Consultant's Corner: Break crop research digs beneath the surface – The research view

Caption: DAFWA researcher Wayne Parker assessing a canola trial measuring biomass growth from nutrient application.

Caption: Canola plants showing symptoms of manganese toxicity where the subsurface pH is below 4.8.

Caption: Assessing and comparing the subsurface pH of healthy and unhealthy canola plants.

A Western Australian research project has found that a high proportion of the State’s grain paddocks have soil acidity at levels that will limit the productivity of break crops.

Information generated from the Putting the Focus on Profitable Break Crops and Pasture Sequences in WA project has highlighted the importance of growers measuring subsurface pH levels, to prevent break crops being seeded onto unsuitable soils.

The Grains Research and Development Corporation (GRDC) supported project, also known as Focus Paddocks, is being conducted by the Department of Agriculture and Food (DAFWA) with assistance from WA grower groups.

“Now in its final year, the project aims to deepen knowledge about the effects of crop and pasture rotations on the farming system,” DAFWA researcher Wayne Parker said.

“Under the project, field data has been collected from the same 184 paddocks, for five consecutive years.

“Although the research is being carried out in WA, some of the project’s findings may be relevant to other grain growing regions in Australia.”

Mr Parker said testing in 2013 showed that potentially one in five WA canola paddocks with adequate surface pH levels would nevertheless have yields limited by poor subsurface pH.

“Many WA paddocks sown to other break crops, such as lupins and chickpeas, are also likely to have subsurface pH levels that limit crop yields,” he said.

“Our data shows that 15 per cent of the paddocks tested have an adequate surface pH of 5.5 or greater, but a pH less than 4.8 at 10 to 30cm below the soil surface.

“Our concern is that break crops are being unwittingly sown in paddocks with an unmeasured subsurface pH that is below critical levels, limiting the yield potential of these crops.”

Mr Parker said incorrect decisions could be made if growers did not have full knowledge of soil pH at depth.

“This is particularly true when the crop is susceptible to low pH or aluminium toxicity (which is exacerbated by soil acidity), as are the break crops chickpea, field pea and canola,” he said.

“Poor yields of these rotation crops may be the result of low pH at depth, even if there is good pH at the soil surface.”

Soil pH targets

Mr Parker said soil pH targets, as established by DAFWA and industry, were set at minimum of 5.5 in the topsoil and a minimum of 4.8 in the subsurface soil.

“At a pH of 4.8 or lower, levels of aluminium in the soil increase to toxic levels,” he said.

“Free aluminium has a big impact on crop yield as it reduces root growth, which in turn reduces the depth of soil the plant has access to.”

Mr Parker said a pH level of at least 5.5 was required in the top 0-10cm of the soil before lime could influence soil below this level.

“Lime applied to the surface will be worked in with the traffic of the seeding implement,” he said.

“This creates a layer where the pH is ameliorated to the depth of the seeding point, but no further.

“Lime must be in contact with the soil of low pH in order to react.

“This ‘layering’ effect has an impact on the yield potential of rotation crops and pastures.

“Where there is an ameliorated surface, with a pH above 5.5, but a subsurface with a pH below 4.8, the yield potential of rotation crops is reduced, as is the efficacy of nitrogen fixation.

“Despite lime having been applied, the subsurface pH remains unchanged until the lime is able to leach through the profile.”

Current recommendations

Mr Parker said current recommendations for field pea and chickpea in particular might be leading growers to sow them onto unsuitable soils.

Recommendations are for field pea to be grown on soils with a pH of at least 5, and chickpea to be seeded on soils with a surface pH of a minimum of 5 if the subsoil rises to above 5.5 within 10 to 15cm of the surface.

The optimum pH range recommended for canola is between pH 5 and 8, with significant yield loss being incurred below pH 4.6.

“However, if the subsurface has not been tested, growers adhering to these recommendations may grow break crops with disappointing yields,” Mr Parker said.

“If growers follow the recommendations and plant field pea or chickpea crops into soils with a pH minimum of 5 on the soil surface, then 42 per cent of these paddocks will in fact be unsuitable for chickpea, field pea, legume pasture and, to a lesser extent, barley and canola, due to a declining pH in the subsurface.”

Break crop decline

Mr Parker said that since the late 1990s, the area sown in WA to break crops, particularly chickpea and field pea, had steadily declined.

“Disease, weeds and crop architecture have often been blamed for this decline in the break crop area,” he said.

“But despite these issues being addressed by breeding and improved agronomic practices, the area sown to break crops has not increased.

“Subsurface acidity may be a significant factor contributing to the reduced popularity of some break crops, as poor soil pH is known to decrease a crop’s competitive ability against weeds, increase disease susceptibility and decrease yield,” he said.

“An improvement of subsurface pH could help mitigate these issues and improve the break crop’s yield potential, potentially leading to an increased area being sown to break crops in future seasons.”

Useful resources

Useful resources to help growers manage soil acidity include:

More information about research into break crops and rotations can be found at:

ENDS

More Information

Wayne Parker, DAFWA
08 9956 8555
wayne.parker@agric.wa.gov.au

GRDC Project Code DAW000213

Region West