Effects of the application or withholding of P fertiliser on bicarbonate-extractable phosphorus

Year of study:


Lead organisation & collaborators (if any):

Department of Natural Resources and Environment

Department of Agricultural Sciences, LaTrobe University


Cameron Gourley

E: cameron.gourley@dpi.gov.au

Best available science assessment:

Overall Score: 32

Overall rating: Very high


Conceptual model

Study design

Soil analysis

Production & financial

Method reporting

Data analysis

Results reporting











Project details


Research was conducted on nine sites with different soil types and phosphorus buffering capacities to assess how the effectiveness of P fertiliser application (expressed as the change in extractable phosphorus per unit of P applied) varied.



Phosphorus is a limiting nutrient for production, but the use of P fertiliser and how it reacts with soil and consequential impacts on plant uptake of N is an important process. Previous trials have considered the impact on P fertiliser effectiveness over time across various fertilisers, but this trial considers the impact of P fertiliser effectiveness across different soil types.

Basis of trial:

The effectiveness of P Fertiliser over different soil types is not well established and is considered in this trial.

Location details

Trial Site Details:

Dry Conditions during trial.

Mean rainfall >800mm/a

Soil Type 1 Outtrim 2 Athlone 3 Yarragon 4 Ellinbank 5 Strzelecki 6 Tynong 7 Caurdievale 8 Glenormiston 9 Terang
Coarse sand (%) 17.8 6.8 4.3 3.1 2.5 12.8 22 10.3 3.7
Fine sand (%) 40.8 37.4 30.2 20.2 18.4 15.3 53.6 15.8 50.2
Silt (%) 18.3 31.3 37.8 27 32.5 17 11.3 22.8 20.8
Clay (%) 5.3 12.3 17.5 35.8 30.8 31 6 32.3 15.3


Pre-trial management:

Sites 2,3,6 and 7 were rotationally grazed (dairy)

Sites 1,4,5,8 and 9 were set-stocked or rotationally grazed (beef).

Various stocking rates

Permanent pasture (perennial ryegrass and white clover)

Methods and Measures

Management practices tested:

P Fertiliser and lime application

Soil sampling method:

Soil from each site was sampled immediately prior to treatment.

Tested every 6 months. Tested for:

  • pH(CaCl2)
  • Olsen P
  • Colwell P
  • Organic C (Walkley Black)
  • Exch H
  • Exch Al
  • ECEC
  • Oxalate-extr Fe
  • Oxalate-extr Al
  • PBC (P sorption curve)
  • PBI


Summary of key findings of trial:

The application of P fertiliser to low P buffered soils was generally more efficient in the short-medium term (6-30 months) (due to greater extractable P/kg of P fertiliser applied). Application of smaller rates of P fertiliser resulted in significantly higher DEP compared to singular applications (smaller applications had stronger results).

The application of SSP, TSP and TSP with lime had little impact on DEP values 18-30 months after treatments were applied (pH changes cannot be attributed to differences in DEP in this trialTable of results

Variations in soil pH with time and management influence:

The application of lime had no consistent effect on DEP values (change in extractable P per unit of P applied).

Other soil treatments:

The application of P fertiliser to low P buffered soils was generally more efficient in the short-medium term (6-30 months) (due to greater extractable P/kg of P fertiliser applied).

Experimental design


The following treatments were applied to each site:

Treatment no. Treatment Treatment no. Treatment
1 Control 10 SSP 35
2 TSP 17.5 11 SSP 70
3 TSP 35 12 SSP140
4 TSP 52.5 13 Control with lime
5 TSP 70 14 TSP 35 with lime
6 TSP 105 15 TSP 70 with lime
7 TSP 140 16 TSP 140 with lime
8 TSP 210 17 TSP 35 (applied every 6 months)
9 TSP 280 18 TSP 70 (applied every 6 months)

Where TSP (triple superphosphate) and SSP (single superphosphate) are provided in kg P/ha, and lime is a 5t/ha treatment.

The treatments were applied in a single application in 1998. Treatment 17 and 18 was reapplied every 6 months.

Trial design/Layout

At each site, 18 treatments were applied. The layout included 3 replicates to 2 x 10m plots (randomised resolvable incomplete block design- each of the treatments were split in 3 blocks with 6 treatments/block)


How results have been reported:

Journal article

Burkitt L, Gourley C., Sale P. (2002): Changes in bicarbonate-extractable phosphorus over time when P fertiliser was withheld or reapplied to pasture soils, Australian Journal of Soil Research, CSIRO, 40, 1213-1229

How can a copy of any relevant reports be obtained:

Online journal article




Level of review of results:

External peer review

Next steps

P fertiliser application at 35 or 70kg P/ha were greater than the soil maintenance requirements of the soils tested in this trial.

Further research is required to determine maintenance P fertiliser requirements.