Fertilisers, Leaching and Eutrophication

Farmers spread fertiliser to replace the nitrate that each harvest carries away. Nitrate dissolves easily in water, which is exactly why roots can absorb it and exactly why rain can steal it. Spreading more than the crop can take up, or spreading just before heavy rain, sends the surplus down through the soil into ditches and rivers. The skill is matching dose and timing to what the plants will actually use. Soil tests, split doses through the season, and keeping off waterlogged fields all cut the loss.

Nitrogen moves in a loop between air, soil, living things, and water. Bacteria and fertiliser add usable nitrate to soil, plants take some up, and whatever dissolves in rainwater leaches downward and sideways into streams. A river a mile from the field is connected to it by this drainage. Once nitrate reaches open water it is still plant food, so it feeds algae instead of crops.

Follow one nitrate ion to see the whole story. It starts in a fertiliser 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. While the algae grow they release oxygen, so the crash comes later. When the bloom runs out of nutrients the algae die all at once, and that is when the trouble starts.

Dead algae rain down and become food for bacteria and other decomposers. As decomposers multiply they breathe, and their breathing strips dissolved oxygen out of the water. Fish need that same oxygen, so they suffocate, and the stretch of river or sea becomes a dead zone. This is why oxygen falls after the algae die rather than while they grow. The killer is not the green water itself but the decay that follows it.

People reshape these cycles whenever they farm, burn fuel, or clear land. For fertiliser the fixes are practical: test soil before spreading, leave unsprayed buffer strips beside streams, store manure under cover, and time doses to dry spells. Each fix has a price in time, land, or lower yield in a wet year.