DIN ISO 11274:2012-04

Soil water content and matric pressure are related to each other and determine the water-retention characteristics of a soil. Soil water which is in equilibrium with free water is at zero matric pressure (or suction) and the soil is saturated. As the soil dries, matric pressure decreases (that means, becomes more negative), and the largest pores empty of water until eventually water is held in only the finest pores. Not only is water removed from soil pores, but the films of water held around soil particles are reduced in thickness. Therefore a decreasing matric pressure is associated with a decreasing soil water content. Laboratory or field measurements of these two parameters can be made and the relationship plotted as a curve, called the soil water-retention characteristic. The relationship extends from saturated soil (approximately 0 kPa) to oven-dry soil (about -10⁶ kPa). The soil water-retention characteristic is different for each soil type. The shape and position of the curve relative to the axes depend on soil properties such as texture, density and hysteresis associated with the wetting and drying history. Individual points on the water-retention characteristic may be determined for specific purposes. The results obtained using these methods can be used, for example: - to provide an assessment of the equivalent pore size distribution (for example, identification of macro- and micropores); - to determine indices of plant-available water in the soil and to classify soil accordingly (for example, for irrigation purposes); - to determine the drainable pore space (for example, for drainage design, pollution risk assessments); - to monitor changes in the structure of a soil (caused by for example, tillage, compaction or addition of organic matter or synthetic soil conditioners); - to ascertain the relationship between the negative matric pressure and other soil physical properties (for example, hydraulic conductivity, thermal conductivity); - to determine water content at specific negative matric pressures (for example, for microbiological degradation studies); - to estimate other soil physical properties (for example, hydraulic conductivity). This International Standard specifies laboratory methods for determination of the soil water-retention characteristic. This International Standard applies only to measurements of the drying or desorption curve. Four methods are described to cover the complete range of soil water pressures as follows: a) Method using sand, kaolin or ceramic suction tables for determination of matric pressures from 0 kPa to -50 kPa. b) Method using a porous plate and burette apparatus for determination of matric pressures from 0 kPa to -20 kPa; c) Method using a pressurized gas and a pressure plate extractor for determination of matric pressures from -5 kPa to -1 500 kPa. d) Method using a pressurized gas and pressure membrane cells for determination of matric pressures from -33 kPa to -1500 kPa. Guidelines are given to select the most suitable method in a particular case. The committee responsible for this standard is NA 119-01-02-02 UA "Chemische und physikalische Verfahren" ("Chemical and physical methods") at DIN.