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How we test compost

By Adele Dyer

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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.  

Find out how we find the best composts and which ones to avoid.

Best composts for raising young plants: 2019 trials

In March 2018, we picked 25 composts to test for raising young plants and 22 to test for sowing seeds. We also grew our seeds in a peat compost made for professional growers to see how our garden centre composts compare. 

Most of the composts were suitable for both sowing seed and raising young plants and so were included in both tests, as we know many people prefer to buy just one bag for both uses. A few were specialist composts for either seeds or young plants. 

Secret shoppers

We send secret shoppers to buy our composts, so we can be sure we're getting the same compost as you. Our shoppers live in four different parts of the UK, so we're able to see whether there is any variation between composts sold in these areas. We bought the composts for garden centres, DIY stores and, this year, several supermarkets and discount retailers. 

Our compost trial site

We carry out our tests at a well-respected horticultural institute, which runs trials for scientists and 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.

Best composts for sowing seeds

Each one of our composts was assessed on whether it had fine or large particles, and any visible pest problems, such as fungus gnats (sciarid fly). We also made a note of any were that were wet, compacted, or contained rubbish such as plastic.  

We then grew basil 'Puck' and Petunia 'Express Rose' seeds in quarter-size trays. We sowed 12 trays of basil and 12 trays of petunia (25 seeds per tray) in each compost. We buy four batches of each compost, so filled three trays per batch. Our tests were run in a temperature- and humidity-controlled greenhouse.

Both these small seeds need to be in close contact with the compost, with access to water and air to germinate and root well. Petunia grow rapidly after germination and so any lack of fertiliser will impede their growth. 

We counted the number of seedlings that germinated and rated them for size and vigour.

Best composts for raising young plants

We tested composts for growing-on seedlings and young plants in a temperature- and humidity-controlled greenhouse. We planted cabbage 'Caraflex' seedlings and plug plants of Antirrhinums 'Express Rose'. We grew 24 tomatoes and 24 pelargoniums in 9cm pots of each compost, with six pots per batch. 

We grew them for six weeks and rated them weekly for growth, foliage colour, health and flowering on the antirrhinums. At the end of the test we counted all buds, spent and open antirrhinum flowers, and then cut both the cabbages and antirrhinums at the base and weighed them. This precise measurement clearly shows how well each one has grown.  

Best composts for patio pots

In our compost for pots test, we look for ones that will grow strong, healthy bedding plants that will flower from early on in the season, and a bumper crop of large, smooth-skinned, unblemished potatoes.

We grew 12 pots of pelargonium 'Designer Salmon' and 12 pots of potato 'Sarpo Axona'. We mixed in a Best Buy controlled-release fertiliser, unless the compost bag stated that it contained enough feed to last for more than the four months that our trial lasted. In these cases, we assessed how well the plants were growing in July and started to liquid feed those that needed a boost, as suggested on the packaging.

The pelargoniums were assessed for flowering impact and vigour, which means leaf colour, the size and bushiness of the plant and general health. We carried out these assessments three times through the summer in July, August and September. The potatoes were harvested in September, when they were weighed and assessed for size and quality.

Our scores are based on how well both trial plants did, weighting the results of both trials equally.

The Which? Gardening difference

We've been testing compost for over 30 years and we use experience to make sure our tests are rigorous, fair and reflect how you will use the compost at home.