The salinity index measures the direct relationship between EC and moisture. It is a ratio, and it tells you the concentration of salinity in available moisture. For instance, with moisture being consistent and EC rising, salinity concentration will go up and salinity index will rise.
We developed the salinity index to offer further insight into how salinity impacts turf performance. We found that even though a soil test report showed proper balances, we still found nutritional issues needed in the turf due to certain stresses that could be offset by sound nutrition. So many people focus on just the EC value.
Remember that EC is a measurement of ions in solution. The movement of + and - ions causes electrical current aka EC, which impacts how the turf responds. More ions moving...more EC. But for those ions to move, they require moisture. As the turf dries, ions bond together and precipitate out into solids that bind to soil particles, root and tissue fibres and organic matter. As the moisture rises, these ions go into solution (hopefully if the chemistry is acceptable) and those begin to drive EC again.
The ideal salinity index is somewhere around 1.0. However, if you have high ion concentrations in your irrigation source, or you fertilize too often, you will see higher numbers on average. You can change your warning appropriately for your course. For instance, if you are on effluent, reclaimed or even brackish influenced well water, most will set this target around 1.6 as it is very difficult to fall below that.
Once set, use POGO to help develop more uniform conditions using the DU score. The higher that number, the better. Once you achieve strong DU, then maintain your turf to consistent moisture through improved irrigation and cultural practices. From that point you can use the salinity index to indicate residual salts or higher or lower influences from your fertility program to gauge salinity as the turf sees it.
Finally, just because EC rises, it does not mean the salinity index will rise. Sometimes just a change in moisture is all the turf needs. So use both numbers to help you dial in your program!