ENVIRONMENTAL INFLUENCES

Temperature

Temperature is the measure of the degree of hotness or coldness of the environment. Temperature is measured in degree Celsius (oC) by an instrument called the thermometer.

Effect of Temperature on Growth of Plants

Both low and high temperatures affect the germination of seeds. The ideal temperature range for germination is 15oC - 30oC.

Freezing temperatures (below 0oC) cause death of cells (cold injuries). High temperatures cause wilting because of increased rate of transpiration. Low temperatures cause reduced rate of nutrient uptake. Low temperatures cause the activities of microbes to seize; this stops the decomposition

of organic matter.

Effect of Temperature on Growth of Animals

Low temperatures cause animals to use energy meant for growth to maintain a constant body temperature.

High temperatures cause loss of mineral salts through sweating. High temperatures suppress appetite, resulting in reduced growth rate. High temperatures confine animals to the shade, greatly reducing grazing time.

Modifying the Effects of Temperature

In practice, the effects of temperature can be modified by:

applying mulch to the soil erecting/providing shade irrigating the soil

Wind

Wind is the movement of a mass of air from an area of high pressure to an area of low pressure.

The speed of the flow of the wind depends on the difference in pressure - the greater the difference in pressure, the greater the speed of the wind.

An anemometer measures the speed of the wind. Unit of measurement is kilometres per hour (km/h).

A wind vane measures the direction of the wind. The wind vane consists of a flat piece of metal that spins to a point in the direction from which the wind is blowing.

Effects of Wind on Plants and Animals- Beneficial

Wind promotes the pollination of flowers. Wind promotes the dispersal of seeds. This allows plants to colonise new habitats. Wind is essential for winnowing, e.g. sorghum. Wind is a source of power to drive windmills. Wind transports clouds that bring rain. Wind lowers atmospheric humidity and thus creates conditions that are not favourable for

disease development.

Effects of Wind on Plants and Animals- Harmful

Wind causes a spread of weed seeds in the field. Wind transports air-borne pests and disease-causing organisms. Strong winds cause lodging of plants. Wind speeds up the loss of moisture through evaporation and transpiration. Strong winds may remove roofs of buildings housing animals. Strong winds cause soil erosion.

Humidity

Humidity is the amount of moisture in the atmosphere.

The temperature of the air determines the amount of moisture that the atmosphere is capable of holding. The higher this temperature, the more moisture the atmosphere can hold.

Relative humidity is the amount of moisture in the atmosphere expressed as a percentage of the maximum it could hold at that given temperature.

Relative humidity is expressed as a percentage and measured by a hygrometer.

Effects of Humidity on Plants and Animals- Beneficial

High humidity reduces loss of water from plants and animals. High humidity brings amount the formation of clouds from which plants and animals

receive rainfall. High humidity keeps plant cells turgid.

Effects on Humidity on Plants and Animals- Harmful

High humidity reduces the rate of evaporation. This causes water logging in soils. High humidity creates conditions favourable for fungal disease development. High humidity stops the transpiration stream. This reduces the rate of nutrient uptake

from the soil into plants.

High humidity reduces evaporation of sweat from animals causing them discomfort.

Modifying the Effect of Humidity

Irrigate using a sprinkler to raise humidity around crops if the level is low. Avoid irrigating the leaves of crops so as not to increase the chance of fungal disease

development.

Rainfall

Rain is the water droplets from clouds that are a product of the process of condensation of moist air.

The amount of rainfall received in an area is measured using an instrument called the rain gauge. The unit of rainfall is millimetres (mm).

Rainfall is a form of precipitation and is the source of almost any available fresh water in the tropics. It is important for all vegetation, crops and animals.

Some aspects of rainfall are:

Rainfall Intensity: is the measure of the amount of rain per unit time. It indicates how heavy the rain is falling. Rainfall intensity is measured in millimetres per hour (mm/h). Rainfall of low intensity is more effective than one of high intensity.High intensity rainfall leads to runoff and hence soil erosion.Rainfall of low intensity has a high infiltration rate, less runoff and is less likely to cause soil erosion.

Rainfall Duration: measures the time the rainfall lasted. Rains of long duration tend to be more effective than those of short duration.

Rainfall Distribution: is a measure of the spread of the total amount of rainfall received in an area across the rainy season.

Rainfall Reliability: is a measure of the chances of receiving a given amount of rainfall that is adequate for growth of crops. It refers to the certainty with which the rains may be expected. In Botswana, rain is unreliable.

Rain Effectiveness: is a measure of the degree to which an amount of rain can sustain plant and animal life without posing any danger.

Effects of Rainfall on Plants and Animals- Beneficial

Rainfall provides essential moisture for plant growth.

Rainfall provides moisture necessary for seed germination. Rainfall supplies water that is important in most metabolic processes in both plants and

animals.

Effects of Rainfall on Plants and Animals- Harmful

Excess rainfall causes water-logging that inhibits the growth of crops. Excess rainfall causes soil erosion. Heavy rainfall leads to the loss of nutrients from the soil through leaching. Rainfall of very long duration creates favourable conditions for disease development.

The Water Cycle

The Water Cycle is also called the Hydrological Cycle. It shows the various processes that lead to the supply of water from the atmosphere as precipitation and its loss back to the atmosphere as evaporation, transpiration or respiration.

The Water Cycle is a solar-driven cycle of evapotranspiration, condensation, precipitation and runoff. The cycle controls water movement between the atmosphere, oceans, aquatic and terrestrial environments.

Water escapes from the Earth's surface into the atmosphere through the process of evaporation. The moist air rises and is transported across the land by wind until it reaches the condensation level.

Moist air at the condensation level condenses to form water droplets. As the droplets become heavy, they fall as rain, dew, hail or fog.

The collective name for rain, dew, hail and fog is precipitation.

The water droplets reaching the Earth's surface enter the soil through infiltration.

Some of the water move along the surface as runoff into streams, rivers, lakes and dams. Water receives as rainfall may be lost from plant use through evaporation, transpiration, percolation and runoff.

Evaporation : Water evaporates from oceans, lakes, plant leaves and land surfaces, and rises into the atmosphere as vapour.

Condensation : Above the condensation line, vapour cools down, condenses to form clouds and returns to the Earth's surface as precipitation.

Precipitation : Water in solid or liquid state is returned to Earth as precipitation. Some of it evaporates before it even reaches the ground. Some lands on plants. The rest reaches the Earth or inland surface water.

Seepage/ Percolation : Much of the water that reaches the ground evaporates quickly and returns to the atmosphere. Most of the rest seeps/infiltrates into the Earth.

Transpiration : Plants often use seepage water eventually returning it to the atmosphere through transpiration.

Recharging of ground water supplies : Some of the seepage water moves beyond the root zone and replenishes supplies of underground water.

Runoff : Water, which does not enter into the soil or evaporate, runs across the land in the form of streams or rivers, which run into lakes, pans or the sea.

Man and animals use part of fresh water, eventually returning it to the atmosphere through breathing, perspiration and decay.

Frost

Frost occurs when the moisture in the air and on the ground freezes on the surface of the soil or on grass.

During winter days, the air cools down quickly and therefore, draws heat out of the soil. The air becomes warm and rises. The heat within the air escapes upwards. This warm air is replaced on the ground by cool air. If the temperature on the ground falls below 0oC, the moisture above and on the ground freezes.

Frost is likely to occur on clear nights with no cloud cover than on cloudy nights. On cloudy nights, the rising heat from the ground is reflected by the cloud cover.

Effects of Frost on Plants

Frost can cause the soil to freeze. Any moisture within the soil will therefore freeze and thus nutrients will not be made available to plants.

The low temperatures cause moisture within the plant cells to freeze. Frost creates a humid atmosphere around the crops.

Modifying the Effects of Frost

Fans could be used to create a wind to disperse the cold air. Small fires could be made to raise the temperature around the plants.

Use of smoke pots to produce a layer of smoke. This layer serves as a cloud cover to reflect the warm air back to ground.

The soil could be mulched to prevent the heat within the soil from escaping into the atmosphere.

Photoperiod

Photoperiod refers to the daily cycle of light and darkness. In the equatorial region, day light is 12 hrs throughout the year, i.e. there are 12 hrs of light and 12 hrs of darkness.

In the temperate region, the day length varies from 8-16hrs, depending on the time of year.

Photoperiodism

Photoperiodism refers to the influence of the daily cycle of light and darkness on the physiology and behaviour of organisms, i.e. it is the reaction of living things to different day lengths.

Plants can be classified into three groups, based on their day light requirements. These are 'short-day' plants, 'long-day' plants and 'day-neutral' plants.

'Short-day' plants- require a period of continuous light less than the period of continuous darkness before flowering can occur, e.g. sweet potatoes, pineapple, rice.

'Long-day' plants- require periods of continuous light more than the period of continuous darkness in order for the plants to flower, e.g. potato, castor oil plant.

'Day-neutral' plants- are independent of the continuous hours of light and will flower at any time of the year, e.g. maize, tomato, and many of the common vegetables.

Cool-season crops- cabbage, lettuce, mustard, onion, peas.

Warm-season crops- squash, snap beans, cucumber, pepper, pumpkin, sweet potato, tomato, sweet corn.

Effects of Photoperiod on Plants and Animals

Longer day length stimulates the process of egg production in poultry. Longer day length stimulates rapid growth in most livestock. Geographical distribution of most plants is determined by the photoperiod. Most crops in

the tropics are 'short-day' plants, while most of the temperate regions are 'long-day' plants.