The hydraulic conductivity is a measure of the rate at which liquid water can move through the soil under the influence of variations in matric pressure from point to point within the soil. The hydraulic conductivity of unsaturated soil depends on the same factors as does the soil water-retention characteristic, also showing hysteresis. As a saturated soil dries, the hydraulic conductivity decreases, and it is convenient to express the hydraulic conductivity corresponding to the soil water-retention characteristic as a function of the decreasing matrix pressure. This International Standard specifies a laboratory method for the simultaneous determination in soils of the unsaturated hydraulic conductivity and of the soil water-retention characteristic. It is applicable only to measurement of the drying or desorption curve. Application of the method is restricted to soil samples which are, as far as possible, homogeneous. The method is not applicable to soils which shrink in the following range of matric head: matric head 0 cm to -800 cm. The range of the determination of the conductivity depends on the soil typ. It lies between matric heads of approximately -50 cm and -700 cm. The range of the determination of the water-retention characteristic lies between matric heads of approximately 0 cm and -800 cm. Examples of how the results obtained by means of these methods can be used are: a) to enable evaluation of the pore size distribution (for example identification of macro- and micropores); b) to determine an index for the plant-available water in the soil and to classify the soil accordingly (for example, for irrigation purposes); c) to determine the drainable pore space (for example, for drainage design, pollution risk assessments); d) to monitor changes in the structure of a soil (caused by, for example, tillage, compaction or addition of organic matter or synthetic soil conditioners); e) to ascertain the relationship between the negative matric pressure and other soil physical properties (for example hydraulic conductivity, thermal conductivity); f) to determine water content at specific negative matric pressures (for example, for microbiological degradation studies); g) to estimate other soil physical properties (for example hydraulic conductivity). This International Standard has been prepared by Technical Committee ISO/TC 190 "Soil quality" and has been taken over by CEN/TC 345 "Characterization of soils" without any modification. The responsible German body is NA 119-01-02-02 UA "Chemische und physikalische Verfahren" ("Chemical and physical methods") of the DIN Standards Committee Water Practice (NAW).