AND HOW DO THEY GET INTO OUR WATERWAYS?
AND WHERE DO THEY COME FROM?
Now that we have had a quick look at Overseer and Farm Environmental Plans, it is timely to take a step back and ask the question “why do we care about nutrient losses from farms anyway?” But you also need to know how they get into our waterways, and where they come from in the first place. This is my simple take on a number of complex and interactive cycles, but at the end of the day, we’re not all scientists, so I hope this helps put some context around why we are where we are today.
What do we care about?
Farming contributes four main pollutants to the environment: the nutrients nitrogen (N) and phosphorus (P), sediment, and faecal matter. So why do we care about these?
- Nutrient (N and P) enrichment of rivers, streams and other waterways can lead to unwanted growth of plants (waterweeds and algae), and this can be toxic to fish, but also other animals (think back to articles about dogs drinking river water and dying), and it’s not so good for humans either.
- Excess sediment may cause siltation (which means that fish and aquatic plants get smothered), and degrade water clarity (makes it look dirty all the time!).
- Faecal matter and its associated pathogens pose a risk to human and animal health through waterborne infectious diseases.
How do they end up in our waterways?
N enters our rivers, streams and other waterways via leaching to groundwater. Whereas sediment, faecal matter and P enter streams mostly in surface runoff. These two distinctions are important.
What are the on farm sources of these nutrients?
N comes primarily from urine patches. P is primarily from P based fertilizer, solid animal excrement or stock having direct access to stream and other waterways.
Sediment and faecal matter is also from stock having direct access to streams, and run-off straight into waterways.
So, let’s put some context around this.
We will take a farm which does dairy support from May to May. Using Overseer, N and P losses can be determined. The following table shows the outputs from this farm, using exactly the same inputs, but for varying topography and soil type.
|
Soil Type and Topography
|
N lost to water (leaching into groundwater) (kg
per ha per year)
|
P lost to water (runoff)(kg per ha per year)
|
|
Light, free draining Lismore soil, flat
land
|
27
|
0.3
|
|
Light, free draining Lismore soil, steep
hill
|
27
|
0.1
|
|
Heavy, poorly drained Timaru soil, flat
land
|
18
|
0.1
|
|
Heavy, poorly drained Timaru soil, steep
hill
|
18
|
0.3
|
Thinking about how nutrients get into our waterways, the Overseer results confirm two things: The first is that P losses are higher on steep land as opposed to the same farm on flat land. The second is that N is more affected by soil type than topography – you get more leaching to groundwater in free draining soils as opposed to heavy, poorly drained soils.
The other thing you will notice is that N losses are much bigger numbers than P losses, but any small increase in either number can have a big impact on waterways – it is generally many small increases that create a big problem – death by 1,000 cuts!
Therefore, when we are looking at how we manage nutrient losses on our farms, understanding the source of the nutrients, and how nutrients get into our waterways is critical. Looking at possible mitigation options will be next time’s topic.
By Keri Johnston, Irricon Resource Solutions Limited.
Keri’s expertise is in the field of natural resources engineering and resource management, primarily in water resources, irrigation and nutrient management. As well as doing this, she farms with her husband and two girls at Geraldine.
Phone: (027) 2202425
E-mail: keri@irricon.co.nz
