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Potting Mix: What Is It?

Potting Mix: What Is It?

Potting Mix- Just what is it?

Well, for a start, it ain't soil! That’s right, there is no soil in potting mixes sold in Australia today. Potting mix usually contains fractions of sand and organic matter like bark, coconut fibre and peat.

These elements are included in varying amounts to get a mix that is free draining and sterile. If you used soil there is a high chance there will be harmful fungal and bacterial spores included.

Better to start with disease-free media, no?

Let's check out the science behind soil….

Soils are made over thousands of years from rocks. The rocks like granite or basalt gradually degrade through the action of temperature, wind, chemical interaction, living organisms and pressure differentials from coarse to fine particles. Into this material, plants begin to grow. When they die, their roots, bark and leaves (what we call peat, humus or charcoal) are left and are combined along with bacteria, fungi and other microorganisms to form soil as we know it.

Soil is great stuff for trees and shrubs and the dizzying variety of natural vegetation we enjoy on our planet but maybe not right for our containerised plants.

Ours is a modified landscape. Each genera, species and variety has it’s own favoured conditions in the natural world that they do best in, but to help them along in our climate modified and the containerised environment we need the right aeration and moisture conditions.

Free-draining Potting Mix: What Is It and Why Is It So Important?

Plant roots need open pore space in soils to get oxygen to perform normal metabolic functions. Limited oxygen means poor root function (moisture and nutrient harvesting. No oxygen means plant death.

Waterlogged soils ‘drown’ roots and lead to invasion of cells by harmful bacteria.

Fine particles, like clay, hold water tightly and restrict air so are therefore excluded from commercial mixes.  Bark, peat and coconut fibre are coarse; they prevent compaction but at the same time hold water and make it available to the plant as required.

Sand does provide some minerals but are basically a structural element; they hold the plant up and their coarse characteristic prevents compaction.

Each land plant species have different water requirements and so differing potting media is called for. Mix for arboreal orchids is really open (chunky lumps of coir) whereas mix for general indoor plants may contain more perlite (expanded rock like pumice). Outdoor species do better in a general purpose mix which may have more sand (and is therefore heavier).

 It’s all about pore space and water holding capacity. Get it right and you are well on your way to luxuriance, horticulturally speaking!