Agriculture Reference
In-Depth Information
on the debris slopes (41%), on the incisions and V-shaped valleys (33%), and on
the amphitheaters (23%). The adverse consequences of landslides in the study area
include a significant reduction by 50% of crop yield and farmers' income. This study
demonstrated the importance of rain-induced landslide, particularly its adverse con-
sequences on cropland and farmers' income. It is an important process of land deg-
radation and should be considered as a major constraint to agricultural development
when planning for soil and water conservation measures on tropical mountainous
areas.
12.2.1.3 Soil Loss Due to Root and Tuber Harvesting
Previous studies have also demonstrated that significant soil masses are lost from
arable land during harvesting of root, tuber, and bulb crops such as carrot (
Daucus
carota
), onion (
Allium cepa
L.), round potato (
Solanum tuberosum
L.), and cassava
(
Manihot esculenta
) (Ruysschaert et al. 2007). Soil sticking to the harvested crops
exported from the field is rarely returned back to the field, and is therefore referred
to as soil loss due to crop harvesting (Ruysschaert et al. 2004; Isabirye et al. 2007).
In East Africa, the amount of soil lost due to the harvesting of cassava roots and
sweet potato tubers under low-input agriculture was assessed on the northern fringe
of Lake Victoria in Mayuge District, Uganda (Isabirye et al. 2007). In this study,
it was observed that the mean annual soil loss for cassava was 3.4 Mg/ha and for
sweet potato was 0.2 Mg/ha (Isabirye et al. 2007). The ammonium acetate lactate
extractable soil nutrient losses for cassava were N = 1.71 kg/ha/harvest, P = 0.16
kg/ha/harvest, K = 1.08 kg/ha/harvest, and for sweet potato were N = 0.14 kg/ha/
harvest, P = 0.01 kg/ha/harvest, K = 0.15 kg/ha/harvest. Many studies of this nature
have been done under highly mechanized agriculture (Poesen et al. 2001), and only a
single research (Isabirye et al. 2007) in Uganda that was conducted under low-input
agriculture is available. This is an area that calls for further research in the region,
particularly with the introduction of root and tuber crops as one of the strategies for
adaptation to climate change.
12.2.2 s
oil
F
fertility
The per capita agricultural production and crop yields per unit area of production
in East Africa are declining (Sanchez et al. 1997; FAO 1999). The main contribut-
ing biophysical factors are nutrient/soil fertility depletion (Vlek 1993) and low soil
fertility particularly N and P deficiencies (Bekunda et al. 1997). Loss of nutrients
through crop harvests, surface runoff, and soil erosion is on the rise for most of
the farming systems. All these have contributed to the negative nutrient balances
reported for East Africa (Stoorvogel and Smaling 1990; Wortmann and Kaizzi 1998;
Walaga et al. 2000). Most of the studied soils in East Africa have very low organic
matter content (%OC <1.2), nitrogen (<1.0%), and available phosphorus (<5 mg/kg)
(Tables 12.5 through 12.7). It is clear that the chemical fertility of soils is low, which
implies that such soils are often poorly structured and therefore susceptible to ero-
sion (Moberg et al. 1982). The observed low soil fertility in the region could also be
attributed to poor land husbandry practices, which include clearing, burning, and
continuous cultivation without proper management (Kimaro 2003). Generally, these
