An assessment of the suitability of home produced green compost as a potting soil

RHS project team
Dr Paul D Alexander
Start date
31/01/2004 12:03:43
End date
31/01/2005 12:03:48
Composting, compost, potting mix, growing media.
The problem

Current scientific knowledge of compost used as a potting soil is largely derived from research into using commercially produced green compost. This has shown that such compost tends to have a high pH, high conductivity, and potential for organic and / or mineral pollutants. It can therefore be problematic for use in growing media at high inclusion rates. Little work has been undertaken at an amateur composting scale and it is unclear whether these problems are prevalent in amateur-produced compost.


Six types of compost were produced in wooden, plastic and open vessels, each type having a turned and unturned treatment. All of the mixes were based on green waste collected at RHS Garden Wisley.

The six experimental composts were all used both undiluted and diluted with sterilised loam at two different ratios. Loam was chosen in order to offer some buffering capacity to the expected high conductivity levels. The final replicate was a pure loam mix (acting as a control treatment).

Tomato ‘Moneymaker’ and lettuce (Cos) ‘Lobjoit’s Green Cos’ were sown directly into the mixes to enable seed germination as well as plant growth to be assessed.

Summary of results

When comparing the effects of the compost mixes, the open vessels exhibited significantly higher weed germination counts than the wooden and plastic vessels. Turning the compost also significantly increased the weed count in the open vessels.

In terms of seed germination, significantly lower counts were initially determined in the undiluted mixes when compared to the diluted mixes. By the end of the assessment period, these differences were reduced. By final harvest, tomato heights and biomass were significantly greater in the undiluted mixes when compared to the diluted mixes and were comparable to the control mixes. Backyard produced compost would appear to have some value as a potting soil but the method of production needs to be taken into account when considering its use.

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