Bags of old compost are still being sold in garden centres and DIY stores, with no indication of the date of production on the packaging, according to a report in the latest edition of Which? Gardening, the Consumers'
The report follows up findings from last June that old stock was on sale, which manufacturers said was down to the previous year's poor summer.
Compost doesn't normally have a sell-by or use-by date on it, so consumers may have trouble distinguishing old stock from new.
Which? Gardening claims that the nutrients in compost can degrade over time, so using 'elderly muck' could prevent plants from thriving.
Guy Barter, chief horticultural adviser with the RHS, said that certainly organic potting compost deteriorates with time.
"Any organic fertilisers incorporated in the compost will break down over time and the soluble nutrients will become too salty and strong which could damage the roots of young plants.
"Potting composts have been formulated to have lots of air in them because roots need to breathe, just like humans.
"As the compost ages, the air space declines and roots find it difficult to grow because of lack of air in the compost."
He says non-organic composts are less likely to be affected by age because the nutrients will almost certainly be in the form of chemicals, which aren't broken down so quickly.
Which? Gardening wants manufacturers to make it easier for consumers to see when the compost they buy was made - for example by printing a production date on packs.
When it suggested this to the industry body Growing Media Association (GMA), it said: "GMA members believe they manage stock well.
"If consumers are dissatisfied with the quality of their compost, they should take it up with the retailer."
When you open the bag, the contents should feel fluffy and fibrous, not wet and heavy. Compost may seem hard because it's been stacked on a pallet but it should fluff up pretty easily.
However, it is possible to use old compost as an effective soil improver when winter digging or just to use as a mulch to conserve moisture and suppress weeds, says Barter, so you could bag a bargain if you see any old stock in the sale.