When used in a filtration basin, Mix E, filter topsoil borrow, probably only sheds phosphorus for a short time, such as one season. I would expect that by season three, the basin no longer sheds phosphorous, maybe sometimes stopping in season two.
The recommendations in the stormwater manual are sometimes taken as fact, with some regulators wanting to reject filter topsoil borrow. I believe sometimes we lost the benefits of a vegetated basin to the complete ecosystem when we are forced to switch to a plantless iron-enhanced sand basin to guarantee phosphorus reduction from the start.
Please continue to promote testing beyond a season so that we can find the long-term effects of filter topsoil borrow in filtration basins. Please link or cite research supporting the opinions of compost in filtration basins and if possible, include the duration of the research.
David Bauer, Professional Soil Scientist with MnDOT Metro District.