Extractable Phosphorus and the Application of Phosphate Fertiliser in Pasture Soils in Southern Victoria

Year of study:


Lead organisation & Collaborators:

Department of Natural Resources and Environment
LaTrobe University


Lucy Burkitt


Project details


Research was conducted to assess how extractable phosphorus concentration response to P fertiliser application at various levels varies between different soil types.


Underlying hypothesis is that increased phosphorus application increases extractable P concentrations

Basis of trial:

Increased phosphorus application increases extractable P concentrations.

Best available science assessment:

Overal score: 32

Overall rating: Very high

Objectives Conceptual model Study design Soil analysis Production & financial
3 4 4 1 4
Method reporting Data analysis Results reporting Publication
4 4 4 4

Location details

Trials were conducted in high rainfall zones of Southern Victoria. All sites where situated in zones where the mean annual rainfall was greater than the 40 year average (>800mm).
The soils examined in this study represented a broad range of soil types used for dairy production in southern Victoria (Sandy Loams, Loams, Clay Loams, and Clays). Soils where selected for low to moderate P concentrations (and most soils were moderately acidic)

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:

All sites had permanent pasture, comprising of predominantly perennial ryegrass and white clover. Stocking rates varied between sites, with sites grazed as following:


Rotationally grazed by dairy cows

Rotationally grazed or set-stocked by cattle cows

1. Outrim

2. Athlone

3. Yarragon

4. Ellinbank

5. Strzelecki


7. Curdievale

8. Glenormiston

9. Terang


Management practices tested:

Fertiliser application

Summary of key findings of trial:

In general, simple, direct measures of soil P sorption could allow the estimation of extractable phosphorus concentration on different soil types.
Simple, direct measures of P sorption capacity, such as PSI800, were closely related to extractable phosphorus values and may be more reliable indicators of extractable phosphorus compared with other soil properties. This is a key finding which could allow the estimation of extractable phosphorus on different soil types, without the need for detailed field studies and time-consuming laboratory measurements.
The application of SSP in this study was found to be more effective for some soils in increasing the extractable phosphorus value in comparison with TSP, at very high P additions, due possibly to competition between the phosphate and sulfate anions.

Experimental Design


At each of the 9 sites, 18 treatments of P fertiliser and lime were applied in three replicate to plots.
Treatments 1–16 received a single application of TSP [containing 20.2% P, 16% calcium (Ca), and 1% sulfur (S)], SSP (containing 8.8% P, 22% Ca, and 12% S), and TSP with burnt lime (CaO).
Treatments 17 and 18 (TSP applied at 35 and 70 kg
P/ha) were reapplied every 6 months

As before – P removed due to grazing and product exports were considered minimal compared with estimates of P sorption.
Summary of treatments (in kg P / ha)

Treatment no.


Treatment no.





SSP 35


TSP 17.5


SSP 70


TSP 35




TSP 52.5


Control with lime


TSP 70


TSP 35 with lime


TSP 105


TSP 70 with lime


TSP 140


TSP 140 with lime


TSP 210


TSP 35 (applied every 6 months)


TSP 280


TSP 70 (applied every 6 months)


Trial design/layout:

Each site included plots 2 m by 10 m, in a randomised resolvable incomplete block design. The blocks contained 9 blocks with six treatments per block with a total of 54 plots per site.

Soil sampling method

Initial extractable P concentrations between each plot of the field experiments were measured immediately prior to the application of the treatments, by taking 15 randomly spaced cores, 10 cm in depth from each of the 54 plots. Soils from each plot were bulked and analysed for P (Olsen and Colwell) Treatments 1,5,13,14,15 and 16 were analysed for pH.

Soils were similarly sampled and analysed for P (Olsen and Colwell). Treatments 1,5,13,14,15 and 16 were analysed for pH.

Samples were taken 6 and 12 months from April 1998.

Soils from each plot were dried at 40˚C for 48 hours and passed through a 2mm sieve before being analysed for P (Olsen and Colwell)

Soils were tested for:

  • pH (CaCl2)
  • pH (H2O)
  • P (Olsen)
  • P (Colwell)
  • Al (Raymen)


Production measurements

P removed due to grazing and product exports were considered minimal compared with estimates of P sorption rotated after trial plots.


Treatment Results

Variation in soil pH with time and management influence:

The application of lime significantly increased soil pH (H2O and CaCl2) and decreased the concentration of exchangeable Al, 6 months after treatments were applied, but generally had little impact on extractable P concentrations.


Other soil treatments:

In general, simple, direct measures of soil P sorption could allow the estimation of extractable phosphorus concentration on different soil types. The application of P in the form of SSP resulted in a trend for higher extractable phosphorus values than occurred with TSP.



How the results have been reported:

Journal Article (Australian Journal of Soil Research)

How a copy of any relevant reports can be obtained:

Australian Journal of Soil Research
CSIRO Publishing
PO Box 1139 (150 Oxford St)
Collingwood, Vic. 3066, Australia
Telephone: +61 3 9662 7628
Fax: +61 3 9662 7611
Email: sr@publish.csiro.au

Level of review of results:

External peer review

Next steps

The results demonstrate the importance of targeting P fertiliser recommendations to specific soil types, in order to improve the economic and environmental efficiency of P fertiliser use on pastures in the region.