Cereal crops following a canola rotation often produce ridges of darker green where the canola trash lay after harvesting the previous season’s swath. The darker green represented a crop health kick capable of delivering a 50 per cent greater yield than the strips between.

A similar phenomenon first occurred in the early 1990s following lupin harvesting, when lupin trash was left in ‘header rows’. These waves were attributed to the green manuring effect of header rows, which boosted soil nitrogen under the row.

No further investigations were conducted until their recurrence with canola crops spurred Grains Research and Development Corporation (GRDC) supported research to investigate the nutrient cycling relationship between wheat and canola.

Supported by growers and the Federal Government, through the GRDC, Ross Brennan of the WA Department of Agriculture tissue tested cereal crops at the crest and trough of the waves to determine the greatest discrepancies in crop properties.

It was the potassium boost which cereal crops growing over a former canola swath received that seemed to give the biggest lift. Potassium levels were, traditionally, not thought to be low in WA’s south west, but the element had gradually been exported from soils over several harvests.

Other nutrients, such as phosphorus, sulphur and copper, also stowed over in canola swaths.

While examining other potential wave producing factors, the GRDC project uncovered a surprising liming effect from burnt canola swaths. On sample paddocks with acidic topsoils (pH <4.2), the pH under the healthier cereal strips was 0.3 to 0.8 points higher.

This reveals that the nutrient boost and soil moderation from canola swaths can significantly enhance subsequent cereal crop performance. With that performance capable of driving yields

30 – 50 per cent higher, it would pay growers to identify specific benefits delivered by the swath and duplicate them across the paddock using lime and fertilisers, or a legume rotation.