Comparison of Manure and/or Fertilizer Recommendations Based on Phosphorus Loss Category as Determined by
the Phosphorus Site Index (PSI) and University of Maryland - Phosphorus Management Tool (UM-PMT)
Farm & field ID
PSI
category
Manure
Application
Recommended
(tons or gallons
per acre)
Commercial
Fertilizer
Recommended
(pounds
phosphate or
P2O5 per acre
UM-PMT
category
Manure
Application
Recommended
(tons or gallons
per acre
Commercial
Fertilizer
Recommended
(pounds
phosphate or
P2O5 per acre
The relative UM-PMT scores (low, medium or high) for the fields on your operation may be quite different than the PSI scores (low,
medium, high and very high) for the same fields. This is due to a change in the formula used to calculate potential for phosphorus loss.
The PSI formula averages the risk of the 3 phosphorus loss pathways (runoff, erosion or particulate and subsurface drainage). This
practice masks a dominant loss pathway if/when the other 2 loss pathways pose little risk. The UM-PMT formula calculates the risk of
phosphorus loss from each of the 3 pathways separately and then adds the risk factors together. The UM-PMT formula is believed to
more accurately reflect actual P loss risk from a field.