Which? reveals why worst performing composts did so badly

We asked a specialist lab to undertake analysis of some of our worst-performing composts from our latest compost for seed sowing and young plants trials. We looked at four bags per compost.

> In Gro-Sure’s Peat Free All-Purpose Compost with 4 months feed, we found sulphates were more than three times the level they should be (1,400mg/l compared to 400mg/l) in each of the four bags we tested. 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. We labelled this compost a Don’t Buy for sowing seeds.

> In J Arthur Bower’s Multi-purpose Compost we found the 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 our plants didn’t 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 we tested had very low levels of fertiliser producing spindly plants that failed to grow as we’d expected.

All of the composts above, with the exception of Gro-Sure’s and Westland’s, will be reformulated next 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? members can check out our test results at www.which.co.uk/home-and-garden/garden/reviews-ns/compost/.

We also discovered some of the composts in our seed sowing and young plants trials have the same name but different formulations in various bag sizes. We found:

> 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 and the 125L is different again, this time with no coir but does contain wood fibre.

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