Welcome to my WTF Gardening series, where I take common gardening terms and explain them for those who are new to the world of plants. Find the entire WTF Gardening series here.
If you’ve bought a new plant, especially a non-native plant, it might have soil requirements that differ to the soil you have in your garden. But soil’s soil, right? Nope. There are sandy soils, clay soils, soils that retain moisture, soils that are acidic – and it’s important to understand the properties of these soils in order to get the best out of your plants.
In this blog post, I talk you through the most common soil types and what they do.
Peat is a dense, soil-like material made up of decomposed organic matter. It is unique to natural areas called peatlands or peat bogs, and develops over thousands of years. Peat has to be mixed with sand and loam to form a soil mixture that we can use in our gardens.
Peat-based soils have the benefits of retaining lots of moisture, making them ideal for use as seed starting mediums. These soils also hold onto nutrients so that they aren’t rinsed out when you water your plants.
However, I would recommend keeping your peat-based soil use to a minimum due to environmental concerns.
Peat-based vs peat-free
Because of the unique flora and fauna found in peat bogs, and the length of time it takes for peat to develop, there is controversy about harvesting it. For the environmentally conscious, there are many peat-free alternatives that have been developed in recent years, including:
- wood fibre
- coir (fibre extracted from coconut husks)
- biosolids (solid organic matter recovered from a sewage treatment process)
- bracken (a type of native British fern)
- green compost (a compost created by recycling green waste from gardens and parks)
The word ‘ericaceous’ relates to the Ericaceae family of plants; this family includes the rhododendron, heather, blueberry and camellia, as well as over 4,000 other varieties of plant. Did you know that Hosta also like a slightly acidic soil too? If you try to grow acid-loving plants in alkaline soils, you’ll get yellow leaves, poor flowering, and your plant will likely die. The reason for this is that alkaline soils don’t allow proper absorption of the nutrients that these plants need to survive and thrive.
Ericaceous soils can get their acidity from organic matter, leaching (stripping of key minerals from the soil due to excessive rainfall), and the use of high-nitrogen ammonia-based fertilisers.
How to test for acidity
Your soil might already be acidic, but you probably won’t know until you test it. You can easily test soil pH with a testing kit, which you can find on Amazon.
The other way to gauge whether your soil is more acidic is to look at what’s growing in your local neighbourhood. Has your neighbour got a thriving rhododendron bush in their border? Are there wild blueberries nearby? This will be able to give you more of an idea of what you could grow.
When you buy compost, you’ll find that most varieties are soil-less. However, there are some advantages to soil-based composts, such as offering more weight, trace element content and the effect of soil content in avoiding undue chemical and water fluctuations. A lot of soil based composts are great for long term planting because of these reasons.
The brand John Innes pioneered soil-based compost in the 1930s, and based their mix on a base of loam; however, loam is in short supply and so current mixes may differ from the mix of the traditional product.
There are several types of John Innes compost:
- Seed compost
- Potting compost
- Compost numbers 1, 2 and 3 (for seedlings, young plants and mature plants, respectively)
- Ericaceous compost
You can find out more about what is in each compost here.
The best compost for containers
Container compost is different to the compost you would use to boost growth in your vegetable patch. Container compost is usually called ‘potting compost’, and there are a few different composts you can choose from depending on the requirements of your plants:
- Multi-purpose compost – this is a reliable compost that suits many different types of plants. However, they can vary from brand to brand in terms of ingredients, so if you’re concerned about whether it will meet your plant’s needs, maybe use a specialist compost instead.
- Soil based potting compost – John Innes potting compost is a good example of this type of compost. These composts tend to contain loam, and dry out slower than many other types.
- Specialist compost – these composts are used for plants that have special requirements, such as ericaceous plants.
You can also add ingredients to your potting compost for extra benefits. For example, you could:
- add a root booster when planting up in order to help the plant develop a strong and healthy root system
- introduce a mycorrhizal fungi to the compost in order to increase the absorptive power of the plant’s roots
- add a controlled or slow release fertiliser. Controlled release fertilisers release vital nutrients when the plant reaches a certain point in its growth, whereas slow release fertilisers release their nutrients over a long period of time regardless of where the plant is in its growth.
Clay soils are usually specific to areas which receive a high annual rainfall. You’re unlikely to have natural clay soil in your garden if your neighbour doesn’t!
This type of soil is heavy, hard to manage, takes longer to warm through in spring, and can be damaged easily if disturbed through digging or even walking on it. However, if treated correctly, clay soil can be very fertile, and it works well for plants that don’t like drying out.
If you have clay soil in your garden, you should work on the soil between autumn and early winter, when the clay isn’t baked hard or too wet. During this time, it’s recommended to encourage better drainage and frost activity (clay needs frost to get in to break up its clumps) by digging ridges in the soil.
Clay soil usually requires ‘improving’, which means slowly introducing a substance that helps the clay in a certain way. For example:
- Adding your own compost will break down the texture of clay soil and open it up, as well as increasing the nutrient value.
- Gypsum will help most clay soils by making it clump together, increasing drainage and making it easier to work with.
- Manure or composted bark can be dug into the soil to make it easier to work and increase fertility.
- Applying organic mulch around trees and shrubs will help the soil to retain moisture in summer and stop it from cracking.
Sandy soils are often found near beaches and heathlands, and contains a high proportion of sand, as the name suggests. The most prominent characteristic of sandy soil is its quick drainage and its inclination to dry out.
To improve sandy soil, add a good amount of moisture-retentive organic matter when preparing. Manure, garden-made compost and bark are excellent materials to mix with sandy soil in order to keep the moisture in and add texture. Regardless of improvement, plants in sandy soil will need regular watering throughout dry periods.
Do you need soil to propagate plants?
Some plants are best propagated in soil, whereas some can be propagated in water! With water propagation, you can cut off the top three inches of a stem of the plant where new growth is coming through, remove the bottom few leaves, and place the cutting upright in a jar of clean water.
With soil propagation, use the same type of cutting, but place it in a soilless potting medium. Do not use garden soil, as this is far too dense to allow a new plant to take root.
We have an entire article about propagation here.
Houseplants require a different soil to what is used in your garden. This is because they grow in containers, and therefore they usually need good drainage in the form of a light soil mix. I say ‘usually’, because there are houseplants that require a very moisture-retentive soil! Read on to find out…
Houseplants that need sharp drainage
Having gained popularity over the last decade, cacti and succulents are excellent houseplants for beginners, as they really don’t need a lot of maintenance. That being said, they are picky when it comes to soil, preferring excellent drainage and good aeration to prevent root rot.
Many garden centres offer dedicated cacti and succulent potting mixes; however, you can also make your own if you know that you’ll be propagating or repotting your plants. An easy recipe comprises three parts potting soil, two parts coarse sand and one part perlite or pumice. Make sure to buy these fresh – especially sand – as you never know what could be lurking in the materials when taken from a beach or garden.
Houseplants that need constant moisture
Carnivorous plants are quickly gaining popularity with modern houseplant lovers! These plants require moisture retentive soil, as they’re usually found in wet, boggy conditions in the wild.
Soil requirements vary from plant to plant, but generally they work well in a mix of peat moss, perlite or sand. Planting them in regular potting soil will kill them!
Orchids – a whole other kettle of fish!
Orchids produce very unique aerial roots that grow up and out of the potting media in order to draw moisture from the air. Orchids do not like to sit in water, which means that excellent drainage and aeration are crucial when it comes to choosing a soil.
Again, you can find orchid potting mixes in many garden centres, but you can create your own mix – taking into account the variety of the orchids you want to plant/repot and their individual needs. Most people who care for orchids will experiment with different mixes, so some common materials to play around with are:
- Fir bark
- Tree fern
- Sphagnum moss
- Styrofoam (yes! It’s great for drainage)
What would you like to learn about next in the WTF Gardening series? Let me know in the comments below!
Michael has been involved with gardening and plants since he was just five years old. He is a self-professed Plant Geek, and was listed in the Sunday Times top 20 most influential people in the gardening world, thanks to his plant hunter role at Thompson & Morgan.
Michael was responsible for new plant introductions such as the Egg and Chips plant and the FuchsiaBerry and keeps busy travelling the world in search of new plants as well as lecturing worldwide, including stints in Japan. He is very active on social media – so why not give him a follow at @mr_plantgeek or Facebook – and writes a plant-focused Substack called Grow This, Not That.