Greenhouse soil warmed by capillary network and its effect on the growth of cucumber

In winter, the air temperature in greenhouse increases quickly after sunrise, whereas the soil temperature changes more slowly due to the much larger thermal capacitance and the shade of plants. Low soil temperature may become one of the biggest adverse factors that depresses root growth and nutrient absorption of greenhouse vegetables. We designed a greenhouse soil warming system, containing a water-heater with temperature controller, a capillary network, a hot water container and a pump. The temperatures of 15 different positions in a column of soil (480 cm length × 120 cm wide × 30 cm depth) were recorded every 10 min. The differences of temperature among eight different positions at a depth of 15 cm were less than 2.8°C. These results indicated that the greenhouse soil warming system could warm the soil uniformly attributed to the well-distributed capillary network. At a depth of 15 cm, the average temperature in warmed soil was 19.2°C, whereas the temperature in un-warmed soil, i.e., control, was only 11.8°C. In addition, we investigated the growth and the nutrient states of cucumber plants grown for 68 days. Compared to the un-warmed control, plant height, stem diameter, leaf area, and soil plant analysis development (SPAD) values were significantly higher in seedling stage, and cucumber yields were 125% higher in fruiting stage in warmed soil. With respect to nutrient uptake, total uptake of N, P, K, Ca, and Mg were increased by 51, 173, 87, 54 and 80% by soil warming, respectively, due to greater biomass despite the concentrations of some nutrient elements being decreased. In addition, soil warming promoted the allocation of N, P, and K from roots to leaves and fruits. In conclusion, the growth of cucumber could be significantly promoted in an appropriate soil temperature, which is maintained by the capillary network soil warming system.