Chemical fertilizers are composed of salts that that dissolve into soil moisture and then are absorbed by the root cells of the plants. Why should you never over fertilize a plant?
When you over-fertilize a plant, you're putting a lot of excess solutes (soluble stuff) outside of the roots in the soil. Fertilizer may form a hypertonic solution in the soil that may draw out the water from the plant cells causing the plant to die. In a hypertonic solution, the total molar concentration of all dissolved solute particles is greater than that of another solution, or greater than the concentration in a cell. If concentrations of dissolved solutes are greater outside the cell, the concentration of water outside is correspondingly lower. As a result, water inside the cell will flow outwards to attain equilibrium, causing the cell to shrink. As cells lose water, they lose the ability to function.