Changes in Ion Balance and Individual Ionic Contributions to EC Reading at Different Renewal Intervals of Nutrient Solution under EC-based Nutrient Control in Closed-loop Soilless Culture for Sweet Peppers (Capsicum annum L. 'Fiesta')

Individual ion concentrations and ionic contributions to EC reading in the circulated nutrient solution are the important factors to be considered for stable EC-based closed-loop soilless culture. This study was conducted to determine appropriate ion-analysis intervals of the circulated nutrient solutions based on ion concentration, ion balance, and ion electrical conductivity under different renewal intervals in EC-based nutrient control systems for sweet peppers (Capsicum annum L. ‘Fiesta’) in early growth stage. Average node numbers of the plants were 13 and 18 when the experiment started and finished, respectively, and three plants were grown in each rockwool slab. Four different renewal intervals of circulated nutrient solutions such as 1, 2, 3, and 4 weeks were used as treatment. Nutrient solutions were supplied to the plants based on integrated radiation. Drainage was collected into drain tanks after irrigation ended in the day and then mixed with fresh water until the EC reaches 2.69 dSㆍm-¹. The replenished nutrient solution was supplied to the plants in the next day. Ion concentrations of the individual ions periodically analyzed in the circulated nutrient solutions showed no significant differences among the treatments during the experimental period. Ion concentrations of K+, Ca²+, Mg²+, Na+, NO₃-, SO₄²-, PO₄³-, and Cl- varied within 5-8, 11-14, 2.0-2.7, 0.5-0.6, 14-19, 4-5, 1-4, and 0.3-0.5 meqㆍL-¹, respectively. Ion balance showed a consistent tendency over all the treatments and especially K+ : Ca²+ and SO₄²- : PO₄³- played great roles in the cation and anion balances in the nutrient solutions, respectively. Activity coefficients of ions such as K+, NO₃-, and H₂PO₄- varied within 0.8-0.9 and those of Ca²+, Mg²+, SO₄²- varied within 0.5-0.6, showing little changes with time. Ionic contributions of K+ and NO₃- to EC reading were the greatest followed by Ca²+, SO₄²-, and Mg²+ in the order. From the results, we thought that allowable ranges in ion concentration, ion balance, and subsequent individual ionic contributions to EC reading would be obtained within 4-week renewal interval of nutrient solution in EC-based closed-loop soilless culture for sweet pepper plants.