One of the basic problems in the industrialized vegetable producing regions of the Soviet Union is directed towards increasing the production of seedlings of good quality and low cost.
The experience of the Moldavian Soviet Socialist Republic certifies that this problem can be solved through the combination of a number of plant-growing structures including greenhouses, hotbeds with electric heating and plastic covered tunnel-type hothouses.
At present great interest is shown for the plastic hothouses for growing vegetable seedlings.
The simplified construction and the low cost have permitted to increase the vegetable seedling areas two and a half times during the last couple of years.
The heating problem of these hothouses, however, has not been solved definitely.
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,