Consumer organisation Which? has described some top-name composts as "shocking" after asking a specialist lab to analyse the composition of some of the worst-performing products in its latest seed sowing and young plants trials.
The analysts looked at four bags per compost.

In Gro-Sure's Peat Free All-Purpose Compost with four months feed they found that sulphates were more than three times the level they should be. Sulphates are used to make stable fertilisers that will feed plants as they grow. But at high doses they increase the levels of salts and cause the compost to become too acidic, burning delicate roots and leading to crisp, lifeless plants. Which? branded the compost a Don't Buy for sowing seeds.

In J Arthur Bower's Multi-purpose Compost chloride levels were more than twice as high as they should be. Chlorides reduce nitrogen, which is vital for green, leafy growth, and this explained why the plants did not flourish.

In contrast, Vital Earth's Chelsea Mix All Purpose Compost had very high nitrogen levels. However, this is also bad, leading to lifeless plants as high fertiliser levels cause water to be drawn out of plants' feeding roots, damaging the roots and the plant.

The four bags of the Westland Multipurpose Compost tested had very low levels of fertiliser, producing spindly plants that failed to grow properly.

All of the composts tested, with the exception of Gro-Sure's and Westland's, will be reformulated this year.

Which? Gardening editor Ceri Thomas said: "Our tests show that gardeners shouldn't necessarily blame themselves if their plants don't grow as they'd hoped, given the shocking compost results we uncovered. It's impossible for anyone to tell what's in each bag as there are no labels telling people the composition."

Which? also discovered that some of the composts in the seed sowing and young plants trials have the same name but different formulations in various bag sizes.

Verve Sowing and Cutting Compost has 75% peat in the 12L size but 40% peat in the 50L bag. Verve Peat Free Multipurpose Compost has three formulations. The 12L bag contains composted bark and green waste. The 20L and 50L bags also contain coir and wood fibre while the 125L has no coir but does contain wood fibre.