Different fertilizers dissolve in water at different rates, and hence have different osmotic potentials. The osmotic potential of a fertilizer is called its salt index. One way to think of the salt index is as a measure of solubility. The more soluble, the higher the salt index.
Salt index values are a measure of a material’s relative tendency to increase the osmotic pressure of the soil solution as compared with the increase caused by an equal weight of sodium nitrate. The salt index of sodium nitrate is 100. The higher the salt index, the greater the potential of a material to increase the osmotic pressure of the soil solution and thus the potential for bum. As indicated in the chart below, there are wide differences in the salt indexes of fertilizer materials.
As a general rule, organic fertilizers and controlled-release fertilizers have a lower salt index. Quick-release fertilizers have a higher salt index.
Use fertilizers with a low salt index in soils with a salt or drainage problem, or if you can’t water it in after spreading. The higher the salt index, the more likely the fertilizer is to burn plants. However, even fertilizers with the highest salt index are safe if diluted properly after spreading.
Salt Index Values for Some Garden Materials
|Material||% Nutrient||Salt Index|
|Ammonium Nitrate||33% N||105|
|Ammonium Sulfate||21% N||69|
|Calcium Nitrate||15.5% N||69|
|Epsom salts||16% MgO||44|
|Potassium Chloride||60% K2O||114|
|Potassium Sulfate||50% K2O||46|