Objectives: By the end of this subtopic learners should be able to;
Identify and define the environmental factors influencing agricultural activities.
List and explain the effects of temperature on water loss.
Explain forms of wilting and their causes.
Give details on the effects of humidity on agricultural activities.
Explain and illustrate the water cycle.
Summarise the distribution, effectiveness, reliability and intensity of rainfall in Zimbabwe.
Identify natural farming regions in Zimbabwe and relate them to environmental factors.
The main environmental factors which influence agriculture are:
Wind
Temperature
Rainfall
Humidity
a. Rainfall:
Rainfall is liquid water formed in clouds as a result of condensed moisture from atmospheric water vapour and then precipitate.
Study the picture below to understand how rainfall forms.
Water cycle
Water is lost from dams, rivers, lakes, ground surface, plants and other water bodies due to heat from the sun.
The moisture escapes into the atmosphere and the air carrying water droplets is cooled and hence it condenses into clouds.
The clouds become heavy enough to fall under gravity, thus giving water which comes back onto the ground and collected again by the different water bodies.
This process will repeat again and again forming a water cycle.
a.Evaporation:
Evaporation is a process where a liquid changes into a gas due to heat.
Liquid water changes its form to steam water/ gas. This occurs as a result of the heat from the sun.
The water then escapes from the ground to the atmosphere.
b. Transpiration:
Transpiration is a process whereby water is lost from trees to the atmosphere through the leaves.
The loss of water from water bodies and plants is known as evapotranspiration.
c. Condensation:
Condensation occurs when water vapour changes from its gaseous state to liquid state.
It occurs when the water vapour reaches high altitudes; it condenses and then cools to form some rain clouds.
d. Precipitation:
Precipitation refers to all forms of water falls from atmosphere to the earth's surface.
Precipitation can be rain, drizzle and hail. When the rainfall gets down, it fills up wells, dams, streams and lakes thereby making water available for different uses.
e. Run-off:
This is the water which flows over the soil surface during the rain time and will collect in water bodies without getting into the soil.
Effects of rainfall:
Rainfall enables:
Seed germination.
Plant food absorption.
Maintenance of plant shape (turgidity).
Rainfall enables photosynthesis, transpiration, translocation and evapotranspiration to take place in plants.
In Zimbabwe the rain season starts from late October to early April.
Most of the rain falls during the summer months, mainly from November to March.
However, long dry spells (a period experienced without rainfall from the last date it fell to the date it resume raining) may be experienced especially in the southern and western parts of the country.
The lowest average rainfall is about 320mm, for example in southern areas such as Beitbridge, Hwange and Gwanda, and the highest is over 1 000mm, in the eastern border mountain areas such as Mutare, Chipinge and Inyanga.
The following are important aspects of rainfall which are of major concern during the rain season.
1. Reliability:
Regular and average rainfall should come consistently at the expected time.
This is preferable to heavy rainfall and periodic drought spells in between.
2. Intensity:
This refers to the amount of rainfall falling at a place at any given time.
A gentle fall is better than sudden downpours.
3. Effectiveness:
This is the amount of rainfall which is enough to enable seeds or plants to grow well.
4. Distribution:
Rainfall distribution is the amount of rainfall received at periodic time intervals such as per season, month or week.
Rainfall must be evenly distributed throughout the rain season, to ensure adequate moisture in the soil at all times.
5. Rainfall recording
Farmers are required to keep a record of the amount of rainfall which they receive within their areas each time they experience rainfall.
The records will assist in determining:
Choice of crop varieties.
Time of planting.
The rainfall pattern of an area.
A rain gauge is an instrument used to measure the amount of rainfall received. We measure the amount of rainfall in millimeters (mm).
Every farmer is expected to have a rain gauge on the farm situated in an open area where there are no trees and buildings that may interfere with its function.
The diagram below shows a rain gauge:
Excessive rainfall:
If rainfall exceeds expected amounts, it will result in some negative effects on agriculture production.
Too much rainfall will affect plant growth and the yield as well.
Effects of excessive rainfall are:
Causes erosion.
Water logging.
Leaching of nutrients.
Stunted plant growth.
Reduced plant yields.
Problems associated with drought are:
Lack of moisture in the soil to support plant life.
Permanent wilting of plants.
Crop failure leading to poor or no yields.
Poor grass development leading to animal starvation.
b. Temperature
Temperature is a measure of coldness or hotness in the atmosphere or within the surroundings.
The instrument used to measure temperature is called a thermometer.
There are two types of thermometers, the mercuryor alcoholthermometer and the maximum-minimum thermometer.
The maximum and minimum thermometer is used to record the highest and the lowest temperatures reached during a period of time.
The left side of the thermometer records the lower temperature than the right side of the thermometer which records the highest temperature.
The productivity and growth of a plant is highly affected by temperature and also depends on whether the crop variety is a warm-season or cool-season crop.
Plants require specific temperatures for flowers to form and develop.
Effects of temperature on plants
Temperatures that are too low or high for a summer crop will prevent the crop from growing well.
Too high temperatures for warm-season crops such as tomatoes, beans and butternuts can cause flower pollination failure.
Unfavourable temperatures also cause stunted growth of plants and poor yield.
Very cold temperatures may result in frost damage on plants.
Warmth, after a period of low temperatures will break dormancy resulting in plants resuming active growth.
Plant sugar content will be induced by warm temperatures. This is the reason why sugarcane is preferably grown in hot temperature areas such as Chiredzi.
Rate of transpiration increases with increase in temperatures.
Respiration becomes rapid with increase in temperature.
Temperature increases evaporation and evapotranspiration rate.
Loss of water through evaporation
Usually water is seen as a liquid, but if heated, it changes into a gaseous state called water vapour. The change of water from a liquid to gas is called evaporation.
Evaporation is the loss of water from the soil surface, dams, lakes, rivers and irrigation canals into the atmosphere as water vapour.
Evaporation occurs when the soil dries out under hot, sunny and windy conditions.
High temperatures and wind increase the rate of evaporation greatly.
Crops dry out when evaporation rate is high.
Effects of evaporation:
Excessive evaporation results in drying up of water reservoirs, such as dams, lakes and rivers resulting in short supply of water for animals and plants.
High temperatures and wind increase the rate of evaporation.
The soil surfaces which are not under mulch or plant cover is more susceptible to evaporation.
Prevention of evaporation:
Ensure that the soil is always under cover through use of plant materials (mulching).
Increase plant population.
Practise intercropping using ground cover crops such as cowpeas (nyemba/indhumba), pumpkins (muboora/ibhobola), watermelons (nwiwa/ikhabe) and other bushy crops.
Benefits of water evaporation
Promotes water cycle.
Reduces moisture content from water logged soils.
Generally makes soils to become workable.
High evaporation rate will:
Increase the rate of loss of water from the entire surfaces of plants, especially from the leaves (transpiration).
Make plants to draw more water from the soil to make up for the loss.
Result in plants wilting. This occurs if the rate of transpiration is greater than the rate of water uptake.
Dry out the surface layer of soil, which is dangerous to young, seedling plants as they have small and shallow roots.
Precautions:
Use mulch on seedbeds to conserve soil moisture.
Protect weaker plants by providing shade and water if necessary.
Evapotranspiration and wilting of crops
Types of wilting
Temporal wilting:
This is when plants drop their leaves downwards resulting in loss of shape (turgidity) due to high temperatures and high water loss.
This normally occurs during the day when temperatures are high. Plants can recover if watered.
Permanent wilting:
This is when the soil is no longer having moisture to sustain the plant due to high temperatures and high evaporation rate.
The soil water available will not be able to support plant survival.
If this situation persists plants may die and watering will not help them recover.
c. Humidity
Effects of humidity on water loss and fungal disease attacks.
Humidity is the amount of water vapour in the atmosphere.
The wet and dry bulb thermometer is used to measure humidity.
Effects of humidity on plants
Favourable effects of humidity:
It slows down the rate of evaporation of water from the soil.
This is good for young roots because it keeps moisture in the soil.
It slows down the rate of transpiration from plant leaves. This makes a plant to keep its turgidity by supporting stems and leaves.
It helps plants to grow by aiding processes such as evapotranspiration.
Unfavourable effects of humidity:
It encourages development and spread fungal diseases.
Fungal diseases thrive best in warm humid conditions.
When the weather is warm and wet, this will result in more incidents of fungal diseases.
Examples of fungal diseases are potato blight, mildews, tomato blight, rust of cereals and spot of beans.
Low humidity increases the rate of evaporation and this can lead to wilting of plants. This will also reduce the rate of photosynthesis and the plants will not be able to grow well.
Measures against unfavourable effects of humidity
Use mulch on seed beds.
Use wider inter-row plant spacing to allow free circulation of air.
Grow cultivars which are resistant to diseases.
Agricultural activities in relation to the distribution and intensity of rainfall
Rainfall is the most important environmental factor which affects agricultural production.
It is also characterised by its distribution and intensity amongst other factors.
Distribution:
Rainfall distribution is the amount of rainfall received at periodic time intervals such as per year, season, month or week.
This is how the total amount of rainfall received per year is spread throughout the whole rain season.
For optimum yields to be achieved, the rainfall must be well-distributed providing adequate soil moisture for the whole growth period of the crop.
If the whole season receives only rain showers, the crops will undergo a long dry period and may even wilt.
Irrigation during dry period is required.
If the water is supplied regularly by little showers, evenly distributed over the rain period, adequate soil moisture is continuously maintained.
This is called a good season quality.
Intensity:
This is the rate at which rainfall precipitates. The more rain that falls in a short time, the greater is the intensity.
If the intensity is high on sloping land, unprotected by vegetation or by adequate conservation measures, then soil erosion will result.
Steady and gentle rainfall is better than heavy sudden downpours.
Effects of rainfall on livestock
a) Excessive rainfall:
This results in formation of swampy areas which can cause foot rot to animals.
Swampy areas or stagnant water bodies promote the breeding of parasites such as liver flukes, mosquitoes and roundworms.
b) Too little rainfall:
This can result in drought conditions leading to poor pastures for grazing, thus reducing livestock productivity.
Agro-ecological zones of Zimbabwe
Zimbabwe has been divided into five natural farming regions, mainly based on rainfall and temperature
This has resulted in the different farming activities carried out in these regions.
Region
Climatic conditions
Farming activities
Region 1: Eastern Border Highlands: Nyanga, Mutare, Cashel, Chimanimani and Chipinge.
Low temperatures, with chances of frost during the cool dry season.
High rainfall zone which receives an average of 1000mm per annum.
Rainfall is highly reliable and evenly distributed.
Temperatures are cool and some ground frost is sometimes experienced.
Intensive and specialized farming.
Favourable for the production of crops like tea, coffee, macadamia nuts, apples, pears, vegetables, peaches, apricots, kiwi fruits as well as forestry plantations.
Intensive livestock production.
Dairy cows, beef cattle and sheep.
Region 2: Harare, Mazoe, Bindura, Chinhoyi, Norton, Chegutu, Marondera and Beatrice.
Moderately high rainfall with averages of 700-1000mm per year.
Rainfall is reliable although some areas at times experience some dry spells which usually occur in January.
Temperatures range between 18-22 in summer and 16-18 in winter.
Intensive farming system for both crops and livestock.
Mixed farming with poultry and piggery production complement crop production.
Crops grown include the following; maize, soya beans, cotton, tobacco, ground nuts, green beans, vegetables and gum trees.
Region 3: Kwekwe, Gweru, Wedza, Mvuma, Mashava, Mhangura and Mt Darwin.
Unevenly distributed rainfall averaging from 650-800mm per year.
Temperatures are on the high side with some mid-season droughts occurrences.
Production systems are based on drought-tolerant crops and semi-intensive livestock farming based on fodder crops.
Cash crops can be grown but with the aid of irrigation.
The region is quite suitable for livestock production.
Suitable for the production of groundnuts and sunflowers as cash crops.
Region 4: Southern and northern parts of the country Kariba, Binga, Kamative, Hwange, Lupane, Bulawayo and Masvingo.
Rainfall is too low, with an average of 400-650mm received per year.
Prone to seasonal droughts and severe dry spells during the rainy season.
Drought tolerant crops such as millet, (mhunga/inyawuthi), rapoko (rukweza/uphoko), sorghum (mapfunde/amabele).
Semi-extensive livestock production.
Region 5: Beitbridge, Mwenezi, Chiredzi and Chirundu, Zambezi Valley and Sabi Limpopo Valleys.
The rainfall is very unreliable with annual average ranging from 350-400mm per year.
The region experiences high evaporation rates due to high temperatures.
Drought resistant crops such as millet and rapoko.
Livestock production is quite extensive with cattle ranching used on beef cattle.
