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Don't Buy composts for sowing seeds

By Adele Dyer

It's always thrilling to see newly germinated seedlings growing, but buy the wrong compost and you'll be lucky to see many of them emerge.

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Avoid our Don't Buy compost for sowing seeds, which will leave you disappointed as your seeds fail to germinate and grow on. 

Worst composts for sowing seeds

A great compost for sowing seeds should have a fine texture that will surround the seed with everything they need: water to kick-start germination and help the plant to grow; air to help the plant to mature, and a small amount of nutrients to fuel growth once the seed has exhausted its own stored nutrients. All our Best Buy composts give seeds all they need for strong, healthy growth, but our Don't Buy compost contained huge doses of fertiliser and was very acidic, meaning the seeds struggled to germinate and grow. The Don't Buy peat-free composts were also very coarse, containing large chunks of wood that make it hard for a seed's fine roots to take up water and nutrients. 

Each year, we sow a large seed that is usually easy to grow, and a small, fine seed that will be a little more tricky to germinate. This year we chose sweetcorn 'Lark' as the large seed and petunia 'Express Rose' as the small seed, sowing 25 seeds of each in quarter-size trays. When the majority of seedlings are at the first two true-leaf stage, we count the number of seeds that have germinated and rate the seedlings for size and vigour.

See how all the composts scored by looking at the full results table, and see which were also Best Buy composts for raising young plants and for containers. 

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Compost Our verdict Sweetcorn seeds Petunia seeds Score
Compost non-member logo M *** ** 38%

Acidic compost

Peat content: 0%

This compost was very acidic, with a pH of 5.2. Composts should ideally have a neutral pH of around 7, and if the compost is too acidic a seedling's delicate roots will be damaged, leading to poor growth. 

The sweetcorn seeds germinated well, but the seedlings stopped growing after a few days, remaining stunted. The leaves also had some dieback around the edges and were very yellow. This was down to a nutrient imbalance, just like the compost below. Few of the petunia seeds germinated and the seedlings were very small. 

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Compost non-member logo M ** *** 38%

Unbalanced fertiliser

Peat content: 0%

This compost mostly contains coir, a natural product derived from coconut husks. It can be naturally very rich in potassium and this can lead to the plant being unable to take up enough magnesium and calcium. 

Our sweetcorn seedlings had stunted growth and yellowing between the veins that is typical of magnesium deficiency, and the edges of the leaves looked 'burnt', a sign of calcium deficiency. The petunias were slightly better, but weren't as robust as those grown in other composts. 

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Compost non-member logo M *** * 25%

Coarse texture

Peat content: 0%

This peat-free compost is made from green waste and was so coarse with chunks of woody material that we found it hard to sow seeds in it. 

It had low nitrogen levels but high levels of other nutrients, creating an unhealthy imbalance for seeds. This explains why so few of our sweetcorn and petunia seeds germinated and grew on. 

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