Salt Index

Scientific research tells us salinity has a negative effect on plant growth; the main issue being excess soluble salts affecting growth by reducing water intake – effectively causing drought symptoms.

You may remember learning about osmosis back in school; water is drawn from low salt concentrations to high salt concentrations through a permeable membrane.  This is exactly what happens when you apply a high salt fertiliser.  When fertiliser salts are added to soil water (as a granular) or directly to the leaf blade (as a foliar), the osmotic pressure changes so there is effectively lower concentrations of water outside the plant; this pulls water out of the cell into the soil water (or leaf surface) to redress the balance – plants cannot efficiently take up nutrients or water in high salinity soils and drought symptoms can be observed even with an ‘acceptable’ moisture content.

Applying a foliar fertiliser with a high salt index can ‘burn’ the leaf.  This can be a noticeable yellowing/browning of grass leaves in bad cases, or the damage may not always be visible to the naked eye but reduced plant health, vigour and growth may result from applying a high salt foliar.  Water will be drawn out of the plant cell leading to a loss of turgor pressure and reduced plant cell health.  There may be no visible damage, but the plant will not be responding efficiently to your fertiliser inputs.

It makes sense then, to be aware of how much salt you are adding to your turf via the products you use and to utilise products that are going to maximise turf health and minimise issues caused by leaf burn and soil salinity.  Why apply excess salt when there are low salt alternatives available?  Salt build up not only causes issues with plant phytotoxicity and drought symptoms, but also adversely affects soil biology.  Good work using organic products and biostimulants may be undone using high salt fertilisers. 

All fertilisers will have a salt index based on the amount of salt within them.  Once you start to get over a salt index of 40-50 there is potential for negative effects on plant health and above a salt index of 80 is where there is a high potential for plant tissue damage.  The salt indices of various common nutrient sources are shown in table 1.

Nutrient Sources:Salt Index
Urea 46% N74
Ammonium Sulphate88
Ammonium Nitrate104
Growth Products Smart Nitrogen™4
Potassium Sources: Salt Index
Potassium Sulphate43
Potassium Nitrate70
Potassium Chloride116
Potassium Carbonate5

Table 1. Some common nutrient sources in turf management and their salt index

Finding out the nutrient sources for your inputs and the overall salt index of the product will give you a good idea of any potential issue you may inadvertently be causing.