Nitrate levels in New Zealand waterways

In 2009 I visited Nick Pyke who was the head of The Foundation for Arable Research at the time.

I wasn’t having any luck convincing a Canterbury dairy farmer to trial my wacky “Happy Cow” dairy farming system. I wondered if cropping farmers would be interested in adding some dairy cows to their system.

What Nick told me formed the basis of our dairy farming system which we loosely call the “Happy Cow Way”. 

Nick explained how mixing cows with crops is a mutually beneficial system. Cows put nitrogen into the ground & plants take nitrogen out of the ground.

I was in Nick’s office in 2009, as it appeared to me that nitrate leaching from dairy cows was becoming a big issue & I wanted to find a solution. 

I felt that the level of scrutiny & criticism that the dairy industry was experiencing was reaching a crescendo. 

10 years later I find myself at a freshwater consultation meeting. John Penno, the chairman of the freshwater leader’s group & Synlait co-founder was telling the crowd that these proposals “have been well signalled”.

Indeed they have.

Nitrate gets into our waterways in 2 main ways, the first is from excess fertilizer leaching through the soil. This tends to be a problem for the horticulture & arable farmers. 

The second way is caused by the urine patches of animals & this is the dairy industries problem.

Note, it is not from cows standing in waterways.

When a cow urinates, it creates a puddle of urine about the size of a dinner plate. This urine patch contains the equivalent to 1000 kg of nitrogen/ha. That means if you covered one hectare with evenly spaced urine patches, it would be like applying 1000 kg of nitrogen fertilizer in one application.

For perspective, an average dairy farmer applies about 100-200 kg of nitrogen fertilizer over an entire year, in 25kg applications.

It’s up to the plants in the dinner plate-sized urine patch to absorb all that nitrogen & of course, so few plants can’t absorb that amount of N at one time.

Nitrogen converts to nitrate in the soil. Nitrate molecules bond to water molecules, which means when water filters through the soil profile, it takes the nitrate with it.

There are a few factors which affect the rate of nitrate leaching. 

Heavy soils leach at a slower rate than light stony soils. That’s because heavy clay soils hold water & nutrients in the root zone for longer.

Light stony soils, allow water & nitrates to drain quickly through the soil profile & below the root zone faster. 

The amount of water in the soil is important too. A large percentage of nitrate is leached during the autumn & winter periods which tend to be the wettest times of the year.

The other key point is to ensure there are roots in the ground. Nitrate can’t be absorbed if there are no plant roots to absorb them. 

The nitrogen loading onto the soils is important too. Obviously the more N applied either by urine or fertilizer will affect the N leached.

Last week I downloaded the river test data from Statistics NZ. I wanted to see how many rivers complied with the new proposed nitrogen limit of 1mg/L.

Of the 1506 rivers tested, 192 (or 12.75%) rivers had average readings over 1mg/L of total nitrogen. 

Of these 192 rivers, 78% were within 1-3mg/L. So most of the 192 rivers over 1mg/L are just a little bit over the limit. Which could indicate minor changes in practices would start to reverse these rivers N levels over time.

There are 41 rivers with average test results ranging from 3-15mg/L & of those 41 rivers, 20 are in the Canterbury region.

That’s not surprising because Canterbury has free-draining soils, we’ve increased the N loading by replacing sheep urine patches with cow urine patches. In fact, Canterbury has a 21% higher stocking rate than the NZ average. Partly, because irrigation combined with lots of sunshine grows lots of grass.

This is all good news. 

Because it means the current state of Nitrogen in Canterbury is exactly what the science predicts. 

Which means we know how to solve it.

The farmers are the experts & by combining creativity & science they will find the economic solutions.

Recently, science has proved exactly what Nick Pyke told me in 2009. 

If you take a deep rooting, Nitrogen hungry plant like wheat & you sow it into the paddocks where the cows spent the winter. You can reduce N leaching by 40%.

Its called a cover crop & it’s just one of the many possibilities available.

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