Waikato Regional Council monitors changes in pasture fertility and nutrient losses to get information on the trends and risk nutrient losses are likely to have for water quality. Monitoring also helps us identify policy responses to avoid or remedy damage to our soil and water resources.
It is important to know whether the region's soils are being mined of nutrient reserves or if excess fertiliser is being applied and risking water quality. Monitoring nitrogen and phosphorus inputs and changes in soil phosphorus levels provides information about the balance of these nutrients in pastoral soils.
We regularly monitor nitrogen and phosphorus concentrations in surface and groundwater throughout the region. Our monitoring information, together with predictive modelling based on stock density and terrain, can be used to help explain any observed increases or decreases in nutrients.
Nutrient levels in water are linked to algal growth. Increasing trends in nutrient levels can be used to predict and manage 'hotspots' where nitrate concentrations in water are approaching or exceed critical limits.
This indicator estimates the average amounts of :
Losses of these nutrients affect water and air quality.
Total nutrient inputs should be close to total nutrient losses. If the inputs are greater than outputs, the soil nutrient levels will increase and higher losses to water bodies are more likely. Alternatively, soil nutrient levels will decrease if the inputs are less than the losses. This is not desirable because over time the soils will become 'exhausted' and unproductive.
The Waikato region contains some of the most productive land in New Zealand and the world.
In 2009-10, average dairy farm nitrogen fertiliser use was 94 kg N/ha/yr. Previously, dairy farm nitrogen fertiliser use was 68 kg N/ha/yr in 1997-98 and 125 kg N/ha/yr in 2002-03. Yearly nitrogen fertiliser use on sheep/beef farms was 6 kg N/ha/yr in 1997-98 and 9 kg N/ha/yr in 2002-03, the only periods for which we have data.
In 2008, nitrogen leaching for dairy was 38 kg N/ha/yr and may have been affected by the drought. Previously, dairy farm nitrogen leaching appeared to be increasing from 32 kg N/ha/yr in 1998, to a maximum of 45 kg N/ha/yr in 2007. Nitrogen leaching for sheep, beef and deer farms remains stable, about 13 to 14 kg N/ha/yr.
Overall, phosphorous fertility on farms is showing improvement, with less soil samples showing excessive fertility.
On average, phosphorus fertility on dairy farms is near the maximum for a high producing farm. About half of the soil samples from volcanic and sedimentary soils show excessive phosphorus fertility in 2012, compared to about one-fifth in 1988-1996.
The results for farms on volcanic soils have remined constant but the results for dairy farms on sedimentary soils have improved. Between 2002 and 2011 about two-thirds of the soil samples from these farms had excessive Olsen P (phosphorous) compared to about half in 2012-2013.
Excessive Olsen P has also decreased on sheep and beef farms since 2011. About 8-14 per cent of soil samples had excess phosphorus fertility in 2013, compared to 15-25 per cent in 2010. Olsen P data in 2013 was similar to the data from 2002 (about 9-18 per cent).
Yearly phosphorus run-off is 1.9 and 0.8 kilograms of phosphorus per hectare for sheep/beef and dairy farms, respectively. The higher value for sheep and beef reflects the hilly nature of these farms. The steeper the slope, the more phosphorous is likely to run off.
This indicator is updated every three years.
Soil Scientist - Science and Strategy Directorate
Updated July 2018