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How to choose the right soil for your plants

Noelle
Super Contributor

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Growing a healthy garden starts with understanding the environment your plants need to thrive. This guide shows you how to care for your plants from the ground-up by choosing the right soil or potting mix for their needs. We also cover the importance of compost for your garden and how to use it.

 

Soil

 

What is soil?

 

Over millions of years of erosion and exposure, rock is ground down to create soil. Soil varies considerably depending on the rock from which it originates, as well as its geographic and climatic location. Some soils are rich red loams originating from historic volcanic action, while others are grey or white sands with little value to horticulture.

 

The ideal soil is a brown-to-black friable (able to break into smaller pieces) loam that has good texture and holds moisture without turning to mud. It can be dug or forked over without forming heavy clods and includes a good percentage of well-broken-down organic material. Few of us are lucky enough to have such a soil in our gardens, so we must be prepared to add fertiliser, manure or compost to soil. Good soil provides anchorage for roots, nutrients to encourage growth, flowering and fruiting, and moisture to sustain life.

 

Where do I use soil?

 

Soil is best for growing plants in the garden, such as trees, shrubs, vegies, herbs and flowers. It is not ideal on its own for plants in pots or raised planters. In pots, soil loses its beneficial characteristics of moisture retention, natural drainage, aeration and thermal mass to maintain a suitable temperature range for plant roots. Soil can overheat quickly and compact in pots, both conditions that are not favourable to good plant health.

 

Premium varieties of soil contain much higher quantities of broken-down organic matter than budget alternatives. Budget soil consists of a large volume of bark used as a bulking material. For optimum results, always choose the best quality soil.

 

Potting mix

 

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What is a potting mix?

 

Potting mixes do not contain any soil removed from land. The main ingredients of a quality potting mix include composted pine bark or a similar renewable resource, coir fibre (sourced from coconut husk), coarse grit or river sand, controlled-release fertiliser, a wetting agent to assist moisture penetration and retention and bio-stimulants.

 

What is the difference between premium and standard potting mixes?

 

Quality mixes from reputable manufacturers do not normally contain composted materials produced from municipal kerbside collections (“green waste”) or bio-solids from sewerage treatments works. They may include animal by-products such as bone meal, blood and bone, feather meal, chicken manure and fish extracts. These additives are listed on the labels of bagged mixes if present.

 

Premium quality mixes that comply with the Australian Standard will carry the Red Tick logo, and standard quality mixes will have the Black Tick. Both demonstrate that the mix they contain is of good quality, but the red premium mark indicates a top performer that lasts longer and needs less initial fertilising. Budget potting mixes consist of a large volume of bark used as a bulking material.

 

Where do I use potting mix?

 

Potting mixes are formulated to support plants in pots of all sizes, raised beds and planter boxes. There is a wide range of potting mix varieties available, designed to meet the specific needs of particular groups of plants. Choose the best-quality mix for the purpose.

 

Common potting mixes:

 

  • Premium or all-purpose mix, which suits most plants.

 

  • Cacti and succulent mix, which is free draining.

 

  • Gardenia, azalea and camellia mix, which has a pH around 4.5 to 5 to suit acid-loving plants.

 

  • Native mix, which has little or no phosphorus and is ideal for Australian plants and proteas.

 

  • Seed and cutting mix, which is fine textured and drains freely so seeds and cuttings will not rot.

 

  • Vegetable and herb mix, which has boosted calcium to help combat a range of diseases.

 

  • Vertical garden and terracotta mix, which is lightweight and with good moisture-retention properties.

 

  • Certified organic or 100% natural mixes, which have no man-made ingredients.

 

Compost

 

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What is compost?

 

Compost is nature’s form of recycling nutrients and minerals. It transforms plant wastes back into a nutrient-rich material. Compost is produced through the breakdown of plant matter (or green waste) by microorganisms that exist in oxygen-rich environments. Good-quality compost looks like dark, loamy soil and has a pleasant earthy smell.

 

By contrast, plant materials that are broken down in an anaerobic (oxygen-deprived) environment will putrefy rather than compost and will have an extremely unpleasant and strong smell of rotting. Where plant wastes are heaped or put into composting bins or barrels, it is extremely important to aerate them regularly either by turning them over with a pitch or garden fork or rotating the barrel to ensure good aeration and mixing of ingredients. For more information on composting, check out How to make compost for your garden.

 

Where do I use compost?

 

Compost is not a planting or growing mix. It doesn’t have sufficient “body” to provide good anchorage for plant roots and it will continue to break down and lose volume in a pot or planter.

 

When added to soil or a potting mix, compost increases nutrition for your plants and improves drainage, moisture retention and aeration. It can be spread as a mulch about 10cm deep over the surface or dug through the top 30cm of soil.

 

Need a hand with getting the most from your garden? Check out more of our helpful guides and feel free to ask the Bunnings Workshop community for assistance. We’re here to help.

 

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