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Home & garden.

Updated: 30 Mar 2022

How we test compost

We’ve been testing compost for over 30 years and we’re still astounded by the variable quality of the bags for sale.
Adele Dyer
How we test composts for raising young plants

Compost can make or break a gardening job. The best will support the healthy growth of plants, veg and seeds, while the worst will lack nutrients and be so coarse it can stop plant roots from growing.   

We carry out independent, scientific tests designed to find out how well different composts cope with a variety of gardening tasks and if they can produce healthy plants. 

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How do we choose and buy our compost?

At the start of each year we ask compost manufacturers which composts they plan to sell that year and the following year. We prioritise peat-free composts when choosing which to test as the government has set out an ambition to ban the sale of peat compost by 2024. We include some widely-available peat composts, but choose those with the lowest percentage of peat per bag and from 2022 we won't test any that contain more than 60% peat. Our compost for containers trial is entirely peat-free. 

Learn more about peat-free compost.

We send secret shoppers to buy our composts. They live in four different parts of the UK, so we're able to see whether there is any variation between composts. In the past we have found problems caused by inconsistent manufacturing techniques, which may not affect all the compost on sale. We buy from garden centres, DIY stores, supermarkets and discount retailers and order some online.  

Where do we test the compost?

We carry out our tests at a well-respected horticultural institute, which carries out research and runs trials for many clients, including horticultural industry bodies. 

The experts there have the knowledge and expertise to ensure our trials are conducted using the best growing techniques, so we know that any differences in performance are down to the compost itself. Our independent assessor is an expert in compost and plant health. 

How we test composts for sowing seeds

We choose 25 composts for the sowing seeds trial. Most are suitable for both sowing seed and raising young plants, and so included in both tests, while a few are specialist seed sowing composts. 

Each of the composts is assessed on whether it has fine or large particles, and any visible pest problems, such as fungus gnats (sciarid fly). We also make a note of any were that are wet, compacted, or contain rubbish such as plastic.  

We sow one vegetable and one flower variety in all our composts, choosing ones that will fail to germinate or grow very slowly in poor compost. We fill 12 seed trays for the vegetable seeds and 12 for the flower seed, sowing 25 seeds per tray, repeating this for all our composts.  

When the majority of seedlings are at the first two true-leaf stage, we count the number of seedlings that have germinated and rate them for size and vigour. 

Discover which seed-sowing composts are Best Buys this year.

How we test composts for raising young plants

We select 25 composts to test. Some are multi-purpose and some are specialist composts for raising young plants and/or sowing seeds.  

We always grow one vegetable and one flower variety, choosing ones that need ideal compost to grow well. Both are grown from seed and the seedlings potted into our trial composts once they reach the two true-leaf stage. We grow 20 pots of veg seedlings and 20 of flower seedlings in each of our trial composts. 

We grow them on for six weeks, assessing them weekly for size, leaf colour and vigour. If the plants flower, we count these. At the end of the trial we cut all the plants at the base and weigh them. 

To find out which are the best, take a look at our Best Buy composts for raising young plants.

How we test composts for containers

We select 25 peat-free composts for containers. These are multi-purpose composts, or formulated for tubs and baskets. 

We grow 16 pots of pelargoniums and 12 pots of potatoes in each compost. Pelargoniums show any nutrient deficiencies by the colour of their leaves, while potatoes are heavy feeders and any problems with wet or dry compost will show in the quality of the tubers we harvest. 

We grow the pots of potatoes from April to September, using a controlled-release fertiliser unless there is enough fertiliser in the compost to last more than three months. The potatoes are harvested in September, weighed and the crop is assessed for size and quality.

We plant our pelargoniums in May and grow them until October. We don't feed all our pots of pelargoniums. 12 are planted without any fertiliser and four have feed added. This is so we can assess how well plants can survive in the compost as it is sold, but also how well they will thrive if fed well. The pelargoniums are assessed for flowering impact, the size and bushiness of the plant and vigour, which means leaf colour and general health. We count the number of flowers produced by each plant. We carry out these assessments every two weeks through the summer, and at the end of the trial we cut all the plants at the base and weigh them.  

Discover our Best Buy composts for containers