GRDC - High P Fixing Soils Project
Project Objectives:
High phosphorus (P) fixing soils (HPFS) exhibit properties that reduce the availability of phosphate to plants. As a result, P-use efficiency by crops is reduced. Approximately 37% of soils in the Southern and Western regions of Australia are characterised as high P-fixing, leading to poor returns on investment (ROI) in P fertiliser management for growers. This project aims to improve growers’ certainty about managing P in high-fixing soils.
To address reduced efficiency associated with HPFS, this project will undertake a coordinated, national program of research, development and extension to address knowledge gaps, identify, validate and demonstrate management practices that can improve PUE on HPFS and build grower and adviser capability in identifying, understanding and managing HPFS. These priorities have been grouped into four key R,D&E areas:
1. Increasing growers’ and advisers’ ability to predict the risk of P fixation across soils.
2. Management practices that improve crop access to currently available P.
3. Fertiliser strategies which protect freshly applied P from fixation.
4. Strategies to mobilise ‘fixed’ P from the soil in HPFS.
Project Outcome:
By 2030, 50% of growers managing high phosphorus-fixing soils in the Southern and Western regions will access improved P fertiliser application strategies that enhance P use efficiency and maximise their Return-On-Investment in high P-fixing soils.
Technical details:
Field trials:
Site selection:
The main aim for site selection was to locate a high P-fixing soil based on the current industry standard PBI test. The criteria for defining a HPFS was established for the project based on location and yield potential. For both the Western and Southern regions with moderate yield potential, the aim will be for a PBI value of greater than 150. For parts of the southern region (e.g. the Southern Mallee) where yield potentials are lower, we’ve set the PBI target of 100. PBI will be driven by soil Al and Fe components in the West (and the high rainfall region of the South) and CaCO3 content in the South.
Importantly the site must be responsive to phosphorus inputs, and we are using the combination of Colwell P (with PBI interpretation) and DGT P to achieve this. The capabilities within the project team have resulted in a high success rate of selecting P deficient sites in the past.
Prior to sowing, sites will be characterised for soil chemistry based on the following list of analyses:
Soil water content
Colwell P
pH (water and CaCl2)
DGT-P
Electrical conductivity (1:5 extract)
Phosphorus buffering index
Mineral N
Exchangeable cations (Ca, K, Mg, Na)
Organic carbon (%)
DTPA trace elements (Cu, Fe, Mn, Zn)
Exchangeable acidity
Boron
Sulfur (KCl)
MIR texture
The site will also be classified according to the Australian Soil Classification System (Isbell) and where appropriate a state-based classification system e.g. the Western Australian Soil Groups.
Locations, rotations and soil types for the output 2 experimental sites across the project:
A comprehensive experimental plan has been developed based on findings from the output 1 technical review. The review identified key knowledge gaps relating to the residual value of liquid forms of P fertiliser as opposed to granular forms, in addition to their well-established potential to improve PUE in the year of application. As a result, a core set of trials will be established across all sites to address a comparison of the residual value of these forms of P. Protocols and treatments will be aligned to enable inter-site comparisons and gain greater insights into the driving factors behind the observed results. In addition to the core experiments, collaborating organisations were asked to investigate options of local interest and relevance to the findings of the technical review which could form annual/ tactical experiments testing further opportunities to improved HPFS management or better understand the risk of HPFS to PUE in their region.
Multi-year experiments to assess the residual value of liquid versus granular P sources on HPFS
Aim:To quantify the residual value of liquid and granular phosphorus fertiliser for biomass and grain yield for up to two years after application. This trial represents the core design to be conducted across all sites and years, addressing an important knowledge gap identified in output 1.
Treatments:The design of this experiment has been structured to assess multi-year responses to P applied in a granular (monoammonium phosphate) or liquid (phosphoric acid) form. To assess residual value, some of the treatments have a fresh P application in 2026 or 2027, with nil P applied in the following years. All plots will be harvested each year. To assess the residual value of applied P, the biomass and grain yield from the nil P applied plots in 2027 and 2028 is used to compare to the yield response to fresh P applied in those years.
Kojonup Trial Design:
Annual experiments to understand the spatial distribution of HPFS and interactions with varying P management strategies on HPFS
Aim:
To quantify which soil properties of HPFS are associated with spatial variation in crop response to P fertiliser application.
Treatments:
For sites located in WA, small plot experiments will be conducted at 4 locations within the same paddock where the multi-year experiment is located. Each experiment will have 4 replicates of nil and 50 kg P ha-1 as MAP to measure PUE across soils with varying fixation capacities.
Annual experiments to test in-season P management strategies on HPFS
Aim:
To assess a range of in season P applications to better match crop demand with P supply and potential P fixation when applied all at sowing.
Treatments:
For sites located in WA, specific in-season management strategies will be tested as part of treatments 25-27 in the residual value of liquid versus granular P trial. This will include comparison of treatments where a reduced rate of granular P is applied at sowing, followed by small rates (<5 kg P/ha) of in-season liquid P during tillering.
Funders
