Control of Mg and P Ion Concentration as a Precondition to Use N, K and Ca Ion Sensors in Closed Hydroponics

Abstract

Recycling nutrient solutions in closed hydroponic production systems is usually accompanied by an imbalance of nutrient solutions when concentration is controlled according to electrical conductivity (EC) levels. This study investigated whether it was possible to automatically control the concentrations of five essential elements nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) using only N, K and Ca ion sensors. N, P, K, Ca, and Mg uptake was measured in the nutrient solution, and relationships between absorbed ions were analyzed through twice-repeated experiments in lettuce. Results confirmed that the pattern of PO4 ion uptake was similar that of N, and the pattern of Mg ion uptake was similar that of Ca. PO4 ion uptake was most highly correlated with N, and Mg was most highly correlated with Ca. Regression coefficients of N and PO4 were significantly different at 1.04 and 0.55, respectively, but were similar between Ca and Mg at 0.35 and 0.40, respectively. Additional experiments were conducted to measure nutrient uptake in pak choi and rose plants, both to confirm the results from the first experiment in lettuce, and to assess possible application to other crops. Coefficients of determination both for N and PO4, and Ca and Mg were considerably high (R-2 = 0.86) in cultured pak choi, and similar results were observed in cultured rose (R-2 = 0.87 and 0.73, respectively). Regression coefficients for cultured pak choi were 0.56 and 0.24, respectively, and for rose were 0.51 and 0.16, respectively. Although the results obtained for N and PO4 were not consistent between the lettuce experiments, N and PO4 have similar regression coefficients for all crops. No common coefficient was found between Ca and Mg.