EFFECT OF SOIL TEMPERATURE ON THE GROWTH AND PRODUCTIVITY OF SEEDLINGS, GROWN IN ELECTRIC-HEATED HOTBEDS AND PLASTIC PLANT-GROWING STRUCTURES
In the course of the investigations primary importance was attached to the soil temperature, as the intensity of physiological and growth processes depends on it. The soil temperature, in a higher degree, lends itself to automatic regulation, assures a stability of the temperature regime determined for a definite period and permits to maintain a positive air temperature of the ground layer, during the night.
The experiments were carried out in greenhouses equipped with electric heating, in which the temperature was kept up automatically. The optimum temperature of the soil was determined for the basic vegetable crops during the different stages of their growth. It was maintained at 15°C, 20°C, 25°C, 30°C and 35°C, day and night, during the heating period. The heating was discontinued somewhere between the 7th and 12th of April. During sunny days, the temperature of the 15°C, 20°C and 25°C variants was 1.2 to 1.9°C higher than the planned temperature; and in the 30°C variant 0.8°C. The humidity of the soil was kept in the limits of 70 per cent of its full moisture capacity.
The experiments showed that the soil temperature influenced the time of germination, the germination of the seeds and the accumulation of dry matter in the sprouts. The highest and lowest permissible soil temperature were determined for winter hardy plants, such as lettuce, radishes and cabbages, as well as for tender plants - tomatoes, peppers, eggplants, cucumbers (see figures 1 and 2).
For the hardy plants the optimum temperature of the soil was 20°C. maximum 25°C and minimum 15°C for the heated plant-growing structures. The germination of the seeds decreased sharply, when the temperature was higher than 30°C. The dry matter weight of the sprouts also decreased. The seeds of the lettuce did not germinate at 35°C.
The optimum soil temperature for the tomato plants proved to be 25°C,