Fertiliser Salt Index

The salt index The fertiliser’s osmotic pressure is a measure of the concentration that the fertiliser induces in the soil solution. It is expressed as the percentage increase in osmotic pressure in the soil saline solution resulting from the application of a specific fertiliser, and is measured as the increase relative to the same weight of Nitrate Sodium, which is used as a reference. The Index of salinity has no dimensions and values above 1.5 are considered high.

The fertilisers They vary in their ability to draw water from the plant and thus cause sunburn.

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, Nitrogen fertilisers o Potassiums 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