It’s not fertiliser, and the cause is probably looking at you.
The secret nitrogen source on your block
One of the key nutrients councils are trying to limit under new freshwater regulations is nitrogen. Many farms and blocks will need a Farm Environmental Management Plan (FEMP) which will set out how you will manage your land to achieve those limits.
Nitrogen comes in many forms in soils. One is a plant-available form, nitrate (NO3-).
If nitrate was a person, you’d say they were charming and free-spirited. Plants love nitrate and take it up easily. However, if plants aren’t able to use all of it, nitrate swans off to find another party the next time it rains.
This generally happens when it’s too cold for pasture to grow, too wet or too dry, or the land is bare between crops. The nitrate leaves the rootzone and makes its way into groundwater. That’s how it ends up in streams and rivers where it starts causing mischief with nitrogen’s significant other, phosphorus (more on that next month).
Nitrogen and phosphorus are key nutrients for rapid plant growth, both on land and in water. When they get into streams and lakes and the water temperature rises, you see algal blooms. Algae becomes a problem when its lifecycle (and death) uses up all the oxygen in the water and starts killing off the freshwater ecosystem. It also puts you off swimming in the slimy mess.
People often think that nitrogen loss comes from using too much fertiliser, and that is certainly one source.
But the most common source of high nitrogen is cattle urine. Cattle are big animals that drink a lot and pee a lot.
Nitrogen fertiliser is generally spread evenly around your paddocks. Cattle pee pours out onto the one spot. These urine spots can have the same concentration of nitrogen as if you’d spread 1000kg per hectare of urea on that small area. This is far more nitrogen than pasture plants can use. The next time it rains, most of the excess nitrogen is flushed away and soaks into our groundwater.