Selection of substrates and management of mineral nutrition in greenhouses

Root-inhabiting media in protected ground are called substrates (soils), as they differ from soils of natural origin.

Substrates are divided into actual soils — well-fertilized natural soils; soil mixtures, which consist of various components, mainly of organic origin with the addition of mineral fertilizers; soil substitutes of organic origin (peat, straw, sawdust); and artificial substrates, which are inert materials (gravel, expanded clay, sand, etc.).

The first three types of substrates are called soil culture. Vegetable growing on artificial (inert) substrates, where plant nutrition is carried out by absorbing nutrient solutions, is called hydroponics.

=> 25) Nutrient substrates for soil-based vegetable culture. Nutrient substrates in greenhouses must be highly fertile with good air permeability, water-holding and absorption capacity, free from disease pathogens, pests, and toxic substances, with a soil solution reaction (pH) for cucumber, lettuce, radish, onion of 6—7, celery, cauliflower — 6.5—7, tomato — 5.5—6.5. The best substrates contain 20—30% or more organic matter and 12—20% or more humus. The optimal bulk density of the substrate for cucumber is 0.5 g/cm³, for tomato — 0.8, for lettuce and transplants for open ground — up to 1 g/cm³. For normal plant life, it is necessary that the air content in the substrate be no less than 10—12%, and total porosity should be 60—70%. The thickness of the nutrient substrate when growing vegetables in greenhouses is 30—35 cm, and for transplants for open ground — 5—10 cm. It houses 85% of the roots.

When growing transplants for open ground, it is important that the mechanical composition of the substrate be light, intermediate between light loam and sandy loam. The ratio of physical clay to sand should be 1:4 (according to the Kachinsky classification).

Actual soils with the addition of organic and mineral fertilizers are widely used in Ukraine for growing vegetables in film greenhouses and transplants for open ground. In autumn, under cucumber, 200 t/ha of manure (after 2—3 months of biothermal disinfection) is applied, and under tomato — 100—150 t/ha of humus.

1 ton of straw requires 10 kg of nitrogen, 1 ton of sawdust — 3—5 kg of nitrogen. Organic fertilizers for transplants are applied in a 5—10 cm layer, as the main root mass is located there; for vegetable crops — in a 30—35 cm layer.

To increase the soil fertility of greenhouses, green manure crops should be used more widely. At the Simferopol Vegetable and Melon Station, at the beginning of August, when the entire harvest in the greenhouses had been collected, a vetch-oat mixture and leaf mustard were sown, and on November 1, the greenery was incorporated into the soil. As a result, the following year, the yield of cucumber and tomato in these areas increased by 1—2 kg per 1 m² compared to the control plot where humus was applied.

To grow transplants, coarse-grained sand is added to the soil in order to bring its mechanical composition to light-loamy, along with organic loosening agents at a rate of 30% of the volume of the 10-centimeter nutrient layer: 27 kg of humus, or 9 kg of peat, or 1.2 kg of air-dry mass of straw chaff per 1 m³.

Soil mixtures are mainly used for making nutrient pots, when growing potless transplants in hotbeds, and for vegetable crops in greenhouses. In Ukraine, in zones where peat is available, pots are made from three parts peat and one part humus. For protected ground, only types of peat with a decomposition degree of no more than 40%, ash content no higher than 12%, total iron content of no more than 5—6%, and without mobile forms of aluminum, ferrous iron, and manganese are suitable. Lime is added to peat pre-moistened to 70% of field capacity at a rate where 1 kg of chalk or lime flour increases the pH of 1 m³ of peat by 0.5—1. In the absence of peat, pots are made from 5—8 parts humus (5 parts for sandy loam soil, 8 — for loamy soil) and 1 part soil.

Soil mixtures for hotbeds consist of 30% humus and 70% sod-podzolic soil for vegetable transplants crops for mass planting dates.

=> c5% planting, and of 50% humus and 50% soil — for transplants for early planting dates.

For growing seedlings, a mixture of two parts humus, one part soil, and one part sand is most often used.

Vegetables in film greenhouses are grown in soil mixtures less frequently than in actual soils. In backfilled soils, peat accounts for 30 to 80%, manure (humus) — from 10 to 30%, and soil — from 20 to 60%.

Soil substitutes of organic origin. The highest requirements are imposed on nutrient substrates in glass greenhouse complexes. Peat moss, transition peat, straw, sawdust, and bark are used as substrates in them. The advantage of these substrates compared to traditional greenhouse ones (sod-podzolic soil or lowland peat) is the absence of silt particles, which contribute to the formation of a surface crust that impairs the air supply to the root system. Their disadvantage is the insignificant content of initial nutrient elements and even harmful substances. To improve the quality of such substrates, mineral fertilizers are applied, peat is limed, bark is composted, etc.

The right choice of substrate in a greenhouse allows for controlling plant nutrition and increasing their productivity. Traditional soil mixtures are gradually giving way to organic materials and hydroponics. Practical experience shows that competent management of substrate properties pays off with increased yield and reduced labor costs.

Organic substrates: peat moss and straw

Experiments show that cucumber and tomato yields on peat moss are 16–25% higher than on a mixture of equal parts of soil, low-moor peat, and manure. For cultivation, peat with a decomposition degree of no more than 10–20% and an ash content of 3–5% is suitable. Before planting transplants, the substrate is compacted by 1.3–1.5 times and mixed with manure in a ratio of 8–9:1–2. Such a peat base can be used for more than 8–10 years if fresh peat is added every three years to a level of 30–40 cm.

The required level of soil fertility of the peat is maintained by irrigation with a nutrient solution or a combination of basal fertilizer and top dressing. Micronutrients are added to the substrate in liquid form during preparation. The application rates of liquid micronutrients per cubic meter of peat are presented in the table.

In greenhouses without technical heating, straw shows high efficiency. From 50 to 200 tons of loose straw per hectare is applied, which is technologically simpler than working with pressed bales. Vegetable yield on a straw substrate is higher than on regular soil with technical heating. This is due to the substrate warming up to 25–27 °C (versus 20–21 °C in regular soil), the active release of carbon dioxide, and the stimulating effect of humic acids.

Hydroponics: aggregate hydroponics and small-volume technology

The hydroponic cultivation method automates nutrient supply and significantly reduces labor costs. At the same time, the technology requires highly qualified specialists and strict control of parameters. Errors in the preparation of the nutrient solution are critical here due to the lack of a natural buffer.

The most widespread among hydroponic methods is aggregate hydroponics — growing crops on solid inert substrates with periodic root wetting. For this, materials with good water-air properties are used. The key physical and chemical parameters of a high-quality gravel or crushed stone substrate are presented below.

• Gravel particle size — 3–5 mm
• Crushed stone particle size — 5–25 mm
• Carbonate content — no more than 20 %
• Bulk density of the substrate — 1.5–1.7 g/cm³
• Water-holding capacity — 9–10 %
• Layer thickness in the basin — 25 cm

For the long-term performance of solid substrates, it is necessary to carry out their maintenance regularly. The care technology includes three mandatory stages that are performed in strict sequence. If these rules are followed, the service life of the filler is practically unlimited.

1. Wash the substrate with clean water.
2. Acidify the working environment to a pH value of 5.6.
3. Disinfect the substrate with a 5% formalin solution.

The most progressive and economically viable form is small-volume hydroponics. Instead of continuous trays, vegetables are grown in separate troughs with plastic lining or in polyethylene containers. The substrate volume is only 3–17 l per plant, and from 1 to 4 plants are placed in one container. The nutrient solution is supplied through drippers, by overhead irrigation, or by the sub-irrigation method from a basin.

A wide range of fillers is used in small-volume technology. Agronomists choose the substrate depending on the farm's capabilities and the specific crop. The main materials used are:

• inert materials — volcanic slag, gravel, perlite, vermiculite, mineral wool;
• organic soil substitutes — peat moss, sawdust, wood bark, coconut fibers.

Growing vegetables and roses on mineral wool with drip irrigation has gained particular popularity. This sterile substrate is obtained by melting diabase at temperatures above 1600 °C. Mineral wool has a neutral reaction (pH = 7), does not contain its own nutrients, and is easily washed of salts. Working with small-volume technology on mineral wool requires the mandatory use of drip irrigation and computerized control systems.

How to control plant nutrition and identify deficiencies

The precise calculation of mineral nutrition in greenhouses is based on regular agrochemical soil analysis and plant diagnostic data. Visual control serves as an operational supplement, but it is laboratory tests that show the actual nutrient availability of the crops in the long term. The supply of nitrogen and potassium in the substrate is inevitably depleted as plants grow, so regular top dressing is a mandatory element of the technology.

• Start top dressing — 1 month after planting transplants
• Treatment interval — 7–10 days
• Root top dressing consumption — 100–150 g of fertilizer per 10 l of water
• Foliar macronutrient concentration — 10 times lower than root application
• Basic dosage in case of data deficiency — 1 matchbox of nitroammophoska per 1 m²

When preparing tank mixtures for foliar treatments of cucumber and tomato, proven dosages of macro- and micronutrients are used. Foliar top dressing allows for rapid compensation of nutrient deficiencies directly through the leaf blade.

To prepare a working solution of micronutrients, strict concentrations of active ingredients are maintained per 10 l of water. The mixture should contain 0.03% boric acid, 0.005% each of magnesium sulfate, zinc, copper, cobalt, and ammonium molybdate, 0.04% iron citrate, and 0.001% potassium iodide.

A nutrient deficiency can be visually identified by characteristic changes in the vegetative mass:

• Nitrogen: lower leaves pale, upper leaves turn yellow and die, stem becomes thin and brittle.
• Phosphorus: leaves turn a dark, bluish color with a purple tint, blacken and die, flowering is delayed.
• Potassium: leaf edges turn yellow, curl downwards and die, internode growth slows down.
• Magnesium: interveinal chlorosis develops (tissues turn yellow, red or purple, but veins remain green).
• Calcium: apical buds, leaf edges, and roots die.
• Iron: uniform chlorosis of young leaves without tissue necrosis.
• Boron: fruit set drop, death of growing points on tops and roots.

Rules for fertilizer application and anti-stress top dressing

The efficiency of mineral nutrition directly depends on the method and timing of fertilizer application. Localized application during sowing or planting allows for reducing the application rate by several times without yield loss compared to broadcast application. At the same time, it is important to physically isolate the seed from fertilizer granules to avoid chemical burns to the seedlings.

1. Apply mineral fertilizer to the bottom of the furrow or planting hole.
2. Thoroughly mix the granules with the soil.
3. Cover with a 5–7 cm layer of clean soil.
4. Proceed with seed sowing.

When planting tomato, pepper, eggplant, and cabbage transplants, one or two handfuls of humus and 1 teaspoon of nitroammophoska are added directly into the hole. Mineral root treatments are necessarily alternated with organic ones — solutions of cow manure, poultry manure, or fermented plant residues. If a deficiency of a specific element (e.g., nitrogen) is identified in the soil, the dosage of accompanying phosphorus and potassium fertilizers is also maintained, but in smaller volumes, as they improve the uptake of the primary element.

The need for nitrogen top dressing increases sharply after prolonged rains and cold spells, when the root system temporarily reduces its activity. In such conditions, to support plant immunity, micronutrients are used as a foliar spray at a rate of 3–5 g per 10 l of water. Foliar treatments become especially important during a sudden shift from cloudy to sunny weather.