New Research By Leading Hamilton Soil Scientist Dr Gordon Rajendram Reveals New Zealand's Nitrogen Fertiliser Use Has Climbed Nine-Fold Since 1990 By Dr Gordon Rajendram
As Canterbury farmers look at their fertiliser spend, one number from the research stands out. Since 1990, New Zealand’s nitrogen fertiliser use has climbed roughly nine-fold, driven mainly by the dairy sector. Average application rates on dairy pasture have gone from about 31 kg N/ha in 1990/91 to around 168 kg N/ha by 2019/20, and as a soil scientist working across New Zealand, I want farmers thinking about what that shift has done to their clover.
What began as a tactical tool for filling feed gaps has become, on many farms, a year-round habit. In my view, that habit has a cost that is easy to miss because it happens below the surface.
Clover is doing a lot for you. Working with Rhizobium bacteria in its roots, it pulls nitrogen out of the atmosphere and feeds it straight into your pasture, with measured fixation rates of 79 to 212 kg N/ha per year where visually clover in pasture was found to be between 9% to 20%. That is free nitrogen, with no spreader and no invoice. On top of that, clover is high-quality feed for your animals with ME values around 13 MJ/kg DM, so it does two jobs at once.
The problem is that fertiliser nitrogen works against it. For every kilogram of N applied, clover’s own fixation can drop by an estimated 0.25 to 0.5 kg. In effect, you pay for nitrogen and then lose some of the nitrogen you were getting for nothing.
Clover is also under pressure from other directions. It often makes up less than 20% of pasture dry matter, it is patchy, and its contribution swings with the seasons. Clover root weevil has made productive stands harder to hold. Meanwhile, about two thirds of our urea is imported and its manufacture is energy intensive, which leaves farmers exposed on both cost and supply.
So where do microbes fit in? Beyond clover’s partnership with Rhizobium, nitrogen-fixing bacteria live free in the soil. The catch is that fixing nitrogen takes a lot of energy, and these bacteria usually have to draw it from plant or soil carbon. My research is asking whether the right microbial partnerships could ease that constraint and add to the nitrogen your pasture already gets from clover.
My view of the bigger picture is that nitrogen should complement clover, not crowd it out. Used strategically, fertiliser fills genuine gaps. Used constantly, it can undermine the free biological nitrogen and feed already growing in your paddocks. With fertiliser costs under pressure, my message is straightforward: look closely at how much nitrogen you are applying and what it is doing to your clover, rather than repeating last season’s programme out of habit.
Watch out for my next column, where I reveal more findings from the research.
For more information, please contact:
Dr Gordon Rajendram
www.gordonrajendramsoilscientist.co.nz
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