Current Fertilizer Salt Index Tables are Misleading

Abstract

In the 1940s, fertilizer materials that were available at that time were evaluated for changes that occurred in the soil solution osmotic pressure upon application. The term “salt index” was then used to compare the increase in osmotic pressure for 45 fertilizer materials against the same weight of sodium nitrate. During the 1950s a simplified laboratory method was developed where salt index was measured by electrical conductance rather than by osmotic pressure. Current salt-index tables in fertilizer references have combined the values from the two methods. The objective of this study was to evaluate five potassium (K) sources by the electrical-conductance method. Sources included potassium chloride, potassium sulfate, potassium nitrate, sulfate of potash magnesia, and potassium thiosulfate. Salt-index values obtained from the laboratory method are significantly higher than values reported in the original soil study. These differences range from 28.6% for potassium chloride to 141.2% for potassium sulfate. These limited results with potassium fertilizer materials indicate that a new system for evaluation should be developed or that current fertilizer materials should be evaluated under the same set of experimental conditions.