 100–200+ cm depth. Decisions were taken accordingly to guide recommended options for restoring landscape. The soil depth discloses how soil material with water infiltration storage can exerts pressure over the sloping land. In addition, it also guides to know the relatively most appropriate crops to be grown over each type of soil.

Very shallow soil depth zones such as the bare rook-covered lands are recommended for area closure. The next shallow depth lands (depth of 0–50 cm) are recommended for the shallow rooting grasses (range land). The utilization of soils with depth between 50–100 cm and 100–200+ was variable regarding the combination with other bio-physical factors. If the same land category was in the moist to dry rainfall regime, one could easily recommend the 100–200+ depth land for trees (deep rooting) and the other for the relatively shallow rooting shrubs. On the other hand, the deep rooting on very steep slopes would encourage excessing waterload on the mass of the deep soil materials, hence landslide occurs. In this case, planting shrubs/trees is recommended.

#### *3.2.1.4 Soil characteristics*

Soils of Gishwati are dominantly underlaid on a bed rock. Shallow soils (0–50 cm depth) are mainl *Leptosols* and *Andosol*s, derived from recent volcanic ejecta along the Bigogwe plain. These soils are very fragile and less structured on steep slopes, hence prone to landlside as typical forest soils. They are called *Ruseseka* in Kinyarwanda by local farmers. *Andosols, Leptosols, and Phaezems* are the dominant soil types in Gishwati.

Regarding fertility potential, Rwanda digital soil dataset revealed that the fertilty of the soil is excellent for crop production in such year-round rainfall regime [19, 20]. In this context, soil analysis was done with top-soil samples collected between 0–30 cm soil depth layer for assessing soil nutrient and acidity status. It showed that Gishwati area has a great potential for agriculture in Rwanda once erosion is controlled. Nutrients were above the critical level except for phosphorus. Soil acidity problem in the area was quite low with soil pH range between 5.5 and 6.6 unlike to other Rwanda soils in the North-Western parts. The soils have high organic carbon content (3.2–5.1%) and significantly high contents of crop nutrients.

The study also recognized the main soil types including *Andosols, Phaeozems, Acrisols and Lixisols* on hillsides; *Leptosols* developed on recent volcanic materials along Musanze-Rubavu national road and *Cambisols* derived from colluvial and alluvial sediments in the narrow valleys. Andisols are developed in mild weathering conditions from volcanic eject while *Phaeozems* are developed in moist conditions under grassland or forest with a mollic epipedon. *Acrisols* are developed in wet tropical or subtropical forests, with acid silicates clays, iron and aluminum oxides. *Lixisols* were developed under moist or mildly acid conditions with acid clays accumulation (called "inombe" in Kinyarwanda); and *Leptosols* referred to as younger or recent soils derived from metamorphic parent materials. Specifically, *Andosols, Phaeozems, Leptosols and Cambisols* are very fertile and suitable for a wide range of crops, namely, Irish potatoes, maize, beans, peas, wheat, variety vegetables, etc.
