Wednesday, 28 October 2015 10:32

Keeping sediments away protects bottom line

Written by  Bala Tikkisetty

Winter's end is a time for cultivating paddocks; this must be done well to prevent sediment and excessive nutrients getting into waterways from the land.

Topsoil erosion of bare or cultivated land, especially in hill country, leads to the loss of valuable nutrients and sediment into rivers and streams. It can also disrupt infrastructure and increases the costs of maintenance, such as cleaning culverts and drains.

Sediment and some nutrients, particularly phosphorus, are carried to streams primarily in the overland flow of water.

Runoff 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. Runoff will move into low-lying areas or to the edge of a field where it can pond for longer periods or move into a nearby surface watercourse. 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 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 x 7mm thick. Exert sufficient pressure to reduce the diameter of the worm to 7mm in 15-20 complete forward and back movements of the fingers. Conditions are suitable for cultivation if the soil cracks before the worm forms. 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% of sediment can be caught in an effectively constructed riparian filter strip. Any faecal bacteria 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 2m 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.

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