Smart control of soil temperature to optimize root-zone conditions for enhancing the physiological performance, growth, and productivity of greenhouse-grown cucumber

Determining the optimal soil temperature in the active root zone to meet the requirements of different plant growth stages is crucial. Controlled greenhouse producers attempt to heat soil sectors in various ways without determining an appropriate or reliable soil temperature. A suitable smart method can be used to adjust the temperature in the active root zone to determine the optimal temperature and improve crop growth and yield. Cucumber plants grown in a plastic greenhouse were subjected to five different soil temperature treatments (13 as a control, 16, 19, 22, and 25 °C) over two growing seasons using carbon fiber smart thermal cable connected to a metal thermostat. The thermal cable was installed 10 cm below the soil surface, extended 20 m apart on three branches per replicate, and then buried and compacted well into the soil. The objective of this study was to determine the optimum soil temperature by smart thermal system, to enhance photosynthesis rate, growth parameters, and yield of cucumber, as well as improving the availability of nutrients within active root zone, and to minimize vapor pressure deficit. The experimental design was a completely randomized design with five treatments and three replicates. The smart soil heating system was a feasible economic option, with a benefit–cost ratio of 1.75% versus 1.1% for the traditional system, resulting in improved cucumber yield and quality, while saving approximately 30% of energy. Plant photosynthesis was positively correlated with total yield but negatively correlated with vapor pressure deficit, suggesting that the initial or mid-season increases in cucumber yield were partly due to the increased soil temperature at 22 °C. In conclusion, plants grown at 19 or 22 °C had significantly similar yields of 3.37 ± 0.09 and 3.71 ± 0.14 kg plant−1, respectively, and most of these plants had higher yields than plants grown at 16 °C, with a yield of 2.33 ± 0.03 kg plant−1. We suggest that a soil temperature of 22 °C is ideal for cucumber growers in the initial or mid-season, but at the end of the season the plant may not be affected, so a soil temperature of 19 °C can be used to save energy. Also, the possibility of presenting this intelligent system as an alternative the traditional one.