O Level Notes : Agriculture - Crop husbandry

In this topic respiration will be explained as an important processes in the plant energy production. Plants have a system of responding to their external environment known as plant tropisms. This will help to increase plant survival chances and productivity. Crops need to be protected against pests, diseases and weeds through the use of Agro chemicals. Some of these chemicals have a high toxicity level which is dangerous to human life. Farmers should know precautionary measures when using the chemicals. The best tool to use when using agrochemicals in small scale farming is the multipurpose sprayer (knapsack). The farmer should be able to calibrate it as well as operating it for successful control of pests, diseases and weeds. 

Respiration

Respiration is the release of energy by the breakdown of glucose in the presence of oxygen. There are two forms of respiration:

Aerobic respiration (with oxygen)

Aerobic respiration is the complete breakdown of food to release energy. The food is burned using oxygen to release energy, carbon dioxide and water. Aerobic respiration occurs in the mitochondria found inside cells. The process can be represented by a simple equation:

Word equation of Aerobic respiration

Glucose + oxygen        →        carbon dioxide + water + energy

Chemical equation of Aerobic respiration

Anaerobic respiration (without oxygen)

In some situations, respiration takes place without oxygen. This type of respiration occurs in the cytoplasm. In anaerobic respiration the food is not completely broken down and very little energy is released from the food. It usually occurs in plants that grow in waterlogged soils with little supply of oxygen. Internal parasites are also other examples of organisms which respire anaerobically in an oxygen lacking environment.  The products of anaerobic respiration in animal cells are lactic acid, carbon dioxide and energy. The equation for this type of respiration is:

Word equation of Anaerobic respiration

enzymes

Glucose              lactic acid + carbondioxide + energy (ATP) Adenosine

Triphosphate

enzymes

Chemical equation of Anaerobic respiration

C6H12O6(aq)

2C2H5OH + 2CO2 + 2ATP (energy)

In some plants and micro-organisms, the products are ethanol, carbon dioxide and energy. People have used yeast over a long period of time to make bread and alcohol. If a yeast is provided with a source of most food and warmth it will multiply and respire actively. The single-celled organism breaks down carbohydrates to produce carbon dioxide, alcohol and energy. The process can be represented with an equation:

Word equation

glucose         ethanol + carbondioxide + energy (ATP)

Chemical equation

C6H12O12

2C2H5OH + 2CO2 + energy

The Plant cell

A plant cell is defined as a cell which has a nucleus and organelles bounded by a thick and ridged cell wall. The organelles include mitochondria and cytoplasm.

Importance of respiration

• It provides energy for plant processes.
• It produces carbon dioxide that can be used for photosynthesis.
• It assists germination of seeds.
• It produces heat that warms the plant.

Comparison between aerobic and anaerobic respiration

Experimenting on respiration

An experiment can be carried out using germinating seeds to show that respiration requires oxygen to take place. We can use apparatus known as the respiroscope to demonstrate respiration.

Some germinating seeds are put in the respirometer bulb. Then the tube is inserted into a beaker filled with mercury. A pellet of potassium hydroxide is placed into the tube.

The controled experiment is set up with the potassium hydroxide pellet but without germinating seeds in the bulb. The mercury level will rise.

Results of the experiment to test respiration

Oxygen in the apparatus is used up in respiration. This reduce the amount of air pressure inside the bulb thereby raising the level of mercury. The essence of potassium hydroxide is to absorb carbon dioxide to remove the assumption that it may be absorbed by the seeds. 

Compare anaerobic and aerobic respiration.

Plant tropisms (responses of plants)

Plants do not respond to their surroundings in the same way as the animals do. They take no notice of loud noises, and make no effort to escape from enemies. But plants do respond to things that are important to them. They respond by growing in a certain direction.Agrowth response of this kind is called a tropism.

Types of tropisms

Geotropism

This is a response to gravity. The roots respond by growing towards the pull of gravity. The shoots respond by growing away from gravity. We say that roots are positively geotropic and shoots are negatively geotropic.

Phototropism

This is a response to light. Plants shoots respond by growing directly towards the light. We say they are positively phototrophic. The main source of light is the sun and the plant shoots grow upwards in order to expose the leaf which contain chlorophyll for trapping light.

Hydrotropism

This is the response to water shown by roots. They tend to grow towards a source of water in the soil.

 Haprotropism

This is the response to touch shown by climbing plants. If they come in contact with any support like the stem of another plant they grow tightly round it and hold on. This gives them support for their own weak stems.

Setting of an experiment to demonstrate hydrotropism

Steps

1. Fill a bucket with soil.
2. Insert another small porous container with water inside the bucket at the corner.
3. Plant a seed in the bucket opposite the porous container.
4. Observe the plant after germination to check the direction of growth.

Importance of plant tropisms

• Tropisms are useful to plants because their response help them to survive for example, geotropism causes roots to penetrate the soil where they will gain support and find water and nutrients.
• Negative geotropism causes shoots of germinating seeds to grow up through the soil to reach the surface.
• Phototropism causes shoots to reach up to where they will receive as much light as possible. This is important to them as they need light for photosynthesis.
• Hydrotropism enable roots to find a good supply of water under the ground.
• Haptotropism helps climbing plants to grow upwards to the light. Without the support of nearby plants they could not do this and they might die.

Crop Protection

Precautions to take when using agrochemicals and storing them

• Wear protective clothing such as overalls, goggles, rubber boots, dust masks and rubber gloves.
• Read instructions carefully before using the chemicals in order to follow recommendations of the manufacturer.
• Do not blow blocked nozzles with your mouth.
• Do not spray against the wind direction for personal safety.
• Do not eat, drink, smoke and snuff during chemical application.
• Do not apply chemicals when temperatures are too high to avoid volatilisation.
• Do not spray on a windy day to avoid wind drift.
• Do not harvest sprayed crops before withdrawal period.
• Do not wear trousers with turn ups where granules or dust particles can collect.
• Avoid inhaling spray and dust in confined places such as store rooms.
• Wash the sprayer and other equipment used in spraying thoroughly using clean water and a detergent.
• In case a farmer has inhaled, ingested or had a skin contact with a chemical, advice should be sought from a medical doctor. The doctor should be told the active ingredient of the chemical or take the original container to the Doctor

Indications of agro-chemical toxicity levels

Agro-chemicals or pesticides are graded depending on their toxicity level that means how poisonous or dangerous they are. There are four main groups of toxicity level and every chemical should have a label with its danger level clearly shown. The labels are colour coded as shown below.

 Showing toxicity level of agro-chemicals

The following colors on triangles are used to demostrate different toxicity levels.

Green                                Orange                                  Red                                   Purple

Practically all pesticides are harmful to humans to some extent. They can cause vomiting, anaemia, headaches, damage to internal organs, breathing difficulties, can be corrosive and can result in death depending on the degree of toxicity and the quantity absorbed by the body. Pesticides can be absorbed through the skin, by inhalation and by swallowing. Some pesticides are very persistent (remain in their original state for long period). Some of them can be taken up by plants which may be eaten later by humans or animals. The chemicals are then stored in the body where their level could build up to a toxic concentration.

Explanation of colours of warning triangles

Green triangle

It denotes the least harmful group of chemicals. Precautions are still necessary in the use of these as they could be harmful if misused.

Orange (amber) triangle

It is the second most toxic group of chemicals. It can be harmful if used incorrectly.

Red triangle

It shows a chemical which is less dangerous than a purple triangle though it is still highly toxic. The label bears the words very poisonous.

Purple triangle

It indicates the most dangerous type and has on it the words extremely poisonous.

The knapsack sprayer

This is a hand operated multipurpose sprayer that can be carried at the back or on the shoulder.

Calibration

This is the calculation done to determine the total discharge of chemical solution per given area. The common chemical application tool used by the farmers is the multipurpose knapsack sprayer.

The factors to consider when calibrating a multipurpose sprayer are:

• Spray width of the knapsack
• Nozzle output
• Speed of applier

Reasons for calibration

• To avoid over-application and under application of the chemical.
• To improve the effectiveness of the chemicals
• To have a uniformity in chemical application.

Calibrating a Knapsack sprayer using the following materials to come up with the required amount of water needed per hector

• Knapsack sprayer
• Measuring cylinder
• Stop watch
• Tap measure
• Water
• Container

Steps

• Fill the knapsack with one litre of water
• Spray on dry hard surface where you can be able to see the area covered. Walk forward with operation speed until the sprayer is empty.
• Measure the area covered by multiplying swath by length.
• Multiply the capacity of a knapsack sprayer by the area covered when spraying one litre. This is done to find area covered by knapsack sprayer.
• To come up with number of knapsack sprayers per hectare divide 10000 square metres (a hectare) by the area covered by one knapsack sprayer.

Here is what we have discussed in this topic

• Respiration is the series of chemical changes which release energy from food materials.
• Aerobic respiration is the complete breakdown of food using oxygen to give out energy.
• Anaerobic respiration is the chemical process which releases less energy from incomplete breakdown of food in the absence of oxygen.
• Phototropism is the growth of a plant in the response to sunlight.
• Geotropism is the growth of a plant in response to force of gravity.
• Hydrotropism is the growth of a plant in response to water source.
• Stimuli responded by plants include light, water, touch and gravity.
• Precautions should be observed to avoid unnecessary poisoning and death during spraying.
• Agrochemicals have different levels of toxicity shown by the colours of the triangle. The effectiveness of a chemical is determined by calibration of the knapsack sprayer before application of the chemical.

Definitions

• Agrochemical - Any chemical used in agriculture for the control of pests, weeds and diseases.
• Toxicity - Degree of danger of a chemical.
• Calibration - Is the calculation of chemical solution required per given area when spraying at a known speed.