Substrate and Soil Temperatures in Passive Mediterranean Greenhouse Crops

Substrate and soil temperatures were analyzed throughout 14 representative fruit–vegetable crop cycles and treatments grown in low-cost Mediterranean greenhouses, mostly around the cold winter period. The mean daily temperatures of most common substrates (perlite and coconut-coir bags) were lower (between 0.9 and 3.0 °C) and more variable than root zone temperatures of the most common soil (enarenado soil) throughout all crop cycles and treatments studied, particularly during the cold period. The mean daily temperature of the perlite and coconut-coir bags was low (around 15 °C) during most of the cold periods, and the minimum daily temperature was very low (around 12 °C) during some crop periods. These low temperatures are generally considered suboptimal for greenhouse production. The seasonal dynamics of the minimum and mean daily temperature of the substrate bags were similar to those observed for the mean daily greenhouse air temperature. The minimum daily temperature of substrate bags was close to the mean daily greenhouse air temperature for all the studied crop cycles. A simple and practical relationship was found for predicting mean daily temperatures of substrate bags from mean daily greenhouse air temperatures (MAE = 0.87 °C; RMSE = 1.15 °C). A substantial spatial and temporal variation in the hourly temperature was found in the cross-section of the coconut-coir bag, but not for the mean daily temperature. No differences were found for the mean daily temperature along the longitudinal section of the bag. In general, representative measurements of the coconut-coir bag can be taken by installing the sensors horizontally and especially vertically around the middle part of the bag.