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The recent market introduction of ion-selective sensors in horticulture removes one of the barriers towards accurate control of individual ions supply in closed water systems in greenhouse cultivation. In previous work, controllers have been designed that are able to compensate for evapotranspiration and nutrient uptake by tracing a set-point for pulse-averaged drain flow and individual ion concentrations in the drain, based on ion-selective sensor information.

The objective of the current work is to investigate the desired operation mode of the nutrient controller under the assumption of successful constant drain flow control. Two cases are distinguished: demand satisfying control and supply regulating control. Using fundamental mass balances and transport equations, conditions are derived to which set-points of the constant drain concentration controllers should obey in order to ensure non-inhibiting nutrient supply. It is concluded that uptake regulation below the demand is most likely difficult to achieve with a drain concentration controller, whereas it is very suitable for demand satisfying control.