Planting soil / Growing Medium mix

Potting mixes

This page describes the characteristics of a good potting mix and outlines some of the common ingredients, as well as detailing simple tests for aeration and water-holding capacity.

There are as many potting mix recipes as there are plants to grow in them. Many recipes have evolved to use local waste such as brewery sludge or jarrah woodchips. Such products may or may not be suitable as components of mixes in various proportions.

Use such a product only if it has the following characteristics:

  • desirable physical properties – wood chips may be suitable for large pots but would be too large for bedding plants
  • does not contain excessive levels of salt or nutrients as some local peats or animal wastes do
  • does not contain any toxic chemicals, such as copper chrome arsenate, PCP or boric acid from timber treatments, phenols as in uncomposted pinebark, or heavy metals as in sewage sludge.

If it might carry disease, sterilise material before use. Gravels, soils and sand from dieback-infected areas are likely sources of Phytophthora.

Characteristics of a good mix

The characteristics of a good potting mix are:

  • well drained, which means an air-filled porosity of at least 15%
  • re-wets easily – some peat and bark media are difficult to re-wet if they dry out
  • does not shrink away from the side of the pot as it dries
  • optimum weight – not too heavy to lift, not so light as to blow over easily
  • suitable pH, between 5.0 and 6.5 is satisfactory for most plants (all pH values quoted are measured in water)
  • free of pests, for example weed seeds, fungal pathogens, or can be sterilised without producing harmful by-products
  • can be stored for short periods without significant changes in physical or chemical properties
  • readily available
  • not expensive.

There is an Australian Standard (AS3743-2003) for bagged retail potting mix. For further information contact SAI GLobal. The standard contains specifications for a range of mixes in both regular and premium grades.

Porosity

Porosity is one of the most important properties of a potting mix. It is the space available within a mix for water, air or root growth. Small pores contribute to water retention whereas large pores promote aeration.

The shape of the container also affects the porosity of a mix. A soil mix in a shallow container will hold more water than an identical volume of mix in a deep container.

Calculating air-filled porosity

A measured volume of mix is saturated with water, then allowed to drain and the drainage water measured.

A milk carton is a suitable container for holding the mix. The container should be equal in volume to the plant pot in use and its shape should approximate that of the pot, since the air-filled porosity of the mix will increase as the height of the container increases. A length of 90mm stormwater pipe can also be used.

Table 1 Interpretation of air-filled porosity (AFP)
AFP% Comments
5 Too low for all but wetland plants
5–10 Suitable for large plants in containers or where watering is infrequent, for example indoor plants
10 An absolute minimum for new mixes
10–15 Bedding plants and those that will receive little attention after planting out
15-20 General nursery lines
20–25 A better starting point for most mixes that allows rapid growth but needs more frequent watering
30 For propagation mixes, indoor mixes and some seed propagation mixes
30–40 Promotes rapid growth, but frequent watering is needed
40–50 For epiphytes

Remove the top of the carton and mark the inside at the same height as the mix will be in the plant pot. Make four drainage holes in the base of the carton.

Fill the carton to the mark with thoroughly moistened mix and leave where it can be watered overhead for several days. The mix should have been at potting moisture content for 24 hours before it is tested.  If necessary top up the mix.  Immerse the carton in a bucket of water so the water level in the carton can be seen just below the surface. Lift the carton vertically from the water and allow it to drain.

Repeat the immersion and draining twice more. This will compact the mix as would happen in normal practice. Finally, place the carton in another bucket of water so that the mix is saturated just to the surface.

Carefully remove the carton vertically, holding your fingers over the drainage holes. Remove your fingers and allow the carton to drain over a tray or bucket. Do not tilt the carton at any stage as this will permit additional drainage and give a false reading.

Cover the carton and tray or bucket to prevent evaporation and leave until drainage is finished.

Measure the amount of drainage water in millilitres, divide this by the volume of mix used (in millilitres) and multiply by 100. This gives the air-filled porosity in percentage. It is  found generally useful to take the average of three readings as you will find that they can vary quite markedly until your technique improves. Table 1 gives a guide to appropriate levels of AFP for a range of nursery situations.

Buffering capacity

Potting mixes that have a higher buffering capacity will resist pH change. This can be useful if you are growing plants that will be in containers for lengthy periods of time. Spagnum peats generally have the best buffering capacity (sedge peats are poor) followed by composted materials and clay minerals such as attapulgite and bentonite. Bark, sand and perlite have little or no buffering capacity.

Cation exchange capacity (CEC)

Mixes with a higher CEC retain fertilisers better. Less will be wasted through leaching and nutrition will be more even. The more decomposed (and generally finer) a material is, the higher its CEC. Humus has a very high CEC whereas raw bark is low. The highest CECs are found in minerals such as zeolite. Clay minerals vary widely. Spagnum peats are moderate.

Potting mix components

Peat

Peat is one of the most common components of potting mixes, and can be quite variable. Peats are graded according to their state of decomposition and particle size.

Broadly speaking, peat falls into three categories: spagnum or light peat (most European peats fall into this category); reed/sedge peat which is darker in colour and finer; and peat humus which is even darker and finer.

The spagnum peats have the highest total porosity. Coarse, light peat has a lower water-holding capacity than fine light peat. Those from England, Scotland and Ireland tend to be similar with a pH of about 3.8 to 4.5 whereas those from Finland and Russia are younger, less decomposed and more acidic with a pH of 3.2 to 4.2. These younger peats can break down more quickly during use.

The reed/sedge peats are brown or reddish-brown to black. The more highly decomposed the peat, the darker the colour. These peats have a higher water-holding capacity than light peat with subsequently lower aeration and a pH of 4.5 to 7.0. Local peat is this type.

Peat humus is a substance derived from the two types of peat in an advanced stage of decomposition. It is dark brown to black and has the least aeration.

Peats should not be allowed to dry out as they can be difficult to rewet. The addition of a wetting agent will help.

Coir peat

Coir peat is a relatively new product derived from coir fibre dust. It is marketed in compressed bales to which water is added. A 30 litre bale reconstitutes to about 150 litres. Coir peat has a pH of about 5.0 making it slightly less acidic than spagnum peat. It has a high water retention capability and rewets easily, unlike spagnum and sedge peats.

Its AFP is 10-12%. Although manufacturers maintain it can be used in high rates (up to 100%) in mixes, it is generally used at about 10% v/v in a similar manner to sedge peats as it will degrade over the long term in a mix thus reducing aeration.

Sand

The particle sizes of sand used in potting mix have important effects on the mix.

To increase the water-holding capacity of the mix, the particle size should be between 0.05 and 1.0mm with less than 10% between 0.5 and 1.0mm.

Particles from 1.0 to 3.0mm increase the aeration of the mix. Sand may also be used to provide weight to the mix.

Perlite

Perlite is a natural mineral of volcanic origin that, when crushed and heat-treated to 1000°C, expands to produce white lightweight particles. The pH is usually neutral to slightly alkaline but may be as high as 9.0.

There are several grades of perlite. The finest has a higher total porosity and holds a reasonable amount of water while the coarser grades hold correspondingly less water but provide greater aeration.

Perlite usually contains dust which should be sieved out where aeration is important, as in propagation, especially when it is being mixed with peat. Perlite dust is highly irritating to lungs so the correct respirator should always be worn when handling the dry product. A dust mask is not sufficient.

Vermiculite

Vermiculite is produced by heat treatment of mica. It is porous and light and has a water-holding capacity of three to four times its weight. Care is needed when handling the material as it is easily compressed, which leads to poor drainage and poor aeration. Vermiculite is a useful material for small container plants.

Zeolite

Zeolite is a mineral with a moderate cation exchange capacity and improves nutrient retention. Its ability to supply nutrients in useful quantities is limited. It can be used in quantities up to 10% v/v.

Jarrah sawdust

Jarrah sawdust is one of the most widely used potting mix components in Western Australia. It is cheap, readily available and relatively sterile if clean.

Jarrah sawdust is best aged or composted before use. When fresh, it has a high carbon to nitrogen ratio and in a mix can cause nitrogen deficiency in plants because of the activity of micro-organisms.

Care is needed not to age sawdust excessively as it will break down to fine particles. In a container, these can move downwards and eventually clog the drainage holes. Six to eight weeks ageing is sufficient.

Although pine and karri sawdusts are available, they degrade too quickly and are not good potting mix components.

Wood chips

Wood chip materials from Eucalyptus diversicolor and E. calophylla are readily available in Western Australia in a range of grades. Experiments by DAFWA have shown these to be suitable for use in potting mixes. These materials should be composted before use, as they contain toxins that may stunt the growth of many plants.

As a compromise, wood chips can be aged for at least six weeks. Many nurseries use fresh material in mixes without apparent problems, but plant growth may not be optimal. A test to calculate the nitrogen drawdown index should be performed and additional nitrogen added to compensate. Given the supply problems with pinebark in Western Australia, it is likely that woodchips will become a standard component of potting mixes in the future.

Pine bark

Pinus radiata and P. pinaster bark are used extensively in the nursery industry both overseas and interstate. Unfortunately, Western Australian supply can be erratic and grades can be inconsistent. Often both species are mixed together. This presents problems as the relative amounts of each vary from batch to batch.

Although many nurseries use fresh and aged bark, this is not generally advisable. Both of these materials contain toxins which can cause problems in sensitive plants, such as the Gesneriads, including African violets and gloxinias. If composting is not done, the nitrogen drawdown index should be calculated and additional nitrogen added to compensate.

Experiments by DAFWA have shown clearly the advantages of composting pinebark (see Table 2). Unlike sawdust, ageing is of little benefit. The procedure for composting pinebark is:

  • Moisten the pinebark thoroughly. If the pinebark is dry and moistening is difficult, it may be necessary to add some soil wetting agent.
  • Add urea at the rate of 3.75kg/m3 of moist bark and mix well. Other sources of nitrogen may be used, but urea has proved to be best.
  • The heap should not be much more than 1m high.
  • Keep the heap moist and turn it regularly during the composting period, to ensure that all the material is composted evenly.
  • Composting will take from four to eight weeks, depending on the temperature. The warmer the heap, the quicker the composting process. The bark is ready when it is almost black and has lost its resinous odour.

To monitor the composting process, use a thermometer and pH test kit. Use the thermometer to ensure the middle of the heap is warm enough to promote the composting process. Temperatures of up to 50°C can be reached in summer but 30 to 35°C is more realistic in winter.

The pH of raw pinebark is usually about 4.5. As the material composts pH rises to a peak of about 8.0 and then falls back to about 6.0 to 6.5, which signals the end of the process.

When using composted pinebark in soil mixes it may be necessary to reduce the amount of lime and/or dolomite in the mix, since it is less acid than most other wood products.

Table 2 Fresh weight of tops (g) of selected plants grown in a potting mix containing 40% sand and 60% wood waste
Material Radish Petunia Gardenia Grevillea Geraldton wax
Pinus radiatabark
 fresh 2.6 5.2 95 291 87
aged 4.6 6.5 118 214 105
composted 9.6 13.4 212 489 197
Jarrah sawdust
fresh 4.4 6.8 139 242 78
aged 5.6 8.9 165 163 111
composted 5.0 8.9 252 320 218

Milled marri and karri bark

Milled bark another waste product from the timber industry in the South-West. When composted it has been shown to have useful properties for suppression of plant pathogens, however, it is suitable for use in small (10% v/v) amounts only in potting mixes due to its rapid degradation. Often it is used as a direct substitute for the local sedge peat.

In large proportions, especially in longer-term (more than six months) mixes, as it breaks down it moves to the sides and the base of the pot as a silty layer, reducing aeration and causing waterlogging. The raw product is acidic (pH 4.0) but after composting the pH rises to about 6.0-6.5.

Phenols in the bark are phytotoxic but composting removes these within about 11 weeks. To completely compost marri and karri bark takes about 14 months so is usually not done.

Polystyrene foam

Polystyrene beads are often used to improve drainage and aeration and reduce the weight of potting mixes. They do not retain any water or fertiliser. The beads tend to blow about in windy weather or float to the surface under heavy watering. They are not biodegradable.

Soil

Soil is a less frequent component of potting mixes. If used at more than 30%, the mix is often heavy and prone to waterlogging. The physical and chemical properties of soil can be variable, so pasteurise it to ensure it is free from weed seeds and fungal pathogens.

Animal manures

Animal manures are generally not suitable components of potting mixes. They are variable in mineral analysis and frequently contain weed seeds and other contaminants. The pH can be extremely high, especially when the manure is fresh when it can often burn the plants. Pasteurisation is necessary to guarantee the manure is free from pests and diseases.

Other materials

Depending on location, a range of other materials may be available. Attapulgite (perhaps most familiar to people as ‘kitty litter’) is useful in small containers to retain nutrients at the rate of about 10% v/v. It comes in a range of grades – 1630 has a particle size of about 1mm and is suitable for potting mixes.

Vermicomposts used at the rate of about 10% can supply a range of nutrients for several months. Larger quantities will reduce aeration and cause waterlogging.

Examples of mixes

Propagation

The main criterion setting a propagation mix apart from a general potting mix is degree of aeration, which should be quite high (see Table 1). Consequently, almost anything can be used in a propagation mix provided it has adequate aeration.

Perlite or coarse sand tend to be the most common ingredients. Make sure you use a coarse grade of perlite and ideally sieve any dust out. Although this seems tedious you will find much better results. Some experimentation may be needed to find the best mix for each individual situation, and for each plant species involved.

Different mixes with different physical characteristics will create roots with differing forms. Some of these forms, for example, more or less branched, may be more difficult to work with when transplanting and this can influence survival rate.

Many growers do not incorporate fertilisers since cuttings do not take up nutrients until they have roots, however with quick rooting species it can be advantageous to use controlled release fertilisers in the mix so rooted cuttings can get an early start. If using relatively high temperatures on the propagating bed though, be aware that fertilisers may dump.

Perlite: peat 3:1
Perlite: peat 9:1
Perlite:peat: coarse sand 1:1:1
Pinebark fines: coarse sand 1:1

Bedding plants

The main parameter that needs to be satisfied for a bedding plant mix is to fit into small plug trays.

Peat: sand 1:1
Fine pinebark: peat 1:1
Peat: sand: vermicultie 2:1:1

General

These mixes are suitable for most nursery crops. The exception is epiphytes such as orchids where the media is primarily a means of support.

Pinebark: peat: sand 2:1:1
Pinebark: peat: sand 4:1:4
Pinebark: sawdust: peat 1:1:1
Pinebark: sand: soil 1:1:1
Sawdust: sand 1:1 to 3:1
Sawdust: pinebark: sand 5:3:2
Sawdust: pinebark: peat: sand 3:3:2:2
Sawdust: sand: peat 2:2:1

Orchids

Mixes can be made from a wide variety of components, but must be extremely well draining (see Table 1).

Coarse pinebark 100%
Composted wood chips: charcoal: coarse river sand 6:1:1
Peat moss: composted wood chips:charcoal: coarse river sand: perlite 1:1:1:1
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