HortFACT - Soilless Media in Horticulture
The most common materials used for soilless media preparations in New Zealand are:
Potential materials for soilless media are:
Compost from organic waste could also be classed as a form of soilless media.
The materials listed above can either be used on their own or as mixes. In this article, we refer to the soilless media as a "mix".
Figure 1: Commercially bagged mixes
Some types of material used for soilless media are the by-products of other industries eg. bark, or are recycled forms of waste matter, eg. compost. Therefore, using soilless media can be relatively inexpensive and environmentally friendly
The air space (also called air filled porosity or AFP) is the volume of air in a mix after it has been watered heavily and then allowed to drain. If the mix is very dense and there is too little air in the mix (less than 10 %) then the roots will not be properly aerated and the plant will suffer.
The plant available water represents the water which is easily available to the plant roots. Although plants can extract more than the available water, they must exert extra energy to do so and this restricts growth. A formal definition of the plant available water is the "volume of water removed from a just-drained mix by a suction of 10kPa, or a 1 metre column of water".
The height and width of the container you use will affect the air space and plant available water. A tall pot will have drier mix in the top layers than a wider, shallower container of the same volume. Therefore, think twice about planting something which needs a lot of water in a tall pot. Often additives such as water-holding gels (eg. Crystal Rain) or surfactants (eg. Slippery Water) can be used to modify the air and water balance.
Figure 2: Soilless media pot trials
The balance between the plant available water and the air space depends on the size and shape of the particles in the soilless mix - or more precisely the pores between the solid particles. Large particles (0.5 millimetres or larger) which have more air space between the pores contribute air space to the mix. Medium-sized particles (0.1 - 0.5 mm) contribute to the available water in the system. Fine material with particles less than 0.1mm in size (dust) will hold some water, but this water is unavailable to the plant. Very little air is held in fine particle mixes. Therefore, the ideal mix will have a balance between medium and coarse particles with a minimum of fine particles.
The exact mix of particle sizes depends on the end use of the media. For example, a higher percentage of large particles is suitable in a glasshouse mix as the mix will be regularly watered so can have a lower available water level. On the other hand, a patio tub mix which may only get watered once a week will need to have a higher proportion of medium sized particles. In general, a good balance is achieved with 2/3 to 3/4 coarse particles, and the remainder medium particles. The volume of fine particles needs to be minimal, less than 5%.
As well as particle size, particle shape is also important to the amount of air space or available water. For example, peat is very fibrous and may have more available water than some other materials such as bark.
However, it is not just a simple matter of adding plant nutrients into the soilless mix. Certain materials, including bark, sawdust and ponga, react with added plant nutrients so the nutrients become unavailable to the plant. To overcome this, the materials are usually composted with added nutrients to stabilise them prior to using in the mixes.
Plant available nutrients are either held on exchange sites on the mix particles by adsorption (the capacity for a mix to hold nutrients is the ‘cation exchange capacity’ or CEC, see below) or dissolved in the mix water. Care needs to be taken that this water does not drain straight out of the container leading to a loss of nutrients and a potential source of water pollution. Such leaching can be minimised by avoiding over-watering, or using additives such as zeolite which allows the media to hold more nutrients in plant-available form (by increasing the CEC).
You are likely to hear the following chemical properties referred to :
Cation exchange capacity (CEC)
The CEC is a measure of the plant available nutrient exchange capacity. The CEC varies between different types of media. A higher CEC is desirable as this means there will be a more even nutrient supply to the roots and less nutrients will be lost through over-watering. Increasing the surface area of the mix, eg. by composting will increase the CEC.
Conductivity (also called salinity, EC or total dissolved solids)
This measures the sum of the soluble nutrients. If the conductivity is too high, (above 3-4 mS/cm (milliSiemens per centimetre)), plant growth rates will decline and sensitive plants may be damaged. High rates of soluble fertiliser or liquid feeds, or high conductivity in the irrigation water will give high conductivity values.
Nitrogen drawdown index (NDI)
By assessing the amount of nitrogen which remains available, you can get an indication of how well stabilised (composted) the mix is. The NDI should be at least 0.7 (in other words, 70% of the nitrogen applied remains available after 4 days), unless you are prepared to supply the additional nutrients to replace those being immobilised by the media.
See Fertiliser Recommendations for Horticultural Crops for more information about fertiliser requirements of soilless media and nutrient testing of soilless media.
New Zealand, unlike Australia, does not have a standard for potting mixes which define a minimum quality in areas such as nutrients, physical properties and toxicity.
Some of the products used in soilless media preparations are thought to have disease-suppressive properties. HortResearch scientists are currently working on disease suppressive composts and soilless media mixes.