2.1 Climate

In the temperate zones of the earth, the seasons are determined by the temperature, and farming takes place in a rhythm which is adapted to spring, summer, autumn and winter. In the tropics, the precipitation determines the timing of sowing and planting, and often also the length of the growing sea son. Sunlight and temperature are important factors in the cultivation of green plants, but other factors such as access to water and nutrients may impose limits on the size of the production. On a global scale, sunlight is at its most abundant at around 15°N and S. In these areas, the temperature is higher than round the equator. The highest temperatures (around 50°C) are found at a certain distance from the equator where abundant sunshine combines with low humidity and long summer days. On the coast, where the air is more humid, the temperatures in the tropics rarely rise above 37°C and only very rarely fall below 16°C. Moreover, the tropical climate is characterized by greater variations in temperature between night and day than between the seasons. The connection between temperature and altitude is significant to vegetation and agricultural production in tropical highlands.

Assumed changes to the global climate are primarily caused by the industrialized countries' burning of fossil fuels and discharge of industrial effluents into the air. In the future, farming in both industrialized and developing countries may be forced to take such changes into account. Local climates, on the other hand, may be affected by land use, such as e.g. deforestation and cultivation.

2.2 Eco-climatic zones

Agricultural systems in the tropics vary greatly depending on their location with regard to eco-climatic zones:

Deserts (arid areas) and semi-deserts (semiarid areas) are often commonly identified as arid areas. A desert is an area with less than 150 mm of annual rainfall, while a semidesert receives an annual average of 150-160 mm. Most arid areas are warm (e.g. Sahel and lowland India), but cold deserts and semi-deserts are also found (e.g. Gobi, Tibet, and parts of Peru and Chile). Not only do the arid areas suffer from a scarcity of precipitation, the precipitation there is also subject to great variations. On a global scale, close to 900 million people live in deserts and semi-deserts, which are two of the most vulnerable agricultural habitats on earth. Despite this fact, the natural and traditional production systems of the arid areas are surprisingly resilient. In recent years, several semi-desert areas which mainly used to serve grazing purposes, have been cultivated. The success experienced by industrial areas in reclamation of arid areas (e.g. Australia, USA, and Israel) has inspired some aid agencies to concentrate on such schemes. Failed agricultural projects in such areas may cause significant damage to the environment such as e.g. changes to the water table, salinization, and accumulation of pesticides (cf. the Aral Sea and its surrounding areas).

Savannas and forest savannas (subhumid tropics) are areas with grass vegetation and scattered trees and bushes. These areas have distinct rainy and dry seasons. Forest savannas have trees in open formation with an undergrowth of grass, but today the trees have all but disappeared in many places. Many savannas are anthropogenous, i.e. the open landscape has been sustained through human activities such as arable cultivation, grazing, and the burning of grass. This kind of farming is usually based on seed crops (cereals, beans, and oilseeds), but with a significant element of root crops and food-producing trees. Traditional migrant farming involving burning has been replaced in many places by intensive farming with only short fallow periods or permanent cultivation.

Rainforest (humid tropics) is found in areas with a minimum annual rainfall of 2,000 mm. The vegetation is dense and rich in species. Two different cultivation systems are found: 1) slash-and-burn farming with a short growing period followed by a long fallow period and 2) continuous horticulture with a main emphasis on root crops and trees. Since most of the nutritional assets are contained in the vegetation, burning releases nutrients and renders possible short term agricultural production. If the landscape remains treeless over a longer period, however, a substantial proportion of the nutrients will be lost, thus reducing the productivity of the land.

Floodplains belong among the most productive agricultural habitats, and occur in all climatic zones. The flood water has a fertilizing effect in that it carries mud which contains nutrients. Such areas have therefore been able to sustain naturally high productivity and continuous cultivation for thousands of years. The first cultivation of rice can be traced back to floodplains. Asian rice originates from the river deltas of India and Bangladesh, and African rice from the river plains of Mali. Different varieties of rice are adapted to different environments such as e.g. floating rice in raising flood waters, wet-rice in controlled flooded paddy fields, mangrove rice in brackish water on the coast, and dry rice in humid and sub-humid climatic zones. Wet-rice is the most productive of these. In areas with sufficient water supplies and possibilities for irrigation, this system of production is expanding in step with the population growth. Cultivation systems involving intensive use of artificial fertilizers and pesticides are common. Traditionally, the cultivation of wet-rice is combined with fish (carp) in some places (see booklet 5 "Aquaculture").

Sub-tropical highlands: Highland areas on the fringes of the tropical areas may have sub-tropical conditions. The cultivation of plants is normally based on annual field crops such as cereals, leguminous plants, and oil seeds, and may be similar to the cultivation systems found in sub-tropical areas. Potatoes and several kinds of vegetables common in temperate agriculture are also found. As a rule, these areas have a high and rapidly growing population density. Farming systems with an emphasis on open fields, annual seed crops, and soil cultivation in the shape of ploughing, result in considerable dangers of erosion. The highland landscape aggravates the dangers of erosion in that the cultivation takes place in sloping terrain. Some parts of the sub-tropical mountain areas may have volcanic rock and fertile soil. In many places, however, erosion and exhaustion of the soil have resulted in strongly reduced crops.

Tropical highlands are situated closer to the equator and are warmer and normally more humid than sub-tropical highlands. These areas, too, tend to be densely populated, and only small residues remain of the original mountain rainforest. The variation of cultivated plants is greater than in the sub-tropical highlands. Annual field crops are combined with root crops, as well as bushes and trees, and the cultivation mostly takes place by means of manual tools. Small units and intercropping of annuals and perennials render ploughing with oxen or tractors impractical. Important crops include maize, beans, and root crops such as potatoes and cassava in lower-lying areas. Among the perennials, bananas and other fruits are important. Tropical highlands are central areas for the production of coffee and tea. Highly intensified agricultural production may lead to considerable erosion of the soil in these areas.

2.3 Soil characteristics

The soil is the upper part of the earth, extending cat 1 metre in depth, where most of the plant roots are found. Its properties are determined by age, geology, climate, and topography, as well as the occurrence of plants, animals, and micro-organisms. There is great variation among the types of soil found in the tropics. The term "tropical soil" expresses few common qualities apart from geographical location. The oldest civilizations in tropical areas developed on the very fertile soils of river plains or volcanic materials. Such soils consist of relatively young materials and are characterized by a steady supply of nutrients either through mud (the flooding of river plains) or through weathering (volcanic materials). On the other hand, surfaces which are old in geological terms have strongly weathered and leached soils with low content of nutrients, limited ability to retain added supplies of nutrition, and a tendency to form compact layers as a result of drying out and compaction. A common denominator for soils in tropical and sub-tropical areas is the rapid turnover (absorption and release) of nutrients. Upon the removal of vegetation, nutrients can rapidly be leached from the soil. Soil which is deficient in nutrients is more vulnerable to erosion and compaction as vegetation is established relatively slowly and remains more sparse.

The content of organic matter is of great importance to the soil's fertility or productive ability. The term humus partly refers to the relatively stable organic components of the soil, and partly to the total content of organic matter. For agricultural soil, the terms mould and mould content are often used. The content of organic matter in the soil will depend on supplies from plant residue, roots and organisms, and on the speed at which decomposition takes place. The latter is determined by the climate, and by the temperature in particular. Too much water may obstruct the air, and the decomposition is inhibited (bog formation). There are also examples of periodic water shortages lowering the speed of the turnover. In tropical areas with a lot of rain, there will be a rich supply of organic matter, but this is counterbalanced by rapid turnover. In rainforests, the organic matter is found in the vegetation. The rapid turnover in tropical and sub-tropical rainforests causes quick changes in the mould content of the soil, and in a matter of only a few years the cultivation of field crops will bring the content down to a level where plant production becomes very hard to sustain. Agroforestry (see booklet 3 "Forestry") represents new systems and methods with the objective of utilizing qualities about trees (deep-penetrating roots) and agricultural crops (shallow roots) in order to achieve a more sustainable plant production.

2.4 Economic and socio-cultural conditions

In addition to the technical-biological production of crops and animals, the agricultural environment also comprises the farmers who carry out the work, and the economic and socio-cultural context in which the production takes place.

Social organization: Basically, traditional farming communities in developing countries have their own social organization with established norms and power structures which it is important to be familiar with during the planning and implementation of projects. Leaders in such communities tend to be elderly men who exert great influence over individuals and over the utilization and management of natural resources. In many farming communities, modernization has brought with it the partial or total disappearance of such power structures, while they may be intact in other communities. Women may have the main responsibility for food production, and their work-loads may be heavy, as they are also responsible for housework, the looking after of children, and the fetching of firewood, water, and fodder. The planning of agricultural projects requires proper knowledge about the distribution of work within the households.

Rights to the natural resources: As a general rule, the local population depends upon the possibility of exploiting local resources. Forested areas are often common property, and the free utilization of firewood, building materials, wildlife, fish, fruits, berries, mushrooms, roots, larvae, insects and medicinal plants has long traditions in many places. Particular groups within the population may have their own rights pertaining to certain areas, e.g. nomads may have particular water and grazing rights.

Conditions of ownership and rights of use: In Africa south of the Sahara, the ownership of land is communal in many places, and the village head distributes the land among the villagers. The right to utilize land may be hereditary, it may be individual, or it may belong to the family. The rules about ownership, rights of use, and rights of inheritance may be complicated and subject to variations between different sections of the population and different villages. Women often have no right of ownership or inheritance. In Asia and Latin America, it is more common for the farmers to rent land from the land-owners. The land at the disposal of each individual farmer may be divided up into small fields scattered over a large area. It is not uncommon for some patches of land to be situated so far away from the family home that the farmer has to stay overnight in the fields during cultivation periods. A great deal of time may be spent walking between the home and such distant fields.

Institutions and infrastructure: Supportive apparatuses such as credit institutions, supervisory services, and marketing organizations may be underdeveloped in developing countries. In addition, the capacity of the national and local agricultural administrations to follow up the tasks allotted to them may be limited. Situations may arise where a jumble of agricultural projects operate more or less on their own without efficient coordination by the authorities. Poor roads and transportation systems, as well as great distances to the markets, are often limiting factors on agricultural development (see booklet 8 "Transport").

Policies: Many developing countries pursue policies which may be unfavourable to agricultural development, e.g. by favouring the consumers in the towns and cities in a way that indirectly inhibits the farmers' production. lust to mention one example, the urban population may be offered subsidized foodstuffs, while the prices offered to the producers are set so low that the motivation for cash crops is reduced.

Culture and diet: Within tribes or villages there may exist norms and taboos which help determine what constitutes an acceptable diet. According to these, certain plants and species will not be used as food. A whole range of preferences with regard to the taste and colouring of food are found in any society. Such conditions should be charted before the planning of food production goes ahead.