Stopping sediment protects bottom line
Post-winter is a time for cultivating paddocks and the quality of practices farmers use is crucial when it comes to
preventing sediment and excessive nutrients from getting into waterways from the land.
Top soil erosion, especially in hill country, of bare or cultivated land leads to the loss of valuable nutrients and
sediment into rivers and streams. It can also disrupt infrastructure and increases the costs of maintenance activity,
such as cleaning culverts and drains.
Sediment and some nutrients, particularly phosphorus, are carried to streams primarily in the overland flow of water.
Run off happens when water infiltration into the soil is slower than the application rate through rain or irrigation.
Because of soil textural and structural differences, in some soils the natural rate of water infiltration is low. But
the infiltration rate can be low due to frequent tillage or other management related constraints like compaction. Run
off will move into low-lying areas or to the edge of the field where it can pond for longer periods or move into a
nearby surface water-course. Avoiding compaction of soils through heavy stock trampling can lessen this risk.
Landowners and cultivation contractors can help mitigate the environmental and operational risks associated with
cultivation, and protect their soil resources, by adopting a range of sound practices.
These include cultivating along contours, sowing at right angles to the prevailing wind, and sediment retention measures
such as detainment bunds for minimising soil loss.
Other conservation cultivation techniques include minimum tillage or no tillage. If soil has been continuously
cultivated for many years, the structure is likely to be poor because cultivation reduces soil organic matter levels.
No-tillage will not repair the damage overnight but it will eventually. Herbicide spraying followed by direct drilling
is an option on light erodible soils.
Soils should be cultivated when the moisture content is neither too high nor too low. To assess if soils are suitable
for primary cultivation, take a piece of soil (half the volume of an index finger) and press firmly to form a pencil.
Roll the soil into a “worm” on the palm of one hand with the fingers of the other until it is about 40mm long and 7mm
thick. Exert sufficient pressure to reduce the diameter of the worm to 7mm in 15 to 20 complete forward and back
movements of the fingers. Conditions are suitable for cultivation if the soil cracks before the worm is made. The soil
is too wet to cultivate if you can make the worm.
As a last defence for those days when you get intense rainfall, an effective filter strip will help to trap sediment and
faecal matter before it can get into water bodies. Healthy riparian vegetation in these areas will also improve bank
stability, reduce stock losses, and provide habitat for wildlife.
Studies show that up to 90 per cent of sediment can be caught in an effectively constructed riparian filter strip. Any
faecal bacteria that are trapped in long grass filter strips will die off.
In the filter strips grasses should generally be kept to a height of at least 10-15cm, with a high density of stems and
leaves at ground level for maximum trapping effect.
Waikato Regional Council has a specific rule in the regional plan which says farmers must not cultivate paddocks within
two metres of a river, stream or lake bed. You should treat this as a bare minimum width, especially on sloping land.
Apart from protecting water quality, good management of soils is also naturally important for the ongoing economic
health of farming. Maintaining the health of our natural capital is therefore a wise investment in our regional future.
• Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council. Contact him on 0800 800 401 or email email@example.com