Fertilizer “burn” is caused by applying too much fertilizer or not diluting it enough after application. The “burn” symptom—leaf tips turning yellow or tan, or even a severe wilt in extreme cases—is caused by the inability of the plant to get water from the soil because of the over-concentration of fertilizer salts. A slight fertilizer burn causes leaf tips to turn yellow or tan and die. A worse burn might kill whole leaves or the whole plant, again turning it yellow or tan. A very severe case might make the plant suddenly wilt and die without turning yellow. These symptoms are caused by osmosis keeping the plant from taking up water or actually pulling water from it.
Osmosis is a force that is used by all living cells to regulate themselves. It draws water from a less salty solution through a membrane (such as a cell wall) into a more salty solution. Most of the time, the saltier solution is inside the cell, and keeps it turgid. Osmosis is the force that keeps leaves erect. When osmosis fails, cells—and plants—lose their turgidity and wilt.
The Nature of Salt
People commonly use the word “salt” to refer to table salt, sodium chloride. However, it also refers to a broad range of mineral elements that dissolve in water. Plant nutrients, in the forms in which they are available to plants, are all salts dissolved in water.
The cell wall that surrounds every plant cell depends on a balance of salts to maintain turgidity and regulate the flow of materials in and out of the cell. Normally, the fluid inside the cell wall is slightly more salty than the fluid between the cells, so osmosis keeps the cells turgid.
Fertilizers contain nutrient salts. When a fertilizer, either as a solid or a liquid, is applied to the surface of the soil, the fertilizer salts must dissolve in the soil solution before the nutrients can enter the roots and be used by the plant. The concentration of salts in the soil solution determines whether the plant can draw up the water it needs.
The solubility of nutrient salts are measured as a . You can use this index to determine how “hot”, or salty, a fertilizer is. The higher the salt index, the more important it is to dilute the fertilizer with adequate water after you apply it.
Fertilizer burn is usually caused by applying a hot fertilizer without watering it in. A light rain or the morning dew dissolves the fertilizer into a concentrated solution which makes it difficult for the plants to get enough water.
Crystals of fertilizer left on leaves for the dew to dissolve will also burn the leaves, but just at the spot they are touching.
If you suspect fertilizer burn, the cure is a good irrigation. To avoid it, water hot fertilizers into the soil with lots of water.
Carefully-formulated commercial fertilizers are not hot and do not cause fertilizer burn.