A homogeneous culture medium for plants comprising microporous wood fibre particles chemically bonded to carbon black particles having a particle size >10????m, obtained by grinding and binding the particles together under pressure at elevated temperature in the presence of steam. The elevated temperature may be above 100????C. The wood fibres may be at least partially rolled up on themselves. The carbon black particles are chemically bonded to the wood fibre particles, and will not separate through normal handling. That is, the colouring effect of the carbon black particles is 'permanent'.
