There occures sometimes growth suppression not only due to low air temperature, but also low soil temperature in the cultivation of fruit vegetables under plastic coverings. In the gravel culture also, the temperature of nutrient solution has an important relation to plant growth.
This preliminary investigation was designed to study the effect of soil temperature on the growth of tomato, cucumber and sweet pepper seedlings, using the apparatus maintained with constant soil temperatures under glass.
1. Tomato seedlings with 10 expanded leaves were treated for 7 days, with different soil temperatures, viz. 10°, 15°, 20°, 25°, 30°and 35°C. Air temperature was maintained at 20°C constant in every treatment. The optimum range of soil temperature for top growth was found to be 20°-30°C. Beyond this range, the top growth of seedlings decreased.
2. Cucumber seedlings with 7 expanded leaves were treated for 10 days with different soil temperatures above mentioned. In this case, air temperature fluctuated between 25°and 15°C. The top growth was the best in the soil temperature range of 25°-35°C, and decreased at the temperatures lower than this range.
3. Under the fluctuating air temperature 25°-20°C, the seedlings of sweet pepper and tomato were treated for 10 days with different soil temperatures, 10°, 15°and 25°C. At lower temperatures as 10°-15°C, the growth of sweet pepper was more suppressed than that of tomato.
4. It is evident that soil temperature had a pronounced effect on increase of stem and leaf length, fresh and dry weight of the top, especially on leaf length and fresh weight in these fruit vegetables.
At lower and higher soil temperatures, their leaf color became darker green than that at the optimum range, and the adventive roots were induced on the base stems. Dry matter percentages of the tops in these temperatures were higher than that in the optimum temperature range.
As for tomato, the total amount of increase in leaf length per seedling was parallel to increase of fresh weight, and was found to be a sensitive indicator for the growth rate in these experiments.