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Home & garden.

13 August 2021

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 healthy growth of plants, veg and seeds, while the worst will lack nutrients and be so coarse you will have to water them every day.  

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 a quality yield. 

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Video: how we test compost

How do we buy our compost?

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. 

Where do we test the compost?

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.

How we test composts for raising young plants

We selected 15 composts to test for raising young plants. Most of the composts were suitable for both sowing seed and raising young plants and so were included in both tests, while a few specialist products were also chosen. We also grew our plants in a peat compost made for professional growers to see how our garden-centre composts compare.

We know many people prefer to buy just one bag for both uses so this offers more choice for gardeners. 

We grew Tomato 'Red Alert' Antirrhinum 'Yellow Appeal' seedlings, starting at the two true-leaf stage, for six weeks. 

At the end of the trial we judged both the young plants on how vigorous they were, which includes looking at size, leaf colour, and whether the plants are stocky and strong or long and straggly. We also noted how well the antirrhinums were flowering. 

How we test composts for sowing seeds

How we test composts for sowing seeds

We selected 17 composts to test for sowing seeds. Most of the composts were suitable for both sowing seed and raising young plants, and so were included in both tests, while a few specialist products were also chosen.

Each of the 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 'Cinnamon' and Petunia 'Express Rose' seeds in half-size trays. We sowed 12 trays of basil and 12 trays of petunia (25 seeds per tray) in each compost. 

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

How we test composts for containers

How we test composts for containers
We selected 25 composts for containers.

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 then 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.