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Which? reveals the best composts

Composts for sowing seeds and growing on seedlings
Compost for Seed

We grew sweetcorn and petunia seeds to find the best seed sowing composts

Which? compost trials have found four great composts for sowing seeds and two for raising young plants. Sadly we also found several composts that had serious problems, in which seeds failed to germinate and seedlings and young plants struggled to grow. 

We tested 25 widely available composts for sowing seeds and 25 for raising young plants and found huge variations between them. Six composts were Don’t Buy composts for sowing seeds, and four were Don’t Buy composts for raising young plants, many of them peat-free composts. Some of these have now been withdrawn, but this still leaves several very poor composts on sale this year. The good news is there are some great composts on sale, including our Best Buy compost for sowing seeds

You can’t choose a good compost over a poor one simply by buying the most expensive option. One of the Don’t Buy composts for sowing seeds scored just 38%, but costs 3p more per litre more than one of our Best Buy composts for sowing seeds that scored 92%. The only way to find the perfect compost is to read our full test results

Best composts for young plants

There are more bargains to be had among our Best Buy composts for raising young plants, where one Best Buy compost costs just 8p per litre and scored 87%. 

Among our Best Buys was one peat-free compost and several that are peat-reduced, with less than 60% of the mix containing peat. Peat harvesting can have serious environmental consequences, and the compost industry has agreed to a target of no peat in compost for home gardeners by 2020, so finding alternatives to peat is vital. For more on this, read our guide to peat-free compost

Problem composts

Our Don’t Buy composts all had one thing in common: the fertiliser was unbalanced, making it hard for plants to grow. Almost all composts had too little nitrogen, which plants need to grow green shoots and leaves, and many had high levels of nutrients such as potassium and chloride, which can prevent plants from taking up other nutrients needed for growth. This led to few of the seeds germinating, while the Don’t Buy composts for raising young plants grew only weak seedlings and plug plants that were small and pale. 

The peat-free Don’t Buy composts for sowing seeds also contained lumps of wood, making it hard to plant seeds and seedlings and even harder for their fine, delicate roots to grow. However, don’t be put off by all peat-free compost, as one of our Best Buys for sowing seeds is peat-free. 

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