You can reduce P loss by carefully managing P in fertiliser and effluent as well as managing sediment and faecal matter losses.
Practical P loss tips
- ✔ Fence streams to exclude stock from waterways
- ✔ Use riparian planting as a buffer between paddocks, races and the water. The plants act as a filter, slowing down runoff and catching sediment and P. See the Riparian Planner Tool.
- ✔ Carefully select your paddocks, grazing routines and crops to minimise disrupting the soil and runoff to waterways, particularly during winter months. See Wintering cows on crops.
- ✔ Maintain races and direct water running off them away from waterways and into paddocks through the use of cut-offs. See Waterways.
- ✔ Control slips and hillside erosion to prevent losing valuable topsoil and P to waterways. See Erosion.
- ✔ Direct any effluent that collects on hard surfaces such as concreted races, bridges or culverts into your effluent system. See Effluent.
- ✔ Create greater effluent pond storage and defer irrigation to land
- ✔ Enlarge areas receiving dairy farm effluent
- ✔ Defer and/or apply effluent at low/variable rate
- ✔ Avoid applying farm dairy effluent to freely draining, stony soils under irrigation
- ✔ Test soil regularly and manage P fertiliser application to ensure Olsen P levels are within the optimum range; this minimises the amount of P leaving your farm in loss events. See P Fertiliser.
- ✔ Use good management practices to apply P fertiliser and effluent to ensure they do not run off to waterways.
Critical source areas
Managing Critical Source Areas (CSAs) is also important for reducing P loss. This concept ranks areas according to their potential to act as a P source and their potential to transport or lose P.
CSAs are those that have a high source and transport potential and can account for the majority of the P loss despite coming from a minority of the catchment’s area. They tend to be small, low-lying parts of farms such as gullies and swales where runoff accumulates in high concentration during rainfall events.
Steeper slopes and steeper stream banks are at greater risk of erosion and collapse, especially during high rainfall events and any activities that disturb soils or streambanks can cause losses of P. The cost-effectiveness of mitigations is on average greater when targeted to CSAs as opposed to untargeted implementation across catchments.
Adopting Good Farming Practice reduces the risk of P losses.
Good farming practice principles
Below are the areas and actions you can take to reduce P loss on your farm.
About phosphorus (P)
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Knowing how P enters and moves through a dairy farm system is important to understand where it can potentially be lost to the environment, e.g. waterways. The diagram below shows the on-farm P cycle including where P runs off into water.
On-farm P cycle
P is typically imported into a dairy farm system via fertiliser and purchased feed. Phosphate is the form of P that can be taken up by plants.
Of the phosphate eaten by the cow in grass or supplement, approximately 30% will leave the farm in products such as milk and meat. The remainder will be excreted by the cow as dung.
P is lost from the farm and potentially to waterways mainly via surface run-off (eroded soils, effluent, fertiliser).
Because phosphate is relatively insoluble and attaches strongly to soil particles, a large proportion of the P added to the system will be retained or “fixed” to soil particles. Therefore, activities which disturb soil can also contribute to losses of phosphate via erosion and surface water runoff. P is also lost by direct deposition or runoff of dung, fertiliser or farm dairy effluent to waterways.
As P readily attaches to soil and organic particles, unlike N, only a very small proportion is leached.
Where phosphorus ends up after entering a farm system
Relative to N, only a small proportion of P is lost or transferred to waterways.
While this may not look too concerning, this small amount can have a damaging impact on the quality of the receiving water body.
Phosphate losses are closely linked to sediment and faecal matter.
P tends to bind to soil particles (sediment). Dung and farm dairy effluent contain high levels of P. Therefore, losses of P on dairy farms are closely linked to losses of sediment and faecal matter.
How phosphates run off into surface water
P loss refers to how much of your phosphorous (P) inputs are lost or leached to waterways.
P loss is mediated through the hydrological cycle. Rainfall and soil type are important factors regulating the rate of P loss.
There are three ways that P is lost from dairy land to water.
Surface runoff – during high rainfall events or when the soil is saturated P in dung, fertiliser or farm dairy effluent can be lost to waterways, particularly if applied on land with a slope.
Sub-surface flow – for example lost via mole, pipe drains, or macropores.
Erosion and soil movement – P is sticky and binds to soil particles. Any activities that disturb the soil, such as pugging, cultivation or erosion, can cause losses of soil particles and therefore losses of P. Careful land use and good management practice is key to soil (and P) loss mitigation (e.g., management of winter forage crops, avoiding overgrazing, pugging, and compaction).
Increased applications of P (fertiliser or effluent) may result in more P being bound to each soil particle. This increases the amount of P that is at risk of running off to water.
