The model separates sources of P losses into two types:
- Background (soil) losses, and
- Incidental (fertiliser and effluent) losses.
Soils supply plants with P but often are also the main source of P loss to surface water bodies. In grazed pasture systems, plants act as a source of P because grazing animals tear forage and expose P present in plant cells. McDowell et al. (2007) estimated that this source could account for on average 20% of the P lost from a paddock grazed by dairy cattle. Another plant source is the breakdown of plant residues left on the soil surface and quantity of dung.
Incidental P losses occur in situations where a flow event (rainfall) coincides with a concentrated source of available P, such as a fertiliser and/or farm dairy effluent application, leading to short-term P losses. Incidental P losses are calculated separately to background losses, but rely on the same transport factors, along with additional management factors such as the concentration, rate and timing of fertiliser/effluent application, the type of P fertiliser applied, and the depth of effluent application.
Factors that could lead to high P losses:
- High P fertiliser use
- High P fertiliser rate
- Use of a more soluble P fertiliser
- Longer duration of grazing e.g. if blocks are only grazed for two months out of the year, this will mean less P loss from plants and dung resulting in the lower overall P losses.
- Blocks with steeper topography. Note: Crop and fodder crop block types are assumed to be typically flat, this assumption is likely to limit estimates of P losses from sloping crop blocks.
- Poorly drained soils - these will have will have greater surface runoff compared to well drained soils.
- Lack of block drainage (mole/tile drains) on poorly drained soils
- High rainfall
- High Olsen P values (above optimum values)
- Soils that have low phosphorus retention (ASC, less