After extensive thinning of beechwood in the Chiltern Hills, Deschampsia caespitosa was seen to have become the dominant component of the vegetation on strongly contrasting soils. At three sites selected for study, the soils were more or less well-drained and ranged from an acid mull (pH 3· 7-4· 2) through a calcimorphic brown earth (pH 7· 0-8· 0) to almost pure chalk rubble (pH 7· 6-8· 3). These soils were characterized in terms of their moisture release curves and their seasonal fluctuations in water availability were monitored during 1971; a computer-based method for the derivation of matric potential directly from gypsum block resistance, employing a polynomial regression model and a temperature correction equation, is described. The chalk soil was found to hold the most available water, but severe water stress proved to be an infrequent contingency on all three sites. However, matric potentials of the order of -10 bar occurred in summer with no apparent ill-effect on D. caespitosa, indicating that its frequent occurrence in waterlogged habitats is not due to a high physiological requirement for water but to its manifestly broad edaphic tolerance coupled with a lack of competitive ability.