Managing the yield of crops

Yield means the amount of useful crop a farmer harvests — for example the mass of tomatoes picked from a field or the number of lettuces grown in a season. Farmers naturally want to get the highest yield they can from their land.

The key idea of this whole topic is simple: a plant grows by photosynthesis, the process that makes glucose (food) using light, carbon dioxide and water. The faster a plant photosynthesises and the more minerals it has, the more it grows — and the more it grows, the higher the yield.

So to increase yield, farmers try to give crops the best possible conditions for photosynthesis and growth:

• enough light
• a warm enough temperature
• plenty of carbon dioxide
• enough water
• the right mineral ions in the soil

Two main methods are tested in this topic: growing crops inside glasshouses and polythene tunnels (which control the conditions), and adding fertilisers (which supply mineral ions).

Exam tip. When a question asks how a method increases yield, always connect it back to more photosynthesis → more growth → higher yield.

A glasshouse (greenhouse) is a building made of glass; a polythene tunnel (polytunnel) is a curved frame covered with clear plastic sheeting. Both work in the same basic way: they let light through but trap heat inside, keeping the crops warm.

How do they increase yield?

• They trap heat, so the air and soil stay warmer, especially in cold weather or at night. A warmer temperature speeds up photosynthesis (up to a limit), so plants grow faster.
• They shelter crops from wind, heavy rain, frost and pests, so fewer plants are damaged.
• They allow crops to be grown out of season or in colder climates where they would not normally survive.
• Inside a glasshouse the farmer can control the conditions very precisely — adding artificial light, heaters, extra carbon dioxide and watering systems.

Because the farmer can keep the temperature, light, water and carbon dioxide near the ideal levels for photosynthesis, the crops grow as fast as possible and the yield increases.

Exam tip. Polythene tunnels and glasshouses are usually credited for the same reasons (trap heat → warmer → faster photosynthesis → more growth → higher yield, plus shelter from weather/pests). Glasshouses allow more precise control of CO₂, light and heat.

Inside a glasshouse, a farmer can increase the carbon dioxide concentration and increase the temperature. Both of these change the rate of photosynthesis, and a faster rate means more food made and more growth.

Carbon dioxide. Carbon dioxide is one of the raw materials for photosynthesis. If there is more carbon dioxide available, the plant can photosynthesise faster — so farmers sometimes add extra CO₂ to the air in a glasshouse (for example by burning fuel). But carbon dioxide only helps up to a point: once it is no longer the limiting factor (because something else such as light or temperature runs short), adding more CO₂ makes no further difference, and the rate levels off.

Temperature. Photosynthesis is controlled by enzymes, which work faster as it gets warmer. So increasing the temperature speeds up photosynthesis — but again only up to a limit (an optimum). If it gets too hot, the enzymes are denatured (their shape is damaged) and the rate of photosynthesis falls. So a glasshouse must be kept warm but not too hot.

You can picture both effects like this:

The result for the farmer: keeping carbon dioxide and temperature at their ideal levels keeps photosynthesis fast, so the crop grows quickly and the yield increases.

Exam tip. The two magic phrases are "up to a limit / optimum" and, for temperature, "too hot → enzymes denatured → rate falls". Don't say the rate keeps rising forever.

Plants need mineral ions from the soil for healthy growth, as well as the glucose they make in photosynthesis. As crops grow, they use up the minerals in the soil. A fertiliser is a substance added to the soil that replaces these mineral ions, so the plants can keep growing well and the yield increases.

The two mineral ions you must know are:

• Nitrate ions — needed to make amino acids, which join together to make proteins. Proteins are needed for growth and to make new cells. Without enough nitrate, plants show stunted growth and yellowing of older leaves.
• Magnesium ions — needed to make chlorophyll, the green pigment that absorbs light for photosynthesis. Without enough magnesium, leaves turn yellow (because they cannot make enough chlorophyll), so they photosynthesise less.

So a fertiliser that contains nitrate and magnesium lets the plant build more proteins and make more chlorophyll, leading to more photosynthesis and more growth → higher yield.

Drawbacks of fertilisers. Using fertiliser is not free of problems:

• Cost — fertilisers (and glasshouses) cost the farmer money, so the extra yield must be worth the extra expense.
• Eutrophication — if too much fertiliser washes off the fields into rivers and lakes (in rainwater run-off), the extra nitrate makes algae and water plants grow rapidly. When these die, bacteria decompose them and use up the oxygen in the water, so fish and other organisms may die from lack of oxygen. This pollution of waterways is called eutrophication.

Exam tip. Learn the two ion → use links exactly: nitrate → amino acids/proteins → growth, and magnesium → chlorophyll. For the drawback, link excess fertiliser → run-off into water → eutrophication.