A review of improvements to methods for the measurement of dissolved oxygen, pH, and soil redox potential and the discovery of convective flow in ponded water of paddy fields

Abstract

To analyze how physical factors vary from hour to hour in ponded water of paddy fields, techniques that allow continuous measurement of these factors are required. Until recently, a lack of proper instrumentation made this type of measurement difficult to do. The development of measurement instrumentation and information processing technology has now made the continuous measurements of these physical factors feasible. Making these types of measurements practical has also required the design of instruments suitable for use under field conditions. This paper reviews the progress in the development of these measurement and instrumentation technologies with respect to four specific physical parameters of ponded water in paddy fields. Dissolved oxygen (DO) in paddy fields is important for the creation of an aerobic environment. DO in ponded water is considered one of the most abundant forms of DO in the world. The improvement of DO-measuring methods is reviewed, ranging from earlier methods requiring periodical manual sampling to current continuous automated sampling methods. The acidic or alkaline nature of the environment is critical for plant growth. The large variation in pH throughout the day in ponded water, factors that affect pH, and the influence of ponded water pH on the surrounding environment are discussed. Methods for measurement of pH in the field are reviewed. Soil redox potential (Eh) has been an area of great interest. Methods for the measurement of soil Eh have been improved. Two types of approaches are used to measure soil Eh: direct and indirect. However, continuous measurement of paddy soil Eh in the field remains difficult. Finally, convective flow is an important physical parameter in ponded paddy water. The discovery of convective flow and the progress made to date in meeting the challenge of developing methods to measure convection velocity in the field are reviewed.