Osmosis

The process by which mineral nutrients dissolved in water are absorbed into plants is called osmosis. Osmosis is the tendency of fluids to pass through a semi permeable membrane and mix with one another. A semi permeable membrane is something that will allow some things to pass through it but not others. In plants the small hairs on the roots allow nutrients dissolved in water to enter the root system but do not allow particles of dirt, for example, to enter the plant. Osmosis is an important process operating in both plants and animals. Digested food enters the bloodstream in animals by osmosis.

The cells in a plant's root hairs contain a dense solution of salts and organic acids. Because this solution is stronger than the weak solution of nutrients dissolved in water in the soil, there is a strong osmotic pressure driving the weak solution in through the cell walls to mix with the dense solution. This process of osmosis continues from cell to cell so that the nutrients dissolved in water in the soil enter the plant's roots, eventually moving through the whole plant.

Osmosis can also work in reverse and kill a plant. Some gardeners when they apply a heavy dosing of soluble fertiliser around a plant create a situation where the solution in the soil is stronger than in the plant. As a result the plant loses its moisture, wilts and often dies. Because the nutrient solution being fed to hydroponically grown plants can be metered, this situation is easily avoided. Plants grown hydroponically, receiving the nutrients they require, may even develop to a degree not normally reached by plants grown in the soil. Their roots become extremely well nourished, accumulating large quantities of mineral salts. Because the solution of salts in the plants root cells is so strong the ability of these cells to take up water is increased. So instead of water and nutrients moving up through the plants xylem by osmosis, so much water may be taken in by the roots that the water is forced up the Xylem. The roots actually act as a pump. This condition has been termed 'root pressure' and accelerates the development of the rest of the plant.

 

Unlike soil grown plants, the roots of hydroponically grown plants do not need to travel far to find all the nutrients they need.