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close this book Animal traction in Rainfed Agriculture in Africa and South America
close this folder C. General factors influencing the use of draft animals
View the document 1. Underlying decision-making components in farm-household systems
View the document 2. Natural endowment
View the document 3. Conditions for agriculture and animal traction in rainfed cropping
View the document 4. Dynamics of farming systems and mecanization
View the document 5. Status of animal traction
View the document 6. Constraints of animal traction

2. Natural endowment

2.1 Climate and vegetation

2.2 Soils and topography

2.2.1 Soils in humid climatic zones

2.2.2 Soils in arid climatic zones

The development of natural vegetation in a region is pre-determined by the given ecological conditions, the climate, the soil and the topography, which are adapted to the respective environment in an optimal manner. In contrast to agriculture in the western industrial countries, tropical and subtropical agriculture must still today comply with the given environmental conditions, due to a lack of agricultural inputs (fertilizers, pesticides).

Thus, the most sustainable cropping methods are being achieved by means of an adaptation to the natural vegetation. Exceptions to the rule are only found in cropping methods which moderate the effect of the given local conditions (e.g. irrigation farming).

2.1 Climate and vegetation

Generally speaking, the tropical region is defined as the area between the tropics of Cancer and Capricorn. This is subdivided into the permanently humid interior and the seasonal wet and dry exterior tropics. The subtropics border the tropics and extend upto 45° latitude, North and South. (Caesar, 1986; Lauer, 1985)

A high thermic uniformity predominates particularly in the interior tropics; the temperature variations between day and night are more marked than seasonal temperature changes (time-of-day climate: see figure C 1). The temperature decreases with increasing elevation; also in mountainous regions (cold tropics) a time-of-day climate is found. In the wet and dry climates of the tropics both the daytime and nighttime temperature fluctuations and the seasonal temperature changes are greater. The seasons are determined by variations in the distribution of precipitation. The subtropics are marked by high summer and moderate winter temperatures. The upper high latitudes of the subtropics are often considered to be the upper variants of the temperate climate at the medium latitudes (Lauer, 1985; Müller-Sämann, 1986).

The differences in the rain regime in the various zones of the tropics and subtropics are depicted in figure C 2. The farther one becomes removed from the equator where precipitation occurs throughout the year, the greater is the marked bimodal rainfall distribution. The two peaks in the precipitation curves become equally high with increasing distance from the equator until in the outer tropics a rainy season only occurs in the summer months. The duration and rainfall quantity are increasingly reduced upto the periphery of the tropics.

In the subtropics humid and dry regions are distinguished as winter and summer humid seasons, depending upon the duration and time of the rainy season. Overall, the variability of precipitation and thus the uncertainty of rainfall supply both in quantity and with time increases with a reduction of average rainfall amounts. The temperatures as well as the total amount of rainfall -with a constant year-round moisture availability -decreases with higher elevation levels, in contrast to the temperate zones. (Weischet, 1984)

In practice an actual demarcation between the tropical and subtropical zones is difficult to establish. Frost may be an approximate borderline for the tropics where balanced temperatures manifest themselves (Lauer, 1985). Independent of the geographic location however the immediate local conditions must be taken into consideration. These are largely determined by rainfall characteristics and the temperature; further factors playing a role are elevation, air movements and the landscape. The requirements of agriculture and the mobilization of technology may be completely different in closely neighbouring regions.

Various systems exist to design exact climatic classifications. Here, a delineation according to characteristic vegetation units appears useful, as it provides information on the agro-ecological zones. The classification according to Troll (Landsberg et al., 1966) and Lauer (Müller-Sämann, 1986) are employed below. The number of humid months is the criterion for a demarcation of the various forms of vegetation, in which the humid period is defined as the time of a water supply surplus. Then, rainfall is greater than potential evaporation.

-arid zones: 0 -2 humid months

-semiarid zones: 2 -4.5 humid months

-semihumid/semiarid zones: 4.5 -7 humid months

-subhumid zones: 7 -9.5 humid months

-humid zones: 9.5 -12 humid months

Furthermore, highland areas above 1000 m altitude are considered separately because of the particular agroecological conditions. In following table, the abbreviation and designation for climatic classes are followed by the number of humid months and the characteristic vegetation:

 

Abbreviation/ Designation

Humid months

Characteristic vegetation

SUBTROPICS

IV1:

Dry-summer, humid-winter climates

> 5

Hard-leaved and coniferous wood

IV2:

Dry-summer, humid-winter steppe

< 5

Grass and shrub-steppe

IV3:

Steppe climates with short summer, humidity and dry winters

< 5

Thorn and succulents-steppe

IV4:

Dry-winter, long humid-summer climates

6-9

Monsoon wood and wooded-steppe

IV5:

Semi-desert and desert climates

< 2

Semi-deserts and deserts

IV6:

Permanently humid grassland-climates

10-12

High-grassland

IV7:

Permanently humid, hot-summer climates

10-12

Humid forest (laurel, coniferous)

TROPICS

V1:

Rainy climates

12-9.5

Evergreen rain forest

V2:

Humid-summer climates

9.5-7

Humid forest and grass-savannah

V3:

Wet and dry climates

7-4.5

Dry wood and dry savannah

V4:

Dry climates

4.5-2

Thorn-succulent wood and savanna

V5:

Semi-desert and desert climates

< 2

Semideserts and deserts

2.2 Soils and topography

Tropical and subtropical soils are, as all soils, a product of parent rock, age, climate relief of the landscape and vegetation. These vary substantially in type and application. Generally, the tropical and subtropical regions have possessed a stable soil surface for millions of years, where the soil could develop untouched and unharmed. Due to the warm and partially moist conditions the soils are fragmentarily weathered very deeply. Various levels of erosion have occurred depending upon the topography, so that different horizons can be found in close proximity in the soil. (Caesar, 1986) In spite of the variations of soil types some generally valid statements may be made, whereby it appears to be more meaningful to consider soils in humid and arid climates separately.

2.2.1 Soils in humid climatic zones

The humus content of soils in humid regions hardly varies from that found in the temperate zones. The high temperatures in conjunction with sufficient soil moisture however lead to a rapid depletion of organic matter as soon as an insufficient replenishment of organic material occurs (Sanchez, 1976). Also, the weathering of soil minerals is considerable under these conditions. These areas have a water supply surplus, i.e. rainfall exceeds the evaporation rate, leading to a downstream direction of water movement. In conjunction with the high mineralization rate and the intensive rainfall there occurs a high nutrient leaching process. As a result the soils have a nutrient deficit (especially P and N). The pH values are low as a rule and plants are subject to Al or Mn toxicity. The cation exchange capacity (CEC), i.e. the ability to store nutrients, is low due to low-absorption clay minerals (especially kaolinite). The biomass production occurring nevertheless is assured by an efficient nutrient exploitation (deep and intensive root penetration, rapid nutrient decomposition) (Caesar, 1986).

In the humid tropics only a few favourable regions exist which do not manifest these soil quality deficiencies. These are regions with neolithic weathered and volcanic parent rock as well as flood plains carrying fertile alluvial soil (Scholz, 1984). The residual mineral content is also higher (Dehn, 1981) in mountainous regions as a result of constant removal of weathering products from water erosion and soil flow (solifluction). Here, weathering as well as the decomposition of organic material is checked, being a positive factor for natural soil fertility. This advantage for soil quality, as opposed to that in humid lowland soils, becomes however a limitation for agricultural utilization, usually due to the rolling relief of the landscape (Scholz, 1984).

2.2.2 Soils in arid climatic zones

In arid and semiarid zones chemical weathering hardly occurs; if so only to a limited extent, since the products are only slightly leached out due to the small amount of precipitation, and thus they retard further weathering processes. For this reason the soils occurring here basically possess a greater residual mineral content and are richer in bases. Thereby, the pH values lie predominantly in the neutral to alkaline level (Caesar, 1986). A calcium carbonate or gypsum accumulation, and thus crusting, can occur in the soil, depending on the soil crumb structure and rainfall quantities. This may hinder soil drainage as well as the root growth. Furthermore, the rising and evaporation of groundwater in some areas leads to salt accumulation in upper horizons of the soil. Thus, surface sealing can rapidly ensue due to the high content of Na+ and K+ in some soils. In regions having marked seasons for rainfall the humus depletion is limited to the rainy season. However, the mineralization beginning with the first rains leads to a sudden liberation of nutrients (MacArthur, 1980). The proportion of humus is all the lower, depending on the quantity of precipitation and accumulation of litter which appears in reduced quantity due to a lack of vegetation (Caesar, 1986). Low humus content is also evident in regions e.g. savannas, where it is common to burn off vegetation (Sanchez, 1976). Life in the soil also diminishes, thus yielding a nitrogen deficit.