South West

Excess Phosphorus In Low pH Soils

This trial is currently in progress

Key Messages

  • Zero phosphorus application over four years did not reduce grain yield in any year on a grey sandy duplex soil with an initial Colwell phosphorus level of 42 mg/kg.
  • A two-tonne per hectare application of lime to soil with a pH (CaCl2) of five in the 0-10 cm zone and 4.5 at 20-30 cm, increased grain yield in years 2-4 of the project.
  • On acid soils with more than adequate Colwell phosphorus, reducing phosphorus to maintenance rates and shifting some expenditure from phosphorus fertiliser to lime can result in increased yield.
  • Trial Rationale
  • Steps Taken
  • Lessons Learnt
  • Looking Forward

Trial Rationale

Nearly 90% of agricultural soils in south-west WA have soil test phosphorus levels above the critical concentrations for crop growth, while about 70% have soil pH values that are lower than recommended levels.

Currently in Western Australia about twice as much phosphorus is applied to grain crops as is removed in harvested grains.

On phosphorus-adequate soils, there is a need to shift phosphorus fertiliser application to maintenance rates, while increasing the application of lime if the soil is also acidic. But before farmers will adopt this practice they need to be assured that the practice will maintain productivity.

Steps Taken

A replicated trial was conducted on Andrew and Rachel Plowman’s farm 21 km South- West of Kojonup by Southern Dirt and Professor Richard Bell from Murdoch University between 2014 and 2018.

The trial aimed to examine the concept of a balanced approach to managing inputs that decrease phosphorus application to a maintenance rate and increase liming to avoid sub-soil acidity.

The soil was a grey sandy duplex that had been under pasture for seven years. Colwell phosphorus level in the 0-10 cm profile was 42 mg/kg, which is well above the critical phosphorus concentration.

Soil pH (CaCl2) was five in the 0-10 cm zone, and 4.5 at 20-30 cm. This compares to minimum recommended soil pH of 5.5 (0-10 cm) to allow enough alkalinity to reach the sub-soil, and 4.8 in the sub-soil to minimise the effect of aluminium toxicity in the subsoil.

Treatments included a range of phosphorus rates between 0 and 27 kg/ha/yr, with half of the plots limed at two tonnes/ha in 2014 only, and the other half un-limed.   

Lessons Learnt

Canola yields in 2014 were high (3.4 t/ha) but were unaffected by either phosphorus rates or lime application. There was no significant effect of the phosphorus rates and lime levels on Colwell P after harvest in 2014. Lime increased pH at 0-10 cm depth from 4.9 to 5.5. The pH of un-limed plots below 10 cm was 4.5 – 4.2. Removal of phosphorus in canola seed was equivalent to 20.5 – 23.8 kg P/ha.

In 2015, there was again no effect of phosphorus treatments on barley yield. However, lime increased mean barley yield from 4.1 to 4.4 t/ha.

In 2016, lime again increased canola yield by 20 % (0.81 to 0.97 t/ha) while phosphorus treatments had no effect on yield.

In 2017, barley yields increased with liming (4.5-4.9 t/ha) but phosphorus treatments had no effect.

The results are in line with the hypothesis for the experiment. Given initial Colwell phosphorus levels of 42 mg/kg, which was well above the critical concentration for a sandy duplex, we predicted that crop phosphorus supply would be more than adequate for canola growth. The Colwell phosphorus levels, even with no phosphorus fertiliser added, have been sufficient to maintain expected yield of canola and barley.

These results, albeit from a single site, suggest that on acid soils with more than adequate Colwell phosphorus, there is scope for shifting some expenditure from phosphorus fertiliser to lime application.

Looking Forward

The trial is continuing and Professor Bell hopes to expand the trial to other sites.


Project Snapshot

Land Manager
Andrew and Rachel Plowman
NRM Region:
South West
Property Size
1600 ha
Average Rainfall:
High - 450-750mm
Enterprise Mix:
Mixed Farming
System Constraint:
Soil Acidity
Murdoch University, NLP, Southern Dirt, SWCC
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