It is best to avoid using ordinary garden soil for use in pots. Some soils can be too heavy and often contain weed seeds. When buying potting mix, look for the Australian Standards Mark. Most good quality potting mixtures are suitable.
Australian Standards have been established for potting mixes (AS 3743). There are two levels of quality of potting mix.
Blacktick or regular grade potting mix is a good quality mix which provides plants with an ideal growing medium. Plants should be fertilised immediately after potting.
Redtick or premium grade potting mix include at least 3 months supply of nutrients from fertilisers such asNutricotecontrolled release fertiliser, and other additives such as water crystals and wetting agents.
To ensure that the mixes conform to the Australian Standards, a number of different tests are completed at the on-site laboratory and throughout the manufacturing process.
An air-filled porosity test is done to check that there are sufficient air ‘pockets’ in the mix to allow the plant roots to breathe. Water holding capacity is another test which gives an indication of how much water the potting mix can hold. Measurements of pH, soil salinity and nutrient levels are also conducted. Potential toxicity is tested by germinating radish seeds and comparing the length of root development against a known base mix to see if any toxic materials have inhibited the growth.
Composted pine bark forms the basis of better quality potting mixes, undergoing a composting process to ensure no weed seeds and plant pathogens remain. Recycled material or green waste is also sometimes used. Screened river sand is also used to promote good drainage. Carefully measured mixes of fertilisers are added during the process. The actual type and quantities are determined by the needs of the plants for which the mix is designed. Some mixes also have wetting agents and water crystals added at this time to improve the future re-wettability of the mix and the water holding capacity. When the mix is ready, it is bagged by a machine and dispatched to garden centres across the State.
Potting up is easy with these mixes. There is no need to cover up the drainage holes in the base of the pot because the texture of the mix will ensure that there will be no loss of medium and additional drainage is not required. Just pour the mix into the pot to the level required, position the plant in the pot and fill around it with mix. Always water the plant in, to bring the roots in direct contact with the mix. Remember to wash your hands after handling any compost, animal manure or potting mixes.
Care with Potting Mix
Avoid breathing in dusts or mists – wear particulate mask if dusty. Wear gloves and keep product moist when handling. Wash hands immediately after use.