Ion control in closed growing systems with inert media: controller settings and modes of operation

G. van Straten, Th. H Gieling


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

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