Biology question about fertilization?

Question

In biology terms: A small amount of fertilizer (mineral salts) will stimulate plant growth, but over fertilization can kill plants? Why? Need to know in regards to diffusion, osmosis etc.

Answer 1

Fertilisers such as mineral salts are soluble in water.

A solution which contains a smaller quantity of dissolved substance (and hence more water) is hypotonic to the cell contents. When the plant roots are surrounded by hypotonic soil water, then water enters the cell by osmosis and the dissolved minerals by diffusion. (High concentration to lower concentration).

A solution which contains a greater quantity of dissolved substance (and hence lesswater) is hypertonic to the cell contents. When the plant roots are surrounded by hypertonic soil water, then water leaves the cell by osmosis and the cells become plasmolysed.

Hope this helps!

Answer 2

Water and mineral enter through root epidermis, cross the cortex, pass into the stele, and are carried upward in the xylem. The cells cannot get enough mineral ions from the soil by diffusion alone. The soils solution is too dilute.

Active transport of these ions must occur. Specific carrier proteins in the plasma membrane attract and carry their specific mineral into the cell.  H+ ions are pumped out of the cell causing a change in pH and a voltage across the membrane. This helps drive the anions and cations into the cell.  Too much fertilizer will load the soil with solute and draw the water out of the plant.  It will also lower the pH of the soil making it more acidic.  This will also kill the root cells.