Nitrate Leaching Overview

Today I give a overview of nitrate leaching.

What is Nitrate Leaching?

What type of farming leach the most Nitrate?

How nitrate leaching from dairy farms is different from cropping & horticulture.

Well, gidday. My name is Glen Herud and today I want to talk about nitrate leaching and basically just give you a run-down on what nitrate leaching is about, and then later on I am going to talk about how we can start farming systems that have low rates of nitrogen leaching.  

Weed & Algae need both Nitrogen & Phosphorus in Order To Grow

So basically, if we look at water quality issues we've got two things. We've got nitrogen and phosphorus and what causes algae blooms and sort of algae build ups in our waterways.  What happens is if you have just phosphorus, pretty much nothing happens. If you've got just nitrate in the water systems, then pretty much nothing happens as well. But if you've got the two together, that's when these algae growths take place.  So, I won't worry about phosphorous today but we'll talk about nitrate today.

Who Leaches the Most Nitrate?

So, a few figures are that if you're a veggie grower, if you're a market gardener, the statistics show us that you will leach around 177 KGs of nitrogen per hectare per year.  If you're a cropping farmer, that's weeds and those sorts of things, you'll do 61 KGs per year.  If you're a dairy farmer, you'll leach around 65 KGs a year. If you're a humble old sheep farmer, you'll do 21 KGs of nitrate per hectare per year.  What that means is that basically this excess nitrogen filters out of those farming systems. and it's basically draining out the bottom of the soil.  When you look at cropping and these veggie growers, basically they are as a result of excess fertilizer, basically nitrogen fertilizer. They put a heap of fertiliser on and that fertiliser is going through the soil profile. 

85% of Nitrate Leaching From Dairy Farms Comes From Urine

But the dairy industry is different.  85% of all nitrates or excess nitrate from dairy farms is from urine.

So that's not urine from cows standing in a river urinating

and it's not from the urine in dairy effluent either

But every time we hear about environmental impacts from the dairy industry, that's all we hear about.  But they are not actually the issues. 

The main one is cows standing out in the paddock, urinating

Urine Patch Contains Equivalent to 800kg Nitrogen per Ha

Here's what happens. Basically, here is a cow. This is a bit of a funny cow.  She's a Friesian so what she does is she urinates into what you call a urine patch, which is about the size of a dinner plate. Within that dinner plate, there are 800 KGs of nitrogen per hectare. So there is an equivalent to 80 KGs of nitrogen per hectare. So the way I think of that, to make sense of it is if you take a hectare, which is a football field and let's say we had a way of getting all these cows lined up so they all urinated at once and all the urine patches covered one whole hectare.  If they all urinated at once, that would be applying 800 KGs of nitrogen per hectare at once. 

Average NZ Dairy Farmer Applies 150-200kg Nitrogen per Ha per Year

To give you an indication of how much that is, your average dairy farmer does around 150 to 200 KGs of nitrogen per hectare per year.  They will do that over multiple small allocations of fertilizer.  So, what I want to sort of get through to you is that there is a heck of a lot of nitrogen in a urine patch.  What happens is that if you think about grass. I don't know, what we got? Five, six, seven grass plants within a urine patch? They've got their roots systems that go like that, probably around 30 centimeters deep.  So what happens is the nitrogen comes in via the urine and while it's in the soil it turns into nitrate.  It can be absorbed by the plants.  If you look at this picture here, you can see these dark green patches in the paddock there. Those are fertility patches.  They are either feces patches or urine patches. What's happened is that those grass plants there, have as much nitrogen as they can get and they've bolted away and they are nice and dark and green. 

The Few Grass Plants in a Urine Patch can't Absorb all the 800kg of N

The problem is that there are 800 KGs of nitrogen being applied. Well, a heck of a lot of nitrogen will be applied and those few little plants, there is absolutely no way they are going to be able to absorb all that nitrogen. There is far too much.

What happens is the nitrate attaches to water molecules, H2O, because nitrate is soluble in water.  Those water molecules filter down through the soil profile and drain away and they take the nitrate with them. Once they get below the roots’ depth of the plants, they can't get absorbed.  It keeps on going, all the way down, until it gets into our groundwater. Then it ends up in our waterways.  So that is very simply how nitrate leaching takes place.  Essentially, excess nitrate that isn't absorbed by the plants ends up filtering through the soil profile and getting into our groundwater. 

There are a couple of things that affect the rate of nitrate leaching.  One of them is your soil profile. If you've got nice, well I shouldn't say nice, if you've got free draining soils that are kind of rocky, the water flows through those soils much more quickly.  So, obviously the nitrate flows through quicker. If you've got heavier clay soils, then the water sort of sits there more and therefore the nitrates sits there more.   The amount of water in the soil profile affects the rate of nitrate leaching. What happens generally is during the summer lots of nitrogen is sort of applied and it sort of sits there in the soil and then winter comes along and we get all this wet weather and it all sort of leaches out through the winter.  What's the other thing that affects nitrate leaching? Oh, your plants, your root depths. If we can get plants that have twice the root depths, then there are obviously twice as much time for them to absorb nitrate. That's a very brief rundown. It will probably horrify a few scientists but that sort of serves the purpose of what we need to explain today. 

So the next little while I'll start talking about how we can farm differently in ways to reduce our nitrogen leaching cap or nitrogen leaching rate. If you have been following the news you'll see that regional councils are starting to sort of propose nitrate caps, so saying that you can only leach around 25 KGs of nitrogen per hectare per year.  Obviously, if you are a potato grower, they would horrify you and dairy farmers are equally worried about how they are going to be able to farm and meet that sort of a target. That's what I want to talk about in the next couple of weeks.