Professional area | Culture factsheet | The growing medium | Description of various materials used for the make-up of a growing medium
Peat in various forms is the main component of a growing medium. Peat holds water well and is favourable to aeration. It is mixed with bulk materials that are inert over time, such as pine bark, marble chips, rock wool or perlite, and the materials complement each other’s properties.
Peat is a material of organic origin, arising from the more or less complete decomposition of plants of the moss class, mainly sphagnums but also rushes and carex.
There are different categories of peat :
Young peat is only a little composted; it is the result of an accumulation of dead sphagnum moss under acid and anaerobic conditions. The sphagnum contains dead cells which play an essential role in water retention.
The fibres it is made of are relatively coarse, hold water well, and encourage aeration. Water is thus held both within the peat particles and between them: this particular feature allows the peat two stages of drying out and enables the plants to withstand drought for longer. Re-moistening after drying out is easy.
Black peat is in an advanced state of decomposition: it arises from the natural ageing of young peat, and is also good at water retention, but not so good at aeration. It shrinks considerably on drying out.
Brown peat is intermediate between the two.
Peats are light mediums. They have a high ion exchange capacity. The main disadvantage of peat is its capacity for acquiring and reacquiring moisture: it shrinks enormously on drying out, as the water in the dead cells is lost, and, excepted for young peat, rehydration is not easy.
Peats are often acidic and require neutralisation by adding calcium carbonate or slaked lime (cf. the specialist literature).
In spite of their organic origin they are disease-free.
They are inert as to temperature.
Peat, and indeed young peat, is obligatory if watering is by irrigation from underneath (subirrigation).
Pine bark may be used, or other by-products of forestry (chippings, sawdust,....) or agricultural and food industry wastes.
Pine bark chippings are not used green since the resins, tannins and other substances present may be poisonous to plants. They are first neutralised and composted, then shredded.
Bark retains little water. When green, their C:N ratio is high and they use a great deal of nitrogen in the process of composting. Once composted their C:N ratio is lower.
They make light mediums, with good aeration and good structural stability; they are also rich in potassium which is released slowly but continuously and becomes available to the plant.
Their water retention, however, is poor, and their cation exchange capacity not high.
We should note that composted bark should be analysed before being incorporated in a growing mix.
These are rocks of volcanic origin. Their composition renders them inert.
They are porous; but the difference in accessibility of the pores means that pozzuolana is better than tufa for aeration, while tufa is better for water retention.
The ion exchange capacity of pozzuolana is negligible, but the same is not true of volcanic tufa.
These materials have great structural stability and so last extremely well.
These materials are graded by particle size. They have little water retention, and what they have is inversely proportional to particle size, while their aeration capacity is directly proportional to it. The addition of coarser material makes it possible to have better-filtering mediums.
Sands containing limestone can upset the medium pH and the balance of the nutrient solution.
On first use, rockwool reacts with the nutrient solution and releases calcium, magnesium, iron and manganese ions. This reaction does the plants no harm provided care is taken to wet the medium with an acidic solution for 48 hours before putting plants in it.
The material has a satisfactory pore structure; it is aerating, but can hamper capillary action.
Perlite is a siliceous sand of volcanic origin, very light and porous and well aerated. Its water retention is a function of its particle size (Brun 1993, quoted in Urban 1997). It is a fragile material: the granules are friable and break up over time, reducing porosity and aeration.
In a mixture it improves drainage without much harming capillary action.
This is light and porous, well aerated and with a strong water retention capacity, of the order of 45 to 50% (Brun 1993, quoted in Urban 1997). Over time, however, vermiculite loses bulk and results in poorly aerated plants unless used in conjunction with better-aerated ingredients.
It has a relatively high cation exchange capacity.
Expanded clay is obtained in granular form by high temperature treatment of nodules of wet clay.
This is a durable material, light and well aerated but with poor water retention and availability. It has a low cation exchange capacity.
“Raw” clay, unfired, is also used.
Trials conducted in 1993 at the pot plant research station of the IFHP (French Horticultural Institute) at Angers (Morel and associates, 1995) investigated the influence of adding clay to mediums for cyclamen under standard growing conditions. The bright scarlet Halios® cultivar (Morel) was used for this work, and various commercial mediums, both with and without clay. Watering was by trough under-pot flow irrigation.
At the end of the trials it emerged that the addition of clay made little difference to the physical properties of the mediums; water availability was slightly lower, while air content was unchanged. This would tend to indicate that the admixture of clay is of negligible importance in these mediums (less than 10% by volume). Agronomic measurements on the other hand recorded an adverse effect of clay on plant growth and flowering; even in small doses, it would appear that clay has some growth-limiting effect.
Polystyrene is not strictly speaking a growth medium, for it does not itself retain water. Used in a mixture with other materials, however, it improves their aeration properties and lightens their structure.
Polyurethane foam is light, flexible, spongy and inert. It has little water retention.
The various materials may be mixed with each other so as to make the properties of some complement those of others. The proportions of each ingredient may be varied depending on the properties required of the medium. Overall we are aiming at a medium with a pH that suits cyclamen (5.6 - 5.8), aerated and light (so that if too many calories are absorbed in the heat of the day the excess can be got rid of at night), free-draining, sterile, with a good distribution of nutrients. The best way to ensure that stressful situations (changes in weather, excessive heat, too much or too little water, salinity, pH) do the least possible harm is to choose a medium that has a coarse fibrous structure, the right degree of wetting capacity, good cation exchange capacity and a high buffering action: and this is what, in the end, makes for good growing conditions.
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