Getting post-winter soil cultivation right

The post-winter period is the time for cultivating paddocks so they are at their best to provide fresh pasture or crops.

In particular, sacrifice paddocks are normally cultivated about now and sown with a summer crop to restore the soil.

Any ploughing can lead to sediment and associated nutrients getting into waterways, affecting water quality. Also, soil erosion generally can be a major contributor to sediment and nutrients getting into water.

Farmers can minimise wind erosion-caused losses to water by providing shelter with well-designed windbreaks. Another way to prevent erosion is to maintain as much vegetative cover as possible to protect bare soil.

Given the amount of preparation of paddocks going on about now, this is one of the times of greatest erosion risk because protective plant cover is lost through cultivation.

Even with well-established shelterbelts, wind erosion can still occur because of inappropriate cultivation techniques.

I recommend that farmers in wind prone areas and with lighter soils consider using “conservation cultivation” measures.  These are cultivation practices aimed at maintaining maximum vegetative cover on the soil surface and which encourage moisture retention in soils.  The aim is to produce an uneven soil surface in as “rough” a condition as practicable and keeping the cultivation period to a minimum.

The use of the chisel plough or grubbers give a fine deep seedbed while still retaining a cloddy surface. Top-working implements, discing and rolling can create a fine seedbed prone to wind erosion even in well-sheltered situations.

Cultivating and sowing at right angles to the prevailing wind and ridging is also good for minimising soil blow.

Satisfactory results are achieved when cultivation is carried out at a suitable soil moisture content and at a suitable depth. If good precautions are observed, two-pass cultivation is often all that is needed to prepare a seedbed.  Repeated passes to get a good tilth are avoided and the risk period for surface erosion between initial vegetation clearance and ground cover by a growing crop is shortened.

Winter-feed paddocks can be different as the top few centimetres of soil has been “puddled” by stock in wet conditions and is very prone to wind erosion once it has dried out.  This is because this top layer of soil has lost all its structure and can lift from the paddock very easily.  Turning this type of ground over as soon as soil conditions permit in the early spring will minimise the risk of losing this fine layer to the north-westerly wind.

Other conservation cultivation techniques include the suite of practices known as 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, with residue retention, it will eventually.  Chemical spraying followed by direct drilling is an option on light erodible soils.

Runoff occurs when the rate of water infiltration into the soil is slower than the application rate (rain or irrigation). Because of certain textural 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.

A range of material from adjacent land can contaminate watercourses. This can include sediment, nutrients such as nitrogen and phosphorous, other chemicals and microbes.

Sediment and some nutrients, particularly phosphorus, are carried to streams primarily in the overland flow of water. Dissolved nutrients such as nitrogen and other materials (including dissolved organic carbon) can also move through the soil in underground flows and contaminate watercourses.

The riparian margin between land and water is a crucial buffer that helps stop sediment, pathogen and other material getting into water.

An effective riparian filter strip is valuable where overland water enters water bodies. Healthy riparian vegetation in these areas improves bank stability, increases water quality, reduces stock losses, filters surface run-off and provides habitat for wild life.

Studies show that up to 90 per cent of sediment can be caught in an effectively constructed filter strip. Any faecal bacteria that are trapped in long grass filter strips will die off in sunlight.

Riparian vegetation also has an important additional benefit in providing shade to the stream, thereby reducing water temperatures and growth of nuisance plants and algae.

It is important to maintain filter strips so that there is almost complete ground cover and a good height of vegetation, which maximises the potential to trap sediment and nutrients.  Designing plantings to provide shade to streams while allowing enough light onto the banks to maintain grass cover is the secret to success. The Clean Streams Guide on our website www.ew.govt.nz can help with this.

In the filter strips, generally, grasses should be kept to a height of at least 10-15 cm with a high density of stems and leaves at ground level for maximum trapping effect.

Also, under the Waikato Regional Plan, farmers should not cultivate paddocks within two metres of a river, stream or lake bed.