Fertilizers are great for boosting crop yield. However, when someone tries to convince me to add loads of a particular fertilizer to my soil, I take what they say with a pinch of salt. That’s because fertilizers are mostly salts. When they dissolve in soil water, they increase the salt concentration (also called osmotic pressure) of the soil solution. The higher the osmotic pressure of the soil solution, the more difficult it is for plants or seeds to extract the water they need for normal growth.
The fundamental principle is that for plant roots to take in water, the water must pass through the root cell membrane. Water can only pass through this membrane when the osmotic pressure of the solution inside the plant cell is higher than the osmotic pressure of the soil solution outside the cell. If the osmotic pressure of the soil solution becomes higher than that of the solution inside the cell, water cannot enter the cell and may eventually move out of it. With time, the plant tissue dries out and eventually dies. Some people call this phenomenon ‘fertilizer burn’.
So, what is fertilizer salt index?
Fertilizers differ in their propensity to create salty conditions in the soil. I dare to say that the brilliant scientist who developed the concept of ‘salt index’ to differentiate fertilizers was definitely worth their salt. Salt index is a measure of a fertilizer’s relative tendency to increase the osmotic pressure of the soil solution. This tendency is compared to the increase caused by an equal weight of sodium nitrate. All fertilizers are compared to sodium nitrate because sodium nitrate is 100% water soluble. It was commonly used when the concept of salt index was developed.
A fertilizer with a high salt index has a high tendency to damage crops compared one with a lower salt index. But salt index alone does not predict the amount of material that will cause injury to crops. It only classifies fertilizers relative to each other and shows which is most likely to cause injury.
It’s important to note that crops vary in their tolerance to fertilizer salts. Moreover, salt toxicity will be more serious in:
- Dry soils, because there is less water to dilute the salts;
- In coarse-textured soils, like sandy soils, because of their low ability to react with the fertilizers;
- Cold soils, because root growth in cold soil is slow. Therefore, the root is exposed to the higher concentration of fertilizer for a longer time.
Furthermore, the risk of injury is higher where fertilizer is banded close to seed or plants.
So, if your soil or plants are showing signs of nutrient deficiencies, there’s no need to ‘rub salt in the wound’. Talk to your local SynergyAG representative for guidance on the right products and agronomic techniques.
-Ikenna Mbakwe, PhD, PAg
Head of Research