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Balancing Phosphorus and Lime

Balancing Phosphorus and Lime

Kojonup, Western Australia

  • Author: SWCC
  • Date Posted: Aug 27, 2014
  • Category:
  • Address: Kojonup, Western Australia

Project Details

Project Delivery: Southern DIRT

Contact: Peter Clifton (Bunbury). Ph: 9724 2469. Email:

Website: SWCC Sustainable Agriculture

Start Date:  June 2014                     End Date: March 2016

Site ID:  IN7

Size Are Ha: 0.1


Project Aim

Based on soil testing, about 87% of WA soils have more than sufficient Phosphorus for crop production but 70% are still too acidic. Current practice continues to build up soil P and often neglects soil acidity. This project aims to increase grower confidence in using maintenance phosphorous (P) fertilizer rates and more lime to increase pH where soil tests suggests that it is safe to do so.

‘We previously had a grant to do soil testing to 30cm depth and discovered that our subsoil pH was scarily acidic. This is an old farm that’s had fertiliser applied for a lot of years. Our phosphorus levels are pretty healthy, so rather than just putting money into phosphorus all the time we’d like to allocate more into our lime program. We want to know if reducing phosphorus fertiliser will adversely affect productivity. We want to be profitable and improve the way we farm so we can keep farming.’ – Andrew and Rachael, participating farmers.


The objective of the trial is to determine the most effective and efficient method of applying P to maintain sufficient levels while ameliorating
soil acidity by increasing the application of lime. The trial will take place on a single farm SW of Kojonup with sandy soils. Phosphorus levels at the site are 42 ppm, above the critical level of 30ppm for these soils.

The trial involves five phosphorus treatments, each replicated in 4 individual plots each (i.e. 20 plots):

  1. Replacing P removed in harvested grains;  2 kg P/ha/year
  2. Replacing P removed in harvested grains plus the estimated P loss by fixation to soils and by leaching and runoff;  13.6 kg P/ha in 2014, 2015 will be determined after 2014 harvest
  3. Small amount of liquid starter P at seeding;  2 kg P/ha/year
  4. No P fertiliser
  5. Double P rate of (2) to verify that optimal P supply has been provided;  27 kg P/ha in 2014, 2015 will be determined after 2014 harvest

The 20 plots will also be divided into two lime treatment: no lime; and liming at 2 tonne/ha, creating 40 plots in total.

Canola will be sown in 2014 and a cereal in 2015.

Key performance measures include:

  1. Greenseeker assessment 6 – 8 weeks after sowing in 2014 and 2015 to determine variations in crop biomass
  2. Shoot biomass in 4 random quadrats per plot in 2014 and 2015. This assessment will take place at early podding in 2014 (canola) and at anthesis in 2015 (cereal).
  3. Soil samples – 6 random 0-10, 10-20, 20-30 cm depth samples for whole site (at sowing); 0-10, 10-20, 20-30 cm depth for each plot (after harvest) from 4 locations per plot in year 2014 and 2015
  4. Crop yield- whole plot harvester will be used over the whole plot minus the border areas in year 2014 and 2015
  5. A grain sample will be harvested from each plot and the grain analysed to determine P removal in 2014 and grain quality in 2015.



19 Nov 2014 – The first season has progressed well with results from early data indicating minimal differences between plots of canola treated with nil and varying rates of P fertilizer at seeding. It will be interesting to see if this trend will follow through with harvest assessments.


This project is supported by Murdoch University and CSBP.

*Join the growing number of farmers and industry advisers receiving trial updates by subscribing to SWCC’s Sustainable Agriculture E-newsletter.  Simply email with “Subscribe to SWAG” as the subject.



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