Fertilizer or Salt Burn
Fertilizers are designed to operate in a broad spectrum of container sizes and soil composition at full recommended strength. There is no danger of 'burning the roots' when used as directed for anything from a seedling with its first true leaves, to a rooted cutting, to four hundred year old trees. Fertilizer 'burn' is a chemical phenomenon which can be described as backward osmosis. Fertilizers are salts, and in solution they have a salt 'value' or concentration. The liquid inside the cells of roots and all other living tissue also have salts and solutes. Osmosis is the phenomenon of water moving across an otherwise impermeable cell membrane into a solution of higher salt concentration. As long as the concentration of the soil solution is lower than the salt concentration of the cell, the soil solution with its dissolved salts (fertilizer and minerals) will be carried into the cell and utilized.
If you do not follow directions and accidently make a solution that has a higher concentration of salt than the concentration in the cell, the water will move out of the cell and into the soil solution, thus dehydrating the cell. This is a backward osmosis for the plant, and the effects are salt or fertilizer 'burn'. In extreme cases it will cause fatal dehydration and the symptoms will be the exactly the same as if you didn't water the plant, wilt, dull blue green color to the leaves, then yellowing or browning and death of the plant. In milder cases of salt poisoning only the margins of leaves will be affected. Usually they will turn yellow then brown.
To achieve a concentration capable of doing this requires mixing a batch many times stronger than recommended full strength soluble fertilizer. Usually you will see it only if too much solid fertilizer is directly applied to the soil surface or in the case of very hard water.
