Understanding soil substrates: The foundation for healthy roots

By Master Horticulturist Cherise Viljoen

With years of experience perfecting soil mixes for growing and propagating plants, I’ve learned that with so many types of substrate for sale, it’s essential for gardeners and growers to choose a growing medium that provides everything their plants need to thrive. Ideally, a good soil mix should offer support for roots to anchor in, supply adequate water and nutrients, and allow roots to both breathe and grow. These elements are key to cultivating healthy plants that flower and fruit abundantly while remaining resilient against pests and diseases.

You might wonder why I use the term “substrate” instead of simply saying “plant soil mix” or “growth media.” Substrates encompass a variety of materials that support plant growth, including water, and many aren’t soil-based. This means they may lack traditional soil components like sand, clay, and naturally decaying organic matter. For more on soil components, check out our previous blog: The importance of soil health and composting.

Choosing the right substrate for successful propagation

When propagating plants from cuttings, I liken the process to performing surgery. Just as in surgery, maintaining sterile and hygienic practices is crucial to prevent infections. Ensure that your tools and work surfaces are thoroughly sterilised  and clean. Additionally, the substrate holding your plant while it roots should be sterile.

When selecting a sterile substrate for your propagation mix, I rely on two cost-effective and readily available options: Perlite and Vermiculite. Both of these substrates are easy to find if you’re looking to buy substrates that offer unique benefits and characteristics.

Inorganic substrates: Perlite and vermiculite

Perlite is made from volcanic rock deposits that are heated to very high temperatures, causing it to expand and become lightweight. It is inert, sterile, and has a neutral pH, making it an excellent choice for providing aeration around the base of the cuttings. Perlite retains sufficient water while allowing excess moisture to drain away, preventing the stems from rotting. This substrate is particularly effective for semi-hardwood and hardwood cuttings due to its ability to maintain structure and support multiple reuses without degrading.

Vermiculite is a silicate mineral similar to clay, with a layered structure that expands when heated. It is also inert, sterile, and lightweight with a neutral pH. Vermiculite excels in water retention, making it ideal for softwood cuttings that require a moister environment to support their delicate stems. However, its soft and spongy nature means it can retain too much moisture, increasing the risk of rot in harder wood cuttings. Additionally, vermiculite tends to disintegrate over time, which limits its reusability.

Both perlite and vermiculite are inert, sterile, and lightweight with a neutral pH, providing the necessary aeration and water retention for healthy root development. If you’re looking for substrates in South Africa, it’s important to match the substrate to the specific needs of your plant cuttings to ensure optimal growth.

Rooting cuttings with inorganic substrates

Perlite is best for semi-hardwood and hardwood cuttings that require excellent drainage and structural support.

Vermiculite is ideal for softwood cuttings that need consistent moisture without the risk of stem rot.

For potting-up cuttings that have rooted in water, opting for vermiculite is best as It allows delicate soft roots to acclimatise gradually as they learn to push through the gentler medium while strengthening.

Choosing the right substrate is essential for successful propagation. By understanding the properties of perlite and vermiculite, you can select the best medium to support your plant cuttings and ensure healthy, robust growth.

Seed sowing with inorganic and organic substrates

Conversely, I often sow my seeds in a less sterile plant soil medium, such as a mix of compost with 6mm milled bark, fine coco peat, or a soil-based substrate mix. Here’s why:

Seeds are naturally sterile. They contain their own protective layers, reducing the need for a completely sterile medium.

Nutrient supply for slow-growing plants: I am often slow to prick my seedlings out and pot them up, or I am cultivating very slow-growing plants. Both scenarios mean that the seedlings are going to stay in that substrate long after they have completely utilised their stored embryonic food. Therefore, I prefer to provide them with a growing medium that supplies nutrient minerals they can use while photosynthesising. If you’re looking to buy substrates that offer long-lasting nutrient supply, this approach ensures your plants grow efficiently and stay healthy until they’re ready for transplanting.

I avoid using regular garden soil for sowing seeds because it introduces too many unpredictable variables, such as inconsistent nutrients, pH levels, water-holding and drainage capacities, as well as the risk of pests and diseases. By crafting my own soil mixes with coco peat and bark, I ensure a controlled and optimal environment for successful seedling development.

Organic substrates: Coco peat and bark

Coco peat: Made from coconut husk fibers. Bark: Milled to sizes between 6mm and 12mm, often referred to by nurseries as seedling mix.

When preparing soil mixes for plants and seeking a relatively sterile yet more organic option, I rely on coco peat and bark. While I seldom use them alone because they can be too dry, they make excellent companions when combined with perlite and vermiculite.

Both organic substrates are lightweight and offer good water and nutrient retention. They maintain their structure well, decompose slowly, and gradually release additional nutrients that plants and soil microbes can absorb. Additionally, both have a slightly acidic pH, which is ideal for most plants’ optimal growth.

I typically combine them in a 1:1 ratio or a 50:50 mixture based on the needs of my plants:

1:1 of perlite and coco peat for rooting cuttings

1:1 of vermiculite and 6mm bark for seed sowing

Potting mixes with natural substrates: Compost and sand

When preparing plant soil mixes with non-sterile and completely natural substrates, I primarily use compost and sand. While these materials offer excellent benefits, they are seldom used alone due to their specific characteristics. Instead, they serve as valuable components when combined with other substrates to create a balanced growing medium.

Compost can be homemade, which is often the best and most cost-effective option, or it can be purchased from reliable sources that offer weed-free, high-quality compost. One of the key advantages of compost is that it is nutrient-rich. Well-decomposed compost provides essential nutrients that promote healthy plant growth. It also excels at water retention, as it holds a significant amount of fine particles, ensuring that plants receive adequate moisture. Another benefit is its slow decomposition rate, allowing it to gradually break down and release nutrients over time, providing sustained plant health.

However, compost does have some disadvantages. One issue is density. Excessively fine particles can cause the compost to become dense, leading to water-logging, which can harm plants. Additionally, compost can contain large, undecomposed bits that make it difficult to work with and may impede root growth. Another potential drawback is that compost may harbor pests and weed seeds, requiring careful management to prevent infestations and maintain the health of your plants.

When selecting sand for soil mixes, River sand or Silica sand are the best options, as they are more suitable for plant growth. It is important to avoid using building sand, as its high pH levels can be toxic to plants. Sand offers several advantages in soil mixes. One key benefit is its excellent water-holding capacity, as sand can retain a significant amount of moisture, ensuring that plants have access to water when needed. Additionally, sand is a completely natural substrate, making it compatible with other organic materials used in soil mixes.

However, sand also comes with a few disadvantages. One major drawback is its weight, which makes handling and transporting soil mixes more challenging. The quality of sand can also be variable, leading to inconsistencies in your soil mix. Lastly, sand’s fine particles are prone to compaction, which can restrict root growth and negatively impact the overall health of plants by limiting air circulation and drainage in the soil.

Selecting the ideal substrate for established roots

Once your seeds have germinated and your cuttings have rooted, the focus shifts from substrate sterility to aeration (drainage capacity) and nutrition that the potting mix can provide or retain for your plants. Choosing the right potting soil at this stage is crucial for maintaining healthy growth and robust plants.

Substrate grades come in three main sizes: fine, medium, and coarse, each offering distinct benefits and drawbacks depending on the needs of your plants. Fine grade substrates can provide a dense medium that holds moisture well, making them suitable for plants requiring high moisture retention. However, the downside is that they can become too stodgy, leading to poor aeration and potential root problems.

Medium grade substrates strike a balance between moisture retention and aeration, making them versatile for most plants. They offer adequate drainage while still maintaining enough moisture, which is why I find this grade to be the most practical and effective. Unlike the fine and coarse grades, there are no significant disadvantages, which is why I prefer using medium-grade substrates in most situations.

Coarse grade substrates excel in providing excellent aeration and drainage, making them ideal for plants that thrive in well-drained environments. However, the primary drawback of coarse-grade materials is that they tend to dry out quickly, which can stress plants that require consistent moisture levels.

In practice, my preference is to use medium-grade substrates for most of my plants. This grade offers the right balance between moisture retention and aeration, ensuring that roots receive adequate oxygen while maintaining sufficient hydration. It simplifies the process and works well across a wide range of plant types.

Experimenting with substrate mixes

Experimenting with different substrate mixes can be a rewarding process, allowing you to fine-tune growing conditions to suit specific plant needs. While medium-grade substrates generally work well for most plants, combining various components can help create more tailored environments. For instance, mixing coco peat with perlite can enhance both moisture retention and aeration, providing a well-balanced growing medium that supports healthy root development.

You can also experiment with varying sizes of substrates to optimize drainage and nutrient availability. For example, adding coarse bark to a medium-grade mix can improve drainage for plants that thrive in drier conditions. This combination allows excess water to drain efficiently while still retaining enough moisture for the roots.

Personally, I enjoy creating different combinations using the six substrates I regularly work with. This approach allows me to customise each potting mix to meet the specific needs of my plants. The flexibility to adjust mixes not only improves plant health but also makes the gardening process more engaging and dynamic, as you learn what works best for each unique plant.

Soil mix recipes

To prepare effective plant soil mixes, I combine various substrates to suit different plant groups. The substrates I use include coco peat, milled bark, compost, sand, perlite, and vermiculite. Below are my general mixes tailored for each type of plant.

Succulent potting mix: 2 parts sand, 1 part compost, 1 part coco peat. Ideal for ensuring excellent drainage and preventing root rot, perfect for succulents that require dry conditions.

Bulbs (shade) potting mix: 1 part compost, 2 parts 12mm bark, 1 part vermiculite. Provides good aeration and moisture retention, making it suitable for bulbs grown in shaded areas.

Bulbs (sun) potting mix: 2 parts silica sand, 1 part compost, 1 part coco peat. Offers great drainage and moderate moisture retention, ideal for bulbs exposed to direct sunlight.

Indoor plants potting mix: 1 part compost, 1 part 12mm bark, 1 part 12mm coco peat, 1 part perlite. A well-balanced mix that promotes good aeration and water retention, perfect for a wide variety of indoor plants.

General outdoor potting mix: 2 parts sand, 1 part compost, 1 part 6mm bark. This versatile mix provides good drainage and nutrient availability for most outdoor plants.

Fynbos potting mix: 2 parts silica sand, 1 part compost, 1 part 12mm bark. Specifically designed for fynbos plants, offering excellent drainage and the right balance of nutrients.

Vegetable potting mix: 2 parts sand, 2 parts compost, 1 part 12mm bark. Rich in nutrients with good drainage, supporting vigorous growth and healthy vegetables.

These recipes allow you to customise your soil mix based on the specific needs of each plant group, ensuring healthy growth and optimal conditions for all your plants.

Enhancing soil mixes with additives

To further boost the health and growth of my plants, I incorporate various soil additives into all of my soil mixes, following the instructions provided by each product. These additives play a crucial role in enriching the soil and supporting plant development. The key additives I use include:

Biocult/Root Builder: Enhances root development and strengthens plants by encouraging beneficial microbial activity in the soil.

Kelpak: Applied regularly as a growth stimulant to encourage healthy root and shoot development.

Organic fertiliser: Provides essential nutrients to support sustained plant growth without the risk of chemical buildup.

Vermicast: A nutrient-rich organic material that improves soil structure and promotes healthy plant growth.

Many gardener’s and growers also use the following organic soil additives to enrich their growing substrates:

Biochar: made from burning biomass under low oxygen conditions resulting in a  product that increases the nutrient holding capacity of soils, making them richer.

Volcanic Rock dust: made from various types of rock and provides beneficial minerals and trace elements

Incorporating additives ensures that your plants receive all nutrients and support they need for optimal health and productivity. Many of them improve the water holding capacity of the soil and improve the soil structure.

Choosing the Right Additives

Understanding what each plant species prefers in terms of growing or rooting mediums—whether pure or in combination, with or without additives—requires a twofold approach

Research: I thoroughly research each individual plant to grasp its basic preferences and requirements. This foundational knowledge ensures that I provide the optimal conditions for each species.

Experimentation: I experiment by planting in varying substrate mixes, allowing the plants to indicate their preferences. Plants can “communicate” their needs through their growth patterns and overall health, provided we know what signs to look for. A stunted plant with yellowing leaves is telling us it is now happy within its growing space.

When experimenting with different substrates and additives, it’s essential to control other variables that affect plant growth to ensure accurate results. These variables include light, water, fertiliser and temperature.

Adapting soil mixes to your unique environment

You might try some of the plant soil combinations above and experience varying results. These differences are often due to your garden or green space’s unique microclimate factors, such as heat, direct sunlight or deep shade, air flow, indoor air-conditioning or heating, humidity, rainfall and watering practices, wind, and exposure to salty sea air.

Remember, gardening is a journey of trial and error, learning from both successes and setbacks. Personally, I often find myself struggling with overwatering and underfeeding, which can hinder my plants’ happiness and health.

Whether you’re experimenting with your own mixes or seeking the best substrates South Africa has to offer, finding the right combination is key to growing healthy, vibrant plants.

You can find a variety of specialised substrate mixes at TerraHub, ranging from aroid mixes and orchid substrates to bonsai blends—crafted by specialised growers and trusted by us for optimal plant care.

We’d love to hear about your best plant potting soil combinations using various substrates and additives. Sharing our experiences helps us all grow better gardeners. Drop your favourite mixes and tips on the comments below—we’re excited to learn from you too!

Happy gardening!