Roots need oxygen for the health and vigour of a plant. The plant can only grow as well as its roots allow and its yield is totally proportional to root growth. The roots struggle to find enough oxygen (in advanced gardens, most of the air for the roots is supplied aeroponically or with multiple aerators). Lack of oxygen at the roots reduces root respiration, and this is one of the factors that shut down photosynthesis. Oxygen at the roots has another very important function. The oxygen changes the electrical charges in the water and nutrients, and allows the roots to extract water and nutrients with much less energy. For this reason, the roots need as much oxygen as they can get. Very cold fresh water can hold up to 0.0014% (14 PPM or 14 mglL) dissolved oxygen. Fresh water at room temperature can only hold about 0.0008% (8 PPM or 8 mglL) dissolved oxygen. Fresh water at 86°F (30oe) holds about 0.0005% (5 PPM or 5 mglL) dissolved oxygen. DO meters (dissolved oxygen meters) are available for measuring the amount of dissolved oxygen in nutrient solutions. The oxygen directly from the nutrient solution is only 1 % of the plants needs, and nutrient oxygenation is not sufficient for any water culture. If the roots in water culture are not given most of their time for breathing air, they will soon brown and become sick. Nutrient water is only a minor source of root oxygen. The main reason we oxygenate the nutrient mix with air pumps and stones is to kill off all the pathogens that can not live in air, called anaerobes. This method keeps the nutrient fresh for a long time. It is also the reason why we add hydrogen peroxide (H202) to the nutrient water. Aerated water will accept 0.0005 - 0.0008% oxygen and hold it for about 24 hours, which is a lot less than that available in air. This is why direct aeration of the roots (aeroponics) is better because it provides so much oxygen to the roots, the plant will actually grow a lot better
