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Slips, Slumps and Subsoil Flow

Slips, slumps and subsoil flows can often arise on hill country after heavy rains or prolonged wet weather. The risk of slips can increase when land has been cleared of bush, new pastures established or when farm tracks are cut in on slopes.

Soil slip erosion is the rapid sliding or flowing of soil and subsoil which exposes a slip surface almost parallel and less than one metre below the original surface. Apart from being unsightly, slips can pollute streams and reduce pasture production for a long time.

Most soils have a high resistance to movement because of the physical friction and chemical bonds between soil particles. This resistance is tested with the downhill "pull" of gravity, and the pressure of water in saturated soil. In Southland slipping is most common in areas underlain by mudstone such as Blackmount and where loessial soils exist around Mataura and north of Gore.

Prevention is the best way to reduce the likelihood of slips occurring on your property. By draining surplus water from the soil surface and lower levels this slows down water infiltration to the deeper soil levels and makes your soil more stable.

However when slips do occur it is best to act quickly to prevent further slipping. By acting quickly you can also avoid burial of your unslipped pastures and to restore pasture production quickly with a desirable pasture species rather than allow weeds to take over.

Restoration steps

Oversowing

Over sow slipped areas immediately before the exposed area has had time to dry out. Where manual over sowing is not possible, or hazardous, over sow by helicopter. If over-sowing has been delayed and the exposed slip plane or eroded surface dries out, then sow in spring or autumn for best results. By mixing your preferred seed choice with super-phosphate and nitrogen based fertilisers you should see good pasture cover within two years.

Fencing the site

It is absolutely essential to exclude stock from over-sown slipped areas to ensure that the plants get well established. You can either exclude them from the whole paddock or temporarily electric fences the site off. Make sure you monitor pasture growth even after it is established because stock will eat the new pasture before the rest of the paddock and may cause further slipping problems if not managed appropriately.

Tree planting

Soil erosion can be minimised in hill country catchments by large scale afforestation with trees or by indigenous forest. Tree roots, particularly lateral ones that spread sideways, greatly reinforce soil's resistance to downhill movement. They act like reinforcing mesh in concrete and if removed from soils with weak resistance it will put extra stress on the soil.

What to plant where...

• On eroded slopes Pinus radiata is a good choice because of its tolerance of a wide variety of conditions, low establishment cost and the possibility of some future cash return from the sale of timber. Recommended spacing is 2.5 x 2.5 m.
• Where erosion is less active, slower growing trees such as macrocarpa or douglas fir are a good choice, however Macrocarpa is suited to higher fertility sites. These longer rotation trees maintain their protective cover for longer periods than with the shorter rotation radiata pine. Eucalyptus nitens, E. delegatensis can also be used for close planting.
• In actively moving wet sites poplar and willow species should be used. Alnus species are expensive but recommended for these wet areas.
• In areas where a permanent grass cover is desirable, such as stream banks, poplars and willows should be planted to minimise shading.