Raised bed trials: top soil, good yields

Narrow raised beds place topsoil in aerated piles in rows leading to good soil structure and more yield per

Even in the recent dry season, the raised beds in Southern Farming Systems trials in south-west Victoria have performed extremely well.

Yield has been as good as or better than expected with conventional techniques even though with raised-bed cultivation up to 30 per cent of the paddock is 'lost' to furrows and drainage.

And, says Bruce Wightman, Regional Agronomist, DNRE, Geelong, the soil in the narrow raised beds is "in such good nick that we are presently employing the State Chemistry Laboratory and the Soils Department from Ballarat University to carry out soil chemical and physical tests".

Mr Wightman noted the drainage demonstration, conducted by farmers, "is not a replicated scientific trial. However, the soil test results do help confirm some of the dramatic effects we can see in the paddock," particularly raised beds with controlled traffic.

The site was onion grass-dominant pasture. The soil was waterlogged in winter and compacted. All experimental areas were disc ploughed and scarified. All were spoon-drained to stop water flowing onto the treatment.

Results:

Control

The soil remains quite hard and is cracked. Normal cropping season traffic traversed this site. Five t/ha of gypsum was applied just before sowing canola in 1996

Wide beds (20 m)

A road grader set up the wide beds with two passes and the area was smudged (surface levelled to reduce undulations) twice before gypsum (5 t/ha) was applied before sowing canola in 1996. Soil in the centre 6-7 m of these beds is in reasonable condition, i.e. the dry clods will break apart relatively easily. The outside 6-7 m near the drain has less topsoil and blocky soil predominates.

Underground drains

Underground plastic pipes (800 mm) and mole drains (350 mm) were installed and 5 t/ha gypsum was applied just before sowing canola in 1996.

The soil is cracking and cloddy but not as hard as the uncultivated area, control or edges of the wide (20 m) beds.

Narrow raised beds with controlled traffic

A rotary hoe bedding machine cultivated and shaped the beds. The beds are 1.5 m wide (centre furrow to centre furrow) and 200 mm high. The furrows are 500 mm wide at the top and the bed 1,000 mm wide across the top. Gypsum (5 t/ha) and subsequent fertiliser were applied to the top of the beds only.

(Mr Wightman said this means higher applications to the area where the plants are growing, even though the rate per hectare and the cost per hectare are the same.)

The soil is friable, crumbly and moist throughout the profile with earthworms in the top 20 cm. Soil moisture has been adequate enough to sow since early February

Result logical

Mr Wightman said the results follow a certain logic. "We have cultivated extensively in the narrow raised beds and placed the topsoil (plus a small amount of clay subsoil) in aerated piles in rows. This created excellent drainage where any excess water only had to find its way a maximum of 0.5 m to the furrow."

Yields

Inputs ended up at a relatively higher rate on the beds. This showed in the yield, said Mr Wightman. The canola crop grown in 1996 averaged about 3.5 t/ha over the whole drainage site. Calculated on a per-square-metre basis, the raised beds themselves yielded around 5 t/ha.

The Franklin barley crop yielded 6-7 t/ha over the whole site but, on a per-square-metre basis, the raised beds themselves yielded close to 9.5 t/ha.

No animals grazed the site and the only vehicles travelling on the beds were a windrower and a harvesting machine (twice) and they were operating when the beds were dry. Some compaction was still observed.

Mr Wightman said the most exciting results are for the dispersion and slaking soil tests. These tests measure whether soils break down in wet conditions.

"The narrow raised beds have been given a perfectly clear bill of health for these physical tests. The soil is in excellent physical condition, where the other treatments show slaking and varying degrees of dispersion."

Where next?

Growers through the GRDC have funded a full systems trial for the next five years. "We are looking forward to learning a great deal more about raised beds," said Mr Wightman.