Critical Colwell P values for wheat and canola in the high rainfall zone

Malcolm McCaskill1, Penny Riffkin1, Amanda Pearce2, Brendan Christy3, Rob Norton4, Andrew Speirs5, Angela Clough6, Jon Midwood7, Debra Partington1

1Agriculture Victoria Research, 915 Mt Napier Rd, Hamilton Victoria 3300, Australia.,
2SARDI, 74 Struan House Road, Struan, South Australia, 5271, Australia.
3Agriculture Victoria Research, 124 Chiltern Valley Road, Rutherglen Victoria 3685, Australia,
4Norton Agronomic, 54 Florence St, Horsham, Victoria 3400, Australia,
5Meridian Agriculture, 32 Henty St, Casterton, Victoria 3311, Australia.,
6Agriculture Victoria, 402-406 Mair St, Ballarat, Victoria 335, Australia.,
7Southern Farming Systems, 23 High St, Inverleigh, Victoria 3321, Australia.


Soil test interpretation criteria for cropping in the high rainfall zone (HRZ) have been derived from empirical experiments in areas of lower rainfall. To derive Colwell P values for the HRZ at which 90% of maximum yield could be expected, a series of P response experiments were conducted in southern Victoria and adjacent regions. These covered 12 site-years for wheat and 10 site-years for canola, and were combined with 4 site-years for wheat that had been conducted previously in the region. The critical Colwell-P value was 37 mg/kg for wheat and 41 mg/kg for canola. These values are higher than those derived from trials in areas of Australia’s grain belt receiving lower rainfall than the HRZ (21 and 20 mg/kg respectively). For wheat the difference appears to be related to a higher phosphate buffering index (PBI) in soils of the HRZ, while for canola possible reasons include a combination of high PBI, high yield potential and its poor waterlogging tolerance.


The Australian Society of Agronomy is the professional body for agronomists in Australia. It has approximately 500 active members drawn from government, universities, research organisations and the private sector.

Photo Credits

David Marland Photography Graham Centre for Agricultural Innovation, Charles Sturt University

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