Choose the right compost
By Adele Dyer
Choose the right compost
Choosing the right compost can seem confusing. Read our guide to find out how to pick a compost that will be perfect for your tasks.
What are the different types of compost?
Our trials test several different types of compost, including multipurpose and those for a specific use, such as raising plants from seed or growing plants in patio containers.
Multipurpose and all-purpose composts
These are claimed to be suitable for germinating seeds, small seedlings and plants in patio containers, so there's no need to buy different types of compost for different stages in a plant’s life. Multipurpose composts are often cheaper than specific-use composts. We have found that some multi-purpose composts, especially peat-based composts, have done well across the board in our tests.
Specific seed and potting composts
These are formulated to optimise plant growth by providing the right amount of nutrients to suit the plant at a particular stage in its life. They often contain additional ingredients to multipurpose composts, such as grit to aid drainage. Specific-use composts tend to be more expensive than multipurpose composts, but plants raised in these don't always grow any better - or even as well.
For our Best Buy composts for seed sowing and raising young plants, read the results of our latest compost trials.
Container composts are formulated to optimise plant growth by providing the right amount of nutrients for plants in containers. Some contain a controlled-release feed and/or water-storing granules. Container composts tend to be more expensive than multipurpose composts.
If a slow-release feed has not been added already, use a Best Buy slow-release fertiliser. Read our review to find out the results of our latest slow-release fertiliser trials.
What makes one compost better than another?
Whether you buy multipurpose or specific-use compost, good structure and aeration are important for encouraging strong root development and healthy plants.
A medium-crumbly texture is ideal. Composts that are too fine are prone to water logging, while those that are too coarse tend to need more frequent watering.
A good seed or potting compost should have the right balance of nutrients to grow a range of plants during this stage of their development, whereas a multipurpose compost should be able to support plants at all growth stages.
How do growing bags compare to other bags of compost?
Traditionally, growing bags have been a low-cost way to buy compost. They have a reputation for containing cheaper-grade ingredients of inferior quality, and although we have found some good-quality bags in the past, this was not the case this year.
How can I reduce the amount of peat I use, and why is this important?
Peat bogs are an invaluable habitat for wildlife, supporting many important native species not found anywhere else. Peat extraction can permanently damage bogs, which means this vital habitat can't regenerate.
There are a number of good peat-free composts available. Our trials found a peat-free compost this year that was a Best Buy for raising young plants and just missed out by a whisker on being a Best Buy for sowing seeds. Discover more about our Best Buy composts for raising young plants.
Some composts have reduced peat levels, and these can be a good way of reducing your peat use. To find out which these are, see our full compost test results.
Caring for plants growing in peat-alternative composts
Materials such as coir, wood fibre and composted bark are now replacing peat in many composts. These all have very different physical structures and this means you have to alter how you care for plants growing in these composts, especially when it comes to watering.
Peat naturally holds a good amount of water, releasing it slowly to plants. It also tends to dry evenly throughout the pot, so the old gardener's trick of feeling the wetness on the surface of the pot used to give a good indication of whether or not it needed to be watered.
Composted green waste or green compost
This is made from the green waste collected by local authorities from door-to-door collections or from municipal waste sites. This material is variable, as in summer green waste is largely made up of grass cuttings, and in winter it's mainly woody material. New standards have been introduced to try to ensure a more stable mix of these two extremes and to eliminate contamination with glass, stone and metal.
Green compost still suffers from several problems. It can rot down quite quickly, creating a compost that slumps, reduces in volume and becomes compacted. It also varies in how nutrient-rich it is - for example it can release large amounts of nitrogen or very little, which makes feeding tricky. And finally it can stay quite wet, so it is worth watching your watering, especially in rainy weather.
Some manufacturers have now decided not to use any composted green waste in their composts, but are using wood fibre instead (see below). Other producers have altered where they source the materials for their green waste compost. They are focusing instead on farming, brewing and carefully vetted landscapers to ensure a steady flow of less variable ingredients.
Coir holds water well, but it does not hold it as evenly as peat. The surface can be dry, but there can still be a good amount of water held further down the pot. To avoid problems of over-watering, it is best to water little and often. Judging when to water again can be tricky, but it is best to pick up the pot where possible. Pots that need watering will be much lighter than pots that those that still contain a good amount of water. You will quickly learn to judge which need water and which do not.
It is also important not to compress coir when you plant. Instead of firming in, overfill the pot and then tap it a few times on the workbench to settle the material and then water to further settle the contents. You may have to hold cuttings, plug plants and seedlings close to the top of the pot while you do this to prevent them from being planted too deeply. Wrap your hand around the pot while you hold the young plant between your thumb and forefinger and then use your other hand to tap the pot on the bench.
Wood fibre is often added to composts and typically makes up around 20-30% of the total volume as too much wood fibre can lock up nitrogen. It can increase drainage in composts that can become wet, such as composted green waste. However, in hot weather it is important to make sure that the wood fibre does not cause your pots to become too dry. Again, water little and often to manage water within the pot.
It's generally made one of two ways. Some is the shavings taken from lumber yards as timber is sawn into planks. Other companies use a wetting and drying technique to convert wood chippings into a light, fluffy material. The result is a little akin to putting a woolly jumper in a tumble drier: the fibres expand and split apart, making them better at holding air and allowing water to drain freely.
Composted bark is often mixed with wood fibre and coir to grow plants. Bark makes quite an 'open' compost with good air spaces within the compost. However, this does mean that it doesn't hold water well and you need to water generously, especially when potting young plants. Check plants regularly to make sure they don't dry out.
All composts have a small amount of fertiliser in them, but you need to add a controlled-release fertiliser to sustain your plants through a full growing season.
What about reformulated composts?
Composts change for a number of reasons, such as availability of ingredients, cutting costs, improving performance or reducing peat content.
One problem we face every year is manufacturers changing their composts after our tests are completed. We can't recommend reformulated brands because there's no guarantee that the new product will perform similarly.
It's possible to find reformulated and old compost being sold at the same time, so our advice is to steer clear of buying compost in faded bags and, if possible, shop somewhere that has a high turnover of stock.
Does quality vary between bags of the same brand?
You could be forgiven for assuming that one bag of compost would be the same quality as any other bag of the same brand. But this isn't always the case.
Our compost trials have repeatedly found dramatic differences in how well plants grow in composts from different bags of the same brand.
Is it true that I can catch Legionnaires' disease from compost?
The bacterium responsible for the potentially fatal Legionnaires’ disease has been found in many composts sold in the UK. The chances of you becoming infected are very low, but it makes sense to take the following precautions when handling compost:
• Use compost in a well-ventilated place; keep the greenhouse or potting shed door open and avoid breathing in dust.
• Wear gloves (and no smoking or snacking!) and wash your hands after use.