WHAT IS GROWING MEDIA? Often also referred to as substrate or potting mix, a growing medium is a material, other than soil in the ground, in which plants are grown.
In nature, plants grow in soil or water. Some plants even grow on rocks or tree branches. A basic requirement for all plant growth is that the medium in which they grow can hold the roots. Fixation is needed to anchor the plant and to provide the plant with water, oxygen and nutrients. If fixation is not needed, like for plants in supporting frames or hanging on ropes, plants can also grow in water or mist systems In soilless culture or hydroponics, soil is mimicked by the use of a growing medium.
In fact, plants can grow in anything, as long as there is water, sufficient nutrients and oxygen available and contains no hazardous components naturally.
Article written for Reiziger by
University & Research Center. The Netherlands.
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too small absolute volume for
should have low pH buffering
important basic requirements
the roots and for air. Moreover,
capacities as well. For
for growing media that are
the relatively thin substrate
logistical purposes is should
different from soil. In the first
height causes a too low
be not too heavy. Since the
place soilless culture is, by
matric suction with almost all
introduction of soilless culture,
definition, a restricted volume
pores to be filled with water.
many types of growing media
in the order of 1–10L/m ,
So substrates must have a
have been tried. Some have
whereas soil has at least a
high pore volume and a lot of
proven to be successful,
rooting volume of 300L/m .
macropores. Furthermore, it
others have not. The most
Soil has a pore space of 40%
must be preferably inert, or at
important and widely used
or even less. In a small volume
least the buffering capacity
substrates are detailed later
of a substrate this involves a
for ions must be low and it
in this article.
2
2
The growing media sector represents an industry with a â‚Ź1.4 billion turnover accounting for 11,000 jobs across Europe.
However, there are a few more
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Media water content and water retention. The behaviour of water and air in soils and substrates are related to
physical properties such as the size and distribution of pores. This can
be seen in the relationship between the water content and the soil water potential, the water retention curve or soil moisture characteristic.
This curve is characteristic for each soil and substrate type. The general features of a water retention curve can be seen in the chart opposite, in which the volumetric water content is plotted against the water (matric) potential. At potentials close to zero, a substrate is close to saturation and water is held in the substrate primarily by capillary forces. As the water content decreases, binding of
•
the water becomes stronger and at small
filled porosity, which is the volumetric
potentials (more negative) water is strongly
proportion of the water contained at
bound in the smallest of pores and as films
saturation (water potential = 0cm in the
bound by adsorptive forces around particles.
coarsest pores) and therefore readily released and replaced by air at water
Sandy soils will involve mainly capillary binding and will therefore release most of the water at higher potentials, while clay soils,
potentials between 0cm and -10cm; •
growing medium by forces compatible
(more negative) potentials.
with root extraction capability (defined for the range of water potential from -10cm
Peaty materials will show much higher
to -100cm); and
moisture contents than clay and sand. The water holding capacity of any soil is due to the porosity and the nature of the bonding in the soil. From water retention curves, the following properties are taken into account: •
Total pore space, which is the total void volume (available to water and/or air) as a proportion of the total volume of the growing medium;
Water availability, which is the volumetric proportion of pore water retained in the
with adhesive, will release water at lower
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Air volume content, also called air-
•
Water buffering capacity, defined as the volumetric proportion of water released by the growing medium between -50cm to -100cm, enabling physiological adaptation of the plant to the changing water potential.
THE WATER HOLDING CAPACITY OF ANY SOIL IS DUE TO THE POROSITY AND THE NATURE OF THE BONDING IN THE SOIL.
Water volume content (% v/v)
100% Solid phase volume (7% v/v)
80% Air volume content or air filled porosity (26% v/v)
60% Water availibilty (33% v/v)
40%
Water buffering capacity (4% v/v)
20% Unavailable water (34% v/v)
0 01
02
03
04
05
06
07
08
09
10
Water potential (-kpa) 93% Total pore space
67% Total pore space
38% Total pore space
34% Total pore space
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Qualities of different types of growing media. PEAT MOSS Peat is an accumulation of partially decayed plant remains or organic matter. In natural peat lands, the annual rate of biomass production is greater than the rate of decomposition.
History
Quality
Treatment
The most common component is
To a large extent quality is also
sphagnum moss peat, although
determined by the production
For growing medium application,
many other plants can contribute.
method. Peat may be gained by
Mankind has used peat for
milling (milled peat), by cutting
hundreds of years as fuel. Today,
and subsequently breaking the
important production areas are
bricks, or by dredging it and
Scandinavian countries, the
compressing it to ‘press peat’.
Baltic States and Canada. There
As a result of the wide range
are many different qualities
of possibilities, the area of
available on the market: from
application is wide as well.
partially decomposed (sphagnum
It is vital that pristine peat
peat and granulated peat moss) to highly decomposed (upgraded black peat) with large variations in particle size. The term particle size has a different meaning than usual in soil science; a chunk of peat is also regarded as particle.
base dressing is required. In addition, for multi-year application extra attention should be paid to the fertilisation with trace elements since these are, for a large part, fixed by the peat, particularly copper. The extent of fixation is closely related to the type of peat. Partially decomposed peat has
products are used so as to
low fixation capacity.
avoid plant diseases and
Natural peat has a low
weeds. Mostly peat contains a lot of water. Air content strongly depends on the fineness of the peat.
pH between 3.5 and 4.0 and requires liming before application. In the course of time the size of the particles may decrease (decomposition). The degree of refining also depends on mechanical tillage and soil life. Furthermore the degree of decomposition is highly determinant for structural stability.
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THE MOST COMMON COMPONENT IN PEAT IS SPHAGNUM MOSS, ALTHOUGH MANY OTHER PLANTS CAN CONTRIBUTE.
Natural peat land in the Baltic States.
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WOOD FIBRE Wood fibre is produced from wood at a temperature of more than 1000ËšC. Certain techniques are used to press wood granules through an aperture, which results in a high pressure and high temperature. As a consequence the product is free from plant pathogens. Pinewood is mostly used as a starting material. The trees are debarked. For some wood fibre products waste wood is used, which of course should be free of impregnating agents and other harmful substances. Wood fibre is applied in mats. The fibres are in a net wrapped in white plastic film. But the fibres can also be mixed with peat in potting soil mixtures. Wood fibre products are manufactured from both fresh wood and from waste wood. A small amount of bark may be present in the final product when the wood used has not been debarked properly. The substrate is often very airy and retains little water. During the first period of cultivation part of the nitrogen will be fixated due to the intensive colonisation of bacteria. As a result the pH also rises. Adapting the nutrient solution can control this. Many wood fibre products may have a high Mn content.
GLASS WOOL Glass wool is made by melting quartz sand in an electric oven at 12,000ËšC. A binding agent is added to the fibres so that the mat keeps its form. As a growing medium, glass wool can be used in all places where stone wool is used. Glass wool consists of long, firm fibres. The fibre diameter can be varied, in contrast to stone wool, so that drier or wetter products can be made. By giving the mat finer fibres in the upper part and coarser fibres in the lower part it is also possible to obtain a uniform moisture distribution in the mat. Glass wool is very light and can obtain much water and, particularly in the upper layer, much air. Chemically speaking glass wool is inert, except that it contains high B levels, which will be released gradually. Nutrient solutions should be adjusted to that. Glass wool hardly dissolves in acids and as a result has greater structural stability than stone wool. It is not biodegradable.
STONE WOOL Stone wool is produced by melting basalt and limestone, after adding cokes, at a temperature of 16,000ËšC. The liquid mass is poured onto a rapidly spinning disk. As a result the liquid mass is flung out after which the fibres solidify. For the manufacturing of stone wool slabs, binders (for firmness) and often surfactants (for the water uptake) are added. Synthetic oil is also added occasionally to make the stone wool water repellent. Stone wool is supplied in the form of slabs, plant blocks, plugs, or granules. Stone wool slabs are available for one-year and multi-year use. The moisture characteristic is determined mainly by fibre structure and fibre positioning. Stone wool is alkaline. At pH levels below 5.0 the stone wool dissolves. In general, stone wool slabs are very wet after complete saturation. Most of the water can be extracted with low moisture tensions. When a stone wool slab is dried out completely it is difficult to rewet it. Stone wool granules are mostly applied as components of potting soil mixtures.
SAND Sand is a collective name for granules consisting for the major part of quartz and with a granule size between 0.05 and 2mm. There are numerous sources. Most likely for substrates river sand is used. Sand is very heavy, has a very limited water buffer and often a low air content. This is due mainly to the fact that sand contains few big pores. In thin layers it contains too little air. As growing medium, therefore, coarse sand is preferred. The problem with sand is that it is mostly not available in specified fractions. This means that for each batch the coarseness may vary. Since the degree of coarseness determines the air content, the air content may fluctuate strongly from batch to batch. Consequently the product is not available in constant quality. Sand is inert. The pH fluctuates, depending on the calcium content. The EC varies, depending on where the sand was dug. Sand can be used in layers of 10cm and upward. It is not wise to grow oxygen-sensitive crops on this growing medium, unless the sand contains little fine parts. With thin layers it should be taken into consideration that the pores may rapidly fill with water. Finally, sand is very durable because it is neither chemically nor biologically affected.
LAVA Lava is a volcanic product. It flows as magma from the volcano during an eruption. Subsequently it slowly solidifies into solid rock. Broken lava is dug up, broken and sieved. In principle lava can be used for many crops on the condition that manual handling is not required. Due to the small water buffer water irrigation must be frequent. Lava is suitable for use in thin substrate layers. Broken lava is a heavy product with a moderate amount of pores. Due to its granular form it is airy, but retains little water. The product is rather sharp and subsequently not suitable for manual handling. In addition lava is very angular. As a result, when the plants are pulled out an amount of substrate will stick to the roots. Alternatively, less sharp lava types are available as well. Since lava is broken and sieved, a constant composition is warranted. Lava does not have any buffering capacity for nutrient elements and has a pH of 7 to 8. Lava is not degradable, chemically or biologically and it has great structural stability. It is therefore very durable and lasts for years.
PERLITE Perlite is a glass-like volcanic rock, which is found in all kinds of places worldwide. It is ground, sieved and then ‘roasted’ at 10,000˚C. The perlite ore contains mineral water, which converts to gas in the high temperatures in the oven. This makes the perlite expand to about 20 times the original volume. Perlite is used often in potting soil mixtures and increasingly as growing medium. Perlite is very porous and light and can simultaneously contain much water and much air. The pH is rather neutral, between 6.5 and 7.5 and the product has a low EC (0.1). Perlite has no buffering capacity for nutrients. It may contain many closed pores, so that not all air-filled pores participate in gas exchange. Perlite is brittle and may pulverise under low pressure. In normal usage this is not a problem. Under proper handling conditions perlite can be used for several years. The material itself cannot be composted but it can be used as additive in the composting process.
PUMICE Pumice is a volcanic product that develops during a volcanic eruption when glowing material is flung away and solidifies in the air. As a result air bubbles are closed in, giving the material a porous structure. Of the volcanic rocks pumice has the lowest bulk density. Some sources may contain clay particles so that the air content is low and blockages may occur for proper drainage. The pH value of pumice lies between 6.5 and 8.0, while the EC value is very low. Pumice is a stable rock and consequently very durable. It can be applied for several years.
VERMICULITE Vermiculite is produced in a similar way as perlite, with the addition that this material has a layered structure. Between the layers there is mineral water. When the ore enters the oven at 10,000˚C the water is converted to vapour which presses the layers away from each other. As a result vermiculite consists of granules with a ‘harmonica’ shape. Vermiculite is mainly used as sowing medium, or as component in potting soil mixtures. Vermiculite is very porous and light and can contain much water and much air. The pH is neutral (7.0) and the EC is low. Vermiculite is less suitable as substrate because the structural stability is limited. Vermiculite easily sags in. As a result the physical characteristics get worse, so that use as a substrate for 2 or more years becomes doubtful.
POLYURETHANE Polyurethane is produced from mineral oil products. The material is made for the furniture industry. The cutting remnants are made applicable for substrate growing. First the cutting remnants are ground to granules. To obtain mats the granules are pressed together with chemicals until a certain density is reached. As a consequence the granules are glued together and form a slab. Polyurethane is applied in mattresses, furniture, foot forms for ski boots and underlay. As a growing medium it has been frequently applied in many crops. Polyurethane mostly contains much air and little water. Water supply should therefore be frequent. In the past there have been problems with mats of polyurethane, because the foam contained harmful substances. Nowadays producers have this under control so that such problems no longer occur. Polyurethane is flexible, has a low EC and no pH buffer. Polyurethane lasts very long, at least 10 years. It is not degradable by the effect of acids or micro-organisms.
PHENOL RESIN Phenol resin foam is made by foaming up mineral oil products. A well known type is flower sticking foam. The size of the pores may vary. The product can be produced as slab but granules can also be made. The granules are used in orchid and anthurium crops. After manufacturing, the pH is very low (3.5). Therefore the material has to be limed before delivery. Before the first water supply an extra lime application is required (0.5kg.m3 Dolokal). Phenol resin foam retains much water but in granular form contains much air due to the cavities between the granules. Phenol resin foam does not have a buffering capacity for nutrient elements. The material is rather soft and not flexible so that it pulverises and loses its structure after being pressed in.
COIR (COCO PEAT) Coir is a waste product derived from coconuts and consists of coir dust, coir chips and coir fibre. Its bark contains fibres and dust. Alternatively, the bark of the nut can be ground to chips. Coir fibres contain oxygen with a low water holding capacity; coir dust absorbs water but retains adequate oxygen. With coir dust and coir chips a small amount of nitrogen is fixed. Coir has a pH of about 5 to 6, yet a light liming is required to buffer the pH. Coir products are not inert; consequently they react to fertilisers and have a high buffering capacity for nutrient elements. Since coconut palms grow along the coast, there may be a high salt content. The cause of possible salt excess is twofold: the coconut palm can take up Na, K and Cl without problems and uses the nut as a storage organ for these salts. In addition, the coir is often insufficiently rinsed in the producing country. Furthermore, fresh material still contains harmful substances. For that reason, the coir substrate must be composted for 4 months and the material must be rinsed and treated with Ca/Mg salts to avoid problems. As a result of such a pretreatment, K and Na are pushed out of the adsorption complex so that K and Na will not be released during the cropping period.
EXPANDED CLAY By baking specific types of dry clay at 11,000ËšC, gasses are released by which the clay expands. It is of great importance that only special types of clay with a low content of water-soluble salts are used for manufacturing. In the process various granule sizes are produced, as well as broken clay granules. For many years clay granules have been applied in hydroculture and in buckets and gutters with various types of plants. The expanded clay granules have a low weight and a porous structure. Clay granules therefore contain a large amount of air and little water. Therefore frequent water supply combined with a constant water layer in the substrate is mostly recommended. The pH is about 7 and the EC low, on condition that a low salt clay is used. It has been demonstrated that expanded clay granules do not show any changes in physical composition after 5 years of intensive cropping. Clay granules are very fit for multi-year use because they are strong and can be well steam sterilised. Used clay granules can be washed and reused.