Compost FAQs
What are the different types of compost?
Our trials test several different types of compost: multipurpose and those for a specific use, such as raising plants from seed or growing plants in patio containers.
Multipurpose composts
These should suit 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.
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
Container composts are formulated to optimise plant growth by providing the right amount of nutrients for plants in containers. They also contain various amounts of other ingredients, such as sand and grit to aid drainage. Some may contain a slow-release feed and/or water-storing granules. Container composts tend to be more expensive than multipurpose composts.
The easiest way to feed plants growing in containers is to use a Best Buy slow-release fertiliser. Read our review to find out the results of our latest slow-release fertiliser trials.
Basil raised in the best and worst compost we tested
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 and 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 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 and are a good choice if you have trouble handling the often heavy and cumbersome bags of multipurpose. They have a reputation for containing cheaper-grade ingredients of inferior quality, but we've found that they can be a good option. One of our 2013 Best Buys for raising young plants is compost from a growing bag. However, there are two brands we don't recommend. Read the results of our latest compost trial to find out what these are.
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 have shown that peat-free alternatives can do just as well for potting on young plants and when used for planting up containers. Sowing seeds in peat-free compost is still a bit of a challenge.
Another way to reduce your peat usage is to switch to a John Innes compost. These contain significantly less peat than many multipurpose composts.
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.
The quality of this compost varied dramatically from bag to bag in our tests
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.
- Read about our Best Buy composts for raising young plants
- Find out which composts are Best Buys for patio pots and hanging baskets
- Subscribe to Which? Gardening magazine