TYPES OF GROWING MEDIA, SOIL PREPARATION AND SUBSTRATE MANAGEMENT IN POLYHOUSES FOR GROWING CROPS

 

Soil mixes used for greenhouse production of potted plants and cut flowers are highly modified mixtures of soil, organic and inorganic materials. When top soil is included as a portion of the mixture, it is generally combined with other materials to improve the water holding capacity and aeration of the potting soil. Many greenhouses do not use topsoil as an additive to the soil mixes, but rather use a combination of these organic and inorganic components as an artificial soil mix.

When managed properly as to watering and fertilization practices, these artificial mixes grow crops that are equal to those grown in top soil.

Media preparation for greenhouse production

The media used in greenhouse generally have physical and chemical properties which are distinct from field soils.

• A desirable medium should be a good balance between physical properties like water holding capacity and porosity.

• The medium should be well drained.

• Medium which is too compact creates problems of drainage and aeration which will lead to poor root growth and may harbour disease causing organisms.

 • Highly porous medium will have low water and nutrient holding capacity, affects the plant growth and development.

• The media reaction (pH of 5.0 to 7.0 and the soluble salt (EC) level of 0.4 to 1.4 dS/m is optimum for most of the greenhouse crops).

• A low media pH (<5.0) leads to toxicity of micronutrients such as iron, zinc, manganese and copper and deficiency of major and secondary nutrients while a high pH (>7.5) causes deficiency of micronutrients including boron.

• A low pH of the growth media can be raised to a desired level by using amendments like lime (calcium carbonate) and dolomite (Ca-Mg carbonate) and basic, fertilizers like calcium nitrate, calcium cyanamide, sodium nitrate and potassium nitrate.

 • A high pH of the media can be reduced by amendments like sulphur, gypsum and Epsom salts, acidic fertilizers like urea, ammonium sulphate, ammonium nitrate, mono ammonium phosphate and aqua ammonia and acids like phosphoric and sulphuric acids.

• It is essential to maintain a temperature of the plug mix between 70 to 75^oT. Irrigation through mist is a must in plug growing. Misting for 12 seconds every 12 minutes on cloudy days and 12 seconds every 6 minutes on sunny days is desirable.

 • The pH of water and mix should be monitored regularly.

Gravel culture

Gravel culture is a general term which applies to the growing of plants without soil in an inert medium into which nutrient solutions are usually pumped automatically at regular intervals. Haydite (shale and clay fused at high temperatures), soft- or hard-coal cinders, limestone chips, calcareous gravel, silica gravel, crushed granite and other inert and slowly decomposing materials are included in the term "gravel". The more important greenhouse flowering crops include roses, carnations, chrysanthemums, gardenias, snapdragons, lilies, asters, pansies, annual chrysanthemums, dahlias, bachelor buttons and others.

 

Desirable nutrient level in greenhouse growth media:

		Concentration (mg/1)
S. No.	Category	NO3	N	P	K
	Transplants	75	125	10-15	250-300
	Young pot & foliage plants	50	90	6-10	150-200
	Plants in beds	125	225	10-15	200-300

 

 

 

Media ingredients and Mix

Commercially available materials like peat, sphagnum moss, vermiculite, perlite and locally available materials like sand, red soil, common manure/ compost and rice husk can be used in different proportions to grow greenhouse crops. These ingredients should be of high quality to prepare a good mix. They should be free from undesirable toxic elements like nickel, chromium, cadmium, lead etc.

Pasteurization of greenhouse plant growing media

Greenhouse growing medium may contain harmful disease-causing organisms, nematodes, insects and weed seeds, so it should be decontaminated by heat treatment or by treating with volatile chemicals like methyl bromide, chloropicrin etc.

Agent	Method	Recommendation
Heat	Steam	30 min at 1800 F
Methyl bromide	10 nil/cu. ft. of medium	Cover with gas proof cover for 24-48 hi Aerate for 24-28 hr before use.
Chloropicrin (Tear gas)	3-5 ml/cu. ft. of medium	Cover for 1-3 days with gas proof cover after sprinkling with water, Aerate for 14 days or until no odour is detected before using
Basamid	8.0 g/cu.ft. of medium	Cover for 7 days with gas proof cover and aerate for atleast a week before use
Formalin	20 m1/1 of water (37%) Apply 2 1/cu.ft	cover for 14 to 36 hr and aerate for at least 14 days.

 

Fumigation in greenhouse:

Physical propagation facilities such as the propagation room, containers, flats, knives, working surface, benches etc. can be disinfected using one part of formalin in fifty parts of water or one-part sodium hypochlorite in nine parts of water. An insecticide such as dichlorvos sprayed regularly will take care of the insects present if any. Care should be taken to disinfect the seed or the planting materials before they are moved into the greenhouse with a recommended seed treatment chemical for seeds and a fungicide —insecticide combination for cuttings and plugs respectively. Disinfectant solution such as trisodium phosphate or potassium permanganate placed at the entry of the greenhouse would help to get rid off the pathogens from the personnel entering the greenhouses.