Biochar study raises sandy soils doubts

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A Western Australian study shows that applying biochar made from wheat chaff and eucalyptus wood may have some negative effects on sandy soils

Key points

  • PhD student finds biochar decreased nitrogen mineralisation and plant growth
  • A high rate of biochar (25t/ha) decreased nitrate and ammonium leaching
  • Different biochar types and soils may produce varying effects

A three-year research project has raised some questions about the viability of biochar as an ameliorant for increasing the profitability of broadacre crops on sandy soils.

Biochar is a carbon-rich solid created by heating organic material such as stubble, animal manure or wood under oxygen-limited conditions. 

It is considered a potentially useful means of increasing soil fertility, as some studies have shown it may enhance the availability of potassium and calcium in soils and lift nitrogen use efficiency.

However, University of Western Australia (UWA) PhD student Daniel Dempster says his research on sandy soils produced insufficient evidence to suggest biochar will improve soil fertility.

Mr Dempster was awarded a GRDC postgraduate research scholarship to study the interaction between biochar and soil nitrogen in 2009. 

Image of a man standing in glasshouse inspecting wheat experiment

PhD student Daniel Dempster used
glasshouse and field trials to investigate
the impact of biochar on the nitrogen
status of WA soils. While the trials
showed biochar reduced nitrate leaching,
it also reduced nitrogen mineralisation
and plant growth.

PHOTO: Nicole Baxter

Working with Professor Daniel Murphy, Dr Zakaria Solaiman and Associate Professor Deirdre Gleeson from UWA, Professor David Jones from Bangor University in Wales and Nadine Hollamby from the Liebe Group at Dalwallinu, WA, Mr Dempster conducted research in the glasshouse, laboratory and the field.

Deep pale sand collected from north-west of Moora, WA, was placed into pots and planted with wheat. Jarrah (Eucalyptus marginata) biochar was added at nil, five tonnes per hectare and 25t/ha. Also applied were organic and inorganic sources of nitrogen.

Contrary to expectations, Mr Dempster says the addition of 25t/ha of biochar decreased plant growth and microbial biomass significantly compared to the nil treatment.

In addition, he says nitrogen mineralisation was decreased significantly in the presence of biochar and might be the reason for the poorer plant growth.

To assess the impact of biochar on nitrogen leaching in a sandy soil from Meckering, WA, Mr Dempster ran another glasshouse trial and compared the results to those achieved by adding clay – another ameliorant being used by some growers to enhance soil fertility.

Jarrah biochar and clay were each applied at 25t/ha. To accentuate the results, large amounts of water were added to the treatments.

Mr Dempster says both the clay and biochar significantly decreased ammonium and nitrate leaching.

He says biochar decreased nitrate leaching more than the clay treatment, possibly because the biochar decreased the conversion of ammonium into nitrates (nitrification).

In a replicated field trial, Mr Dempster investigated the effects of adding 4t/ha of wheat chaff biochar as a banded treatment into the subsoil and compared it with 4t/ha of wheat chaff biochar spread on the soil surface. On top of the biochar treatments, urea was applied at nil, 20kg/ha and 40kg/ha.

The results showed yield decreased significantly from 2.1t/ha to 1.7t/ha when biochar was banded into the subsoil. Yield was also decreased to 1.8t/ha when biochar was spread on the surface but the result was not statistically significant. There was no significant difference in grain nitrogen content between the treatments.

“On the basis of my results it would be difficult to justify a case for adding nutrient-poor biochar made from wheat chaff or eucalyptus wood to sandy soils,” he says. “But the results may be different on other soils types or using nutrient-rich manure sources of biochar, or different rates of application.”

More information:

Daniel Dempster

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GRDC Project Code GRS153

Region West, North, South