the growing of plants there are fundamental aspects which need to be fulfilled to allow
the plants to grow and develop to their optimal genetic potential:
• light (typically sunlight as a source of energy for photosynthesis)
• temperature (suitable range for plant type)
• space (density to allow air, light and growth and development of the plant)
• Carbon dioxide (for photosynthesis)
• water (for essential plant processes)
• oxygen (supplied to root zone for optimal root function)
• nutrients (growth and development), and
• the provision of a growing environment devoid of plant pests, pathogens and weeds.

An understanding of the physical, chemical and biological characteristics of a growing media can improve plant growth and your bottom line. In this article we look at the physical characteristics of growing media.
Air Filled Porosity - AFP.
AFP is the percentage of the growing media occupied by air after the media has been saturated with water and allowed to drain. More air in the media will improve root growth, but a higher AFP may require more frequent watering and consequently nutrient leaching may be increased. Container depth has a significant effect on AFP as, in shallow containers, the AFP is reduced due to the greater proportion of media occupied by the saturated media in the bottom of the container. AFP will also decrease during the growing cycle, but this can be compensated, to a degree, by the increased ability of the plant to extract water from the growing media, reducing the amount of pore space occupied by water. A range of 13-30% AFP is considered to be acceptable under EcoHort guidelines for general nursery stock.

Nursery irrigation should be applied at a rate that matches the ability of the growing media to absorb and hold the water that is applied. The best designed irrigation layout operated at the correct pressure cannot deliver the required efficiency if a significant portion of the applied irrigation is quickly draining through the growing media without being absorbed and held in the container. Ensuring the application of water matches the absorption rate of the growing media will take full advantage of the water holding capacity of the growing media. This application of water is known as the Mean Application Rate (MAR) and the past results from a national survey of over 200 nurseries showed an average MAR of 20.2 mm/h. Production nurseries should be seeking to achieve a MAR of between 5 & 12mm/hr, but there are many variables that can have an impact on the absorption rate such as the makeup of the growing media, the physical components, the air-filled porosity, the addition of wetting agents and polymer gels and scheduling techniques. A well designed premium growing media can greatly improve irrigation efficiency.

Last month’s article was concerned with the physical properties of growing media and how to go about analyzing these characteristics. This month’s article will continue this theme but will look at the chemical properties of growing media. Understanding the chemical properties and their possible interactions is actually quite complex. Hence this article will only touch on a few key points, other key areas such as the buffering capacity of mixes will not be covered in this article.

Why do you need to know the physical properties of your growing media?? Having visited more than a couple of nurseries over the years it becomes readily apparent that there are as many growing media recipes as there are growers who use them. This may become an issue if a grower from one nursery decides that they prefer the growing media of another nurseryman or if a particular grower decides he wants to change their media for whatever reason.

The air filled porosity (AFP) of a growing media is the percentage of its volume that is air after being saturated with water and just after it has stopped draining. Below is the method used in the Australian Standard for Potting Media for testing AFP.

Air filled porosity (AFP), water holding capacity (WHC), bulk density, shrinkage, wettability and water retention efficiency (WRE) are all physical properties of growing media that influence plant growth and ultimately plant health.

The pH measures the hydrogen ions in the growing media, while EC (electrical conductivity) measures the soluble salts in the mix. For optimal plant health, the pH and EC of the growing media should be in accordance with the plant’s requirements because different plants can have different preferred pH and EC ranges (see Container Media Management; K. Bodman & Dr. K.V. Sharman which can be purchased from NGIQ).

In response to requests by production nurseries in Far North Queensland (FNQ) for industry hands-on workshops about the use and testing of growing media in production nurseries, two Growing Media Workshops were held in May 2013 in Cairns and in the Atherton Tablelands. Both workshops were presented by Thea Pobjoy, the Northern Queensland Farm Management Systems Officer (FMSO) for the Nursery & Garden Industry Queensland (NGIQ).

Coir or coconut fibre is the fibrous material that exists in the middle layer (mesocarp) of the coconut fruit. The long coir fibres are extracted from the coconut husk and used in the manufacture of brushes, twine and other products, while the short fibres have been shown to have excellent physical properties which can contribute to significant growth benefits for nursery plants.

The air filled porosity (AFP) of a growing media is the percentage of its volume that is air after being saturated with water and just after it has stopped draining. Below is the method used in the Australian Standard for Potting Media for testing AFP.

Water Holding Capacity Testing
The Water Holding Capacity (WHC) of a growing media measures the amount of water that a growing media can hold without causing plant stress. The WHC is largely determined by the amount and type of pore spaces available in the growing media, however the height and shape of the growing container also influences the growing media’s WHC (see Container Media Management; K. Bodman & Dr. K.V. Sharman which can be purchased from NGIQ). EcoHort guidelines recommend a WHC of greater than 40% (see EcoHort Guidelines from NGIQ).

The pH measures the hydrogen ions in the growing media, while EC (electrical conductivity) measures the soluble salts in the mix. The pH and EC of the growing media should be in accordance with the plant’s requirements because different plants can have different preferred pH and EC ranges (see Container Media Management; K. Bodman & Dr. K.V. Sharman which can be purchased from NGIQ).

Water retention efficiency (WRE) is a measure of the ability of a growing media to hold water applied from irrigation and this influences how long the irrigation must be run to wet up the media. An Australia wide survey
has shown that the WRE of commercially available growing media varies considerably, and suggests that the choice of growing media can influence water consumption and the quantity of nutrient runoff produced from a sprinkler or drip irrigation system.

The Grow Me Instead project has provided a wonderful opportunity for the Nursery and Garden Industry to put our environmental credibility on show. It is a real feather in our cap to take this proactive stance on invasive plants; showing government and the community we care about the environment and are prepared to take positive action to reduce the impact of invasive plants.

Last month’s article was concerned with the physical properties of growing media and how to go about analyzing these characteristics. This month’s article will continue this theme but will look at the chemical properties of growing media. Understanding the chemical properties and their possible interactions is actually quite complex. Hence this article will only touch on a few key points, other key areas such as the buffering capacity of mixes will not be covered in this article.