The natural environment

The natural environment consists of the climate and weather, the land and soils, and the ecology (the interaction among crops, weeds, insects, animals, diseases, and people).

Weather refers to the daily changes in temperature, rainfall, sunlight, humidity, wind and barometric pressure. Climate is the typical weather pattern for a given locality over a period of many years. To quote one definition, people build fireplaces because of the climate, and they light fires in the fireplaces because of the weather.

The climate and weather factors that have the greatest influence on crop production are solar radiation (sunlight and temperature), rainfall, humidity, and wind.

Solar Radiation

Solar radiation markedly influences crop growth in several ways:

· It provides the light energy needed for photosynthesis, the fundamental process by which plants manufacture sugars for use in growth and food production. Sugars are made by this process in the green cells of plants when carbon dioxide from the air combines with water from the soil using sunlight and chlorophyll (the green pigment in plants) as catalysts.

· The daily duration of sunlight (daylength) and its yearly variation greatly affect time of flowering and length of growing period in some crops.

· Solar radiation is the primary determinant of outside temperature, which strongly influences crop growth rate and range of adaption.

Regional and yearly variations in solar radiation

Unlike the temperate zone latitudes, the region between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S) has relatively little seasonal variation in solar radiation, since the sun remains fairly high in the sky all year long. Measurements above cloud level show an annual variation in solar radiation of just 13 percent at the equator versus 300 percent at a latitude of 40°. However, this supposed advantage of the tropics may in some cases be largely offset by cloudiness, which can be excessive in the higher rainfall zones, particularly near the equator (cloudiness can reduce solar radiation by 14-80 percent depending on depth and extent of the cloud cover). For example, due to heavy cloud cover, the equatorial Amazon Basin receives only about as much total yearly solar energy at ground level as the Great Lakes region of the U.S.

Daylength

The length of time from plant emergence to flowering as well as the actual date of flowering can be strongly affected by daylength in the case of some crops. Among the reference crops, soybeans and the photosensitive varieties of millet and sorghum are the most affected.

Maize is less influenced by daylength unless a variety is moved to a latitude where daylength is markedly different from that of its point of origin (see Chapter 3). Daylength is usually not a critical factor with peanuts, beans and cowpeas.

As shown by the table below, both latitude and season influence daylength. Note that the annual variation in daylength markedly decreases as the equator is approached.

Table 1 Length of Day in Various Northern Latitudes

Temperature

Temperature is the major factor controlling a crop's growth rate and range of adaption. Each crop has its own optimum temperature for growth, plus a maximum and minimum for normal development and survival. Even varieties within a crop differ somewhat in their temperature tolerance. Excessively high daytime temperatures can adversely affect growth and yields by causing pollen sterility and blossom drop. In addition, the hot nights common in the tropics can reduce crop yields. This is because plants manufacture sugars for growth and food production by the daytime process of photosynthesis, but "burn up" some of this at night through the process of

respiration. Since high nighttime temperatures increase the respiration rate, they can cut down on the crop's net growth. Several factors affect an area's temperature pattern:

· Latitude--Seasonal temperature variations are pronounced in the temperate zone where solar radiation and daylength fluctuate considerably over the year. In the tropics, this seasonal temperature difference is much smaller. Nighttime lows are seldom below 10-30°C near sea level and are usually above 18°C. Seasonal variations become more pronounced as the distance from the equator increases.

· Elevation--Temperature drops about 0.65°C for each 100meter rise in elevation. This greatly affects a crop's length of growing period as well as its adaptation to the area. For example, at sea level in Guatemala, maize matures in three to four months and the climate is too hot for potatoes; however, about 50 km away in the highlands (above 1500 m), maize takes five to ten months to mature and potatoes thrive.

· Topography, or the shape of the land surface, can cause differences in local weather and climate (micro-climates). A work area may have two or more distinct micro-climates.

· Cloud cover has a definite buffering effect on diurnal (daily) temperature variation. It will lower the daytime high but raise the nighttime low.

· Humidity exerts an effect similar to cloud cover on temperature. Humid air takes longer to heat up and cool off and therefore is subject to considerably less daily temperature variation than dry air. Maximum shade temperature rarely exceeds 38°C under high humidity, while maximums of 54°C are possible under dry conditions.

Rainfall

In dryland (non-irrigated) areas of the tropics with year-round growing temperatures, rainfall is the major environmental factor that determines which crops can be grown, when they are planted, and what they will yield. Rainfall varies greatly from place to place (often within surprisingly short distances), especially around mountainous or hilly terrain. The dryland farmer is keenly aware of his area's seasonal rainfall distribution. This includes deviations from the normal cycle such as early or late rains, or unseasonable droughts. Too much rain, which can drown out the crop, delay harvest, and accelerate soil erosion, can be just as serious as too little. It may be too wet for plowing one day, yet too dry the following week for good seed germination.

When gathering rainfall data for an area, one should keep in mind that annual rainfall averages have little meaning. Seasonal distribution and reliability are far more important in terms of crop production.

For example, Ibadan, Nigeria is located in the transition zone between the humid and semi-humid tropics and receives about the same annual rainfall (1140 mm) as Samaru Nigeria, which is located to the north in the savanna zone. Ibadan's rainfall is spread out over nine months from March to November in a bi-modal pattern (i.e., two rainy seasons with a drier period in between). The first season is long enough for a 120-day maize crop, although there is some periodic moisture stress. The second season is shorter, and soil moisture is usually adequate for only an 80-90 day crop. On the other hand, Samaru's equal rainfall is spread out over five months in a uni-modal pattern, providing for a single maize crop not subject to moisture stress.

From the example it is apparent that annual rainfall averages alone are not a dependable gauge of the rainfall in an area. The same goes for seasonal rainfall distribution. Although it gives a good general indication of the amount of moisture available for crop production, it does not tell the whole story. The amount of rainfall that actually ends up stored in the soil of a farmer's field for crop use depends on other factors such as water run-off and evaporation from the soil surface, and the soil's texture and depth.

When interpreting the rainfall pattern of a work area, it is good to remember that averages are somewhat misleading. Variations to the average can be expected even though the general seasonal distribution curve usually maintains a consistent shape (Figure 1). Cropping cycles and how they relate to the rainfall pattern:

Cropping cycles are determined by using the cropping calendar (planting and harvest dates for crops involved), and are closely tied to the seasonal rainfall distribution. This can be seen by comparing the cropping calendar in the next column with the rainfall chart in Figure 1.

Crop Calendar, Managua Area of Nicaragua

A primary source of rainfall information in a given area is the local farmer. Although official weather station rainfall data is handy to have if it is reliable and representative, it is not essential. Most of the information needed about rainfall distribution can be found by talking to experienced local farmers.

Figure 1 Monthly Rainfall Pattern, Managua, Nicaragua, 1958-67

Humidity

Relative humidity affects crop production in several ways:

· Daily temperature variation is greater under low humidity; high humidity exerts a buffering effect on temperature.

· High humidity favors the development and spread of a number of fungal and bacterial diseases (see the disease section in Chapter 6).

· The rate at which crops use water is highest under hot, dry conditions, and lowest when it is very humid.

Wind and Storm Patterns

High winds associated with thunderstorms, hurricanes, and tornados can severely damage crops. Among the reference crops, maize, sorghum and millet are most prone to damage from heavy rain. Hot, dry winds can markedly increase the water needs of crops. The frequency of high winds is also a factor that warrants investigation when surveying a work area's climate.

Topography

The shape of the land surface influences agriculture by causing local modifications in climate and weather and often is the major factor that determines the suitability of land for various types of farming. A work area may include several topographic features such as mountains, hills and valleys. Individual farms, too, often have significant topographic variations that affect crop production. Mountains and hills can greatly alter rainfall, and it is not uncommon to find a drier, irrigated valley on one side of a mountain range and a wetter, rainfed valley on the other side. Cold air usually settles in valleys, making them considerably cooler than the surrounding slopes. Steep slopes drain rapidly, but are very susceptible to erosion and drought, while flat or sunken areas often have drainage problems. Slopes angled toward the sun are warmer and drier than those angled away from it.

Soils

After climate and weather, soil type is the most important local physical feature affecting cropping potential and management practices. Most soils have evolved slowly over many centuries from weathering (decomposition) of underlying rock material and plant matter. Some soils are formed from deposits laid down by rivers and seas (alluvial soils) or by wind (loess soils).

Soils have four basic components: air, water, mineral particles (sand, silt and clay), and humus (decomposed organic matter). A typical sample of topsoil (the darker-colored top layer) contains about 50 percent pore space filled with varying proportions of air and water depending on how wet or dry the soil is. The other 50 percent of the volume is made up of mineral particles and humus. Most mineral soils contain about two to six percent humus by weight in the topsoil. Organic soils like peats are formed in marshes, bogs and swamps, and contain 30-100 percent humus.

Climate, type of parent rock, topography, vegetation, management and time all influence soil formation and interact in countless patterns to produce a surprising variety of soils, even within a small area. In fact, it is not uncommon to find two or three different soils on one small farm that differ widely in management problems and yield potential. Important Soil Characteristics

There are seven major characteristics that determine a soil's management requirements and productive potential: texture, filth (physical condition), water-holding capacity, drainage, depth, slope, and pH.

· Texture refers to the relative amounts of sand, silt and clay in the soil.

· Tilth refers to the soil's physical condition and capability of being worked.

· Water-holding capacity refers to the ability of the soil to retain water in its spaces.

· Drainage refers to the soil's ability to get rid of excess water and affects the accessibility of oxygen to roots.

· Depth is the depth of the soil to bedrock and the effective soil depth is the depth to which plant roots can penetrate.

· Slope is the inclination of the land surface, usually measured in percentage (i.e., number of meters change in elevation per 100 m horizontal distance).

· pH is a measure of the acidity or alkalinity of the soil on a scale of 0 to 14. These characteristics are discussed in detail in Soils, Crops and Fertilizer Use, U.S. Peace Corps Appropriate Technologies for Development Manual #8, Parts I & II, by D. Leonard, 1969, and Crop Production Handbook, U.S. Peace Corps Appropriate Technologies for Development Manual #6, Unit I, 1969.

Ecology

For our purposes, ecology refers to the presence of, and interaction among, the reference crops, weeds, insects, diseases, animals (humans, wildlife and livestock), and the environment in general. Agriculture is a perpetual contest with nature and farmers have developed many preventative and control measures, as well as special cropping systems, to give agriculture the advantage over natural succession. Each area will have its own combination of weeds, insects, diseases, and wildlife (including rats and graineating birds) that affect crop production. Identifying these and learning how farmers cope with them is crucial to understanding and dealing with the agricultural environment. The effect of people and agriculture on the overall environment

Modern technology, land shortages, and increasing populations have increased agriculture's ability and need to "beat back" and manipulate nature. Often little thought is given to the possible environmental consequences of agricultural development. Potential ecological impacts of agricultural projects include:

· Deforestation
· Soil erosion
· Desertification
· Laterization
· Salinization
· Agrochemical poisoning of soil water, animals and people
· Flooding.