VIII. Greenhouse soil culture:
1) Seedling production facilities:
a) Germination chamber:
It is a chamber used to facilitate the seed germination process by providing optimal environmental conditions. It is equipped with a heating/cooling system for temperature control and a misting/fogging system for humidification. Air circulation is important because it ensures a uniform temperature and humidity in the chamber. Once the germination process has finished, the seedlings should be removed outside the germination chamber.
b) Tray support system:
Trays should be placed on benches to avoid the direct contact between the roots and the soil, eliminating the risk of disease transmission to the plants.
Stages of filling and sowing trays:
• Tray cleaning
• Tray filling with growing medium
• Removing the surplus of growing medium by hand or by brush
• Pressing to firm the growing medium
• Sowing the seeds
• Covering the seeds with the growing medium
• Again removing the surplus of growing medium by hand or by brush
2) Factors affecting seeds germination:
a) Seed quality:
Good quality seeds play an important role in increasing production, as they provide vigorous growth, good germination and rapid emergence. The seeds used should be viable, certified, and free from diseases.
Components of seed quality are:
• Genetic quality: cultivar purity and longevity
• Physical quality: analytical purity, moisture content, size and appearance
• Physiological quality: germination capacity, viability, vigor, vitality and dormancy
• Pathological quality: health
The quantity of light required during germination process depends on seeds type. Some seeds require high light level, some seeds are indifferent to light and other seeds require low light level.
Water is essential for seed germination and it must be uniformly distributed. It is important never to let the surface of the substrate dry out until the germination is completed.
Temperature should be stable and regularly monitored for good seed germination.
Humidity is helpful during the germination process.
3) Growing medium:
A growing medium is a porous material that provides functions essential for plant growth:
• Physical support to the plants
• Water, oxygen and nutrient to the roots
The growing medium should have the following desired characteristics:
• Good water holding capacity
• Good air capacity
• Stable structure
• Slow decomposition
• Pathogens and weed seeds free
• Easy handling
• Low cost
a) Physical properties of a growing medium:
Plants need macropores and micropores for their growth. A good growing medium is characterized by the total porosity which is a combination of different particle sizes that gives good water holding capacity as well as good air capacity.
Bulk density is defined as the dry mass of the substrate per unit of substrate volume in a dry state. Excessive bulk density indicates compaction. Bulk density and porosity are indirectly related, porosity decreases when bulk density increases.
Oxygen is necessary for good healthy roots. It is supplied through the larger macropores, which allow the dissipation of carbon dioxide during respiration. A good growing medium is characterized by a high percentage of macropores.
Water holding capacity:
Water holding capacity is the maximum water content that can be retained by a particular containerized substrate which is allowed to drain after saturation.
b) Chemical properties of a growing medium:
The pH indicates the acidity or alkalinity of a solution and it is measured by means of a pH meter which is a portable instruments. The pH also plays a role in determining the nutrient availability for plants. pH in the root zone should be between 5.5-6.5.
The growing medium should contain all essential nutrients, macronutrients (nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) and micronutrients (manganese, zinc, copper, boron and molybdenum).
Cation exchange capacity:
CEC is the ability of growing medium to exchange cation for charged nutrient ions that help in growth and development.
c) Types of growing medium:
• Growing medium with organic ingredients: peat moss, compost, coconut noir, rice hulls, sawdust and bark
• Growing medium with inorganic ingredients: vermiculite, perlite, pumice and cinder, and sand
Fertilization begins when the set of true leaves appears. Fertilization plays an important role in increasing crop production. Fertilizers found in solid or liquid forms are added to provide plants with macronutrients and micronutrients essential for their growth. Nitrogen, phosphorus and potassium are the most essential nutrients that the plants may need. Nitrogen helps leaf of young plants to grow. Phosphorus and potassium promote fruiting and flowering, and guarantee robust stem growth and water movement. Other nutrients may also be added depending on plants nutrient requirement.
Seedlings should be hardened prior to transplanting. Hardening is to slowly acclimate seedlings to the new environment they will be grown in by reducing water supply, reducing fertilization, reducing temperature and by gradually exposing seedlings to outside climate condition (light, moderate wind, etc).
Care is needed when extracting the seedlings from the trays to minimize root disturbance. Seedlings can be transplanted to soil either by hand or by machine. The soil should be disinfected in order to be free from pests, pathogens and weeds. Soil solarization is a nonchemical soil disinfection method that is based on trapping solar radiation by covering the wetted soil with a transparent polyethylene sheet for a month or more. Soil preparation begins by ploughing or turning the soil and smoothing its surface. The soil is then watered, a wet soil has the ability to conduct heat making soil organisms vulnerable. Then a plastic sheet is laid on the soil to be treated. After the soil solarization is completed, the plastic sheet is removed.