Improving crop phosphorus supply on highly calcareous soils

The project team conducted glasshouse pot experiments in 2025 prior to establishing two replicated field trials on a grey calcareous soil on the upper Eyre Peninsula of South Australia, near Port Kenny.

“The first field trial focuses on identifying an optimal rate of CCM for cereal crop performance, while the second trial seeks to assess the impact of other P sources on wheat P status and productivity relative to CCM,” Dr Wilhelm said.

Results from these two trials in 2025 showed that:

• P nutrition was an important driver of wheat performance on a highly calcareous grey sandy soil at Port Kenny, with yields nearly doubling from very low to very high rates of CCM.
• CCM performed well, but it appeared that was largely due to the added phosphoric acid not to its biochar component.
• Highest yields were obtained with phosphoric acid as the P source.
• Supplementing biochars with phosphoric acid performed no better than applying phosphoric acid alone.
• Wheat grew poorly when supplied only with biochar.

Unfortunately, the results from 2025 are not encouraging. They did not show that CCM, or biochars supplemented with phosphoric acid, are a superior source of P for crops growing on highly calcareous sandy soils, despite CCM performing well in previous studies.

Grain from the 2025 trials has been sent to a commercial laboratory for analysis of P content as a further assessment of effectiveness of P strategies.  Grain P was 11% higher in wheat treated with CCM than with wheat treated with DAP at the same rate of P/ha. This suggests that CCM is improving P supply relative to DAP.

However, Dr Wilhelm noted that a valuable aspect of CCM as a novel P fertiliser is that it has shown residual benefits to the crop after the year of application in previous trials.

For this reason, in 2026 the two field trials have been re-sown with barley and without P fertiliser to assess residual benefits of all P sources from 2025. Prior to reseeding, assessment of P reserves in the topsoil were taken by soil sampling key treatments from 2025. It was found that while high rates of CCM (1 t/ha or greater) substantially increased reserves of plant-available P, CCM applied at the same rate of P as DAP resulted in similar and low reserves of plant-available P to DAP.

“If CCM continues to show residual benefits in crop performance, then the economic value of its use could improve substantially compared to a single season costing,” Dr Wilhelm said.

“In addition, there might be other long-term benefits for soil ecosystems after the application of CCM, such as the improvement in soil physiochemical properties, which would require further investigation.

“While one season of trials does not eliminate CCM, it is a setback for its prospects as a novel and more effective source of P for crops growing on very highly calcareous sandy soils.”