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Choosing a Potting Mix

There’s a saying in the nursery industry advising gardeners not to “put a $10 plant in a $2 hole”.  Put simply, it is a reminder that plants are not a cheap investment and to place them in an inferior potting mix is a gamble.  They may never thrive and may not even survive and so the $10 investment in the plant is soon lost.

Potting mix does more than just help a plant to stand up.  A good potting mix will have just the right size air pockets for the plant’s feeder roots to move through, contain all the moisture and nutrients that the plant needs and have the ability to be re-wet easily.  Unfortunately, not all potting mixes can do all of this and almost certainly, soil from the garden won’t either.

It’s important to remember that for a plant in a pot, the soil that they are growing in is their world.  They don’t have the luxury of spreading their roots into other areas of the garden to find water and nutrients and so, if the essentials are not in the pot, the plant soon becomes hungry and thirsty.  This of course leads to a stunted and possibly dead plant and probably a very disappointed gardener as well.

There is absolutely no benefit in taking soil from the garden and putting it in a pot to grow plants in.  Garden soil is almost totally sand and will compact hard in the pot.  The roots won’t have access to the air pockets that they need to grow into and the plant will be prone to root rot diseases and will eventually die.

The quality and price of potting mixes differ immensely but choosing a good one is made easier using the Australian Standards logo.  This has been developed by a peak non-government body and can only be applied to the bags of potting mixes that have been independently tested to meet their exacting standards.

The black Australian Standards logo is applied to the bags of “regular grade” potting mixes which have met a basic standard but which do not contain fertilisers or wetting agents.  These can be used for growing short-term plants such as annuals but a controlled release fertiliser must be added immediately.

The red Australian Standards logo can only be used on “premium potting mixes” that have achieved high standards of nutrient levels, excellent water holding capacity and effective wettability as outlined in AS 3743-2002.  Premium potting mixes cost a bit more but do not need to have anything added to them to grow a healthy, vibrant plant for many months.  These are the mixes most often recommended by horticulturists and the results definitely justify the extra few dollars that they cost.

Using a premium potting mix is an almost ‘fool proof’ way of growing a strong, lush plant in a container.  It already has the fertiliser to get the plant off to a great start and to feed it for at least two months.  It also has a wetting agent to make sure that the container holds water well and can be re-wet easily.  All that is needed is a regular supply of water and a good level of sunshine and the plant should grow beautifully.

Adding water saving crystals to potting mix can be worthwhile when pots are in full sun or where annuals and vegies are planted which must always stay moist.  When the soil is watered, the water crystals soak up any excess and gradually release it as the plant needs it over the coming day or two.

However, some gardeners feel the urge to mix manure or compost through commercially made potting mix thinking that it will make it more effective.  This is, without doubt, a waste of time and money and will probably do more harm than good.

The roots of plants need oxygen to respire so premium potting mixes are designed with both small and large spaces between the particles.  The small spaces hold the water and the larger spaces carry the oxygen.

Commercial potting mixes are carefully prepared in scientific laboratories to have just the right balance of air holes, water holding capacity and fertiliser content for optimum plant growth.  The materials used in these mixes are of a very high grade and every batch is tested before being bagged and made available to the consumer.  Adding manures and compost to these mixes will alter the balance of nutrients as well as the pH and add expense that is not necessary.

In addition, manure is made up of very fine particles which, when mixed through potting mix, clog up the larger holes that hold the oxygen that the plant needs to respire.  This then causes the pot to hold far too much water and take away the air pockets that the plant’s roots grow into.   The bacteria in the manure combined with this stagnant water will eventually smell, potentially cause root-rot diseases and almost definitely stunt the growth of the plant.

Deciding on the quality of the potting mix to purchase is not the only decision for gardeners since, although a general purpose premium potting mix will suit almost all plants, there are those that have different needs.  Orchids, for example, require a very open potting mix meaning that the bark pieces making it up need to be very large allowing the water to drain through quickly.  Australian plants, which can grow well in containers, require low phosphorous levels but azaleas and camellias are acid-loving plants so need a pH of between 5.0 and 6.0.

For many gardeners, this will all sound very daunting but in fact, with specialty potting mixes available for these as well as palms and ferns, succulents and cacti and even pond plants, the guess work has been taken out of choosing the right one.  None of these specialty mixes need to be further adjusted and after placing the plant into the mix, it is simply a case of just adding water.

Specialty potting mixes are available for the following plant groups:

African Violet Mix: Fine, light mix for indoor flowering plants.
Bulb Growing Mix: Has extra drainage to prevent bulbs from rotting.  Also contains added nutrients.
Camellias & Azaleas: Low pH to suit acid loving plants including blueberries and rhododendrons.
Orchid Mix: Open, free draining mix particularly suitable for Cymbidium orchids & bromeliads.
Seed & Cutting Mix: Very fine mix & low fertiliser level to encourage germination & root growth.
Native Plant Mix: Contains a low level of phosphate which most natives are particularly sensitive to.
Palm and Fern Mix: Although drains well, designed to hold moisture for optimum growth.
Aquatic Mix: Suitable for pond plants and aquatic plants in fish tanks.
Bonsai Mix: Designed to suit species such as conifers commonly grown in the tiny containers.
Cacti & Succulents: High level of grit and sand to ensure very good drainage.

Some people dig potting mix through their garden beds or spread it as mulch but it offers no real benefit for these uses at all.  It is far more effective and much more economical to use garden products for what they are designed for.  Manure is fantastic mixed through the soil, pine bark is a brilliant mulch and, as the name suggests, potting mix is perfect for pots! 

Planting potted colour and seedlings into pots and containers is really simple.  It all comes down to choosing a decent size container with plenty of room for the plant to send its roots into (at least 20cm deep) and using a premium potting mix which already contains soil wetter and slow release fertiliser.

To start, place a square of shade cloth over the drainage holes at the bottom to stop the soil from dropping out and fill the pot with the soil.  Position the plants and then fill with more of the soil finishing at about 4cm from the top of the container.  Don’t press the soil down too much as this takes away the air pockets that the roots need.  Water with seaweed extract to encourage the roots to establish quickly and then stand back and wait for the plants to take off and burst into colour.

If premium potting mix has been used, most plants won’t need any further fertiliser added for at least two months.  After that, further controlled release fertiliser can be added for ongoing nutrients and, in the case of potted colour, vegies and seedlings, a fortnightly application of liquid fertiliser will give them the extra food that they need to produce flowers and fruit.  Ensure that the soil in the container doesn’t dry out by using drippers on the reticulation or regular hand watering.

Potting mix is made from organic materials and contains living organisms, including bacteria and fungi, as well as minerals and fertilisers.  When using potting mix, always avoid contact with eyes and skin and breathing in the dust to prevent any diseases, in particular Legionnaires.  Wear a dust mask and dust resistant eye protection as well as standard duty gloves.  Wash hands and clothes thoroughly after handling.  Keep the mix damp when using and be extra careful when opening a bag as this is when the spores and bacteria tend to puff up into the air.


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