21 December 2001

How to retain soil structure and prevent river pollution

By Hannah Velten

HARVESTING maize early, establishing another crop before winter and avoiding slurry spreading until spring are the keys to retaining good soil structure and preventing soil wash during winter.

Loss of topsoil and nutrients from maize fields reached a peak last winter and if sustained could lead to reduced crop yields. But when soil wash occurs over time from a number of fields, it causes diffuse pollution.

This concerns the Environment Agency so much that an environmental and agricultural partnership – the Landcare project – has been established to reduce the problem of diffuse pollution affecting the Hampshire Avon river – an EU designated conservation area.

Project officer Chris Westcott says a significant proportion of diffuse pollution comes from agricultural land in the rivers catchment area.

The EAs Richard Smith adds: "Compacted and sealed bare soil following maize is particularly prone to soil wash during rainfall." As a result, the Landcare project has sponsored a pilot approach towards stemming the problem involving the EA and the Maize Growers Association.

Producer workshops aim to highlight the mutual benefits of reducing soil wash through good animal husbandry and soil management. "We hope producers will voluntarily practise techniques to avoid soil wash, without having to resort to regulations," says Mr Westcott.

In August, MGA members attended a workshop at Andy Drakes farm in Wilts to discuss solutions to run-off from maize fields on the silty clay land.

Following the workshop, demonstration maize plots of 0.2ha (0.5 acres) received different treatments. These included early and late harvesting, minimal and conventional cultivation, winter cropping or leaving as maize stubble.

Returning to Blackhouse Farm in early December, inspection of the plots revealed which strategies had reduced soil wash.

The best performing maize plot was harvested early on Sept 13 at 25% dry matter, but at this DM did not reduce his 70-cow herds milk yields.

However, MGA agronomist Simon Draper advises sowing an early maturing variety when harvesting early.

The plot was then ploughed and Except winter wheat sown for whole-crop, using a combination drill at 150kg/ha (60kg/acre). "After recovering from a cold October, the wheat crop has established extremely well and will need to be slowed down," says Mr Draper.

"This plot is acting as a sponge, soaking up rainfall. This is because of good soil structure," adds Mr Smith. "The maize crop was taken off early when the land was relatively dry, so we could sow a winter crop without fear of compacting the soil. The conventional cultivations helped break up existing pans in the sub-soil, improving soil structure. The wheat then had time and optimum soil conditions to set down roots."

The plot left as stubble following maize harvesting was most affected by soil wash. Plots that were worked down into too fine a tilth by power-harrowing also showed soil erosion. "Rain can compact fine particles of soil, leading to seed-bed capping in the top 1cm of earth.

"Plots that were roughly ploughed, sub-soiled, mole ploughed and left uncropped showed little soil wash because of good drainage," says Mr Smith.

He also warns producers not to allow stock to graze in wet conditions, particularly those practising extended grazing.

"Stock grazing in wet fields can lead to surface compaction. Consequently, water will stand on the surface, but when dug with a spade, soil will be hard underneath. Where compaction has occurred it may be best to rip out grassland and reseed it."

Another source of diffuse pollution discussed at the workshop was manures and slurry spread on fields in winter. To make optimum use of nutrients available, they should be applied at times of maximum crop uptake, particularly February to April, says Mr Draper.

But Mr Drake spreads slurry at Blackhouse Farm every month because his lagoon becomes full. During the workshop, it became evident the lagoon could hold four years worth of slurry, but clean water was absorbing capacity.

This is because the lagoon and yards are uncovered and there is a lack of guttering and well-maintained down pipes around the farm yard. "Installing an effective guttering system may cost about £1000, but will solve winter slurry storage problems and costs will be recouped within a year through not having to move slurry every month," says Mr Draper.

Other sources of diffuse pollution, according to the EA, include waterlogged tracks and supplementary feeding of out-wintered stock near streams and watercourses which can lead to pollution. &#42

Andy Drake (left) and the EAs Richard Smith examine optimum soil structure to prevent soil wash from maize fields.

&#8226 Voluntary solutions.

&#8226 Maintain good soil structure.

&#8226 Avoid slurry spreading in winter.