Food production and crop yield

Types of agriculture: what the words mean

Farming is described in several different ways at once, and the words are not alternatives to each other. Arable farming grows crops on cultivated land; pastoral farming rears livestock, usually on land that cannot be ploughed, such as hill pasture, moorland or semi-arid grassland; mixed farming does both on the same holding, which spreads the farmer's income across two products and lets the manure from the animals be returned to the arable land as fertiliser. A farm in Devon with dairy cattle and barley fields is mixed, and so is a smallholding in Malawi with a few goats, a maize plot and a vegetable patch.

Subsistence farming produces mostly for the farmer's own family. The plot is usually small, the work is done with hand tools or draught animals, few inputs are bought and there is little or no surplus to sell, so one bad season means the family goes hungry. Commercial farming produces mainly to sell, so it needs a market, roads, transport and storage as well as the land itself, and it usually means larger fields and more bought inputs such as seed, fertiliser, machinery and irrigation. The two are not exclusive: a smallholder may grow food for the family and sell a small surplus of vegetables for cash.

Intensive and extensive describe how much is put in and how much comes out of each hectare. Intensive farming applies very large amounts of labour, capital, water, fertiliser or machinery to a small area to obtain a high yield per hectare, like a glasshouse of tomatoes in the Netherlands or a market garden on the edge of a city. Extensive farming covers a very large area with few inputs per hectare, which gives a low yield per hectare, like cattle ranching on the dry grasslands of Patagonia. This is the commonest confusion in the exam: intensive means high inputs per hectare, not a large farm.

Intensive farming, monoculture and sustainable food production

Intensive farming exists for a good reason: a growing population needs more food, and the land available for farming is not increasing. If each hectare yields twice as much, the same harvest comes from half the land, leaving grassland and forest unconverted. Intensive systems also cut risk, because irrigation, fertiliser and a controlled environment remove much of the uncertainty of the weather. The cost is that they depend on bought inputs of fuel, fertiliser, pesticide and machinery, and that the environmental damage they do is concentrated in the areas where they are practised.

Monoculture is the growing of a single crop over a large area, season after season. It is efficient: one set of machinery, one herbicide programme, one fertiliser recipe and one harvest date can be planned for the whole field, which lowers the cost of every tonne. The same uniformity is the weakness. A pest or disease that can attack the crop finds an unbroken field to spread through with nothing to stop it, the same nutrients are removed in every harvest, the soil lies bare between harvest and drilling, and a field with only one crop offers almost no habitat for the pollinators and the natural predators of pests that a mixed farm keeps around.

Sustainable food production is defined as the production of sufficient food for the present generation using methods that ensure future generations can grow food from the same land. The definition has two halves and both are examinable. The first is about demand: people alive now must have enough to eat. The second is about time: the soil, the water, the biodiversity and the nutrient store of that land must still be able to produce a crop for the people who follow. Neither half alone is a complete answer, and it is the second half that rules out practices which raise the yield this year by using up the land's ability to yield next year.

• Intensive farming: high inputs per hectare, high yield per hectare, high dependence on bought inputs.
• Monoculture: a single crop over a large area, cheap to manage but easy for a pest or disease to sweep through.
• Mixed cropping: several crops in the same field with no fixed pattern, so the risk of a failed harvest is spread.
• Sustainable food production: enough food now, from methods that leave the same land able to grow food later.

Cropping systems and irrigation: more from the same land

Mixed cropping is the growing of two or more crops in the same field at the same time in no fixed pattern; if one crop fails, the other still gives a harvest, and the ground is covered for more of the year. Intercropping is more deliberate: the crops are arranged in rows or strips, often a cereal with a legume, so that the legume fixes nitrogen for its partner, the canopy shades out weeds, the roots hold the soil and the farmer gets two products from one field. Neither system needs a single gram of bought fertiliser to work, which is why both are common on smallholdings where cash for inputs is limited.

Crop rotation takes the sequence rather than the spacing. Different crops are grown one after another on the same land, so a pest or disease that lives on one crop starves or is left with no host when its crop is absent. Crops that feed at different depths use nutrients from different layers of the soil, and a legume such as beans or a clover ley adds nitrogen and organic matter instead of taking it out. The opposite of rotation is growing the same crop every year, which is how soil-borne pests such as root eelworm build up to the level where the crop fails.

Water is usually the largest single limit on yield. Where rainfall is seasonal, the growing season stops when the water runs out, so irrigation both raises the yield of the existing crop and can allow a second crop in the same year. The traditional method is flood irrigation, sometimes called surface irrigation, in which water is released to run across the whole field. It is cheap and needs little equipment, but much of the water evaporates from the wet surface or runs off the lower end, and on heavy soil it can fill the pore spaces so completely that the roots cannot get oxygen.

Trickle irrigation, also called drip irrigation, delivers water slowly to the base of each plant through a network of pipes and small emitters. Almost none is lost to the wind or to the bare ground between the rows, so the same crop can be grown with a small fraction of the water used by flooding, and fertiliser can be added to the water so it reaches the roots directly. It was developed in Israel for the dry Negev and is now used on high-value crops worldwide, but it costs money to install and needs pipes, filters, power and maintenance, and the emitters block if the water is not clean. Rainwater harvesting collects runoff from roofs, roads or a prepared catchment into tanks, ponds or bunded fields for use in the dry season; it is cheap, locally built and keeps water out of the drains, but it depends on the rain falling, the water must be stored somewhere, and open tanks lose water to evaporation and silt up. Automated watering systems use soil moisture sensors and timers to water only when the crop needs it, which saves both water and labour, but they rely on electricity and technical skill, and a sensor fault can either starve the crop or flood it.

• Mixed cropping: several crops together with no fixed pattern, which spreads the risk of failure.
• Intercropping: crops in rows or strips, often cereal with legume, so nitrogen is fixed, weeds are shaded out and the soil is covered.
• Crop rotation: a different crop each season, so soil-borne pests lose their host and a legume returns nitrogen.
• Drip irrigation: water and nutrients delivered to the base of each plant, with very little lost to evaporation or runoff.
• Rainwater harvesting: roof and ground runoff stored in tanks, ponds or bunded fields for the dry season.
• Automated watering: sensors and timers apply water only when the soil is dry enough to need it.

Mechanisation, controlled environments and genetically modified organisms

Mechanisation means replacing human and animal labour with machinery. A tractor and seed drill sow in a few days what would take weeks by hand, which allows a larger area to be farmed and gets the seed into the ground at the right moment, and a combine harvester brings in a crop before the weather breaks. The limits are the cost of the machine, fuel, spare parts and a trained operator, and the fact that heavy machinery compacts the soil when it is driven over wet ground, squeezing out the pore spaces the roots need. Mechanisation also removes paid work from the countryside, so fewer people earn a living from the land.

A controlled environment removes the weather from the equation altogether. A greenhouse holds the temperature and humidity steady, keeps out pests and extends the growing season into the colder months. Hydroponics grows plants with their roots in a nutrient solution, with or without an inert medium such as gravel or rockwool, so no soil is needed at all and the water and nutrients that are not taken up can be collected and reused. Aeroponics goes further and suspends the roots in air, misting them with nutrient solution, which uses even less water. All three give very high yields per square metre and can be placed on land that could never be farmed, but they need large capital investment and a reliable electricity supply, they need technical skill because a failed pump can kill a crop in hours, and they cannot economically produce staple cereals such as wheat, rice and maize.

A genetically modified organism is one whose genetic material has been changed by removing, changing or inserting individual genes to give it favourable characteristics. The best-known example moves a gene from the soil bacterium Bacillus thuringiensis into cotton, so the plant itself produces a protein that kills the larvae of certain moths; this is Bt cotton, grown widely in India and China. Other modifications give resistance to a herbicide, tolerance of drought or of salty soil, or a longer shelf life. The advantages are a higher and more reliable yield, less pesticide sprayed and a crop that can be grown where it previously failed.

The objections are practical as well as ethical. The farmer must buy new seed every season rather than saving it from the harvest, which ties a smallholding into a cash cost it may not be able to meet. Pests can evolve resistance to a toxin that is present in every plant all season, and the pink bollworm has done exactly that in parts of India, so the advantage is not permanent. Genes can move by pollen into wild relatives and into neighbouring farms, and a field of one modified variety has the same uniformity problem as any monoculture. Critics add that a higher yield does not by itself put food on the plate of a family with no money to buy it, so the technology does not answer the question of access.

• Mechanisation: larger areas, timely sowing and harvesting, less drudgery; but costly, fuels soil compaction and reduces rural employment.
• Greenhouse: temperature, water and pests controlled, growing season extended; needs capital and energy.
• Hydroponics and aeroponics: no soil, water and nutrients recycled, very high yield per square metre; but high capital and skill, and unsuitable for staple cereals.
• GMO: genes removed, changed or inserted to give favourable characteristics such as pest resistance; needs bought seed each season and resistance can develop.

Fertilisers: inorganic NPK and organic matter

Every harvest removes nutrients from the field: nitrogen in the grain and leaves, phosphorus and potassium in the crop that is carted away. If those nutrients are never replaced, the store in the soil falls year by year and the crop gets smaller, whatever else the farmer does. Inorganic NPK fertiliser supplies the three main nutrients in a form the plant can take up at once: nitrogen as nitrate ions, phosphorus as phosphate ions and potassium as potassium ions. The farmer can choose a mixture to suit the crop and the soil analysis, and the response is visible within days, which is why NPK fertiliser is the single biggest reason cereal yields rose so steeply in the twentieth century.

Organic fertilisers work in a different way. Crop residue is what is left in the field after harvest and is either ploughed in or left as a surface mulch; manure is animal dung and bedding, which recycles nutrients that would otherwise be wasted and is the reason mixed farming is more efficient than arable farming alone; mulch is a layer of straw, cut grass, leaves or compost spread over the soil surface. Organic material holds its nutrients in complex molecules, so they are released slowly by soil organisms rather than dissolving all at once, and it adds the organic content that gives soil its crumb structure, its water-holding capacity and the living organisms that break down residue.

The two are not rivals so much as different tools. Inorganic fertiliser gives a known concentration, an immediate effect and a mixture that can be matched to a deficiency, but it adds no organic matter and is manufactured using large amounts of energy, mainly natural gas. Applied at more than the crop can take up, its soluble nitrate is carried away by rain and irrigation, which wastes the farmer's money and pollutes the water. Organic fertiliser builds the soil itself, feeds the organisms in it, covers the ground and reduces both evaporation and erosion, but it is bulky to move, its nutrient content varies from load to load, and it may carry weed seeds and pathogens unless it has been composted properly. The best results come from using both: return organic matter to keep the soil in condition, and top up with inorganic nutrients when a soil test shows a shortage.

Controlling pests: chemicals and biological control

A pest is any organism that reduces the value of the crop. Three groups are named in the syllabus: insects that eat or suck the plant, weeds that compete for light, water and nutrients, and fungi that rot the plant or the stored harvest. Pesticides are the chemicals used against them, and the names follow the target: insecticide for insects, herbicide for weeds and fungicide for fungi. Their advantage is speed and certainty. A farmer who sees an insect infestation spreading through a valuable crop can stop it within days, and a herbicide allows a large area to be kept weed-free without weeks of hand hoeing.

Overuse of pesticides creates exactly the problems that make the farmer need more of them. Pesticide resistance develops because a few individuals in the pest population carry genes that let them survive the spray; they breed, and the next generation contains a higher proportion of survivors, so the dose that once worked stops working. Pest resurgence happens because a broad-spectrum insecticide kills the pest's natural enemies as well as the pest, and the predators often take longer to recover; the pest then returns without anything to hold it down and reaches even higher numbers than before. Pollinators such as bees are killed by the same sprays while they work the crop, so the flowering crop and the orchards nearby set less fruit, and the chemicals can drift into hedgerows, ponds and streams where they harm species that were never the target.

Biological control uses a living organism instead of a chemical. Ladybirds, lacewings and parasitic wasps feed on aphids and are either released or encouraged by planting strips of flowers and leaving hedges in place; Bacillus thuringiensis is a soil bacterium that kills the larvae of certain moths and is sprayed like a pesticide but is far more selective; ducks herded through rice paddies eat insects and snails; pheromone traps either catch male moths or flood the field with the scent so that females cannot find a mate. The advantages are that the pest cannot easily develop resistance to a predator, the control agent does not persist in the water supply and the method is cheap once the population is established. The limitations matter just as much: biological control is slower than a spray, it depends on the predator surviving the winter and on the farmer leaving it somewhere to live, it works poorly in the middle of a sudden outbreak, and the agent can attack species it was not meant to. The cane toad was introduced to Queensland to control a beetle in the sugar cane and became a major pest itself, which is the classic warning.

Because no single method is reliable on its own, the usual recommendation is integrated pest management, sometimes shortened to IPM: rotate and intercrop so pests cannot build up, grow resistant varieties, monitor the crop with traps and count the pests, spray only when the pest passes the level at which the damage would cost more than the spray, and then choose the most selective chemical available and use the biological controls alongside it. The farmer spends more time, thought and training on the decision, and keeps the crop, the pollinators and the soil organisms at the same time.

• Insecticide kills insects, herbicide kills weeds, fungicide kills fungi.
• Resistance: survivors breed, so the chemical stops working and a higher dose is needed.
• Resurgence: the spray kills the pest's natural enemies too, so the pest returns in greater numbers.
• Biological control: ladybirds and parasitic wasps against aphids, Bacillus thuringiensis against moth larvae, ducks in rice paddies, pheromone traps.
• Integrated pest management: monitor, act only above the threshold, and combine the methods.

When farming damages the land and the people who depend on it

The same strategies that raise yield can damage the resource they depend on if they are applied without care. Irrigation is the clearest case. Water applied faster than the soil can drain fills the pore spaces and excludes the air the roots need, which is water logging; in a dry climate the water that evaporates from the surface leaves its dissolved salts behind, and the salt accumulates in the topsoil until the crop will not grow, which is salinisation. Bare sloping fields, cultivations that follow the slope and cleared vegetation together allow the topsoil itself to be washed and blown away, and it is the topsoil that holds the nutrients and the organic matter. The Aral Sea has become the textbook case: irrigation for cotton across the basins of the Amu Darya and the Syr Darya has drawn down the lake, waterlogged and salted the fields around it and left a dry, salt-crusted plain behind.

Fertiliser overuse damages water rather than land. Nitrate is very soluble, so the surplus the crop cannot take up is carried below the root zone by rain or irrigation water, which is leaching. It reaches groundwater, and it drains from fields into rivers and lakes, where the extra nutrients cause rapid growth of algae, an algal bloom on the surface. The algae die and are decomposed by bacteria that use up the dissolved oxygen, so fish and other animals suffocate; this whole sequence is eutrophication. Where a large river drains a heavily farmed basin the effect reaches the sea, and the seasonal low-oxygen area off the mouth of the Mississippi in the Gulf of Mexico is monitored for exactly this reason.

Land can be exhausted as well as polluted. Continuous cropping with no fallow, no rotation and no return of organic matter removes the inorganic ions faster than weathering and decay can replace them, and the organic content of the soil falls with it, so the structure crumbles and the soil holds less water. Removing the natural vegetation makes all of this worse: deforestation takes away the roots that held the soil and the leaves that broke the force of the rain, overcultivation works a small plot until it can produce nothing more, and overgrazing strips the grass that binds the surface so that wind and water carry it off. Monoculture and the intensification that goes with it add pollution from fertiliser and pesticide and remove the habitats and food sources that wild plants and animals need, which is why farmland birds and pollinators have declined steeply across Europe.

The people side of the topic is food security: whether everyone can obtain enough safe, nutritious food to live an active life. Demand rises with population and with income, and it is met or not met by what the land produces, what is imported and what is wasted. A very large share of the food that is produced is never eaten, lost between the field and the plate through poor storage, damage in transit or being thrown away in shops and homes, so a country can be short of food while still wasting it. Cash crops complicate the picture further: coffee, cotton, soya beans and palm oil earn export income that can pay for food imports and for fertiliser, but the land, water and labour given to them are not available for food grown locally, so a country can export valuable crops and still have households that cannot afford to buy food. Biofuel crops such as maize for ethanol in the United States or sugar cane in Brazil compete for the same good land, and when food crops are diverted to fuel the price of food tends to rise.