MEDIA FOR THE CULTIVATION OF GREEN HOUSE PLANTS

Dr. P. Syam Sundar Reddy

Assistant Professor (Hort.)

MEDIUM
A base that enables and helps the seed to get good germination or rooting is known as a medium. A good medium : Must be sufficiently firm and dense to hold the seeds or cuttings in place during rooting and germination. Its volume must be fairly constant when either dry or wet. It must have good water holding capacity and sufficiently porous so that excess water drains away. pH must be suitable for the plant being propagated, be free from excess salts and be relatively free from disease causing organisms. It should withstand sterilization without undergoing any chemical change.

SOIL-BASED MEDIA:
Generally soil based media have the same characteristics such as soil itself, except that the bulk density and aeration is increased using amendments such as perlite, peat moss, bark or polystyrene. A medium is considered as soil based if it contains at least 25% soil. One advantage of soil based media over the soil less media is the high CEC that soil contributes to the mix. Nutrient retention is increased which makes soil based media especially useful for long term crops such as stock plants, cut flowers planted in ground beds. Traditionally ,a soil based medium has been composed of equal parts by volume of loam field soil ,concrete grade sand and sphagnum moss amended with phosphorus. Selection of a root media is made on the basis of economics and the physical situation in which it must serve.

SOIL LESS MEDIA

They have been proved popular with majority of the producers because of consistency, excellent aeration, reproducibility and low bulk density which reduces shipping and handling costs of the medium. Soil less media however generally have a lower CEC than soil based media which requires constant attention to the nutritional program. Soil less media are also harder to wet than dry Consequently most premixed soil less medias contain a wetting agent to facilitate wetting Micronutrients must be applied to soil less mixes either prior to potting or sometime thereafter.

PROPERTIES OF A MEDIA
MOISTURE RETENTION AND AERATION: A medium must be able to provide plant roots with both water and air Pores or spaces of various sizes allow a medium to accomplish both functions at once Total porosity refers to all the pore space within the medium The water in the medium after irrigation is of two types: available and unavailable The available water is loosely held by the medium particles and can be absorbed by the plant roots The unavailable water is so tightly bound to the surface of the medium particles that the root is not able to absorb the water. A suitable growing medium contains a balance of capillary and non capillary pores that allows optimal root growth.

 Another consideration is the movement of water within media. Water moves within the media from the point of application by capillary action, which consists of adhesion, the ability of water molecules to bind to other materials such as media particles, and cohesion, the ability of water to bind to itself. The rate of water uptake by a dry medium can be increased with the incorporation of a wetting agent. Wetting agents decrease water surface tension. Wetting agents are especially useful for increasing the amount of water uptake during the first irrigation by media components that can be difficult to wet such as dry peat moss or bark.

It strongly influences the availability of nutrients to plant roots and is a measure of the concentration of hydrogen ions in media water solution. If pH 7.0 is neutral. above 7.0 is basic and below 7.0 is acidic . The recommended media will vary with the species being grown but general recommendations have been developed. For soil less media the should be 5.4 to 6.0 and 6.2 to 6.8 for soil based media(25% or more soil) Media pH often changes over time in response to fertilizers and water alkalinity If the water alkalinity is high ,the grower may want to start the crop at the low end of the recommended pH range

BULK DENSITY
: Bulk density refers to the dry weight of the medium component relative to the volume. Media such as peat moss are very light and have the ability to absorb a large amount of water and become heavy. For most of the crops ,a medium with a low bulk density of 0.1 to 0.8g/cc is generally used. Media for flat beds can be especially light weight because there is no chance the flats will fall over.

CATION EXCHANGE CAPACITY

Root media components such as peat moss, vermiculite and bark have a negative electrical charge which attracts positively charged ions in the medium water solution. The CEC indicates the strength of that electrical charge for a medium and capacity of the medium to hold positively charged nutrient ions. The greater the CEC ,the more the nutrient ions the medium will hold. The CEC is based on volume for soil less media and should be high to allow the medium to retain the nutrients. Most of the plant nutrient ions are cations:NH4+,Cu++,Mn++,Fe++. Anions are negatively charged ions and include NO3-,H2PO4-, 2,Cl- of which the later two are usually limited in supply. Media components with a high CEC include soil, peat moss, and vermiculite and components with a low CEC include perlite, styrofoam

STABILITY:
The properties of a medium continually change from the time the plant is planted till the plant is harvested. Stability is especially important for the production of long term crops such as cut flowers in ground beds or stock plants that will remain in the same medium for a long time. The main reason for instability is biological degradation & the natural decomposition of organic matter Although all organic matter eventually begins to decompose, not all organic matter decomposes at the same rate. One reason sawdust is not used in media is due to the rapid decomposition which alters the structure of a media. Peat moss and bark, however decompose more slowly, allowing the media to retain its original properties much longer. Vermiculite is also unstable due to physical compression in media over time. The natural compression from watering and plant root growth will cause the plate like structure of vermiculite to collapse.

CARBON:NITROGEN(C:N) RATIO:
Any media with organic components will decompose due to the action of microorganisms. Most organic media components ,such as peat moss, do not break down rapidly and so nitrogen depletion is minimal. The optimum ratio of organic matter to nitrogen is 30lb C to 1lbN. Sawdust has a carbon to nitrogen of 400:1 and decays rapidly, resulting in significant nitrogen depletion. Bark also has a high carbon to nitrogen ratio ,but is usually composted before use.

Commonly used soil less mixtures for greenhouse crops.

Volume/Volume Ratio 2:1 2:1:1 2:1 3:1:1 2:1:1 2:1:1 3:1:1 Components Peat, Perlite z Peat, Perlite, Vermiculite * Peat, Sand 3:1 Peat, Sand Peat, Perlite, Vermiculite * Peat, Bark, Sand Peat, Bark, Perlite Peat, Bark, Sand
* Most commonly used combinations in commercial soil less media

DIFFERENT TYPES OF MEDIA

Perlite Bark Vermiculite Calcined clay Rock wool Styrofoam Peat moss Sand Soil Coir dust Saw dust Selected crop by products

PERLITE:
Alumino silicate rock is ground and heated to 1800F resulting in white ,popcorn like pieces. It has many characteristics similar to sand and is commonly used as a low weight replacement for sand. Perlite contains fluoride which can be detrimental to fluoride sensitive species such as Easter lilies (Lilium longiflorum) Moisture retention and aeration :Low water retention and excellent aeration. CEC: Very low pH: Approximately 7.5 but has little influence on media Stability: Very stable and already sterile. Carbon: Nitrogen ratio :0:0,no carbon Cost :High

PERLITE

BARK:
The type of bark available varies by area. The two types are soft wood with a pH of around 5-6.and hard wood with a pH of 7 Moisture retention and aeration: Moderate water retention and excellent aeration after composting. CEC: High composting after
FINE ORCHID BARK

PINE BARK

VERMICULITE
Vermiculite is a very desirable component of soil less root media because of its high nutrient and water retention ,good aeration, and low bulk density .It is commonly included in soil less media. The water holding capacity of expanded vermiculite is high because of the extensive surface area within each particle. Its light weight property makes it desirable in pot plant media. Vermiculite is usually not used with soil. The predominant fertilizer nutrients in vermiculite are potassium ,magnesium, and calcium. Expanded vermiculite can be compressed easily between the fingers. The magnesium content of African vermiculite is high and has been known to provide the total needs of a green house crop

 CEC: High

pH :

African vermiculite-9.3 to 9.7,

American/U.S Vermiculite-6.3 to 7.8

Stability: Chemically stable and sterile ,but the particles compress easily, especially when wet and should be avoided for long term crops. Bulk density: Low,497g/dm3 Cost :High Moisture retention and aeration: Excellent, do not use with the soil that plugs the pores of vermiculite particles.

Vermiculite
VERMICULITE

African vermiculite U.S Vermiculite

CALCINED CLAY
Clay particles baked at 1300 F can vary with the type of clay used, but are generally very porous. Calcined clays are also used as litter ,industrial spill absorbers, and other functions. Moisture retention and aeration :Excellent

CEC: High
pH: Varies from 4.5-9.0 Stability :Very stable and already sterile Bulk density: Moderate to high Carbon: Nitrogen ratio: 0:0,no carbon Cost: High

ROCK WOOL
Rock wool is manufactured by melting basalt rock, steel mill slag, or other minerals at 2700F and spinning the material into fine fibers similar to fiberglass insulation. Moisture retention and aeration: Excellent CEC: Low

pH: Neutral to slightly alkaline

Stability :Very stable and already sterile Bulk density: low 264g/dm3

Carbon: Nitrogen ratio: 0:0,no carbon

Cost: Moderate

PLANTS GROWN ON ROCKWOOL

STYROFOAM
Styrofoam (polystyrene) is the lightest commonly used component, and the small beads float to the top of the medium with overhead irrigation. The beads often turn green with algae growth and can blow out of the containers and become a nuisance. Moisture retention and aeration: Low water retention and excellent aeration CEC: Very stable and sterile. Bulk density: Very low.25g/dm3 Cost :Low Larger particles may be used in bench media and for epiphytic plants such as orchids.

STYROFOAM

Expanded Polystyrene
Polystyrene flakes, a bi-product of polystyrene processing, are highly resistant to decomposition, increase aeration and drainage, and decrease bulk density. Polystyrene may be broken down by high temperatures and by certain chemical disinfecting agents.

PEAT MOSS
Peat is a natural accumulation of organic matter that can be divided into four types: Sphagnum peat moss is the partially decomposed remains of various Sphagnum moss species .It has a pH range of 3.0 to 4.0 and a fiber content of at least 66% and is the major peat used commercially. It is considered weed free ,insect ,pathogen free. It is light in weight and has the ability to absorb 10 to 20 times its weight in water. Sphagnum moss contains specific fungistatic substances which accounts for its ability to inhibit damping-off of seedlings The most important type of peat moss is the sphagnum peat moss Hypnum peat moss is the partially decomposed remains of various Hypnum moss species. It has a pH range of 5.0 to7.0 and fiber content of at least 33%. it decomposes more rapidly than some other peat types, it is suitable for media use.

 Reed sedge peat is the partially decomposed remains of reeds and sedges and has a ph range of 4.0-7.5. These fine textured materials are generally less acid and contain relatively few fibrous particles. The rapid rate of decomposition, fine particle size and insufficient fiber content make reed and sedge peats unsatisfactory for media use Humus or Muck - consists of the decomposed debris of finely divided plant materials of unknown origin. Humus often contains large quantities of silt and clay particles, and when mixed with soil does not improve drainage or aeration. Due to its rapid rate of decomposition and particle size, humus is considered to be undesirable for growing media use.

 Moisture retention and aeration :Excellent water retention and moderate aeration. Sphagnum peat moss can absorb up to 60% 0f its total volume in water. CEC : Medium to high pH: 3.0-4.0 Stability: Very stable Bulk density :Low, 352g/dm3 Carbon: Nitrogen ratio: 50:1 for Sphagnum peat moss Cost: High

Peat moss

Sphagnum peat moss

Sphagnum moss
Peat humus

Hypnum moss

SAND
Sand is used in soil based media to develop large diameter pores for good aeration. Sand was once used as one of the most commonly used components in media but use has declined as low weight components such as perlite and styrofoam were developed with similar characteristics as sand Moisture retention and aeration :Low water retention and low to moderate aeration. CEC: Low pH :Inert and neutral Stability :Very stable Bulk density: 1600-1760g/dm3 Carbon: Nitrogen ratio: 0:0, no carbon Cost: Low

SOIL
It generally contains little organic matter{1.5) Moisture retention and aeration: Excellent water retention and poor aeration CEC :Very high pH: Variable and depends on the source of the soil Stability: Very stable Bulk density :High(1364g/dm3) C:N ratio :Low, in fact there might be no carbon if there is no organic matter

Cost: Generally low if there is own source but can be expensive to purchase and ship

Coir dust
It is similar to peat moss in appearance and is a bye product of coconut husk fiber processing industry.
Coir is relatively recent addition for the list of possible amendments and quality varies with the source.

In particular total soluble salts Na, chloride, K may be injuriously high

Coir dust does not contain weed seeds pathogens and absorbs water more easily than dry peat moss.

Moisture retention and aeration: Excellent

C:N ratio: Moderate Stability: Excellent

Bulk density: Low

CEC: medium to high Cost: High

OTHER COMPONENTS
Several other organic and inorganic components are occasionally used in mixing the media. Besides bark. sawdust, and wood chips are used. Sawdust and wood chips are used after being composted. A second type of organic amendment is the various composted products such as composted paper, straw, spent mushroom, composts, manures. A variety of inorganic materials such as waste tire components can be used in growing media. Water absorbing polymers and starches are occasionally added to media to reduce the number of irrigations and to delay wilting during marketing. Composted products have excellent moisture retention and, good initial aeration and a high CEC.

Wood shavings charcoal

Sugarcane bagasse

SAWDUST
In many aspects it is similar to bark

It should be partially composted because in fresh state ,its rate of decomposition is excessive and it may contain toxic substances like resins ,tannins and turpentine.
Saw dust decomposes at a faster rate than bark and because of its wider C:N ratio(1000:1) a greater amount of nitrogen is tied up in the root substrate. Sawdust like other plant materials ends up close to neutral in pH when thoroughly composted, however this is well beyond the stage at which it is initially used in greenhouse substrates.

Selected crop bye products

Various relatively stable crop bye products are occasionally used including rice hulls ,cocoa hulls, groundnut hulls Moisture retention and aeration :Medium to low water retention but excellent aeration CEC :Excellent

Stability :Usually stable

Bulk density: Low C:N ratio: High

cost: often low

p H-5-7

Bagasse
Bagasse is a waste bi-product of the sugar industry. It may be shredded and/or composted to produce a material which can increase the aeration and drainage properties of container media. Because of its high sugar content, rapid microbial activity results after the incorporation of bagasse into a media. This decreases the durability and longevity of bagasse and influences N levels. Although bagasse is readily available at low cost, its use is limited.

Rice Hulls
Rice hulls are a bye product of the rice milling industry. Although they are extremely light in weight, rice hulls are very effective at improving drainage. The particle size and resistance to decomposition of rice hulls and sawdust are very similar. However N depletion is not as serious of a problem in media amended with rice hulls.