Fertilisers, Leaching and Eutrophication
Nitrate added to a field does not stay in the field, and once it reaches a river it feeds algae rather than crops. The damage is done by the decomposers that follow, because the oxygen they use is the oxygen the fish needed.
What a learner can do afterwards
- Trace the sequence from fertiliser application through leaching to an algal bloom to fish death
- Explain why the oxygen falls after the algae die rather than while they are growing
- Suggest a change to farming practice that reduces leaching and say what it costs the farmer
1 · Read
Farmers spread fertiliser to replace the nitrate each harvest carries away. Nitrate dissolves easily in water, which is why roots can absorb it and why rain can steal it. Spreading more than crops take up, or spreading just before heavy rain, sends the surplus down through soil into ditches and rivers.
Follow one nitrate ion. It starts in a granule, dissolves in rain, and slips past the roots into field drains. In the river it meets sunlight, and algae use it to multiply into a green bloom that clouds the water. A river a mile from the field is connected to it by this drainage.
While algae grow they release oxygen, so the crash comes later. When the bloom runs out of nutrients the algae die all at once and rain down as food for bacteria and other decomposers. Their breathing strips dissolved oxygen from the water, fish suffocate, and the stretch becomes a dead zone.
The fixes are practical and each has a price. Test soil before spreading, split doses through the season, leave buffer strips beside streams, and time doses to dry spells. The trade is honest: testing time, set-aside land, or lower yield in a wet year buys rivers that still support fish.
Leached nitrate feeds a bloom, the bloom dies, decomposers spend the oxygen, and fish pay for the fertiliser.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.