I have studied of Phosphorus and degree of phosphorus saturation in some soils in the Vietnam and Olsen'S test was used to analysis the available phosphorus in soils.
What is the threshold of Degree of phosphorus saturation to runoff Phosphorus into the environment?
Most of P present in soil is sorbed to clay particles. The Olsen soil test extract only part of P that is desorbed at pH 8.5. However, most part of total P in soil is not dissolved in Na bicarbonae.
To estimate losses of P by soil erosion you have to collect the water flow with soil particle during rain event and evaluate the content of clay in the runing water and total water flow from a soil area.The analysis of total P in the clay and the solution is needed to calculate the total P loss from a contributing area to the water flow.
A large part of the P released in the environment is not soluble P, but P adsorbed on soil particles. The run-off of these fraction of P is not strongly dependant of the soil P content but on the amount of soil released in the water (rivers, lake…) by the run-off. Afterward in the water the P adsorbed on particles is (more or less fastly) released and can affect ecosystem functionning. So, I think that collect run-off and measure the P concentration of this run-off could be a good solution.
Mr.Long , you have to estimate an additional parameter to represent soil solution P-0.01 M CaCl2 extractable P(1:5 soil:solution ratio).Then you have to plot the relationship between Olsen'P and CaCl2 soluble P.Above a certain level of Olsen P there will be a rapid increase in CaCl2 soluble P.If Olsen P in soil exceeds that threshold limit ,water soluble P will be subjected to the risk of leaching losses.Percent P saturation values can also be ploted with CaCl2 P values to know at what percent P saturation the solution P rises rapidly in different soils.
The following two papers may be consulted.
The leaching of soil phosphorus:a hundred years of getting it wrong. by Hesketh Nick and Brookes Phil Scientific registration no:2033 Symposium:08 Presentation :poster.pages 1-4.Available online.
Heckrath,G.,Brookes,PC.,Poulton,PR, and Goulding,KWT (1995).Phosphorus leaching from soils containing different phosphorus concentrations in the Broadbalk experiment.Journal of Environmental Quality,24,904-910
Mr Long, you also need to factor in the phosphorus buffering capacity of the soil/s you're studying i.e. the size and strength of P sorption on to soil surfaces. Take a look at a couple of my papers that discuss this:
Hart and Cornish 2014. An agronomic test to manage phosphorus environmental risk from pastures with good grazing management
Hart and Cornish 2016. Soil phosphorus status and environmental risk of phosphorus run-off from pastures in south-eastern New South Wales.
The first is on my profile, send a request if you would like the full paper of the 2016 one.
Mr.Long you have to distinguish or separate the leaching losses from runoff losses.Degree of P saturation and threshhold level of P saturation are more relavant to leaching losses of P through soil profile.As explined by Drs.Kafkafi and Fort,run off losses depend on soil erosion.Both particulate and solution P can move with run off.It may not depend much on percent saturation of P adsortion sites rather depends on rainfall intensity ,duration and number of run off events.
The following brief article provides details of how phosphorus moves from land to water and what processes and factors influence it.
Phosphorus movement from land to water.By Ebeling,A.M.etal.
http://ipcm.wisc.edu/download/pubsNM/P_Movement_landwater.pdf
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