Our Wa­ter

It’s not fer­tiliser, and the cause is prob­a­bly look­ing at you.

NZ Lifestyle Block - - Contents - Words Emma Buchanan

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.

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