The present work compares grape marc compost (GMC) with rockwool (RW) as a substrate for greenhouse tomato production in open and closed systems. GMC exhibited a high total pore space (84.3 v/v) and air capacity (59.0% v/v) but a low easily available water (1.2% v/v). Most of the total water content (25.9% v/v) of GMC was retained under a tension above 100 cm of water column (23.7% v/v). Rockwool (Grodan ® ) has a higher total pore space (96.7%) and total water retention (81.8%), most of it (77.8%) retained between 10 and 100 cm of water column, but a lower air capacity (14.9%). Tomato 'Sinatra' (Sluis & Groot, Holland) was grown on 15 L rockwool slabs and on 30 L compost bags, in a heated plastic greenhouse, from November to June, in two consecutive years. In the first year (1997/98) tomato production was compared on RW (open and closed systems) and on GMC (open system). No significant differences were observed in commercial yield on RW (15.6 kg m -2 and 16.2 kg m -2 , respectively in closed and open system) and on GMC (16.6 kg m -2 , open system). In the second year (1998/99) RW and GMC were used on a closed system and tomato yield was compared using new substrates (1 st crop) and substrate from the previous year (2 nd crop). Commercial yield reached the standard of the region and no significant differences in between RW and GMC, and between 1 st crop substrate (new substrate) and 2 nd crop substrate (reused substrate). The results show that grape marc compost can be used as a rockwool substitute for greenhouse tomato production in open and in closed systems.

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