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Understanding P loss

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  • Updated

The model separates sources of P losses into two types: 

  1. Background (soil) losses, and
  2. Incidental (fertiliser and effluent) losses.

 

Background (soil) losses

Background or soil P losses arise from P that has had an opportunity to react with the soil and is lost in flow events that may occur throughout the year. It is affected by how much P in in the soil (for example, Olsen P), the ability of the soil to retain P (for example anion storage capacity), structural integrity of the soil (clay and carbon content of soil), rainfall, overland flow potential (affected by soil properties determined by soil group, soil order, drainage class), topography, deer behaviour (fenceline pacing, wallows). 


Incidental (fertiliser and effluent) losses 

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 form and solubility of P fertiliser applied, and the depth of effluent application, effluent management.
 
Factors that could lead to higher P losses:

  • High P fertiliser use
  • High P fertiliser rate
  • Use of a more soluble P fertiliser
  • Fertiliser/effluent applied during high (rainfall) risk months
  • Increase in effluent application depth
  • High effluent rate
  • Longer duration of grazing e.g. if blocks are only grazed for two months out of the year they will have less exctreta deposited on them than blocks grazed year round.
  • 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.] 
  • More 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
  • Higher rainfall
  • Higher irrigation rates
  • High Olsen P values (above optimum values)
  • Soils that have low phosphorus retention (ASC, less
    than 15%)

 

 

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