Irrigation strategies for greenhouse tomato production on rockwool.

To address the concern that irrigation provides sufficient water to match the crop needs, while not impeding oxygen availability to the roots, we conducted an experiment to develop suitable irrigation schedule(s) for greenhouse tomato (Lycopersicon esculentum Mill.) on rockwool. The experimental treatments incorporated the electrical conductivity (EC) of the nutrient solution in the rockwool slab (slab-EC) along with the water content (WC) in the rockwool slab (slab-WC) as the irrigation decision-making variables. They were: slab-WC <= 70% or slab-EC >= 1.4x normal or more (T1), slab-WC <= 70% or slab-EC >= 1.7x normal or more (T2), slab-WC <= 80% or slab-EC >= 1.4x normal or more (T3), slab-WC <= 80% or slab-EC 1.7x normal or more (T4), and the combined weight loss (WL) 700 g or more (T5) and WL 500 g or more (U), in which "normal" means the feed solution EC as recommended in the seasonal fertigation schedule for a spring-summer tomato crop. The data on early-season marketable yield, total seasonal marketable yield, and fruit grades indicated the superiority of treatments T1, T2, and T6 over T3, T4, and T5. Better root growth was observed with T1, T2, and T6 and this was also associated with minimized nutrient solution leaching; furthermore, these plants had an abundance of coarse and fine roots, higher photosynthesis and transpiration, higher marketable yield, and a higher water use efficiency. Our results thus established that irrigation based on either a slab water content 70% or less or a 500-g weight loss is the best strategy for rockwool-grown greenhouse tomatoes in the spring-summer season. A variation in slab-EC between 1.4 and 1.7x normal, at a slab-WC of 70% or less, would have no significant effect on root growth, water use, marketable yield, or fruit grades.