Lesson 2 - Growing media for containers
Growing media is the key to growing plants successfully in containers. Just a quick recap, just like soil, to support successful plant growth a growing media needs to provide five things: enough water, adequate oxygen, sufficient nutrition, physical support for plants, and hygienic conditions (i.e. low toxicity, no pathogens, no weed seeds). As container ‘environments’ are limited by their dimensions, these resources are also limited, so choosing and using a good quality growing media is critical to growing success. You also need to factor in the short-term nature of container growing (weeks, months, years) into the type of growing media (soil mix) that you use (more on this later)
Container mixes are largely ‘soil-less’ (although ‘loams’ were traditionally the basis to potting mixes) and are based on mixes of different organic and mineral components. There are six main requirements for high-quality container growing media mixes. These include:
1. Lightweight but still provide good plant anchorage
2. Good drainage and infiltration
3. High water-holding capacity
4. High nutrient availability and nutrient holding
5. Resistant to decomposition/shrinkage, water erosion, wind displacement and compaction
6. Free from weeds, pests and diseases
A big challenge with container growing is the shrinkage of the growing medium over time, caused by the breakdown of the organic components as the mix ages. This is a common problem in many container growing mixes which can be comprised with as much as 80 or 90% (by volume) of organic components. Decomposition of the organics dramatically changes the physical properties in the container, increasing the proportion of fine particles, in turn changing the porosity, the balance of air to water, and leading to a loss of overall volume. IN some cases, there can be as much as a 40% reduction in the total container volume over 24 months of growing.
This may not necessarily be a problem where the growing media has a ‘short lifecycle’ – i.e. where the food plants are only short-term or where they will be potted annually. But it can be a problem in larger containers where plants are grown for many years, such as perennial plants or fruit trees in containers. Under these circumstances it’s a good idea to reduce the proportion of organic matter in the growing medium and increase the proportion of more stable substrate materials, such as mineral or processed/synthetic components. It’s also a good idea to choose an organic component that is more stable and breaks down more slowly, such as some of the aged or composted products.
Something to keep in mind with container growing mixes is wettability. This can be seen where after watering the water pools on the surface or drains down the sides of the containers, not through the pot and plant roots. This is called hydrophobicity and is mainly caused by organic materials, such as pine bark and peat, becoming waxy as they decompose, especially when dry, repelling water from their surface. A good mix of components in a growing medium, one with a range of different particle sizes and components, will generally prevent this from happening. Also, ensure that the growing medium never becomes excessively dry before using or potting up. In severe situations wetting agents can be used to counteract hydrophobicity. These are soluble and can be added by drenching plants well after potting-up.
The other consideration with container growing media is its bulk density – the mass per volume – and is calculated by dividing the weight of the growing medium by its total volume. In many container situations we want a low bulk density mix – one that doesn’t weight too much, so we can move our pots around easily without effort and has high porosity, so it drains freely. But the problem with this is that the plants can easily blow over and need to be watered very frequently. By contrast, a growing media with high bulk density is heavy, providing good support to plants but has less porosity and drains slowly. So, we tend to need a balance between these extremes, a growing media with a moderate bulk density that provides sufficient support, enables containers to be moved around freely but drains and also retains water well.
The components used in growing media include organic, mineral and synthetic and processed materials. Organic components are the most common used and include aged and composted products such as pine bark, coir (coconut fibre), peat moss, rice hulls and different composts. Organics often make up to 60% of the volume of a short-term growing medium (< 12-24 months), but much less, say 20-30 % of the volume, for longer-term growing situations (> 2 years). Organic components are light and help with water and nutrient retention in a mix. However, they do degrade with time, with some becoming very hydrophobic (i.e. water repellent) when they dry out. When making up a growing mix, make sure you choose a range of particle sizes, this helps increase the porosity of the mix and reduces the chance of too many fines being produced as the component ages. This could include a blend of larger-size aged materials like pine bark, together with finer materials, such as coir or peat moss. Some caution is needed in the use of composts. Not all are high-quality materials, and many are unknown in terms of their makeup, composting process or properties. Always chose a compost that meets the Australian Standard (AS 4454-2012, Composts, Soil Conditioners and Mulches) to ensure you have a quality product.
Another group of components used in container growing are mineral materials such as sand, granite, scoria or pumice. These products are produced by processing that involves mining, crushing and screening. Some are aggregates, with a defined particle size (e.g. 8 mm), while others include all the particles below a certain particle size (8 mm minus). Mineral components are stable in a mix, but can be heavy, particularly sand, although pumice, can be very light in weight. The other main group of components used in container growing media are synthetic or processed materials. These include processed clay materials such as vermiculite, perlite and LECA (Lightweight Expanded Clay Aggregate); and recycled waste materials such as plastic chips and polystyrene beads. These lightweight materials are expensive and vary in terms of their physical properties. Vermiculite and perlite have excellent water and nutrient retention properties, which is why many growers add small proportions of these to a mix (i.e. 20%). Some care is needed when using lightweight, uniform size materials, such as plastics and LECA. Without finer and heavier components in a growing mix these components can slowly rise to the surface of the container, changing the properties of the mix completely. As stated earlier, make sure you have a growing medium with a mix of particle sizes and moderate bulk density (weight per volume) to create the ideal growing environment for your container plants. Most container growing mixes do not use soil. Soil is not used because it is unknown in terms of its exact properties, but also because many soils can be full of pathogens and weed seeds, leading to later problems for container growing.
Of course you can purchase growing media as well, although be aware, the quality varies enormously. The best way to determine quality of a growing media to look for the Australian Standard. The Australian Standard for Potting Mixes (AS 3743-2003) covers bagged products and includes two categories of mixes – regular and premium mixes. The standard has minimum requirements for physical, chemical and biological properties and is a good way to ensure you are gaining a quality growing medium.