Compost buying guide

Compost, or potting mix, is a growing medium used to support plants as their root system develops. The ingredients contained in potting compost provide all the nutrients required for growth and actively help plants to absorb these nutrients.

Most plants can cope perfectly well without compost. However, using this type of medium provides plants with the best possible growing conditions. There is a wide range of potting soils or compost on offer, all of which vary in terms of ingredients (for example, peat, compost, and so on). With this in mind, there are big variations in terms of quality.

If you want to achieve optimal plant growth, your first step – before you pick up your gardening tools – should be to choose the right type of compost for the plant varieties you want to grow.

As mentioned, compost comes in a variety of different types and quality can vary. So before you get stuck into cultivating the soil or filling your pots, let's take a look at the most common and the most effective types of compost for each application.

Multi-purpose compost

Multi-purpose, or all-purpose, compost is the most common type of compost and the easiest to find. It contains a high amount of composted materials and is perfect for tasks like planting trees and shrubs. However, it is not at all suitable for potting flowering plants or crops that require an acidic soil.

Seed starting potting mix

Seed sowing composts are very light and airy. This type of compost contains sand, or even perlite, which is integral in helping young plants to develop root systems. It can also be used to encourage transplanted plants to quickly resume growth.

Flower compost

These potting mixes are specifically designed to support flowering plants or shrubs and often contain a high percentage of peat or organic matter. Flower compost contains all the nutrients required for plants to bloom, so do be careful if you also plan to add fertiliser to your plants.

Ericaceous compost

Featuring similar ingredients to flower compost, ericaceous compost is acidic which makes it suitable for plants that require a low pH level such as azaleas, rhododendrons, camellias, hydrangeas and magnolias. Ericaceous compost is made up of materials with a high acidic content such as pine bark or needles, and compost. This type of mix often contains peat, though peat-based composts are best avoided.

Potting mix

A potting mix is often better quality than a generic all-purpose compost and also contains fertiliser.

Vegetable compost

Vegetable compost is made up of organic and composted materials. It contains all the required nutrients for vegetable growth including nitrogen, phosphorus and potassium.

Lawn dressing

Lawn dressing is a very fine compost designed to be spread very thinly over newly seeded lawn or bare patches in a lawn. This process is known as top dressing.

Cactus potting mixes

These potting mixes contain a high percentage of sand to help with pot or soil drainage in order to prevent the roots getting waterlogged.

Finally, you will find a multitude of various plant-specific composts for things like orchids, citrus plants, and so on. Most of the time, these potting mixes will be fairly similar to all-purpose compost with the addition of certain ingredients.

Most garden soils have a pH level of 4 to 9. As a reminder, pH stands for 'potential hydrogen'. A pH level of 7 or below is considered acidic while a pH level of 7 to 14 is considered basic or alkaline. A pH level of 7 is netural. Unfortunately, garden soil can become more acidic due to acid rain which is associated with high levels of pollution. Some composts are used as soil conditioners, or amendments, to rectify the pH level by adding hydrogen ions (H+). To find out whether you have acid or alkaline soil, you have a few options:

• analyse the types of plants that naturally grow in your soil: heather, ferns and foxgloves will grow in acidic soil while primroses tend to grow in alkaline conditions;

• take soil samples from different spots around the garden from a depth of 20 to 30 cm. Mix the samples in two different glass jars. Pour white vinegar into one and observe. If it reacts (for example, produces bubbles) you have basic or alkaline soil. In the second jar, add some water to create a mud mixture and add some bicarbonate of soda. If you see any bubbling, you have acidic soil. Of course, if you don't get results from either test, you have neutral soil.

• analyse your soil using a test kit sold for this purpose. These kits usually contain a test tube and strips, and are really easy to use – just read the instructions!

• if you're really curious, send a soil sample off to a soil testing lab.

Potting compost can be made up of a range of ingredients, each of which provides different properties:

• sand: helps to promote drainage;

• clay: helps to regulate water content and retain nutrients;

• compost: helps to promote growth through nutrient provision;

• bark and pozzalana: helps to aerate the soil;

• blond peat: helps to retain water and retain nutrients; blond peat is acidic, fibrous and has a low mineral content;

• brown peat: also helps to retain nutrients; brown peat is neutral, less fibrous and has a higher mineral content.

It is easy enough to pick the right type of compost as long as you stick to the criteria set out above and apply them in the right order. If you want to get growing down to an art form and achieve the best results, start by:

• finding out the pH level of your soil;

• define your requirements (transplanting, planting, etc.) ;

• analyse the compost ingredients which will be noted on the bag and adapt according to your soil condition or plant requirements.

When it comes to storage, be sure to stack your compost bags in a dry area. Seal the bags properly and avoid placing them directly on the ground. If possible, raise the bags on a palette. Do not hesitate to compost your own organic materials. Garden compost can be used as mulch or added to commercial compost to boost nutrient content.

Avoid using compost containing peat as this material is considered a solid fossil. It is not renewable as its high extraction rate exceeds its very slow growth rate. In other words, there is no point in buying an 'organic compost' that contains peat! It is extremely important to protect the diverse ecosystems that are peatlands, which are home to a diverse range of flora and fauna. Even worse would be to purchase peat sourced from other countries like Chile: there's nothing eco-friendly about shipping a non-renewable from another continent!

• How to identify your soil type

• How to reuse your coffee grounds

• How to mulch your garden and why