A system (10,11) for controlling plant growth conditions in hydroponic growing systems, the system for controlling plant growth conditions comprising: at least one detector (7,1101) for measuring at least one property of a plant growth substrate; first (9,1103) and second (9,12, 1107) data processing means; data storage means; and the or each detector (7,1101) being arranged to measure a property or properties of a plant growth substrate and to transmit a detector identifier and the measured property or properties over a communications link to the first data processing means; the first data processing means (9,1103) being arranged to: hold in a memory predefined irrigation data defining a relationship between: plural values for one or more of temperature, pH level, water content, nutrient content, oxygen content, and plant parameters of the substrate; and plural desired irrigation parameters; process measured properties received from each detector to obtain processed properties of the substrate; provide an output indicative of a desired irrigation input for the growth substrate, based upon the processed properties and the predefined irrigation data; and send processed data to the data storage means, the data storage means arranged to store the sent data as logged data; the second data processing means (9, 12, 1107) being arranged to: receive data from the data storage means; calculate predicted properties of the substrate based on the logged data; determine a difference between the processed properties of the substrate and the predicted properties of the substrate; receive an alert condition input for outputting an alert based on said difference; adjust the alert condition to an optimum alert condition, the optimum alert condition being determined by optimum desired irrigation parameters obtained by an analytical evaluation of the logged data; and output an alert when said difference meets the adjusted alert condition. at least one detector (7,1101) for measuring at least one property of a plant growth substrate; first (9,1103) and second (9,12, 1107) data processing means; data storage means; and the or each detector (7,1101) being arranged to measure a property or properties of a plant growth substrate and to transmit a detector identifier and the measured property or properties over a communications link to the first data processing means; the first data processing means (9,1103) being arranged to: hold in a memory predefined irrigation data defining a relationship between: plural values for one or more of temperature, pH level, water content, nutrient content, oxygen content, and plant parameters of the substrate; and plural desired irrigation parameters; process measured properties received from each detector to obtain processed properties of the substrate; provide an output indicative of a desired irrigation input for the growth substrate, based upon the processed properties and the predefined irrigation data; and send processed data to the data storage means, the data storage means arranged to store the sent data as logged data; plural values for one or more of temperature, pH level, water content, nutrient content, oxygen content, and plant parameters of the substrate; and plural desired irrigation parameters; process measured properties received from each detector to obtain processed properties of the substrate; provide an output indicative of a desired irrigation input for the growth substrate, based upon the processed properties and the predefined irrigation data; and send processed data to the data storage means, the data storage means arranged to store the sent data as logged data; the second data processing means (9, 12, 1107) being arranged to: receive data from the data storage means; calculate predicted properties of the substrate based on the logged data; determine a difference between the processed properties of the substrate and the predicted properties of the substrate; receive an alert condition input for outputting an alert based on said difference; adjust the alert condition to an optimum alert condition, the optimum alert condition being determined by optimum desired irrigation parameters obtained by an analytical evaluation of the logged data; and output an alert when said difference meets the adjusted alert condition. receive data from the data storage means; calculate predicted properties of the substrate based on the logged data; determine a difference between the processed properties of the substrate and the predicted properties of the substrate; receive an alert condition input for outputting an alert based on said difference; adjust the alert condition to an optimum alert condition, the optimum alert condition being determined by optimum desired irrigation parameters obtained by an analytical evaluation of the logged data; and output an alert when said difference meets the adjusted alert condition.