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Summary A new measuring technique based on a capillary rise method has been developed to quantify the wettability of more or less decomposed peats (the main materials used as horticultural growing media) in relation to their water content. A preliminary study of their physico‐chemical and biochemical characteristics enabled us to study the variation of their physical properties (particularly their rewetting capacity) in relation to their degree of decomposition. Analysis of these physical properties during two drying and rewetting cycles showed that the greater their degree of decomposition, the greater and more irreversible was their shrinkage. The change from hydrophilicity to hydrophobicity was measured by the increase of the water contact angle during desiccation and by the progressive change from bipolar to apolar surfaces. A strong correlation between the water retention of peats (related to their degree of decomposition) and their free energy (also called hydration energy) at the interface between solid and water ΔGSW is also shown.

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