The phosphorus cycle
Applying phosphorus (P) fertilisers to pastures with a legume base has been consistently shown to increase pasture production and significantly increase carrying capacity. Grass-dominated pastures are less responsive to phosphorus.
Phosphorus is essential in plant and animal cell structure and function.
Many Australian soils are naturally low in available phosphorus - native plants have adapted mechanisms, such as low growth rates, associations with soil organisms and a focus on internal nutrient cycling, resulting in efficient but less productive systems.
Phosphorus is frequently cited as the primary limiting nutrient in Australian grazing systems.
Establishing commercial livestock systems on these soils with low phosphorus fertility, where products are grown, harvested and transported away, means this self-sustaining system requires additional input of nutrients.
Knowing how (and how much) phosphorus moves through a grazing system helps us understand why we need to keep applying phosphorus to most pastures, and also to realise the optimum level of application to reduce the risk of harmful on-site or off-site impacts from too much, or too little, phosphorus application.
The biggest concern is phosphorus losses from the system, especially in surface run-off. Appropriate ground cover levels will avoid soil erosion, protect riparian zones and minimise surface run-off.
The off-site negative impacts of phosphorus loss need to be balanced against the two-fold to three-fold increases in carrying capacity of high phosphorus input systems relative to unimproved pastures.
Fortunately, there are strategies to assist with managing this balance.
Applied phosphorus also can be ‘lost’ over time to the system through fixation in the soil. How much phosphorus is fixed depends on soil chemistry.
In most of the soils in the high-rainfall zone, the rate at which phosphorus is fixed is far greater than the rate at which it will return to the soluble pool that plants can access. The net effect is that continued use of phosphorus fertilisers is building a bank of essentially inaccessible phosphorus in soil under pastures. The fixed phosphorus remains in the soil, but because it is strongly held by the soil, it may take many years for this phosphorus to become available again and is effectively unavailable (’lost’) to the grazing system.
Some key findings include:
- Phosphorus lost in run-off from grazed sheep pastures represents only a small financial cost to production (around 1kg P/ha/yr), but can be a risk to waterway health.
- A high proportion of surface run-off is generated from small areas of the farm that are often wet for extended periods after rain. This is more likely on dairy farms than extensively grazed livestock production systems.
- Soil and stream-bank erosion is potentially a more significant source of phosphorus in waterways than soluble sources, so give riparian area management a higher priority than controlling soluble phosphorus losses from the paddock.
To achieve the best total result, it is important to understand how phosphorus moves through the system (the phosphorus cycle); how that movement is impacted by the things you can’t control (eg soil type and rainfall); and the things you can manage (eg fertilisers, fixation, livestock losses and grazing management).