We compared the effects of quantitative control in closed system versus electrical conductivity (EC)-based control in open system of nutrients on the growth and yield of tomatoes in rockwool cultivation. Quantitative control supplied all nutrients once a day based on criteria for three application rates (1.0 ×, 1.25 ×, and 1.5 × the standard). These criteria were previously developed in the deep flow technique (DFT) to estimate their nutrient requirements, which used the three-day average amount of water absorbed by tomato plants. We compared the quantitative control in a closed system with two EC treatments in an open system, in which we supplied nutrient solution at high and low EC ranges. The 1.0 × quantity standard reduced the nutrient supply to 30-37% of the levels used in the low-EC treatment. N, P, and K concentrations in the 1.0 × substrate solution remained low and stable for two months after transplanting. The 1.0 × treatment had 27% lower total leaf dry weight and 23% higher total fruit yield than the low-EC treatment. However, greatly reduced growth and nitrogen content of upper leaf and slightly lower soluble solids content in the fruits suggest that the 1.0 × standard might provide insufficient nutrients during late growth stages. Excessive nutrient supplies, which were much lower than the low-EC treatment, increased substrate salinity owing to the accumulation of nutrients during the latter half of the growth period in the 1.25 × and 1.5 × treatments. The stable EC and nutrient concentrations in the 1.0 × substrate solution over time suggest that the three-day adjustment interval would be short enough. The quantity criteria developed for the DFT system could generally be applied to long-term rockwool culture, but the optimum range of nutrient supply was found to be from 1.0 × to 1.25 × the DFT standard in the later growth stages.

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Daily nutrient additions reduced nutrient supply and concentration in the hydroponic solution without affecting fruit yield and quality of tomato 'Renaissance', grown during the winter of 2003-2004 and the summer of 2004. The results were compared to those obtained by the conventional EC-based plots. In the daily nutrient addition plot, all nutrients were supplied according to water uptake by tomato plants under three different application rates (Low, Medium, and High nutrient solution). The nutrient levels in the Medium treatment were reduced, except for sulfur, to 53-66% with respect to the EC-based treatment in the winter experiment and 73-77% in the summer experiment. Nitrogen and phosphorus concentration in the Low and Medium nutrient solution did not exceed 10 mg·L −1 and 3 mg·L −1 , respectively, except for 30 days after the transplanting. The total amount of macro nutrient absorbed and dry weight accumulated by leaves increased with increasing nutrient supply. The yields from the Medium and High treatments were equal to the EC-based treatment. Daily nutrient addition increased soluble solids content and ascorbic acid, but it did not influence titratable acidity in fruits. It was estimated that the addition of 50 mg N·plant −1 ·day −1 is adequate to maintain normal growth by tomato plants, provided the water uptake was kept at 0.5 L·plant −1 ·day −1 . N could be increased to 130 mg·plant −1 ·day −1 with an increase in water uptake. Thus, this system of nutrient additions that can be easily regulated could increase productivity of tomato fruits by preventing excess vegetative growth, and simultaneously reduce the environmental impact of wastewater.

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