Managing erosion risk during cultivation

By Bala Tikkisetty, sustainable agriculture coordinator at Environment Waikato

It’s the time of year when farmers are generally cultivating their paddocks for next season’s crop or pasture.

So called sacrifice paddocks – particularly damaged after being used for standing-off cows in winter – can need special remedial attention (they’re normally cultivated and sown to a summer crop to restore the soil).

But it’s also a time when erosion and sedimentation risks are higher.

The time of greatest risk is right about 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.

So what can farmers do to minimise soil loss through wind erosion?

Providing shelter from well-designed windbreaks is one key method of control. The other is maintaining a good vegetative cover to protect bare soil.

Farmers in wind prone areas, and with lighter soils, should consider 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 possible, to help prevent wind scouring of flatter land, and keeping the cultivation period to a minimum.

It is very important to manage the soil structure. Good soil structure is vital for plant growth. It regulates soil aeration and gaseous exchange rates, the movement and storage of water, soil temperature, root penetration and development, nutrient cycling and resistance to structural degradation.

Using chisel ploughs or grubbers are best 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”.

It’s also best to cultivate soil when the moisture content is neither too high nor too low.

To assess if soils are suitably moist for primary cultivation, take a piece of soil (half the volume of your index finger) and press firmly to form a pencil with your fingers. Try to 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.

Meanwhile, winter compaction - the compression or squeezing of a soil by stock, reducing the number and volume of large soil pores and increasing soil density – can badly affect productive capacity.

Generally, 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.

But compacted winter-feed paddocks can be different as the top few centimetres of soil has been “puddle” 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 erodable soils.

A risk associated with erosion is that of various forms of sediment getting into waterways via overland water flows.

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.

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.

Maintaining adequate vegetation cover, use of conservation cultivation techniques, as well as maintenance of wide buffer zones, remain critical components of maintaining good water quality and soil health. Narrow riparian filter strips alongside streams will not be effective on their own if poor management practices leading to excessive soil erosion elsewhere are permitted.

There is no need to take filter strips completely 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 you can maintain productivity while at the same time keeping grass cover for sediment trapping.

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

Remember soil is the farmer’s most precious resource. Good management can avoid losing it.

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