Specifics of growing transplants in hotbeds and film greenhouses

Soil preparation and sowing dates in hotbeds

Growing seedlings in hotbeds requires adjusting the usual dates due to the specific temperature regime. Physiological processes in these structures proceed more slowly, therefore seed sowing must be carried out 7–10 days earlier than for greenhouses. This rule should be followed particularly strictly when growing seedlings in the winter months.

The thickness of the soil layer in bio-heated hotbeds depends on the cultivation technology of the planting material. A layer 6–7 cm thick is applied for potted seedlings, and 14–16 cm for non-potted seedlings. A mixture of humus and sod soil in a 1:2 ratio, or equal parts of humus, peat, and sod soil, is usually used as a nutrient substrate.

In hotbeds on natural soil without additional heating, soil preparation does not differ from that in greenhouses. When using biofuel, it is important for the agronomist to carefully monitor the gas regime. Premature sowing or pricking out before the end of the active heating phase of the biofuel can lead to ammonia poisoning of the plants.

Soil temperature regime in film greenhouses

Film greenhouses are recognized as the best structures for growing vegetable seedlings. However, high-quality plants can be obtained only if there is an effective ventilation system for hardening and equipment for maintaining the microclimate. For each crop, it is critically important to monitor soil temperature at all stages of cultivation.

For early white cabbage and cauliflower, soil temperature directly determines the development rate of the root system and future yield. Any deviations from optimal values lead to undesirable growth of the aerial part to the detriment of the roots or to a general delay in plant development. Detailed data on the influence of temperature regimes on cabbage yield are given in the table.

When growing tomato seedlings, soil temperature in the period from emergence to hardening determines plant viability. Maintaining an average daily soil temperature of 15–17 °C inhibits excessive stem elongation, promotes a lower setting of the first flower cluster, and thickens the leaves. As a result, strong seedlings with a developed root system are formed, which take root faster in the field.

• Optimal soil temperature — 15–17 °C
• Increase in early tomato harvest — 29%
• Growth in production profitability — 35%
• Cultivation period without pricking out — 50 days
• Cultivation period in pots — 65 days

The choice of the greenhouse heating scheme depends on the age of the tomato seedlings and the method of growing them. When growing 50-day non-potted early tomato seedlings without pricking out, simultaneous heating of the soil and air is necessary to keep the soil temperature within 15–17 °C. For 65-day extra-early tomato seedlings with pricking out into pots, soil heating can be turned off if air heating is sufficient to maintain the required soil temperature.

For mass planting dates, it is acceptable to grow tomato transplants in plastic greenhouses without artificial heating. Lower temperatures and good light levels in such structures have a beneficial effect on the physiological processes in the plants. Nevertheless, without air heating, there is a high risk of obtaining non-standard transplants in cold years, especially when growing without pricking out. Air heating is necessary for drying out the soil, its rapid warming upon emergence of seedlings, and maintaining the temperature regime at night.

Adjusting the temperature regime and heating systems

The temperature regime in a greenhouse directly determines the rate of seedling emergence. For example, when growing tomato transplants for mass planting dates, the soil does not need to be heated, as its average daily temperature is 13–15 °C. However, it should be borne in mind that at such a temperature, seedling emergence is delayed by almost two weeks.

In the spring, daily rhythm plays an important role. The daytime increase in temperature activates the vital processes of the plants and compensates for the influence of cool night and morning hours, when the temperature drops below optimal values.

For mid-season white cabbage transplants in unheated greenhouses, the temperature is usually kept at 13–14 °C, which fully complies with the biological requirements of the crop. However, for the first sowing period (March 20 in the forest-steppe zone), it is advisable to provide emergency air heating — it is necessary for drying out and warming up the soil.

It will not be possible to grow high-quality 45–50-day pepper and eggplant transplants without pricking out in cold soil. For these crops, a combined system of soil heating and air heating is mandatory. Heating only the air will lead to delayed emergence, and the transplants will not be ready for the optimal planting dates.

To plan the greenhouse engineering systems, rely on the following crop requirements:

• Soil and air heating: necessary for peppers, eggplants, early white cabbage and cauliflower, as well as for early tomatoes grown without pricking out.
• Air heating only: sufficient for mid-season white cabbage, tomatoes for mass sowing dates, and extra-early tomatoes grown with pricking out.

Greenhouse equipment directly affects the intensity of its use. Greenhouses with air heating usually operate in two cycles from March to September (after harvesting the cabbage and tomato transplants, peppers or melons are planted in them). Areas with combined heating (soil and air) can be operated year-round in 5–6 cycles — after the transplants, cucumbers and tomatoes are placed there, and in the autumn-winter period, green crops are grown.

Soil preparation and optimization of its physical properties

The technology of soil preparation depends on the transplant growing method. For the pot method, the soil is prepared from autumn according to the scheme of the subsequent vegetable crop: it is cultivated to a depth of 20–25 cm and the appropriate fertilizer rates are applied. The main task here is to perfectly level the surface for the pots using electric tillers and manual grading.

Work on preparing the soil for bareroot cultivation is divided into several stages:

1. Autumn bed preparation. Before applying organic matter, cut beds 1.6–1.8 m wide depending on the greenhouse design.
2. Soil leveling. Carefully grade the surface to avoid water stagnation. An even relief ensures uniform snow melting and simultaneous soil maturation. Uneven soil matures 7–10 days later.
3. Application of organic matter. For tomatoes and mid-season cabbage, organic fertilizers are applied in autumn and worked in with a tiller. In year-round heated greenhouses for peppers and eggplants, organic matter is applied in the spring immediately before their cultivation.
4. Spring mineral nutrition. 3–4 days before sowing, apply mineral fertilizers and incorporate them with a tiller to a depth of 10 cm.

In the conditions of Ukraine, where fertile chernozems prevail, there is no need to use purchased bulk soil. The hydro-physical properties of the local soil can be improved with accessible loosening materials to reach optimal levels:

• Soil bulk density — 0.8–1 g/cm³
• Total porosity — not lower than 60 %
• Organic matter — 10–15 %
• Size of soil aggregates — 3–5 mm

The mechanical composition of the ideal soil should correspond to light loam or sandy loam. The presence of up to 15 % of large soil aggregates up to 10 mm in size is acceptable.

Deviations in soil density from the norm reduce plant survival in the field. If bulk density exceeds 1 g/cm³, seedling growth is delayed. Decreasing density to 0.35 g/cm³ stimulates rapid growth of the above-ground part, but impairs the ratio of root mass to leaf mass. In such loose soil, fragile lateral roots develop actively and are easily damaged during transplanting.

To obtain uniform emergence and high-quality bare-root transplants, a fine-crumb soil structure with particle sizes of 3–5 mm is optimal. In such soil, seed is placed at the specified depth and covered evenly. To improve the physical and water-related properties of the soil and save on scarce humus, low-lying peat or chopped straw is added to the soil mixture in a volume of up to 30% of the soil mass. Peat and straw have a stable chemical composition, which allows for more precise control of plant nutrition.

• Optimal soil crumb size — 3–5 mm
• Consumption of low-lying peat (10 cm layer) — 9 kg/m²
• Consumption of chopped straw — 1.2 kg/m²
• Savings of humus — 270 t/ha

Light soils are best suited for growing transplants, as they warm up and become ready faster in the spring. Heavy soils crust over and remain cold longer, which shifts sowing dates, leads to uneven emergence, and causes growth delays. Furthermore, when pulling seedlings from heavy soil, roots are frequently torn, the proportion of stunted plants increases, and machinery operation becomes more difficult. To lighten heavy soil, sand should be applied in the autumn when the soil is in a state of physical maturity and thoroughly incorporated using a rotary tiller.

The main rule of spring preparation is to work the soil only when it is in a state of physical maturity. Hurrying in this matter is destructive, as tilling damp soil destroys its structure for a long time. Covering the greenhouse with film must be done in advance, combining warming with active ventilation.

To timely prepare and dry the soil before starting work, follow this sequence:

1. Two weeks before the start of greenhouse operation, cover it with film and turn on the heating to accelerate soil warming.
2. 10–15 days before the start of sowing or transplanting, be sure to open ventilation openings to dry out the top layer.
3. Do not keep the greenhouse completely closed during warming; otherwise, moisture will evaporate from the soil, settle as condensation on the film, and drip back down, creating a closed cycle.

Mineral nutrition of transplants

Balanced nutrition shifts the balance toward the accumulation of phosphorus and potassium relative to nitrogen. This improves the survival rate of tomato, pepper, and eggplant transplants, stimulates photosynthesis, and increases early harvest. In the absence of chemical soil analysis results, fertilizer application rates are selected according to technical guidelines; however, it is more reliable to monitor the concentration of elements in a water extract.