The environment is the total of surrounding conditions which influence the development and growth of every living organism.
In the wild state, plants are found in certain habitats where the environment is favourable to their growth. If plants are to be grown successfully by a farmer he must simulate or even improve upon the natural conditions to be found around a plant.
Wild animals live in places where their natural food and shelter are obtainable. Domesticated animals require protection and a carefully selected diet if they are to thrive.
In their natural conditions, plants and animals are found in small colonies scattered throughout the countryside. Plants on farms are gathered together in relatively small spaces. As many as 400 000 plants may be grown on a hectare of soil and under conditions like this the natural pests and diseases of both build up rapidly.
Successful animal and plant production depends upon controlling the environment within the limits of tolerance for satisfactory growth. A study of the environment helps us to know what we can do to improve output. It also helps us appreciate the many things in the air and soil which we cannot alter and must therefore accept as natural limitations.
Meteorology is the study of the Earth’s atmosphere, especially factors which cause weather forming processes and weather forecasting.
The surface of the earth consists either of air, water and soil. These are held to the earth by gravitational force, but may be moved by the earth’s rotation by the gravity of the moon or the heat from the sun.
Air becomes heated by the sun and rises by convection. Hot air can hold more water vapour than cold so the rising air can pick up moisture. The air movement creates winds therefore the moist air will move. As the air cools again it can no longer hold its additional moisture, so rain falls. This process occurs continually over the surface of the earth and is the source of water for plant growth.
Figure 1: Shows how typical thunderstorms are formed
Rain enables plants to grow and provides clean drinking water for man and animals. Many farming operations are best carried out when the weather is dry. Ploughing, seed bed preparation and seed drilling all require dry weather, especially on clay soils. Hay-making and harvesting in warm dry weather cuts down the need for artificial drying which can be expensive. Any help which the farmer can receive from instruments or cloud formations, which might indicate changes in the weather must be accepted and appreciated.
Farmers are usually reasonably good weather prophets. This ability stems from long experience of local signs including the behaviour of the animals, birds and insects and the movement of the clouds.
To obtain the complete picture of weather conditions during a growing season several instruments must be consulted and recordings taken. The following records might be needed: pressure, rainfall, evaporation, humidity, maximum air temperature, minimum air temperature, wind direction and speed, sunshine duration and cloud cover.
It is important for the farmer to do a brief study of these records.
- 2. AIR PRESSURE
The air is a mixture of nitrogen gas, oxygen gas, argon gas, carbon dioxide, water vapour and small amounts of other gases. All the constituents are in gaseous form and their individual chemical particles, called molecules, are extremely active. The molecules are constantly on the move, bumping into one another, bombarding the objects on the earth, penetrating every crevice in the soil and even attempting to fly off into space. Just as a stone thrown into the air will fall again, so a molecule moving upwards is eventually slowed down and drawn back to earth by gravitational force. Even so, some air has been detected as high as 135km up, but most of the air or atmosphere is concentrated above the earth in the first 11 kilometres.
The result of this constant molecular bombardment is called pressure. Since most of the molecules are nearest the earth, that is where pressure is greatest, therefore at higher altitudes pressure falls.
Air pressure may be very powerful as it can lift water like in a syphon or lift pump. It cannot move downwards because the earth becomes too dense, it cannot escape upwards because gravity pulls it back, so it presses sideways around the world pushing equally in all directions. Should there be a drop in pressure anywhere the air will flow in to fill up the space.
The pressure of the air remains fairly constant but even small fluctuations affect the evaporation of water. Reducing the pressure of air above water helps the water to evaporate. Increasing the pressure of the air restricts the evaporation of water. Sensitive instruments register pressure changes however slight they may be. The human eye is sensitive to air pressure; one’s eyes seem to protrude further when the pressure is low. The human ear is sensitive to pressure and a sudden change of altitude can cause pain in the eardrum if the pressure on either side of the drum becomes unequal.
Figure 2: A mercury barometer
To measure air pressure accurately, a barometer is used. Air pressure will support a column of water about 250mm high and a column of mercury 760mm high. A simple barometer can be made by filling a thick glass tube about 840mm long and closed at one end with mercury liquid. Invert the glass tube and place the open end in a dish of mercury without allowing mercury to escape, until the open end of the tube has been submerged. The mercury will drop a little when released as the column exceeds 760mm. The space at the top of the tube is a vacuum. As the pressure varies from day to day, the height of the mercury column varies and can be measured with a ruler and recorded in millimetres of mercury. Over 760mm is considered High Pressure and 750mm and below is considered Low Pressure.
The value of the barometer and the pressure record becomes obvious when the separate records for wind, speed, rainfall, frost and sunshine are compared with the pressure record.
The aneroid barometer is a cylindrical metal container which may be compressed by air pressure. A lever which indicates the amount of compression is attached to a dial and shows a high or low pressure.
Figures 3: Shows what an Aneroid Barometer looks like on the Inside.
The chief cause of pressure change is the expansion of air caused by the heating of air by the sun or the radiation of stored-up heat from the sun from a large land mass. This heated air is forced to rise by convection leaving behind it a reduced pressure.
Areas of low pressures are called depressions and are called cyclones if the pressure is extremely low. Areas of high pressure are called anticyclones. The weather is settled and fine in an anticyclone but is often cold in winter and spring. In a depression, there is likely to be wind and rain.
AGRONOMY & PLANT NUTRITION –LEARNER GUIDE
Figure 5: Shows a Cyclone (low pressure) and an Anticyclone (high pressure)
- 3. WIND
Air movement is the direct result of pressure change. Air will flow into a depression causing strong winds to develop. In a very deep depression, called a cyclone, the winds may exceed gale force and may reach typhoon speeds of 150km per hour or more.
In low pressure areas the weather is bound to be unsettled but is usually mild in the winter.
In anti-cyclones (where pressure is high) the winds are gentle, moving away from the area as cool air falls from above. The weather is settled, usually cold in winter, although sunny by day. During the summer, high pressure brings warm sunny days.
MEASUREMENT OF WIND
Wind speed or pressure can be measured by a moving flap with a pointer attached indicating a reading on a calibrated scale. The speed of rotating cups can be measured by a speedometer. This type of instrument is called an anemometer.
The direction of wind can be measured by a rotating vane above compass bearings.
AGRONOMY & PLANT NUTRITION –LEARNER GUIDE
Figures 6 and 7: Show an Anemometer which measures wind speed
Figure 8: Shows a wind vane which shows the direction of wind.
THE EFFECT OF WIND ON CROPS AND STOCK
Winds are unpleasant for stock when they are unable to find protection. Cows give less milk; hens go off lay and fattening or growing stock do not put on weight if exposed to a biting wind.
A strong wind on a newly planted crop in light sandy soil can blow the soil and seed off the field. Well-grown cereals are subject to lodging if a strong wind beats through the crop before harvest.
Eddy: move in a circular way.
The force of the wind can be temporarily diverted by an obstacle in its path which completely or partially resists its passage.
- Solid barriers: e.g. walls or close-boarded fences. The characteristic wind-flow pattern caused by a solid barrier across the direction of flow leaves a sheltered
area behind the barrier diverting the air up and over or around the barrier.
- The overall result is a great wind reduction for a short distance which is very suitable for stock protection.
Filter barriers: e.g. tree belts, hedges, or wattle fences; the result is to restrict the force of the wind by much less than a solid barrier but since eddy formation is restricted by the leakage of wind through the hedges etc., the effectiveness of the barrier stretches much further across the field.
Pulling out the hedge takes a short time but growing one takes much longer. It is worth considering the loss of wind protection before pulling out a hedge on an exposed site.
Figure 9: Shows the effect of trees and shrubs as a windbreak on air flow
Figure 10: Shows how natural windbreaks affect the distance of protection from air flow.
|Saturated: holding as much water or moisture as can be absorbed; thoroughly soaked.
As air becomes heated its ability to hold water is increased. The amount of moisture in the air is called humidity of the air. When air is fully saturated with water vapour, drops of liquid can remain suspended in the air and a mist or fog is thus formed. If drops become large they will fall and is called rain. Clouds are suspended drops of water. Fully saturated air is at dew point if cooled rapidly by contact with cold grass; leaves or cold water surfaces, drops of water are deposited from the air and is known as dew formation. In limestone country where water for
stock is scarce, many dew ponds can be found. These collect rain and also encourage dew to form on their cold surfaces.
MEASUREMENT OF HUMIDITY
Relative humidity is the amount of water held by the air expressed as a percentage of the maximum which it could hold at a particular temperature. For example, fifty per cent (50%) relative humidity means that the air is half saturated with water. An instrument for measuring the percentage of moisture in the air is called a hygrometer.
Country people often suspend fir cones at the door of their house to indicate the humidity of the atmosphere. The cone closes up when it becomes damp and then opens again when the weather is dry.
The curls of people and animals with curly hair will become tighter in damp weather.
A twisted hair attached between two fixed points with a lightweight pointer on the hair can indicate changes in atmospheric humidity by twisting and untwisting.
This is the principle behind the homemade weather house. These instruments can give fairly accurate readings if properly made and carefully calibrated. A hygrometer of this type is used to measure the humidity of a grain sample after harvest. From the humidity, the moisture content can be found fairly accurately.
Figures 11 and 12: Show a wet and dry bulb hygrometer.
Two thermometers side by side, one with a dry bulb and the other with a wet one can be used as a hygrometer.
If Water evaporates from the surface of the wet bulb it will be cooled by the removal of latent heat for the evaporation of water. When evaporation is occurring the wet bulb thermometer has a lower reading. Since evaporation only takes place when the air is unsaturated the difference between the two readings gives an indication of the relative humidity of the air.
THE EFFECT OF HUMIDITY:
In cold weather when the air is moist, mist and fog can form. In warm weather during the summer, humid conditions lead to the rapid spread of fungal diseases through crops. Mildew spreads rapidly through barley, rusts spread quickly through wheat, and blight can spread through potatoes.
Many insect pests of crop plants and stock thrive under humid conditions. Leather jacket larvae of the crane fly feed at the surface when the weather is humid and the green-bottle fly strikes sheep when the weather is moist.
A farmer drying corn will find that air at a low relative humidity will dry his grain well, but moist air will not dry it at all until its relative humidity decreases. At least one type of grain-drying unit refrigerates air to precipitate its moisture content: The air then warms up again with a lower humidity more suitable for grain drying.