What's in the mix?
Potting mixes are composted bark blends designed to provide the best planting medium for pots and other containers. They are different in structure from soils or composts.
These potting mixes (also referred to as 'potting composts') are designed to provide air, moisture and support to plant roots while also allowing water to flow through. Potting mixes provide the basic nutrients required for plant growth (nitrogen, phosphorus and potassium), and some are designed to suit the special needs of a range of garden plants.
Potting mixes have evolved over the past 50 years as containerised growing – both for home gardeners and nursery production – has increased. Up until the 1960s, most nursery plants were grown either in the ground, in terracotta pots or in improvised containers such as large tins (prune tins were popular). These containers were filled with a mix of soil, peat moss and sand.
As plastic pots became the norm for plant production, growers and gardeners began to look for lighter and more reliable materials to fill them.
The first potting mixes were developed in the UK in the 1930s as John Innes composts. These were made from composted materials, peat and sand.
Australian potting mixes are produced from composted pine bark, which is a byproduct of the timber industry.
Potting mixes have changed over recent years with the addition of a wide range of composted waste products, soil wetting agents (often indicated with a WaterSmart symbol) and slow-release fertilisers. Modern mixes rarely contain peat moss.
One of the issues with potting mixes, which led to the introduction of the Australian Standard, is their level of composting. When woody materials break down, they use nitrogen in a process called nitrogen drawdown. This nitrogen is used by ‘good’ bacteria, which are assisting the composting process. However, plants also use nitrogen as they grow, so if the medium they are growing in is still composting, nitrogen may be in short supply for growing plants. Yellowing leaves and stunted plant growth may indicate that nitrogen is in short supply. If this occurs, repot plants into a potting mix that meets the Australian Standard.
Selecting potting mixes
Potting mixes vary in price and quality. When purchasing a potting mix, check that the mix you select meets the Australian Standard (look for the ticks on the bag).
Potting mixes are sold for general purposes but may also be customised for specific plants or uses. Specialised potting mixes include those for acid-loving plants, propagation or seed raising, orchids, succulents, native plants and for terracotta pots.
Tip: When potting mixes are used in containers, there is no need to fill the base of the pot with drainage materials.
Potting mixes, along with all composts and soils, may contain bacteria. Always wash your hands after using potting mixes, follow warnings on the bags and wear gloves and a mask when using potting mixes.