How we test compost
By Adele Dyer
How we test compost
We’ve been testing compost for over a quarter of a century and we’re still astounded by the variable quality of the bags for sale.
Read our guide to find out which brands you can trust and which you should avoid.
Best composts for raising young plants: 2017 trials
In March 2016, we picked 25 composts to test for raising young plants and 25 to test for sowing seeds. Most of the composts were suitable for both purposes and so were included in both tests, as we know many people prefer to buy just one bag for both uses, but a few were specialist composts for either seeds or young plants.
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.
Seed-sowing compost trial
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 sweetcorn 'Lark' and petunia 'Express Rose' seeds in quarter-size trays. We sowed 12 trays of sweetcorn 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.
Sweetcorn has large seeds that should be quick to germinate and grow strongly. If any failed to do this, it indicated a problem with the compost. Petunias have very small seeds that are slow to germinate. They need even moisture, so the compost can't dry out too much. Once germinated, they need plenty of nitrogen to get going.
Raising young plants compost trial
We tested composts for growing-on seedlings and young plants in a temperature- and humidity-controlled greenhouse. We planted pepper 'Arianne' seedlings and plug plants of fuchsia 'Shadowdancer Amelie'. We grew 20 peppers and 20 fuchsias in 9cm pots of each compost, with five pots per batch.
We chose pepper as it needs a moisture-retentive but well-drained compost, so it's important that the compost has a mix of small and slightly larger particles. It also needs a fertile compost that lets plants grow rapidly. Fuchsias are less fussy and should grow strongly, but they still need a good balanced fertiliser and a neutral pH to avoid stunted growth and to help the plant flower well.
Our compost test scores
As our seeds started to germinate, we noted which were the first to emerge. After 10 days for the sweetcorn and four weeks for the petunias, we counted the number of seeds that had germinated and assessed their size and quality.
In the raising young plants test, the plants were assessed after six weeks and marks were given for size, quality and leaf colour, plus flowering on the fuchsias.
We give separate marks for the sowing seeds and raising young plants tests, even though many composts are in both tests. We think it's fairer to highlight which composts are good or poor at certain tasks. This year, no composts were Best Buys in both tests, although several did well in both.
Best composts for plants in containers: 2016 trials
In our compost for containers 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 begonia 'Belleconia Rose' and 12 pots of potato 'Sarpo Mira'. 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 begonias 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 October. The potatoes were harvested at the beginning of October, 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 2016 test results of our container composts trial will be published in the April issue of Which? Gardening and online, at the same time, at the end of March, so look out for these before you buy your compost for pots.
Best growing bags 2016
Growing bags are most commonly used to grow salad crops, such as tomatoes and cucumbers, so this is what we do in our tests.
We bought two of each brand of growing bags from four areas around the country. In early May, we placed them in a greenhouse and planted three 'Shirley' tomato plants or two 'Passandra' cucumber plants into each one. We watered them using an automatic drip-feed system and gave them a liquid feed at each watering after the first few weeks, following the instructions on the bags.
We assessed the plants every week for the first six weeks while they were establishing, and then every two weeks. We harvested the cucumbers three times a week until mid-August and the tomatoes twice a week until October, grading the fruit and weighing it. The score is calculated by combining these three assessments equally.
The Which? Gardening difference
This year, we tested a total of 37 composts and 10 growing bags for our three tests, growing a combined total of 3,600 plants over six months. We use our 25 years of compost-testing experience to make sure our tests are rigorous, fair and reflect how you will use the compost at home.