Soil preparation and technologies for growing high-quality vegetable transplants

Soil and Bulk Substrate Preparation

Soil preparation is the initial stage of seedling cultivation, whether in open nurseries and under temporary covers, or in heated greenhouses. The quality of substrate preparation directly impacts plant establishment and their development rate. Depending on the facility type, agronomists use field soil or specially prepared bulk substrates.

For open nurseries and plots under temporary plastic film covers, tillage begins in the autumn. Follow this sequence of operations:

1. Apply organic and mineral fertilizers, immediately followed by deep ploughing in early autumn.
2. After the first flush of weeds appears, treat the soil with cultivators or shallow ploughs to keep the field clean until winter.
3. In spring, if necessary, perform cultivation or rotary hoeing to a depth of 10–12 cm.
4. Compact the soil immediately before sowing.

• Organic matter (humus or compost) — 80–100 t/ha
• Mineral fertilizers (NPK by active ingredient) — 90–100 kg/ha
• Spring tillage depth — 10–12 cm
• Proportion of sod or field soil in bulk substrate — 40–50%
• Proportion of sand or sawdust in bulk substrate — 10–20%

For hotbeds and greenhouses, bulk substrates are prepared in the autumn. In southern regions, a mixture of equal volumes of sod (or high-quality garden) soil and humus is most popular. If the soil in the greenhouse is not replaced, the same volumes of fertilizers as in open nurseries are applied in the autumn, the soil is loosened to its full depth, steamed, and the elements of the greenhouse structure are disinfected.

Container Growing Technologies and Mixture Recipes

Seedling cultivation in pots or containers is a distinct technological branch. Its specificity lies in the variety of containers used and the ability to fully mechanize planting. Vegetable growers use several types of containers:

• cubes and pots made of peat-humus and humus-soil mixtures;
• factory-made peat blocks;
• hollow pots of various shapes made of peat-cellulose mixture or polymers;
• mineral wool cubes;
• multi-cell polymer and paper trays.

Tray-type containers are selected to fit specific models of transplanters. Using seedlings grown in nutrient pots or cubes allows for harvesting 5–15 days earlier compared to conventional seedlings. The root system is not damaged during transplanting, the plant does not lose its growth "head start," and the nutrient supply in the cube stimulates the rapid formation of productive organs.

Peat-humus or humus-soil mixtures are used to make nutrient cubes and fill hollow pots. In the south of the country, a recipe consisting of equal parts humus and sod (or field) soil with the addition of 10% sawdust for fluffiness and 5–10% cow manure slurry as a binder is popular. If the soil is heavy and clayey, its portion is reduced by half, adding an equivalent volume of humus. When filling rigid trays or hollow pots, cow manure is excluded from the mixture composition.

Precise dosage of mineral fertilizers is determined only through agrochemical monitoring. According to field research data, optimal threshold levels of nutrients in seedling substrates should fall within strict ranges.

Preparation of Nutrient Cubes and Fertilizer Application

For growing high-quality vegetable seedling crops, it is important to accurately dose mineral fertilizers when preparing the soil mixture. An excess or deficiency of nutrients directly affects the development of the root system and plant establishment in the field. Application rates of fertilizers for major crops are provided in the table.

When producing pressed nutrient cubes, it is necessary to strictly control the physical properties and humidity of the prepared soil. An overly wet mixture compacts significantly during pressing, which hinders oxygen supply to the roots. The optimal parameters of the finished substrate for forming high-quality cubes should meet the following values:

• Bulk density — 0,4–0,9 g/cm³
• Porosity — 50–70 %
• Water holding capacity — 48–50 % of field capacity
• pH level — 6,0–7,0

To form nutrient blocks, an STM-8-20 mixer or a mortar mixer is used in conjunction with an IGT-10 potting machine. This equipment allows for pressing cubes with edges of 5, 6, 8, and 10 cm. In vegetable growing, block sizes of 6x6x6 cm and 8x8x8 cm are in highest demand. For cassette technology and growing young transplants, cells with a size of 4x4x4 cm or less are used.

Seedling cultivation technologies: with and without pricking out

The choice of cultivation technology for transplants determines the greenhouse operating schedule and the timing of obtaining marketable produce. The traditional method involving pricking out assumes an initial dense sowing in a seedbed with a density of 2,0–2,5 thousand units per square meter. The seedbed is created on ridges or in special boxes in heated greenhouses.

The method with pricking out is valued for the opportunity to obtain a strong fibrous root system. During transplanting, the taproot is broken, stimulating branching, and the burying of the hypocotyl gives additional lateral roots. The main organizational advantage is the saving of greenhouse space: while the seedlings grow in the seedbed for 1,5–3,0 weeks, the agronomist has time to prepare the main areas. However, the method has disadvantages: pricking out delays plant development by 5–10 days, requires high manual labor costs, and is practically impossible to mechanize in small enterprises, although on modern automated lines, the transfer is carried out by robots.

When growing transplants without pricking out, seeds are sown directly into pots, cubes, or permanent beds. In this case, the feeding area of each plant initially corresponds to the standard seedling stand density. This method eliminates root trauma and shortens the total growing period.

Seeding depth of seeds">The seeding depth during sowing depends on their fraction. Small seeds are sown to a depth of 0,5–1,0 cm, medium and large ones — to 1,0–1,5 cm. Immediately after sowing, irrigation is carried out, and subsequently, soil moisture is maintained within the specified range.

1. Sow to a depth corresponding to the seed size using a PRSM-7 manual seeder or by hand, then perform irrigation.
2. Mulch the surface with a dark film or paper to maintain the substrate moisture at the level of 70–75 % of field capacity.
3. Maintain the optimal air temperature until the first emergence.
4. Immediately after emergence, lower the air temperature by almost half and keep it at this level for 4–7 days.

Reducing the temperature after emergence is a critically important technique. Such a drop inhibits the growth of the aerial part and prevents stretching of the hypocotyl. This stimulates active development of the root system and lays the foundation for full-fledged mineral nutrition of the plants during the growing season.

Growing seedlings in small-cell cassettes in a nutrient solution or in sawdust with periodic or constant supply of mineral substances of low concentration is very effective. Such seedlings retain their roots completely during pricking out and can be transplanted even if they have become overgrown.

When pricking out, the feeding area of each plant is increased several times. The number showing how much the area increases after pricking out compared to the seedling seedbed is called the area expansion coefficient. It is usually equal to 5–7 for cabbage, 8–10 for tomato, pepper, and eggplant. If this coefficient is less than 3–4, it makes no sense to carry out pricking out for economic reasons. It is not recommended to prick out seedlings of crops that do not tolerate transplanting well (cucurbits), but some farms practice growing seedlings in sawdust, from which extraction ensures complete root preservation, and they get a good effect from using pricking out for cucurbit plants. The seedling feeding area depends on the crop and its age at the time of planting in the field or on the type of protected soil facility and the calendar growing dates. To avoid excessive crowding of seedlings (mutual inhibition of plants) and thin stands (inefficient use of space), it is necessary to take into account existing patterns: 1) each crop is characterized by a specific plant size at the seedling age; 2) the older the seedling, the larger the feeding area required for each plant; 3) the later the seedlings are grown (from winter to summer), the faster their growth and development, therefore early cabbage seedlings with a feeding area of 6×6 must be 50–60 days old, while those grown in middle and late terms — 40–45 days. Pricking out is carried out manually in two ways: under a peg in soil marked with a multi-tooth marker and under a bar. The latter method is more productive, but the quality of root-to-soil contact is worse. Immediately after pricking out, plants are watered abundantly (fine-droplet sprinkling) so that the soil settles and connects tightly with the roots, and shaded for two to three days. In such conditions, illumination is limited and good moisture of the environment is maintained, which at the optimal temperature (see Table 14) ensures the survival of pricked-out seedlings within the specified time. Further care for the seedlings consists of maintaining optimal temperature and humidity of the air and soil (pre-irrigation moisture 60–65 % of soil field capacity).

Irrigate moderately during morning hours, in clear sunny weather with mandatory ventilation. The number of irrigations (4—6 times or more) depends on the age of the seedlings, their planting dates, weather conditions, and the type of structure. Water consumption per square meter is up to 10 liters. Seedlings grown in nutrient blocks are irrigated with the same dose, but in two stages, so that the water is absorbed by the entire mass of the pot. In addition, two top dressings are performed — after the transplanted seedlings have rooted and one and a half to two weeks later; if growth is vigorous, the second top dressing is postponed to a later date — 10 days before planting. Fertilizer application rates differ for each crop and must be strictly maintained so as not to change the concentration of the soil solution in an undesirable direction. If necessary, treatment against diseases and pests is applied.

Figure 4 — Methods of pricking out vegetable seedlings: a — using a dibber; b — using a lath; c — pricking-out lath (board), dimensions in millimeters

An increase in air temperature in spring sometimes leads to seedling overgrowth. To prevent this phenomenon, irrigation is restricted or retardants are used.

Growing seedlings without pricking out differs little from the technology described above. First of all, the sowing rate is several times lower (see Table 15). If grown in soil without isolation from the underlying soil, root pruning at a depth of 4 cm is sometimes practiced at the three true leaf stage. This is necessary because, with restricted irrigation, plant roots grow deep into the soil and break off when the seedlings are lifted. Pruning ensures the formation of a fibrous root system concentrated in the nutrient layer. When growing seedlings in open nurseries, after sowing, special attention is paid to maintaining optimal soil moisture, or timely measures are taken to break the soil crust that forms after rolling. In open nurseries, SLN-20, SZL-3.6, and other seed drills are used for sowing, and herbicides are applied for weed control.

The growing conditions for seedlings without pricking out are the same as for previously pricked-out seedlings. The humidity regime, temperature regime, and soil nutrition conditions are no different. The timing of the first top dressing, which is performed during the formation of 2-3 true leaves, may not coincide, which is slightly ahead in calendar terms compared to the timing of this operation when growing seedlings with pricking out.

The methods for growing high-quality seedlings described above and various types of work involve manual labor. Therefore, the problem of producing seedlings on an industrial basis, without the use of manual labor or with its minimal use, has been addressed for a long time. The most promising technologies are those involving the use of small-cell trays (16-30 cm3) made of polymer materials or special paper and production lines. According to these technologies, with the help of appropriate mechanization means (automated or using manual labor), the nutrient soil mixture is prepared, trays are filled with it, sowing or pricking out into cells is performed, trays are stacked in a specific order convenient for transport to greenhouses and placement on the floor, and after the seedlings are grown, they are loaded onto transport for delivery to planting machines. There are already special small-cell trays for growing seedlings, from which it is convenient to perform pricking out using equipment on production lines.

For mechanizing the pricking out of seedlings when growing bare-root seedlings, a technology using paper tapes is suitable. This method allows for completely preserving the formed root system of the plants and avoiding the loss of "lead time" during transplanting. The seed is placed on an automated line, after which the transplanter places the young plants directly into the soil together with the paper base.

1. The seed is placed between two glued paper tapes 5–10 cm wide on an automated line.
2. The resulting tape is loosely rolled into coils.
3. The coils are placed in a greenhouse on trays to which a nutrient solution is supplied.
4. In the phase of full cotyledon expansion (at the very beginning of the emergence of the first true leaf), the coils are installed on the transplanter.
5. Mini-seedlings are planted into the soil directly along with the paper.

• Paper tape width — 5–10 cm
• Size of nutrient blocks — increased by 1.5–2 times
• Water temperature for irrigation — 22–26 °C

Features of growing seedlings for protected ground

The technology for preparing greenhouse seedlings has its own features and differs significantly from the rules for open ground. Plants are grown exclusively in nutrient blocks of increased size. The temperature regime after emergence, as well as during night hours, is maintained a few degrees higher than for open-ground seedlings. Irrigation is carried out strictly with warm water of a specified temperature.

Growing conditions and plant density depend on the specific production cycle. During the winter period, it is essential to provide supplementary lighting for transplants to prevent stem elongation, and as they grow, the pots are spaced out. During the spring-summer and summer-autumn periods, natural lighting is sufficient, so supplementary lighting is not used, and plants are grown at a younger age and planted more densely.