The History of Salt Index Measurement.

The researchers who defined many of today’s commonly-used salt index (SI) figures for fertilizer materials worked  between 1935 and 1942. Their methods used soil solution extraction techniques developed two decades earlier. Osmotic pressure (OP) changes were determined by using two even earlier procedures (6,7).

After the original list appeared giving 45 benchmarks for fertilizers in common use around 1940, commercial sources of more modern plant nutrients have published additonal SI numbers. Some have cited a different OP method, others failed to identify their methods. Several authors have published warnings about these inconsistencies. Recent textbooks carry conflicting information.