Reduce P Loss
1 min read
Phosphorus (P) is essential for plant and animal growth but can harm the environment if managed improperly. This page discusses ways to minimise P loss on your dairy farm. Suggestions include managing fertiliser use, effluent, controlling erosion, and installing buffers between water sources and paddocks. Critical Source Areas (CSAs), high risk parts of your farm for P loss, should receive special attention. Good farming practices help mitigate P loss and involve monitoring soil P levels, ensuring accurate farm records, and managing operations to minimise nutrient loss. Remember, reducing P loss not only protects the environment but also optimises your farm's productivity.
There are many options to reduce the amount of P that enters the environment. This page will help you identify opportunities on your farm to reduce P loss.
You can reduce P loss by carefully managing P in fertiliser and effluent as well as managing sediment and faecal matter losses.
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.
Below are the areas and actions you can take to reduce P loss on your farm.
P is a chemical element used by plants and animals for growth. It is found naturally as rock-phosphate, in sedimentary and igneous rock. Rock-phosphate is mined and processed in the manufacture of inorganic chemical fertilisers.
Supplies of P are finite and come from just a few key countries globally, with Morocco, Western Sahara, USA, China and Russia responsible for more than 75% of the worldwide raw material production. Food production in New Zealand is therefore potentially vulnerable to global P supply shocks.
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.
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). As P readily attaches to soil and organic particles, unlike N (nitrogen), only a very small proportion is leached.
Because phosphate tends to be 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.
P loss refers to how much of your phosphorous (P) inputs, either from fertiliser or effluent, are lost via surface runoff, and/or leaching into waterbodies.
P loss is mediated through the hydrological cycle. Rainfall and soil type are important factors controlling 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, and leaching on freely draining soils that have a low ability to bind P.
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 being lost to water. This is why soil testing is important.
The loss of P to water from dairy-grazed crop and pasture systems and catchments can vary widely from 0.5 – 4.2 kg/ha/yr, respectively.
Eutrophication is caused by excess nutrients (P and N) in water and can cause algal blooms and oxygen depletion, which is detrimental to freshwater ecosystems. Furthermore, human health issues can also arise due to toxic blue-green cyanobacterial blooms.