Salinity Index of Common Fertilizers

The salt index (SI) is the ratio of the osmotic potential created by a given fertilizer compared to the osmotic potential created by an equivalent weight of sodium nitrate. The higher the salt index, the greater the increase in osmotic potential caused by that fertilizer in the soil, which can make seed germination and plant growth more difficult due to increased salinity. For reference, the salt index of sodium nitrate is set at 100.

According to Mortvedt (2001), the salt indices of common fertilizers are as follows:

It is clear why the use of blended NPK fertilizers is often discouraged: blended NPK fertilizers commonly contain 2 to 4 of the above compounds, which results in a high interfering salt index. In contrast, synthetic NPK fertilizers, produced from mineral ores and elemental precursors, have a much lower interfering salt index.

Note that a high salt index alone does not indicate whether a fertilizer is good or bad. In Emami’s nutritional principles, it is better to call this the interfering salt index. For example, chlorides have a very high interfering salt index due to the presence of chlorine as an interfering element. This is also seen in sodium-containing compounds such as sodium nitrate, sodium chloride, and sodium sulfate.

To understand the concept of interfering elements, consider ammonium sulfate (salt index 69) and sodium sulfate (salt index 74). Both have similar salt indices, but ammonium sulfate is not harmful to the soil; its high salt index is due to its high nutrient concentration. On the other hand, sodium sulfate mainly causes soil dispersion damage.

[1] Salt Index (SI)