The Phosphorus Site Index (PSI) and the Phosphorus Management Tool (PMT) are tools used to determine the risk for phosphorus runoff into nearby waterways. The PSI has been used for many years on fields with a Fertility Index Value (FIV) of 150 or greater. This is used to determine the safety of applying manure or phosphorus-containing fertilizers on fields with an FIV-P of 150 or greater.
Additional scientific studies determined that the PMT was a more accurate method then the PSI, although in some situations the PMT is more restrictive. For this reason, Maryland regulators decided that the PMT would be implemented in order to give farmers time to transition to the PMT.
Currently, when a field has a FIV-P of 150 or greater, the PSI and the PMT are both reported in the Nutrient Management Plans. The PSI is used, but the PMT is also provided to show the different expectations. This gives operators advance notice of what will be required in the near future.
MDA is using a tiered system to determine when the PMT requirements will replace what is dictated by the PSI. As your nutrient management plan is updated, if you had fields with an FIV-P of 150 or greater, you will be placed in a tier A, tier B, or tier C category. The tier you are placed in is determined by the average soil phosphorus FIV result of the fields in your operation that have an FIV-P value of 150 or greater.
Your tier category determines when you will begin using the PMT instead of the PSI. The following shows PMT start times for each tier group.
Take a look at your nutrient management plan to see what your tier is. Can’t find a tier determination? If you don’t have fields with a FIV-P of 150 or greater, you do not need to be placed into a tier.
Reference source: “Farming with Your Nutrient Management Plan”