10 - Measurement of Oxygen Diffusion Rate

Abstract

Air and water comprise a large part of the soil. For an average soil, air and water take up 50% of the space. Organic matter and mineral matter take up the other 50%. At optimum moisture content for plant growth, the air and water space are about equal, each about 25 percent of the soil volume. With so much of the soil volume taken up by air and water, it is obvious that air and water must play a major part in soil and plant–water relations. The chapter looks at the most widely used method to monitor aeration status of the soil, the oxygen diffusion rate (ODR) method. Respiration by plant roots depends on soil oxygen. Roots, like animals, do not photosynthesize, and they give off carbon dioxide and take in oxygen during respiration. Diffusion of gases in the soil practically stops when the fraction of air-filled pores is less than 10%. Therefore, roots need at least 10% by volume air space in the soil to survive. Evaluation of the aeration conditions at the interface between the root and the soil system presents the greatest possibility of defining the influence of aeration on plant growth. In the process of respiration, plants quickly take up the oxygen surrounding the roots in the rhizosphere, and an increasing oxygen concentration gradient develops between the soil atmosphere and the atmosphere next to the root surrounded by the water film. Movement of oxygen from the atmosphere to a respiring root involves diffusion through the following three phases: 1. the gaseous phase of the soil; 2. the gas–liquid phase boundary; and 3. the liquid phase of the water film around the root.