Scientia Horticulturae, 11 (1979) 9--17 9 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands WATER RELATIONS OF NURSERY POTTING-MEDIA D.V. BEARDSELL, D.G. NICHOLS and D.L. JONES Department of Agriculture, Victoria Horticultural Research Institute, Knoxfield, P.O. Box 1 74, Ferntree Gully, 3156, Victoria (Australia) (Accepted for publication 12 February 1979) ABSTRACT Beardsell, D.V., Nichols, D.G. and Jones, D.L., 1979. Water relations of nursery potting- media. Scientia Hortic., 11: 9--17. The available water-holding capacities (AWHC) of different potting-materials varied widely; coarse sand has a very low AWHC, and peat moss, sawdust, poppy straw and brown coal each have a high AWHC. The AWHC determines the length of time taken for plants to wilt, except in the case of peat moss and pinebark. Much of the available water in peat moss is rapidly lost through evaporation and easy exploitation by the plant. Pinebark, because it resists water loss through evaporation, and because its available water is not readily accessible to the plant, is able to maintain plants unwilted for a longer time than peat moss despite its lower AWHC. The AWHC and wilting-time of mixtures of 2 potting materials can be predicted from their individual properties. INTRODUCTION Plants grown in containers have only limited access to water. After aeration, the next most important physical property of a potting-medium is a high water-holding capacity. Different potting-materials are known to vary widely in their water-holding capacities (Boggie, 1970). A previous paper (Beardsell et al., 1979) showed that experimentally determined available water-holding capacities (AWHC) of mixtures of 2 potting-materials could be predicted from their individual AWHC. This work also showed that peat moss had the highest AWHC of the range of potting-materials that were tested. Nursery experience, however, indicates that although peat moss contains a large amount of water, it cannot prevent plants from wilting for very long. This has been attributed to water loss by evaporation due to the "wick effect" of peat moss (Laurie and Ries, 1950). A wilting-experiment was set up to study the water relations of 8 potting- materials alone and in certain combinations. This experiment was designed to determine the water loss from the media due to both evaporation and transpiration. We also wanted to determine whether the water relations of 1:1 mixtures of the materials could be predicted from the water relations of the individual components.