If you were to ask ten people what they are using as a potting mix for fruit trees, you would probably get ten different combinations. All the trees would probably grow in these 'brews' but there would be a significant difference in growth rates and quality of trees produced.

What is the ideal mix? Well, there is no one ideal mix. However, there are a number of essential characteristics that a potting mix should have. Plants need air, water and food, so when making up a mix, it is important that it supplies adequate quantities of air, water and nutrients.

The two most important characteristics that need to be considered in making up a potting mix are the physical characteristics (this includes air space and water availability) and chemical properties (includes pH and nutrients).

Air space (or air-filled porosity) should be in the range of 15 to 20%. How do you know what the air-filled porosity is in your mix and how can you measure it? The DPI and the Nurserymen's Association have a simple technique which you can follow to measure the air-filled porosity of your own mix. However, there is no ideal figure for the percentage porosity your mix should have, as it depends on your specific situation and plant types grown. If the air space is too high, water will drain too rapidly from the pots, causing the mix to dry out too frequently, and also causing a rapid loss of nutrients through leaching. The other extreme of low air space (less than 5%) leads to the mix becoming waterlogged, which results in poor plant growth.

Size and shape of a pot has a very interesting effect on air-filled porosity. At the bottom of all pots, there is a saturated layer where there isn't enough air for proper root growth. The height of this saturated layer is the same in all shaped pots. This means that a potting mix in a shallow or squat pot will have a higher readily available water content then the same volume of the same mix in a tall pot. Therefore, when you measure air-filled porosity, measurements should be made in the same pot as that used in your nursery.

Chemical properties of a mix is the other important factor to get right when designing your mix. The pH level should be correct for the plant species concerned. A pH between five and six is ideal for most plants. Most potting mix ingredients are acid by nature, for example, sawdust, peat. As a result, dolomite or lime is required in the mix to achieve the preferred pH. Salinity can also be a problem in potting mixes. This can be a problem due to excess fertiliser use, use of salty water or if the original constituents of the mix were high in salts. Symptoms of salinity are leaf burn on the tips and margins of older leaves, with premature leaf drop. The leaves are smaller and growth rate slower. Sedge peats, mushroom composts and fly ash are all ingredients which tend to be naturally high in salts.

Nutritional requirements of pot plants vary depending on the type of plant. This whole area of nutrition in pots deserves an article on its own.

It takes a lot of trial and error to develop a suitable potting mix. Its qualities have to be balanced with the watering and fertiliser systems, to ensure that the particular mix works in your situation.

There are a huge range of materials that can be combined to form potting mix. However, it is sensible to keep things simple by combining no more than two to three ingredients. Some examples of ingredients that are useful in potting media are: peat, sawdust, sand, perlite, vermiculite, polystyrene beads, peanut shells, rice hulls and soil.

Hardwood sawdust is one cheap, readily available local ingredient. For the best results using hardwood sawdust, it should be composted with urea, followed by leaching to remove toxins before use. It should be thoroughly wetted with a solution containing 2.6 kg of urea per cubic metre. The heap should be covered with plastic for about eight weeks and ideally should be turned at least once during that time. The treatment will turn the sawdust black and the leaching will remove black liquid containing the natural toxins from the heap.

Nick McLeod
Extension Officer DPI Rockhampton
Article from Capricornia Branch Newsletter Vol. 5 No.4

DATE: September 1989

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