Soil & Potting Mixes

The majority of information in this article is attributed to Victoria Lee Blackstone who is a qualified Horticulturist. She has been writing professionally for more than 20 years, regularly writes technical papers for newspapers and has developed several training manuals. She studied botany and microbiology at Clemson University and gained her master gardener certification from the University of Georgia.


The perfect soil or potting mix may not yet have been invented but there is quite a lot of information available that describes some of the essentials for a reasonable chance of success in growing gerberas either in an ‘in situ’ spot in your garden or in a pot.


The interest and pleasure in developing a mix to you own liking is one of the reasons for experimenting with various mediums so when you attain one to your liking then you might say this is the perfect potting mix – for you at least and you are the one that needs to be happy.


The following information has been sourced from several reputable publications and it is intended to show the basics required to have a good medium to grow your Gerberas. The Society’s handbook Gerbera Growing in Western Australia has two excellent sections dealing with growing gerberas in the ground and also in pots. Beginners would do well to read these sections and members can refresh their knowledge when soil problems arise. The basic principles to apply in the growing medium are given below.


Water- Holding Capacity

Because potting soil in containers dries more quickly than soils in the garden, it’s important to choose a mix that can hold water so roots don’t dry out. Balancing this, is the need to have ingredients that don’t hold too much water as gerberas don’t like wet feet. Organic sphagnum moss is used in many commercial potting mixes because of its ability to retain up to 30 times its weight in water. Another organic choice is coir, which are shredded coconut husks. Compost, whether homemade or purchased, is an organic additive to potting mixes that helps to retain some water, although it may not be sufficient as a stand-alone potting mix.


Aeration

Air is as important to plant roots as water. Without adequate aeration plants can suffocate to death from lack of oxygen. When potting mix is confined to a container, the particles in the mixture become squeezed together, which may keep air from reaching roots. Peat and humus-based potting mixes can retain water but they can become matted if looser ingredients are not incorporated in the overall mix. Adding river sand, perlite or vermiculite can help overcome the problem of heavy potting soils by loosening the mix and improving aeration.


Drainage

Plant roots must also drain freely. With the exception of bog and water-garden species, which can tolerate waterlogged roots, most plants will drown if their roots sit in water. This is especially so with gerberas. Over time organic potting mix components decompose, which compacts the soil and causes drainage problems. Some drainage problems go beyond the actual potting mix you use and include the container itself. Pots must have adequately sized weep holes for drainage, which means checking these to make sure roots do not clog the holes and hinder water from draining.


Soil pH

The Society’s handbook has an excellent article on what the pH levels mean and explains these in detail and how to remedy any deficiencies detected. It is almost essential to acquire a good measuring tool to test pH.


Potting Mix Characteristics

This matrix endeavours to explain the various characteristics of the ingredients used in a combination of potting mixes and is designed to give you some guidance on what and in what combination you might like to consider using.

Ingredients - Pine Bark, Peat/Coir, Sand, Vermiculite, Perlite, Limestone Derivatives, Compost

XXX -Very good in this characteristic

XX - Good in this characteristic

X - Weak in this characteristic

NA - Not applicable

Increase Air Space

Pine Bark: XX

Peat/Coir: XX

Sand: XX

Vermiculite: XXX

Perlite: XXX

Limestone derivatives: NA

Compost: NA

Promote Drainage

Pine Bark: XXX

Peat/Coir: NA

Sand: XX

Vermiculite: NA

Perlite: XXX

Limestone Derivatives: X

Compost: NA

Hold Water

Pine Bark: X

Peat/Coir: XXX

Sand: NA

Vermiculite: XXX

Perlite: NA

Limestone Derivatives: NA

Compost: XXX

Acidity

Pine Bark: XX

Peat/Coir: XXX

Sand: NA

Vermiculite: NA

Perlite: NA

Limestone Derivatives: NA

Compost: X

Neutral

Pine Bark: NA

Peat/Coir: NA

Sand: X

Vermiculite: X

Perlite: X

Limestone Derivatives: NA

Compost: X

Alkalinity

Pine Bark: NA

Peat/Coir: NA

Sand: NA

Vermiculite: NA

Perlite: NA

Limestone Derivatives: XXX

Compost: X

The famous Dutch gerbera propagating company, Terra Nigra, gives some advice on growing gerberas in soils and in pots. They say the most important texture is one that provides 1/3 of the medium in which they will grow, 1/3 of air access for the roots and 1/3 for moisture. The percentage of oxygen in the soil or mix is very important to the gerbera plant. It needs to be relatively high as it is necessary for the aeration of the roots. The roots are sensitive to a growing environment lacking enough oxygen. In the summer (and particularly in WA) the plant needs a higher proportion of oxygen due to the increased respiration.


Each type of soil has its own structure. To improve airiness (aged) organic material such as bark, straw, peat, perlite or sterilised rice husks can be used in the top layer of beds to accommodate in-ground plants up to a depth of 30 cm. Whatever additives you decide upon must be well dug in to provide a consistency of soils around the growing area and the extent of the root system.


Terra Nigra which grows it gerberas in hot houses and others who do their growing in shade houses highlight the optimum growing temperatures. Their research also shows that a temp below 7°C stops the formation of buds.

Summer

Average Day Temperature°C: 24 -26

Average Night Temperature °C: 18 - 20


Autumn

Average Day Temperature°C: 21 – 24

Average Night Temperature °C: 16 – 18


Winter

Average Day Temperature°C: 19 – 22

Average Night Temperature °C: 14 – 16


Spring

Average Day Temperature°C: 21 – 24

Average Night Temperature °C: 16 – 18


In Western Australia’s summers, it could be beneficial to provide our outdoor plants with some filtered shade during the hottest periods of our summer and also consider placing some mulch around the roots


These observations are intended to provide as much information as possible on the factors that influence a successful growing programme.



Contribution: Tom Polich

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