Quantitative determination of the extent and impact of soil erosion by water in the tropics have been sketchy. However all available evidence indicates that accelerated erosion is a problem of serious magnitude and with a multitude of negative effects in many tropical countries. The awareness to conserve soil began approximately nine thousand years ago when human civilization shifted from “nomadic hunting and gathering experience” to a more permanent, settled and intensive ” soil-dependent plant and animal” farming systems (Miller, Rasmussen and Meyer, 1985). Soil provides the medium from which most of the sustenance for humankind is derived. This thin, complex, crustal carpet uniquely integrates many attributes of the lithosphere, atomosphere, hydrosphere and biosphere (Miller, Rasmussen and Meyer 1985).
Soil is regarded as a nonrenewable source since its formation from the parent rock material to an agriculturally productive growth medium is a very slow process (Lal 1990). The top soil is where most of the living activities of both flora and fauna take place. According to Brady and Weil 1999, the top soil is the upper most part of the soil ordinarily moved in tillage or its equivalent in uncultivated soils which range in depth from 7 to 25 cm (the plow layer). Removal of this plant nutrient enriched topsoil due to soil erosion would result in lowering of soil fertility through losses of both organic matter and nutrients which would result in decline of crop yields (Lal 1986; Rose and Dalal 1988). This loss in soil productivity due to erosion is serious in caribbean countries where fertilizer substitute for the lost plant nutrients is not affordable. This would lead to poorer soil profile characteristics such as low organic matter levels (Rose 1989;Rose 1998).
Soil Erosion is perhaps the most serious form of land degradation throughout the world. For the tropics specifically, while it is acknowledged that erosion is more serious than else where, no analytical or systematic studies have been undertaken to document the problem, it consequences or potential solutions. Soil erosion results in lost water and plant nutrients at rates even greater than those occurring naturally through leaching ( Brady and Weil 1999). If there are no conservation practices present, soil erosion will occur and land degradation, reduced productivity due to loos of top soil, increased runoff and off site sedimentation problems (such as siltation of bodies of water and damage to crops and roads) will occur. The consequent socio-economic costs of sometimes more obvious off site damage are commonly easier to identify and quantify (Chrisholm 1987; Rose 1993).
1.2 Soil Conservation Strategies
The caribbean is characterized by steep slopes. The degrees of steepness and the proportion of the land area classified as steep may vary. In Trinidad and Tobago, a larger propotion of steep slopes is still covered with natural vegetation which reduces the soil washed off the surface. Trinidad is marked with a long dry season in the early part of the year and then followed by an intense wet season. Most erosion occurs when the land is cleared of vegetation for farming. This usually occurs just before the rainy season. It is important to note however that most food crop production in Trinidad as well as the rest of the Caribbean, is carried out by small farmers on sloping land with no attention to soil conservation practices. This is the main reason for severe soil erosion in Trinidad and Tobago and by extension the Caribbean.
Soil conservation is understood as not only involving the control of loss of soil material due to erosion but also the decline in fertility (chemical, physical and biological breakdown of the soil). (Young 1984). The adverse effect of soil erosion is not confined only to decrease in soil depth but ultimately to the loss ” loss of organic matter and plant nutrients” and consequently to ” degradation of soil physical properties” and crop yield decline ( Young 1984). It is important to note however that treating the benefits of soil conservation in isolation from other agricultural improvements does not assure adoption of the soil conservation practice (Young 1984). Integration of soil conservation is important ( Douglas 1988; Shaxson 1988).
Sheng and Meiman (1988) stated reasons for the difficulty of farmers in adopting soil conservation practice. The reasons are as follows: i) long time for the result of soil conservation to be realized, ii) identification of benefits from soil conservation and iii) the need for big investments by farmers. It is important to note however that Williams and Walter (1988), in a terracing project in Venezuela, found that improvement in the living incomes and opportunites for employment, increases the difficulty of motivating the farmers to adopt soil conservation practices. Apparently when low income and underemployment prevail, farmers easily participate in soil conservation programmes (Williams and Walter 1988; Liao et al 1988).
The adverse effect of soil erosion is not confined only to loss of soil particles but also the loss of organic matter and plant nutrients. As a result of this crop yield decline will occur (Young 1984). Conservation farming should serve as the basis for counteracting the problem of soil erosion. To carry out this approach in developing countries, novel approaches in extension and research are needed. The majority of the soil conservation techniques were developed in the United States of America under condtions different from the other parts of the world experiencing erosion (Sheng, 1982; Hudson 1988; Sheng 1988). The techniques that worked well in the United States were extrapolated to the tropics in the 1930’s and 1940’s. It took fifty years to realize that these methods were unsuitable for the tropics ( Hudson,1987; Hudson 1988). The dominant traditional farming system in the tropics are the “shifting cultivation” and related bush fallow systems (Okigbo and Greenland, 1976). The non-acceptance of the many recommendations on soil conservation methods is attributed to their in appropriateness and incompatibility with the farmer’s operating environment (Douglas, 1988).
The following agricultural conditions in the United States of America paved the way for the development of soil conservation activities (Hudson,1982;Hudson 1987):
Combined good topography and favourable climate
Low population pressure for intensive land use
Strong and sustained government support
” well educated and informed” agriculture sector
Readily available credit and financial support
Reliable prices and market outlets for agricultural produce
Highly developed and highly mechanized agricultural industry.
According to Hudson (1987), the absence of these conditions in most developing countries made the ” North American approach to soil conseration” in appropriate. The absence of political will, or the limitations in or lack of resources often hindered soil conservation programmes in developing countries. The main aim of soil conservation should be preventation rather than cure. Soil conservation activity should be focused mainly on preserving good land rather than reclaiming damaged land. Without waiting for visible soil erosion damage, farmers should be encouraged to understand that there is a need for soil conservation due to continuing reduction in agricultural yield (Hudson, 1987).
Loss of agricultural productivity should be emphasized in any soil conservation programme. For sometime, soil erosion research activities had almost been exclusively directed towards uantfying soil loss, hence data on the effect of soil erosion on agriculture productivity is seriously lacking (Crosson and Stout, 1983; ASAE,1985; Follet and Stewart, 1985; Stocking, 1985). Removal of topsoil resulted in decline of yield of a variety of agricultural crops (El-Swaify, Dangler and Amstrong 1982). Factors like type of soil, depth of soil, fertility status, topography, and type of crop affected the size of the decline in agricultural yield (Frye et al, 1982; Langdale and Schrader, 1982; Schertz 1983). There is limited data for soil and crops in the tropics (Lal, 1977). The effects of soil erosion on soil productivity in the tropics are move severe than for temperate countries (Moberg, 1972). This is because of highly weathered soils, fragile fertility status and most crop nutrents are found in the topmost layer of the soil. There is also significant deterioration of physical qualities of the soil as a growing medium after soil erosion (Lo, 1990). “Higher erosion rates, the more severe changes in chemical qualities resulting from erosion and the inability of the farmer to provide the necessary inputs for restoring those qualities to a sufficient level” were the primary reasons given for these conclusions (El-Swaify, 1990). The benefits of soil conservation ” are not immediately realized in every case and may initially result in crop yield reductions” (John 1988).
Improvement in agricultural production, rather than mitigating soil erosion, is of more significance and desirable for the farmer, while prevention of soil loss is an “unreal concept” for them (Hudson, 1987). The new approach of soil conservation will be a mixture of both agronomy measures as well as mechanical works ( Tracy, 1988). Mechanical systems are frequently expensive, consume space and time, need regular maintenance and do not assure improved crop production (Roose, 1988). Too much emphasis put into mechanical works discourages effective soil conservation policies (Rose, 1989). Conservation farming systems include improved farming, with mechanical protection works being a component of last resort. This approach is consistent with the principle that improved agricultural production should lead to better soil erosion control (Hudson 1988). Biological measures provide immediately recognizable short term benefits to farmers. Any mechanical work involved in soil conservation must maximize the use of locally available experts, minimize, the use of structures and required labour. Community involvement must be involved (Hudson, 1987).
To gain acceptance by farmers, any proposed soil conservation strategy should offer short-term, apparent, rapid or immediately recognizable, directly effective benefits and positive results, particularly for subsistence farmers who work in a “short-time scale” and who comprise a large percentage of farmers in less developed countries (Harper and El-Swaify 1988; Hudson,1988; Lovejoy and Naiper, 1988; Sanders, 1988; Thomas 1988; Tracy, 1988; Wenner, 1988; Williams and Walter, 1988). Soil conservation techniques recommended for farmers should be simple, easily understood and demonstrated, low cost, productive, sustainable and acceptable (Douglas, 1988;Vonk 1988). Conservation measures should involve principles that can be applied to more than one situation. However, methods and techniques are often site specific and caution should be exercised in extrapolating them to other sites (Saunders 1988).
Adoption of soil conservation technology is assured if the farmers have full understanding, support and participation in all the stages of the project, from planning, implementation to maintenance (Harper and El-Swaify, 1988; Sanders, 1988; Vonk 1988; Tracy 1988). Farmer’s full participation is ensured if they are convinced that their relevant and important needs can be adequately met (Sanders 1988). Thus the farmer needs to be recognized as part of the solution rather than a part of the problem (Hudson,1987; Hudson,1988).
The three (3) main objectives of this study are:
To investigate the effect of slope angle and rainfall intensities on soil erosion under controlled conditions using four distinct soil types
To compare this data with that for a cropped plot.
To highlight an approach at estimating erosion risk and nutrient loss for Trinidad and Tobago.