Fertilizers and natural productivity at Higher Level: Different examples from SL: China's fertilizer use, Lake Taihu, the Murray-Darling basin, urine fertilizer in Vermont, Faidherbia trees and organic Sikkim. At HL, go on to regenerative farming and permaculture (5.2.15).
Practise this as you read
- Explain why intensive systems depend on synthetic fertilizer.
- Evaluate natural ways of restoring productivity.
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Put back what the harvest takes: Every harvest carries nitrogen (N), phosphorus (P) and potassium (K) off the farm. Intensive farms that crop the same land every year, with no animals, replace them with synthetic fertilizer to keep their high commercial yields.
The points to remember
- Crops take N, P and K from the soil; selling the crop removes them for good.
- Intensive systems crop every year, with no fallow and often no manure.
- Synthetic fertilizer (made in factories) puts nutrients back fast and in exact amounts.
- It keeps commercial productivity high, but at the expense of sustainability.
Example (HL): China. China uses roughly a third of the world's synthetic fertilizer on under a tenth of its farmland. Much is wasted: studies found farmers often used far more nitrogen than their crops could take up.
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Synthetic fertilizer keeps yields up, but much of it never reaches the crop. What is lost harms water, soil and air.
The points to remember
- Water: runoff and leaching add nitrate and phosphate to rivers: eutrophication.
- Drinking water: nitrate in wells and rivers must be removed at a cost.
- Soil: acidification, salt build-up, less organic matter and fewer soil organisms.
- Air and climate: soils release nitrous oxide, a powerful greenhouse gas.
- Energy: making it burns natural gas, so it is fossil-fuel dependent.
Remember it as: Fertilizer lost = algae in water, acid in soil, gas in air.
Farms
- Fertilized rice and vegetable fields and fish ponds surround Lake Taihu, China's third largest lake.
Nutrients
- Nitrogen and phosphorus wash in from the fields, with sewage and factory waste.
Bloom
- In May 2007 a huge bloom of cyanobacteria covered the lake.
Harm
- The city of Wuxi stopped its tap water for about a week; around 2 million people had to buy bottled water.
Synthetic fertilizer is part of a wider intensive system. That system affects soil, water and wildlife too. In the exam, link each cause to its effect.
Soil
- Bare fields lose topsoil to wind and rain: erosion.
- Cropping the same field every year depletes nutrients.
- Heavy machines compact the soil, so water cannot soak in.
- Ploughing speeds decay: organic matter falls.
Water
- Fertilizer runoff: eutrophication.
- Irrigation drains rivers, lakes and aquifers: water scarcity.
- Irrigation in dry areas leaves salt: salinization.
- Pesticides wash into streams and groundwater.
Wildlife
- Clearing land for farms is the biggest cause of biodiversity loss.
- Monocultures leave little habitat; hedges are removed.
- Pesticides kill non-target species, such as bees.
Example (HL): the Murray-Darling basin, Australia. Clearing native trees for wheat and sheep and heavy irrigation raised salty groundwater to the surface. Large areas became too salty to farm, and salt has to be pumped out of the rivers to protect Adelaide's water supply.
Cause, then effect: Write each impact as a chain: the farming activity, what it does to the environment, the result. 'Fertilizer runoff adds nitrate to rivers, causing algal blooms and oxygen loss' scores; 'pollution' alone does not.
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Food production releases about a quarter of the world's greenhouse gases. Each gas has its own farm source. Cattle make methane by enteric fermentation.
The points to remember
- Methane: cattle and sheep belching (enteric fermentation), rotting manure, flooded rice paddies.
- Nitrous oxide: released from soils given synthetic fertilizer or manure.
- Carbon dioxide: fuel for tractors, irrigation pumps, processing and transport.
- Land-use change: clearing forest for pasture removes a carbon sink.
- Eroding and ploughed soils release their carbon as CO2.
Remember it as: Cows and rice: methane. Fertilizer: nitrous oxide. Machines and clearing: CO2.
Example (HL): Denmark's livestock tax. In 2024 Denmark agreed to become the first country to tax the greenhouse gases from farm animals, starting in 2030. The money helps farmers cut emissions, for example by changing feed or planting forest on old farmland.
Sustainable farms keep soils fertile in ways that rebuild natural productivity, rather than buying it in a bag. One is humanure.
The points to remember
- Fallowing: rest a field for a season or more so nutrients and organic matter build up again.
- Organic fertilizer: manure from farm animals, compost, or humanure.
- Legumes (beans, peas, clover): bacteria in their roots fix nitrogen from the air.
- Crop rotation: change the crop each year so no nutrient is used up and pests cannot build up.
- Herbal mixed leys: a field of grasses, clovers and herbs grazed for a few years before crops.
Remember it as: Rest it, feed it, fix it, rotate it.
Example (HL): urine fertilizer in Vermont, USA. The Rich Earth Institute collects urine from volunteers, makes it safe by heating it, and spreads it on hay fields. Urine holds most of the nitrogen and phosphorus people eat, so it can replace some synthetic fertilizer.
Fallow must not be bare: A field left bare between seasons can lose soil to wind and rain and nutrients to leaching. A fallow helps only if plants, crop stubble or a cover crop protect the soil.
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Some methods work with living things in and above the soil, so the soil stays covered and fed all year. Mycorrhizae are one of them.
The points to remember
- Mycorrhizae: root fungi that pass phosphorus and water to the plant in return for sugars.
- Ploughing and heavy phosphate fertilizer harm mycorrhizae; less disturbance helps them.
- Continuous cover forestry: trees are cut singly or in small groups, so the soil is never bare.
- Agroforestry: trees grown with crops or animals; leaves add organic matter and roots hold soil.
Example (HL): Faidherbia trees in Malawi. Faidherbia albida sheds its leaves just as maize is planted. Maize grown under these trees can give up to two to three times the yield of maize grown without them, with no fertilizer bought.
Synthetic fertilizer
- Fast, exact, high yields; easy to spread.
- Feeds the crop, not the soil: organic matter and soil life decline.
- Losses cause eutrophication and nitrous oxide.
- Made with natural gas; price follows gas prices.
Natural methods
- Build organic matter, soil life and water-holding.
- Use local, cheap inputs: manure, legumes, trees.
- Release nutrients slowly; yields are often lower at first.
- Need more land, labour and knowledge.
The points to remember
- To evaluate, give both sides, then a conclusion.
- A sudden switch can cut yields and food supply.
- Many farms combine them: less fertilizer, plus manure, legumes and rotation.
Example (HL): Sikkim, India. In 2016 Sikkim became India's first fully organic state after phasing out synthetic fertilizer over more than ten years. Its soils and organic produce gained, but yields of some crops fell and the state buys much of its food from elsewhere.
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How this comes up: Paper 2 Section A gives a graph and asks you to describe it [2], then explain a change [2].
The graph shows the chemical fertilizer used on farms in China from 1980 to 2020. In 2015 the government set a plan for no further growth in fertilizer use.
(a) Describe the change in fertilizer use shown. [2]
(b) Explain why the government wanted to reduce fertilizer use. [2]
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