Potting mixes not just for pots

By John Fitzsimmons

By now, most of us are familiar with the key performance criteria of potting mixes but it doesn’t hurt to refresh our knowledge periodically, particularly with the growth in specialist lines available to industry and for resale. It’s also worthwhile to consider the expanding list of applications; potting mixes are not necessarily just for pots any more.

Vertical and rooftop gardens, as examples, are literally ‘growing’ markets for growing media. The usual criteria apply, of course, but there can be a much greater emphasis or concern with other characteristics of the mix, such as weight or density, and drainage and moisture-holding in particular. These growing situations can be subject to significantly higher temperatures and other evaporation factors which also must be taken into account.

With the emerging plant selections now appearing in these situations, there might also be some broader and deeper knowledge needed of particular genera and species’ requirements. Therefore, adjusting mixes accordingly, and making them more species-specific might be needed and worthwhile.

Because routine maintenance and care of plants in these situations post-installation might also be ‘variable’ (ranging from very good and regular to not-so-good and intermittent), the ability of the growing media used to sustain the plants over time is also of higher significance. This is related to both physical and chemical properties of mixes. As examples:

• the mix should hold a suitable reserve of plant nutrients without restricting the plants’ ability to access and take them up as needed (not ‘locked up’)

• mixes should be composed of materials that do not readily result in shrinkage (‘slumping’) which affects porosity, water holding capacity and plant anchorage. This is especially relevant to plants grown in longer ‘crop cycles’.

pH and EC should also be in acceptable ranges, while the inclusion or application of slow or delayed release nutrients is always a consideration.

Depending on sourcing and processing, some mixes can occasionally have undesirable levels of ag-chem residues (pesticides, herbicides, sterilants), heavy metals or organic toxins.

A number of surveys have revealed occasional variabilities in Standards compliance of potting mixes. These usually unintentional events can be due to:

• variabilities in raw material

• changes in supply chains

• changes in processing methods and settings

• lapses in monitoring and maintenance of standards

Therefore, regular independent monitoring of potting mix quality is advised.

Particularly in vertical and rooftop gardens, overall weight, related to the volumes required and potting mix density (BD – Bulk Density), is a prime consideration. Exceeding the physical engineering limits of the containers or supporting structure/s, wet AND dry, newly planted AND as greenlife matures, is obviously to be avoided. This is especially relevant where mix components with high moisture holding capacities (e.g., cocopeat, perlite) are involved. (*Perlite is very porous and can hold 3-4 times its weight in water. Due to its very low bulk density, perlite is a commonly used substrate for green roofs, so high weight of the moist substrate must be considered.)

In the case of vertical gardens, cutting corners with unsuitable or light gauge materials in either support structures or containers, will quickly reveal itself in deformation or outright failure!

With expanding construction of high-rise residential living, the growing demand for balcony plants, notably those in coastal locales exposed to heat, wind and airborne salinity, places yet another load on the performance of potting mixes utilised (after careful plant selection of course).

Basics revision

Soils are generally unsatisfactory for the production of plants in containers because they don’t provide the requisite aeration, drainage and water holding capacity required, hence various potting mixes and growing media have been developed.

These media provide capacity for water holding, nutrient holding and exchange, gaseous exchange and anchorage for plant root systems.

They have physical and chemical characteristics.

The physical characteristics reflect the components used and the proportions in which they are blended together. The three foremost factors are bulk density, water holding capacity and air porosity.

• Water holding capacity is the volume percentage of water retained by a saturated growing medium after it is allowed to drain.

• Air porosity is a measurement of the volume of pore space occupied by air after a saturated growing medium is allowed to drain.

The two key chemical characteristics of growing media are pH (acidity-alkalinity) and EC (Electrical Conductivity, an indication of the amount of nutrients available for the plants to take up).

• For general purpose growing media, pH should be between 5.2 – 6.2 with a target of 5.8.

• EC for general purpose growing media is recommended between 1.0-2.0 mmhos/cm.

• For seed germination and rooting of cuttings, the desired pH range will be slightly lower (5.0 – 6.0). pH can tend to rise in response to minimal fertiliser applications and irrigation water alkalinity from constant misting.

• Desirable EC for germination and propagation growing media has been put between 0.5 – 1.1 mmhos/cm.

Growing media components can be organic or inorganic. Examples of the former include coconut coir, peat moss, bark or other timber processing by-products, rice hulls etc. Inorganic examples include perlite, vermiculite, sand, scoria, hydrogel etc. Some components hold very little, if any water, while others hold water on their surface, or on their surface and within their structure.

Different components can vary in their water holding capacity and physical structure depending on their origin and processing method/s.

Greenlife enterprises can formulate and mix their own growing media or source media in bulk or bag from specialist producers and suppliers. Plenty of information is available on how to calculate which option might be best for individual situations. If mixing in-house, your source materials must be consistent to produce a predictable growing medium to the required specification. Know the chemical and physical properties of the ingredients you use to ensure the growing medium you produce is consistent, batch after batch.

Ultimately a growing media should be:

• porous and well drained yet capable of holding sufficient moisture to meet the water requirements of plants between irrigations

• relatively low in soluble salts but with an adequate exchange capacity (CEC) to retain and supply the elements necessary for plant growth

• standardised and uniform across batches to allow consistent and standardised nutrition and irrigation programs for each successive crop

• free from pathogenic soil pests and organisms, insects, nematodes and weeds/seeds

• biologically and chemically stable; free of organic matter that releases ammonia when subjected to heat or chemical treatments

In Australia, potting mix Standards (AS 3743-2003) reference ‘Regular’ (black ‘ticks’) and ‘Premium’ (red ‘ticks’) products.

Specialty potting mixes have been formulated to provide particular plant types with the most appropriate growing medium. There is a now steadily expanding range of specialty mixes available ex-catalogue from a range of suppliers. They include:

• African Violet (fine, light)

• Bulb Growing (extra drainage)

• Camellia/Azalea (low pH)

• Orchid (good drainage)

• Succulent-Cacti (good drainage & aeration, +Ca and K)

Maintain regular contact with your supplier/s regarding innovations and variations.