Using soil bulk electrical conductivity to manage saline irrigation in the production of potted poinsettia

Monitoring root substrate electrical conductivity (EC) is critical for the better management of irrigation water and the effective control of soil salinity. The availability of soil moisture sensors that are also capable of measuring permittivity, temperature and soil bulk EC (ECB), such as the Hydra Probe II (HPII, Stevens W.M.S. Inc.), has opened up new possibilities for the automatic control of saline water irrigation schemes. From such measurements, models have been developed relating the ECB to pore water EC (ECPW), because the latter is the EC that directly affects the plant. However, previous results have shown that the variability of the HPII-sensor output increases with increasing salinity, affecting the accuracy and reliability of the estimation of ECPW. The purpose of this paper was to assess whether the measurements of the ECB in saturated substrate can be used to maintain different substrate saline levels, given that ECB is a function of both water content and ECPW. The greenhouse study evaluated the growth and physiological status of potted poinsettia irrigated with a saline solution (4.5 dS m−1), according to three ECB thresholds (1.5, 2 and 2.5 dS m−1). In each treatment, when the ECB threshold was exceeded, the supply of irrigation water was doubled (flushing). Damage to plants increased (lower bract area, aerial part DW and evapotranspiration) as the ECB threshold increased. Therefore, despite not being a real reading of the soil solution salinity, it is a closely related parameter, which can be regarded as a useful tool for mitigating the negative effects of saline irrigation in the production of potted ornamental plants.