Updating the "Salt Index" of Fertilizers.

Updating the “Salt Index” – Richard Yudin Salt index numbers are widely used to predict the potential for seed damage when applying specific fertilizer materials. They are frequently given in textbooks and online, and in recent years, are seldom properly attributed. The methodology used to assign index numbers has been inconsistent. The original calculations were made around 1940 using technology then available. Unfertilized topsoils were dosed with different materials, incubated for a period, and soil column extractions made to measure osmotic pressure changes caused by addition of each fertilizer. Using sodium nitrate as a benchmark with an assigned index of 100, the results for 45 materials were ranked, and a list published in 1943. A derivative, the “partial salt index” was included in this publication to aid agronomists in predicting the effects of compound fertilizers. Fertilizers coming onto the market since 1943 were given numbers according to the electro-conductivity readings of a saturated paste of unspecified soils by commercial laboratories, without peer review. Some recently published lists show variances from the original figures, without explanation or citation of sources. A few publications have confused the salt index and the partial salt index. Farmers continue to need guidance on what fertilizers are best suited to their individual circumstances. Replication of the original soil column extraction methodology using modern materials and instruments shows that the 1943 ranking of fertilizer materials appears correct. However the method is extremely time consuming, requires purpose-built equipment, and is based on the availability of now-rare unfertilized soils. Several simpler alternative methods are under evaluation, and any that appear capable of confirming the hierarchy of fertilizers in the original list will be described.