Raising yield from a fixed area of land

1. Yield is the amount of useful product harvested from a given area of crop or from a given number of animals.
2. The area of land that can be farmed is limited, and clearing more wild land destroys habitats, so the food supply is raised mainly by getting more from each hectare already in use.
3. Anything that is holding growth back acts as a limiting factor, so raising yield means finding what is in short supply and supplying it.
4. For a crop the usual limits are light, carbon dioxide, temperature, water, mineral ions, and losses to pests, disease and weeds.
5. Agricultural solutions attack those limits directly, by controlling conditions, replacing mineral ions, removing pests and weeds, and reducing disease.
6. Selective breeding and gene technology raise yield by a different route, changing the plants and animals themselves rather than the conditions they grow in, and each is set out in its own article.

Fertilisers replace lost mineral ions

1. A fertiliser is a substance added to soil or to a nutrient solution to supply the mineral ions that plants need in order to grow.
2. In a natural ecosystem, dead material is decomposed and the mineral ions it contains return to the soil, but on a farm the crop is harvested and carried away, so those ions leave the field for good.
3. Without replacement the soil is gradually stripped of the ions the crop needs most, and each year's yield is lower than the last.
4. Nitrate ions are supplied because a plant uses nitrogen to make amino acids, and therefore to make the proteins needed for new cells and growth.
5. Phosphate and potassium ions are supplied alongside nitrate, and a fertiliser sold as NPK contains all three.
6. Magnesium ions are needed as well, because magnesium is part of the chlorophyll molecule and a shortage of it lowers the rate of photosynthesis.
7. Fertiliser can be applied as manure and compost, which release ions slowly as decomposers break them down, or as manufactured granules, which dissolve quickly and act fast.
8. Applying more fertiliser than the crop can take up wastes money, and rain washes the surplus into rivers and lakes where it causes the water to become choked with algae.

Pesticides kill the organisms that damage crops

1. A pesticide is a chemical applied to a crop to kill the organisms that feed on it or damage it.
2. Insecticides kill insect pests, fungicides kill fungi that cause plant disease, and herbicides kill weeds that compete with the crop for light, water and mineral ions.
3. Killing the pest means more of the crop survives to be harvested and more of its biomass goes into the parts people eat, so yield rises.
4. Pesticides are quick to apply over a large area and act within days, which is why they are still the most widely used method of crop protection.
5. A pesticide rarely kills only the target species, so pollinating insects and the natural predators of the pest are killed alongside it.
6. A pesticide that does not break down easily is eaten with the tissue of the organisms it is in, and its concentration builds up along a food chain, so top predators receive the largest dose.
7. Pests also vary, and any individual with an allele giving resistance survives the spray and passes that allele on, so repeated use selects for a resistant population and the pesticide stops working.
8. Pesticide applied close to harvest can leave residues on food, so there are legal limits on which chemicals may be used and how long before harvest they may be applied.

Biological control uses a natural enemy

1. Biological control is lowering the number of a pest by introducing a natural predator, parasite or pathogen of that pest.
2. The control organism eats or infects the pest, the pest population falls, and less of the crop is damaged.
3. Ladybirds and their larvae are released to feed on aphids, and the parasitic wasp Encarsia formosa is released in glasshouses, where it lays its eggs inside whitefly larvae and its own larvae kill them from within.
4. Because the control organism reproduces, one release can keep working for a whole growing season without further cost.
5. No chemical is sprayed, so there are no residues on the food, no damage to the wider community of insects, and no selection for chemical resistance.
6. Biological control is slower than spraying, and it never removes the pest completely, because the control organism needs some pest left to feed on.
7. The pest and its enemy settle into a cycle in which each population rises and falls after the other, and the grower accepts a low level of damage rather than none.
8. Introducing a species also carries a risk, because a control organism with no natural enemy of its own may attack harmless native species or become a pest itself.

Hydroponics grows plants without soil

1. Hydroponics is growing plants with their roots in a nutrient solution instead of in soil.
2. The roots sit in a shallow flowing film of solution, or in an inert material such as rockwool that the solution is pumped through, and the plants are held upright by a support.
3. The concentration of every mineral ion is measured and adjusted, so the plant is never short of nitrate, phosphate, potassium or magnesium, and none is wasted.
4. There are no weeds, so nothing competes with the crop for light, water or mineral ions, and no herbicide is needed.
5. Soil-borne pests and diseases are absent because there is no soil, which cuts the need for pesticides.
6. The solution is collected and recirculated, so hydroponics uses far less water than irrigating a field, where most of the water drains away or evaporates.
7. Crops can be grown on land with poor or contaminated soil, and can be stacked in layers indoors, so a large growing area fits into a small footprint.
8. Hydroponics is usually combined with a glasshouse or polytunnel, where the temperature is kept near the optimum for the enzymes controlling photosynthesis, artificial lighting extends the day, and the carbon dioxide concentration is raised above its level in the air.
9. The costs are high, because pumps, lamps, heaters and monitoring equipment all use energy and the equipment is expensive to install.
10. Disease is also a serious risk, because every plant shares one body of solution, so a pathogen that reaches the tank is carried to every root in the system.