Development of Fertigation Control System Based on Measuring Plant Weight Using Load Cell for High-brix Tomato Hydroponics.

A fertigation control system based on plant weight using a load cell was developed to improve the stability of high-brix tomato fruit production with restricted root-zone hydroponics. In this control system, plant weight (Wx) was measured by hanging a tomato plant and a coconut fiber in a pot with nutrient solution dripping from a load cell. The maximum plant weight (Wmax) as Wx without plant water stress was measured after the first sufficient fertigation of the day. Relative plant weight (Rw) was used as an index of fertigation timing, which is calculated from a percentage of Wx towards Wmax. After the second fertigation, the fertigation pump starts when the Rw is less than the value of the start point for fertigation (SP) during the control period. Relative stem diameter (RSD) was used as an index of plant water stress by measuring tomato stem diameter using a laser sensor and was more closely correlated with Rw than with substrate water content and amount of evapotranspilation. The monitoring of Rw can be used to non-disruptively evaluate plant water stress. These results suggest that the fertigation control system based on measuring plant weight using a load cell can be used to avoid excessive plant water stress in tomato hydroponics producing high-brix fruit.