Plant Parenting: Digging deeper into growing media

SINGAPORE – Readers who have been following The Straits Times’ Root Awakening column may have noticed that I use the words “growing medium” to describe the material in which plants are grown.

Gardeners are familiar with soil, which is a traditional growing medium found in the ground outdoors.

Those who grow plants in high-rise gardens use various potting media, which may or may not contain soil. Media that do not are called soil-less growing media.

A growing medium provides physical support for roots to anchor themselves, so the plant can stand upright and grow. It should be loose enough to permit root development and growth. The growing medium also plays an important role in ensuring the roots get sufficient air, water and nutrients. 

The type of growing medium used will vary depending on the plant species, its growth requirements and prevailing growing conditions, as well as the gardener’s care regimen and budget.

It is important to understand your growing medium’s characteristics to ensure you select the most suitable one for your plant. 

What is a growing medium?

A growing medium consists of inorganic and organic components. The proportion of these two components and how they combine determine the medium’s characteristics.

In soil that is found naturally outdoors, the inorganic component consists of a mixture of particles of varying sizes, from large sand grains to minute clay. Sandy soils have a large proportion of sand, making them porous, and water will easily drain away.

Clay-based soils, which are common here, are known for their density and high water retention. The clay particles are very small and closely packed together.

The organic portion of soil consists of partially to wholly decomposed material from once living organisms. Organic matter keeps clay aggregates apart, ensuring drainage and aeration. It serves as a food source for beneficial organisms that help plants with nutrient intake, nutrient cycling and fighting disease-causing pathogens.

A common organic material that is incorporated into soil to improve its properties is compost, which is derived mostly from decomposed plant material. Use good-quality compost that is mature and stable so it does not continue to break down, which will rob nutrients from the soil and plants.

Drainage and aeration are key to good growing media

One of the most important physical properties of a good growing medium is its porosity.

The size of spaces within the growing medium matters. They should be small enough to be able to hold air and water, but not too small that they impede the movement of air and water.

Although the roots of plants are buried in the ground, they are living organs and require oxygen to stay alive. Numerous small air pockets in a growing medium provide oxygen for plant roots. Similar pockets in the growing medium retain and store water for the plant.

The roots of terrestrial plants should not be left to sit in water for long periods. Roots can become waterlogged, suffocate and die.  

Conversely, soil should not be too porous, as this can lead to enhanced drainage and aeration. It will drain too efficiently and dry out quickly during hot weather, and will consequently have to be watered more.

Plants absorb nutrients that are dissolved in water. A very porous growing medium will lead to a situation where nutrients are washed away whenever a plant is watered or there is a heavy downpour.

Acidity, alkalinity and nutrient retention

pH is a chemical property that determines how acidic or alkaline the growing medium is, and affects nutrient availability.

Most plant nutrients are available for absorption within the 5.5 to 6.5 range. Aim for a similar pH range for your growing medium so that essential nutrients are available for your plants. You will need to use a soil pH metre to measure your growing medium.

Whether you are improving the properties of your existing growing medium or making a new one, check the pH characteristics of the components you intend to use, as it can affect the final pH value of the resultant growing medium. pH cannot be quickly or easily corrected if it falls out of the optimal range. 

A growing medium should retain nutrients well, so that plants do not need to be fed as often and less fertiliser is lost to leaching.

Clay in soils that have small pores limit nutrient loss by leaching. Particles of clay, due to their minute size, also have a large surface area. Coupled with a negatively charged surface, they can hold on to a range of dissolved nutrients which exist as positively charged ions.

Organic matter has a similar feature. When incorporated into clay soils, it will not only help to enhance drainage and aeration, but also improve the clay-based medium’s ability to retain nutrients and moisture.