Professional area | Culture factsheet | The growing medium | The growing medium : physical properties
Cyclamen need a medium which both retains water and drains well.
It is very important that the medium is suited to the watering system and the grower’s management practices.
What needs to be taken into account is the known water retention capacity, the ease of passage of water through the medium, and its air content so as to allow a good oxygen supply to the roots.
Cyclamen prefer a medium that is always damp but also drains well, with the best possible aeration or oxygenation and good heat conductivity. It must provide properly for the supply of water and air to the roots and tuber, both in overall quantity and in terms of regularity. It is therefore necessary to know:
The medium plays a role as support for the plant and at the same time the role of a relay, supplying water, air and warmth. These physical properties are bound up with the texture (particle sizes & proportions) and the structure (assembly of the solid ingredients in the medium). Texture may be judged by feel: structure is assessed indirectly, by the study of porosity.
This measure is the volume of empty space as a percentage of the total volume of the medium. These are the spaces that may be occupied by air or water. The higher the porosity, the more space the substrate has that is capable of allowing air and water to circulate, and the larger the volume for the roots to spread into and search.
The medium’s capacity to provide a good supply of water is determined by its ability to hold water and then to relinquish it.
It is vital to know the quantity of water that a medium holds. In horticulture the maximum amount of water that a medium can hold is known as its reservoir capacity. Mediums with a high reservoir capacity have a certain advantage : the less water there is in the medium the more quickly the plant may use up its reserves and be checked by want of water. All the spaces are occupied by water, which does not flow away but is held in the soil pores by surface tension. The quantity is an average for the whole of the medium; in fact the water gathers towards the bottom of the pot by gravity, and this region is therefore damper than the upper one.
It is mediums with fine ingredients (small particles) which have a high water retention capacity.
However we also have to reckon with the fact that the medium is not able to give up to the plant all the water it contains. A proportion of the water is held onto strongly by the soil, and though present it is unavailable to the roots. This gives us the concept of the biological threshold moisture content: the degree of humidity in the medium below which the plant begins to droop. Moreover, these retentive forces increase as the medium gets drier. A higher proportion of fine particles means a higher droop point.
The amount of water that the plant can in fact make use of is what constitutes the water availability.
The water which arrives in the medium is subject to movement due to its weight (percolation or drainage) or otherwise (movements due to differences in moisture content of different regions of the medium). The percolation speed (or permeability) depends on texture and structure. The more uniform and rounded the component particles, the more permeable the medium; the more irregular in size and entangled the particles, the more impermeable the medium.
The air content of the medium is the difference between total porosity (amount of space) and the water content. The more water held in the medium, the less air is available to the roots, which can become asphyxiated. Now cyclamen need a good air circulation in the soil if they are to grow and flower as they should: and it is therefore necessary to find a good balance between oxygenation and drainage capacity.
The existence in the medium of a gradient of humidity, and conversely a gradient of air supply, allows the roots to situate themselves just where water and air content are best.
The medium absorbs heat from solar radiation. The amount of heat received depends on many factors: the nature of the pot, the colour of the medium and its humidity.
Finally, we need to remember that mediums of organic origin are ‘alive’. Their physical properties are liable to change over time, as the seasons pass, as irrigation affects them, and as they are colonised by roots. Physical analysis of the medium is important because it allows us to know how it changes from time to time as the plants grow, and this lets us respond quickly so as to keep the plant in ideal conditions for growth (for instance by altering irrigation if the water availability decreases).
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