Fertiliser Salt Index

The salt index of a fertiliser is a measure of the concentration that fertiliser induces in the soil solution. It is expressed as the percentage increase in osmotic pressure in the soil salt solution produced by the application of a specific fertiliser, and is measured as the increase over the same weight of sodium nitrate, which is taken as a reference. The Salinity Index is dimensionless and values above 1.5 are considered high.

Fertilisers vary in their ability to attract water from the plant and thus cause scorching.

NaNO₃ --> Na⁺ + NO₃- ↑↑ Solubility. Measurement reference

NH₄)H₂PO₄ --> NH₄ᶧ + H₂PO₄- ↓↓ Low Solubility. Low Salinity

This can be avoided by using fertilisers with low salinity rates, especially on hot days and/or by applying fertilisers and watering afterwards.

Sodium nitrate was chosen as the standard because it is 100 % water soluble and was one of the most common nitrogen sources when this concept was proposed in 1943, J. J. Mortvedt et al.

Thus, nitrogenous or potassic fertilisers have higher salinity indices. than phosphates. It is also interesting to understand the different acid or basic reactions that their contribution produces in the soil:

Fertiliser Ind. salt. Reac.

• Ammonium nitrate 3 acid
• Ammonium Sulphate 3.3 very acidic
• Potassium nitrate 1.6 basic
• Calcium Nitrate 4.1 low acidic
• Urea 1.6 acid
• MAP (NH4)H2PO4 0.5 acidic
• DAP (NH4)2HPO4 0.6 acidic
• Potassium chloride 2.2 neutral
• Potassium sulphate 0.9 neutral

Mc Carty, L.B, Best Golf Course Management Practices, 2001