Plant Parenting: Sizing up soil-less growing media for containers

SINGAPORE – Many gardeners use basic potting soil to grow plants in flowerpots and troughs indoors.

The disadvantage of this is that, with time, this soil-based growing medium compacts and becomes a heavy mass. This is due to the breakdown – and subsequent lack – of organic matter, coupled with the settlement and aggregation of the soil’s heavier clay component.

Gardeners who use potting soil will have to regularly fork in compost to keep the soil porous for proper aeration and drainage. This needs to be done carefully to avoid damaging roots. 

Enter potting mixes – man-made growing media designed for container gardening. Many home gardeners buy and use such mixes without knowing that some of them contain no soil.

These are called “soil-less growing media”, and are much lighter than a similar volume of actual soil. Soil-less media are designed to stay fluffy and porous with use, thereby overcoming the issue of compaction that is typically associated with potting soil. 

What makes up a soil-less potting mix?

The bulk of soil-less growing media consists of an organic component, usually peat moss or coconut coir. Other common materials are compost and burnt rice husks.

Such organic materials have a fine particle size, which makes for small pores and a mix with moisture-retentive properties.

Peat moss is acidic, and retains nutrients and moisture well. However, it is gradually being phased out due to sustainability issues, as harvesting it involves disruption to natural habitats. 

Finely chopped coconut coir, also known as cocopeat, is widely available in this part of the world. It is an upcycled plant waste material that is often used to replace peat moss.

On the flip side, unwashed or poorly washed cocopeat may have high salt content, which can be harmful to plants. This material needs to be soaked in water for several rounds to lower the salt content if you want to use it to make your own potting mix.

Improving drainage and aeration of the mix

The bulk of organic material used to make soil-less growing mixes can hold a lot of water. This can be useful for gardeners who are growing plants in a hot or windy environment.

However, it can become an issue if the growing area is shaded and humid, where the evaporation rate can be low. This can lead to wet feet, root rot and disease.

To address this issue, manufacturers of soil-less growing mixes incorporate coarse, inorganic gritty components such as vermiculite and perlite to aerate the mix and improve drainage.

Other inorganic components that can improve porosity are coarse sand, pumice, volcanic sand and zeolite. Finely chopped pine bark, an organic material, is sometimes used as well.

Gardeners can select from this range based on their price, availability, weight, pH levels, and nutrient and moisture retention. 

Things to note about soil-less growing mixes

When you buy a new potting mix, figure out how much water it can hold by putting a sample in a flowerpot and watering it thoroughly.

After excess water has drained away, the material should look and feel moist – like a squeezed sponge – and not soggy.

You can improve the drainage and aeration of the mix by slowly adding coarse inorganic components to better suit your growing conditions.

Soil-less mixes typically do not retain nutrients well, so more frequent fertilisation is needed. Slow-release fertilisers can be added to provide plants with a sustained release of nutrients. 

The organic portion of soil-less mixes is biodegradable and will break down over time. As it breaks down, you will notice the mix settling, with possible changes to drainage and aeration.

When organic materials break down, the process can sap the growing media of the nutrients the plant needs. This often results in slower growth and pale green or yellow leaves, a sign of nitrogen deficiency. When this happens, you will need to buy or concoct a fresh batch of potting mix.

The degraded potting mix need not be discarded. Instead, it can be used as a soil amendment material to improve the soil of outdoor plants.

Speciality mixes

Generic soil-less mixes are mainly for terrestrial plants in containers, which typically include large flowering and foliage shrubs, as well as edible plants.

Such mixes may not always be suitable for other plant groups such as cacti, succulents and aroids, which have more specialised needs that will be explored in a future article.