Fertilisers and Eutrophication Revision Notes | A Level AQA Biology

1. Why Soils Run Out of Nutrients

Picture a wheat field harvested every year. Each time the crop is cut and removed, the nitrogen and phosphorus locked inside those plant tissues leave the field permanently. The same happens when livestock are taken to market — the nutrients they consumed from the pasture go with them. Over time, the soil becomes progressively depleted.

In a natural ecosystem, nutrients cycle continuously: organisms die, decomposers break them down, and minerals return to the soil. Farming interrupts this cycle at the point of harvest. Without intervention, crop yields fall year on year as the soil's mineral content declines.

The two most critical minerals lost are nitrates and phosphates. Nitrates are essential for synthesising amino acids and nucleic acids; phosphates are needed for ATP, DNA, and cell membranes. Replacing these is the core purpose of fertilisers.

2. Natural and Artificial Fertilisers

Farmers have two broad categories of fertiliser to choose from, and the distinction matters both agronomically and environmentally. Natural fertilisers include animal manure, slurry, compost, and sewage sludge. They release nutrients slowly as microorganisms decompose the organic matter, and they also improve soil structure and water retention.

Artificial fertilisers are manufactured inorganic compounds — ammonium nitrate ($NH_4NO_3$), ammonium phosphate, and potassium salts are common examples. They are highly soluble and release nutrients rapidly, giving farmers precise control over the concentrations of nitrogen, phosphorus, and potassium delivered to the crop.

Both types replace the nitrates and phosphates lost by harvesting plants and removing livestock, which is the fundamental reason for their use. The choice between them involves balancing cost, convenience, crop response time, and environmental risk.

3. Leaching — When Nutrients Leave the Field

Here is where the environmental story begins. Nitrates are particularly vulnerable to leaching because they carry a negative charge and are repelled by the negatively charged clay particles in soil — meaning they cannot bind to soil and are freely mobile in water. When rainfall exceeds the soil's capacity to absorb it, water carrying dissolved nitrates and phosphates drains through the soil profile and into groundwater, streams, and rivers.

Leaching is most severe when fertilisers are applied to waterlogged soils, on steeply sloping land, or immediately before heavy rainfall. Artificial fertilisers carry a higher leaching risk than natural ones precisely because their nutrients are already in soluble ionic form — there is no decomposition step to slow their movement into waterways.

4. Eutrophication — A Step-by-Step Cascade

Once nitrates and phosphates reach a body of water, a chain of events unfolds that can devastate the aquatic ecosystem. This process is called eutrophication, and understanding each stage in sequence is essential — exam questions frequently ask you to describe or explain the full chain.

Worked example: The Eutrophication Sequence

Describe and explain the sequence of events that occurs when excess nitrates and phosphates enter a lake or river.

1. Nitrates and phosphates leach from agricultural land into the waterway, raising the concentration of mineral ions in the water. — These minerals act as limiting factors for algal growth. When their concentration rises, the constraint on growth is removed.
2. Algae and cyanobacteria reproduce rapidly, forming a dense surface layer called an algal bloom. — Nitrates and phosphates are the key nutrients for algal growth. With the limiting factor removed, population growth is rapid. The bloom covers the water surface.
3. The dense algal bloom prevents sunlight from reaching submerged aquatic plants. — Without light, submerged plants cannot photosynthesise. They die and sink to the bottom.
4. Aerobic bacteria decompose the dead algae and plant material, multiplying rapidly. — The large quantity of dead organic matter provides a substrate for decomposers. Their population increases dramatically.
5. Aerobic decomposers consume dissolved oxygen in the water during respiration, causing the biochemical oxygen demand (BOD) to rise sharply and dissolved oxygen levels to fall. — Aerobic respiration by decomposers uses up dissolved $O_2$ faster than it can be replenished. This is the critical step that kills aquatic animals.
6. Fish, invertebrates, and other aerobic organisms suffocate and die due to the lack of dissolved oxygen. — Without dissolved oxygen, aerobic respiration is impossible. Biodiversity collapses. Anaerobic bacteria may then dominate, producing toxic gases such as hydrogen sulphide.

Answer: Excess nutrients → algal bloom → light blocked → submerged plants die → aerobic decomposers multiply → dissolved oxygen depleted → aerobic organisms die. Biodiversity is severely reduced.

Notice that the algae themselves do not directly kill the fish. The lethal mechanism is the depletion of dissolved oxygen by aerobic decomposers breaking down dead organic matter. This indirect chain is the part students most often get wrong in exam answers.

5. Evaluating the Environmental Impact of Fertiliser Use

Fertilisers are not inherently harmful — the problem arises when nutrients reach waterways in excess. The severity of eutrophication depends on several factors: the type of fertiliser used, the timing and quantity of application, local rainfall patterns, soil type, and proximity to water bodies.

Natural fertilisers carry a lower leaching risk because nutrients are released gradually as organic matter decomposes. However, they are not risk-free — poorly managed slurry storage or application to saturated soils can still result in significant nutrient runoff. Artificial fertilisers, applied in excess or at the wrong time, pose a greater immediate risk because their soluble ions are immediately available for leaching.

Beyond eutrophication, high nitrate concentrations in drinking water drawn from affected groundwater sources pose a human health risk, particularly for infants. This adds a public health dimension to what might otherwise seem a purely ecological problem.

6. Exam Technique and Mark-Winning Strategies

Questions on eutrophication are among the most predictable in AQA A Level Biology papers, yet they are also among the most poorly answered. The reason is almost always the same: students describe the algal bloom and then jump straight to 'fish die', missing the critical intermediate steps involving decomposers and oxygen depletion.

When a question asks you to explain eutrophication, the examiner is looking for a causal chain with mechanistic links. Each step must be connected to the next with a reason. Listing stages without explaining why each leads to the next will cost you marks at A Level.

For a 6-mark extended response on the environmental issues arising from fertiliser use, structure your answer as follows: open with the mechanism of leaching, explain how excess nutrients reach waterways, then walk through the full eutrophication sequence with causal links, and close with a judgement on the relative severity of the impact or a comparison of natural versus artificial fertiliser risk.

Taken together, the two specification requirements for this topic — the use of fertilisers to replace lost nitrates and phosphates, and the environmental issues of leaching and eutrophication — are two sides of the same coin: the agricultural need to maintain soil fertility creates a risk of nutrient pollution that cascades through aquatic ecosystems with serious consequences for biodiversity.