CHARACTERIZATION OF THE WETTING PROPERTIES OF AIRDRIED PEATS AND COMPOSTS

The hydric behavior of various samples of peats and composts which are commonly used in horticulture were studied. The wettability was assessed from contact angle measurements performed on air-dried pellets. Void index and water index curves were given in the process of first dehydration and then after air drying and rewetting, in a second dehydration process. Concerning the solid-liquid contact angle on air-dried material, peats turned out to be hydrophobic; this character was more obvious for woody (122.1°) and herbaceous (116.8°) peats than sphagnum peat (110.9°). These results were correlated with the humification degree (von Post index and pyrophosphate index). In comparison, fresh sphagnum is quite hydrophilic. The contact angles measured on composts are lower than 90°, whereas the woody materials used as litter are hydrophobic. Contact angle decreased during the composting process. The hydrophobic character of herbaceous and woody peat, together with their lack of rigidity, explains the irreversibility of the shrinkage. The opposite sphagnum peat, due to its hydrophilic character and elasticity, presented a reversible behavior after air drying. After air drying, the hydric behavior of composts, which are made of rigid and hydrophilic materials, was not greatly modified.