Fertilisers and Eutrophication

Introduction

Fertilisers are one of chemistry's great gifts to humanity. By supplying nitrogen, phosphorus and potassium, they have roughly doubled crop yields and help to feed billions of people. But nutrients do not always stay where they are spread. When rain washes them into rivers and lakes, they feed an explosion of algae that ends, strangely, with fish dying for lack of oxygen. This chain of events is called eutrophication , and it links farming chemistry directly to water pollution.

Core explanation

Why fertilisers are needed. Plants need three main elements from the soil in large amounts:

Element Taken up as Needed for --- --- --- Nitrogen (N) Nitrate ions NO₃⁻, ammonium ions NH₄⁺ Making proteins; leaf and stem growth Phosphorus (P) Phosphate ions PO₄³⁻ Root growth; energy transfer in cells Potassium (K) Potassium ions K⁺ Flowers, fruit and resistance to disease

Harvesting crops removes these elements, so farmers replace them with NPK fertilisers . Common compounds include ammonium nitrate, ammonium phosphate and potassium chloride. These compounds are chosen because they are soluble in water , so plant roots can absorb the ions. Manure and slurry from animals are also nutrient-rich.

How nutrients escape. If more fertiliser is applied than crops can use, or heavy rain falls soon after spreading, soluble nitrate and phosphate ions are carried away in run-off or leach down through the soil into groundwater, streams and lakes. Sewage and some detergents add phosphates too.

Eutrophication: the sequence.

1. Nitrates and phosphates enter a lake or slow-moving river. 2. Algae and water plants grow rapidly, producing an algal bloom that turns the water green. 3. The dense layer of algae blocks sunlight from plants lower down. 4. Plants beneath the surface cannot photosynthesise and die; algae also die in huge numbers. 5. Bacteria feed on the dead material and multiply. 6. The bacteria respire aerobically, using up the dissolved oxygen . 7. Fish and other animals suffocate and die, adding more dead matter.

The key point is that the oxygen is used up by decomposing bacteria, not by the algae themselves. In the worst cases, "dead zones" form where almost nothing can live.

Health concern. High nitrate levels in drinking water are also a concern, especially for young babies, so water companies monitor and limit nitrate concentrations.

Reducing the problem. Farmers can test soil and apply only the fertiliser needed; avoid spreading before heavy rain; leave buffer strips of grass and trees beside rivers to trap run-off; and use slow-release fertilisers. Sewage works can remove phosphates, and many detergents are now phosphate-free.

Step-by-step reasoning

To answer a six-mark question on eutrophication, include each link in order:

1. Fertiliser run-off adds nitrates and phosphates. 2. Algae grow rapidly and block light. 3. Plants below die. 4. Bacteria decompose dead matter. 5. Bacteria use up oxygen; fish die.

Visual explanation

Picture a cross-section of a lake in three stages. First: clear water, green plants and fish. Second: a thick green scum on the surface and shadows below. Third: brown dead plants on the bed, swarms of bacteria, and fish floating at the surface, with an oxygen meter reading close to zero.

Real-world analogy

Eutrophication is like a party where too much food is delivered. The guests (algae) multiply, block the windows and then collapse; the cleaning crew (bacteria) that arrives to deal with the mess uses up all the air in the room, and everyone else is left gasping.

Real-world example

Every summer a large dead zone forms in the Gulf of Mexico where the Mississippi River enters the sea. Nutrients from farmland across a huge area of the United States feed algal blooms, and the decomposition that follows leaves thousands of square kilometres of seabed with too little oxygen for fish and shrimp.

Why?

Why do fertilisers pollute water more easily than many other farm chemicals? The compounds are deliberately chosen to be soluble so plants can absorb them. The same solubility means rainwater dissolves them easily and carries them into streams and groundwater.

Common misconception

"The algae use up all the oxygen." Living algae actually produce oxygen by photosynthesis during the day. The oxygen is used up mainly by bacteria decomposing the dead algae and plants.

Worked example

Question: Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃ (Aᵣ: N = 14, H = 1, O = 16).

Reasoning: Mᵣ = 14 + 4(1) + 14 + 3(16) = 80. Mass of nitrogen = 2 × 14 = 28. Percentage = (28 ÷ 80) × 100.

Answer: 35% nitrogen by mass.

Quick check

1. Which two ions from fertilisers are mainly responsible for eutrophication? Answer: Nitrate ions and phosphate ions.

Exam focus

Examiners reward a complete, logical chain: nutrients → algal growth → light blocked → plants die → bacteria decompose → bacteria use oxygen → fish die. Missing the bacteria step, or blaming the algae for using the oxygen, is the most common way to lose marks.

Advanced insight

In many freshwater lakes, phosphate is the nutrient in shortest supply, so it limits algal growth; adding even a little phosphate can trigger a bloom. In many coastal seas, nitrogen is the limiting nutrient instead. Knowing which nutrient is limiting helps scientists decide which source to control first.

Summary

Fertilisers supply nitrogen, phosphorus and potassium as soluble compounds so plants can absorb them. Excess nitrates and phosphates are washed into water by run-off and leaching. They cause algal blooms that block light, plants die, bacteria decompose them and use up dissolved oxygen, and fish suffocate — eutrophication. Careful application and buffer strips reduce the problem.

Practice questions

1. State the three main elements supplied by NPK fertilisers. Answer: Nitrogen, phosphorus and potassium. 2. Why must fertiliser compounds be soluble in water? Answer: Plants absorb nutrients as dissolved ions through their roots. 3. Explain why fish die in a lake affected by eutrophication. Answer: Dead algae and plants are decomposed by bacteria, which use up dissolved oxygen by respiration, so fish suffocate. 4. Suggest two ways farmers can reduce eutrophication. Answer: Any two: apply only the fertiliser needed, avoid spreading before heavy rain, leave buffer strips by rivers, use slow-release fertilisers.