Getting the best out of paddocks

By Bala Tikkisetty

The post-Winter period is a time when farmers are cultivating their paddocks to ensure they can best provide fresh pasture or the next crop.

Also, so-called sacrifice paddocks are normally cultivated about now and sown with a summer crop to restore the soil.

It is important to remember that any ploughing for re-grassing or other cultivation can lead to sediment and associated nutrients getting into waterways and affecting water quality. Wind erosion, in particular, can be a major contributor to sediment and nutrients getting into water.

Generally speaking, farmers can minimise these wind erosion-caused losses to water by providing shelter using well-designed windbreaks. Another method is to maintain as much vegetative cover as possible to protect bare soil.

But the time of greatest risk can be at times like now when the protective plant cover is lost through cultivation of soils for pasture renewal and crop establishment. Even with well-established shelterbelts protecting paddocks, wind erosion can still occur because of inappropriate cultivation techniques.

Farmers in wind prone areas and with lighter soils should consider “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 restricting the cultivation period to the minimum time.

The use of the chisel plough or grubbers is recommended as these 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 recommended for minimising soil blow.

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 50 mm long and 4 mm thick. Exert sufficient pressure with your fingers to reduce the diameter of the worm to 4 mm 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. Clods that are too dry won’t break down with cultivation to give a good seedbed.

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.

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

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.

Runoff results 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 near-by surface water-course.

The area beside waterways that forms the interface between water and land is called the riparian margin. This area is a crucial buffer between land use activities and the natural waterway.

An effective filter strip needs to be established and maintained where overland water enters water bodies. Healthy riparian vegetation in these areas should be maintained to improve bank stability, increase water quality, reduce stock losses, filter surface run-off and to provide habitat for wild life.

The main role of a grass waterway margins is to improve water quality and control bank erosion. Suitable riparian species are the effective filter for removing sediment, bacteria and nutrients from surface runoff. Good riparian vegetation slows runoff down so that sediment, phosphorous (which binds to soil particles) and faecal matter can settle out before the runoff reaches waterways. 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 as well as providing bank stability.

There is no need to take filter strips out of production, but it is important to maintain them so that there is almost complete ground cover and a good height of vegetation, which maximises the potential to trap sediment and nutrients. As these areas are often highly productive, it is important to work out how to maintain productivity while at the same time keeping grass cover for sediment trapping.

Remember though that Environment Waikato has a rule in the Waikato Regional Plan, which says farmers must not cultivate paddocks within two metres of a river, stream or lake bed.

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.

For further advice on best management practices, please contact Bala Tikkisetty, sustainable agriculture coordinator on 0800 800 401.