Most fertilizers are salts containing macro and micro nutrients which, when added to soil, cause an increase in the osmotic pressure of the soil solution. Fertilizers vary with regards to their osmotic effects and their potential for crop injury. They are measured in comparison to a (w/w) standard reference material, sodium nitrate, giving a ratio referred to as the salt index .
The Jackson (1958) method  is the most recent employed and was used to assess a range of potash fertilizers. Results from seven independent laboratories show a lower salt index for POLY4 than MOP, SOP and SOP-M (Figure 7).
POLY4’s relatively lower index supports a plant’s ability to absorb water and the nutrients contained within the soil solution.