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soil, result in loss of soil aggregates stability (see Section 3.3.2) and soil structure, which in turn results in soil compaction, increased bulk density and decreased infiltration rate and water holding capacity of the soil. This leads to an increase in overland flow and further increase in erosion and degradation of topsoil. The practice of tillage also destroys the topsoil thin (3-5 cm) A 1 organic horizon, which is present under the virgin forest (see Section 4.3.1). This A 1 horizon is humic, very rich in fine roots, very friable, very porous, with granular subangular structure induced by a network of fine roots. It is within this A 1 horizon that farmers plant with zerotillage, cucumber (Cucumeropsis mannii) and maize (Zea mays) seeds when they clear an old forest. Erosion. The accelerated erosion is considered as man-induced erosion. The change of the soil chemical and physical properties under land cropping leads to an increased soil loss hazard due to detachment and transportation. In general, agricultural land in the study area are mostly located in dissected plains and rolling uplands landform units (van Gemerden and Hazeu, 1999) where the existing gentle slopes limit the effects of soil erosion processes. This partially explains the low sediment yields in the rivers reported by Waterloo et al. (2000). However, the soil type and the cropping systems / farming systems are the most important factors affecting erosion hazard variation in the area. From our field observation in the study area, sheet erosion occurs under the impact of the early rains at the beginning of the cropping period in annual crop fields. The consequence of this erosion is the rapid deterioration of the physical condition of the soil surface and leaching of nutrients from ashes. This process is lessened when the first crops cover the ground. When the first crop has been harvested, the soil surface is again exposed to some extent, but intercrops, weeds and the residues from the harvest provide some protection. Soil fauna. Soil micro-organisms together with soil macrofauna act as agents of nutrient cycling by regulating the retention and flux of nutrients in the soil system through processes of decomposition, mineralisation and immobilisation. Soil micro-organisms also enhance the amount and efficiency of nutrient acquisition by plants through the role of plant symbionts such as mycorrhiza and nitrogen fixing bacteria (see also Onguene, 2000). These functional relationships might be most evident in the natural ecosystems but also play a key role in determining the productivity and sustainability of agricultural systems. The activity of soil microbiota can be measured by what is called ‘microbial biomass’. This aspect should not be left out when one is to deem thoroughly the effect of shifting cultivation on soil degradation. Other factors Weeds, diseases, pests and insects. Weed growth is often of profound hindrance to crop production in tropical agriculture. Weed competition to crops is considered a factor that limits the effectiveness of soil available nutrients in increasing crop yields. However, weeds invasion has a positive effect as soil cover and protect soil against high erosive rainfall. Nematode and insect fauna and pathogenic microbes are also diverse in the tropics. High population of these parasite species often cause significant damage to crops. The parasite population, which might have built up during the cropping phase, is likely to be greatly reduced in numbers after several years of fallow. Phytomass reduction. In selecting a site for clearing, shifting cultivators, in general, judge the suitability of a prospective cropping site on the basis of existing vegetation rather than on soil characteristics (see Tables 2.11 and 2.15). In the study area, soils are classified in general lower chemical fertility levels (van Gemerden and Hazeu, 1999). A large part of nutrients is stored in the phytomass (vegetation biomass). The mature forest has adapted to the low nutrient level of soil by maintaining an almost closed nutrient cycle. Through recycling, these nutrient stocks 44
emain almost constant. When clearing the forest destroys this nutrient equilibrium, the nutrient cycle is severely interrupted and through crop harvest, leaching and erosion, part of the nutrients disappears out of the system; and the land becomes unproductive. Effects of burning. The direct effects of burning vegetative materials are: - concentration of mineral nutrient ions contained in the vegetative material into ash spread over the soil surface; - loss of carbon, nitrogen and volatile sulphur from the burned material to the atmosphere; - high temperatures of soil surface layer affect the physical and chemical properties of soil colloids, biological populations, nutrient availability and destroy some weed seeds. Sanchez (1984) and Andriesse and Schelhaas (1987) reported in tropical Latin America and tropical Southeast Asia striking changes in soil properties due to burning. According to Andriesse and Schelhaas (1987), successive burns result in 20-25% decrease in organic carbon, 5-10% decrease in CEC and less than 10% decrease in total phosphorous. Mainly affected are the minerals in the top 25-cm of the soil. Fertility improvement from ash as a result of burning was observed at the same depth with increases in available Sulphur (2-60%), available Phosphorous (50-300%), Calcium (50-200%), Magnesium (15-45%), and Potassium (6-80%). The increase in bases led to higher pH. Burning, on one hand, improved the soil fertility, but on the other hand, left the soil bare leading to intense leaching and erosion of bases during cultivation resulting in subsequent pH decrease. 45
Page 2 and 3:
Farming systems in the evergreen fo
Page 4 and 5:
ABSTRACT Nounamo, L. and Yemefack,
Page 6 and 7: 4.3.4 Relationship between commerci
Page 8 and 9: PREFACE This report presents the fi
Page 10 and 11: stability and structure, and the de
Page 12 and 13: (calcium), le pourcentage de satura
Page 14 and 15: Figure 1.1. Location map of the TCP
Page 16 and 17: 600 28 500 27 Rainfall (mm) 400 300
Page 18 and 19: low pH (3.8 to 5) and a high clay c
Page 20 and 21: Characteristics of the four sub-sit
Page 22 and 23: 2.2.7 Household labour study The st
Page 24 and 25: Roads may temporarily be closed, du
Page 26 and 27: swamps and valley bottoms are also
Page 28 and 29: Table 2.6. cont’d Species Ewondo
Page 30 and 31: dispose of land near the village, o
Page 32 and 33: The heat may, however, reduce the g
Page 34 and 35: is an activity practised by nearly
Page 36 and 37: The crop production activity takes
Page 38 and 39: Table 2.17. Priority socio-economic
Page 40 and 41: 3 ASPECTS OF SOIL DEGRADATION UNDER
Page 42 and 43: 3.2.3 Laboratory soil analyses Soil
Page 44 and 45: The magnitudes of changes are inver
Page 46 and 47: as it is practised in shifting cult
Page 48 and 49: 15 years. The vegetation evolves th
Page 50 and 51: Organic matter. Organic carbon incr
Page 52 and 53: Microbial Biomass Carbon C (ugC/g s
Page 54 and 55: processes on soil chemical properti
Page 58 and 59: 4 TRENDS IN FIELD EXPANSION AND SOI
Page 60 and 61: Greenland, 1960; Sanchez, 1976; Vol
Page 62 and 63: 5 SOLUTIONS TO FARMERS’ PROBLEMS
Page 64 and 65: Integrated Nutrient Management meth
Page 66 and 67: REFERENCES Almy, S.W., Besong, M.T.
Page 68 and 69: Prat, C. (1990). Relation entre ér
Page 70 and 71: ANNEX 2: CHECKLIST FOR THE EXPLORAT
Page 72 and 73: ANNEX 4: AGRICULTURAL LAND EXPANSIO
Page 74: ANNEX 6: AGRICULTURAL LAND EXPANSIO
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