9.1 - Food Security

Fertilisers

• Plants need mineral ions from the soil to grow properly - in particular nitrogen, phosphorus, and potassium.
• When a crop grows, it takes these mineral ions out of the soil, leaving the soil short of them for the next crop.

Replacing lost mineral ions

• Farmers add chemical fertiliser to replace the missing mineral ions.
• This boosts plant growth, which raises the crop yield.
• Fertiliser is usually rich in nitrogen, phosphorus, and potassium (NPK) - the ions a growing crop needs most.

Pesticides and bioaccumulation

• Pesticides are a form of chemical pest control - sprayed onto crops to kill the pests that damage them.
• Fewer plants are lost, so the yield rises.

Types of pesticide

• Insecticides - Kill insects
• Herbicides - Kill weeds
• Fungicides - Kill fungi

Drawbacks of pesticides

• Often poisonous to humans, so they must be used carefully to keep the amount left in food below a safe level.
• Many harm wildlife that aren't pests, such as bees and ladybirds. This can leave less food for animals higher up the food chain.

Bioaccumulation

1. Some pesticides are not broken down and persist in the bodies of organisms that take them in.
2. When a sprayed plant is eaten, the pesticide passes into the animal, then into whatever eats that animal.
3. At each stage up the food chain, the concentration of pesticide in the organisms increases.
4. Eventually organisms at the top can receive a lethal dose.
5. This build-up of a toxic substance in living organisms along a food chain is called bioaccumulation.

Biological control

• Biological control is an alternative to chemical pesticides: instead of spraying poison, farmers use other living organisms to reduce the numbers of a pest.
• This is done either by encouraging organisms already in the area or by adding new ones.

Types of helpful organism

• Predators - Such as ladybirds that eat aphids
• Parasites - Such as flies whose larvae develop on or in a host insect and eventually kill it
• Disease-causing organisms - These infect the pest

Advantages over pesticides

• No chemicals used - Means less pollution and less risk to people eating the food
• No toxic chemicals passing along food chains
• Longer-lasting effect - Spraying often has to be repeated, but biological control can last longer

The risk of introducing new organisms

Biological control does carry a risk: introducing a new organism into an ecosystem can cause problems of its own.

Example: cane toads were brought into Australia to eat the beetles damaging crops, but the toads became a major pest themselves - they poison the native animals that try to eat them.

Glasshouses and polytunnels

• A glasshouse is a large greenhouse. A polytunnel is a big tube-shaped structure made from polythene.
• Both grow crops inside a controlled, enclosed space rather than out in the open.
• Enclosing the plants lets the farmer control several of the things that affect how well a crop grows.

What a farmer can control inside

• Pests and diseases - Easier to keep plants free from these when they're shut inside.
• Water - The water supplied to the crop can be controlled.
• Light - Artificial light can be supplied after the Sun goes down, giving plants more time to photosynthesise.
• Temperature - The structure traps the Sun's heat to keep plants warm, and in winter a heater can hold the temperature at the ideal level.

Boosting the rate of photosynthesis

• The rate of photosynthesis depends on how much light, carbon dioxide, and warmth a plant has.
• Inside the glasshouse the farmer can deliberately raise the carbon-dioxide concentration and the temperature to make the crop grow faster.
• A paraffin heater does both at once: burning the paraffin warms the glasshouse and gives off carbon dioxide as a by-product.
• With more light, a higher temperature, and a higher carbon-dioxide concentration, the rate of photosynthesis goes up - so the plants make more food and grow bigger and faster.

The effect on yield

• Controlling all these conditions together keeps the crop growing well all year round.
• As a result, the yield is higher than it would be growing the same plants out in the open.