Ash Horticultural Society – Compost Comparisons Meeting-27th February

At our meeting in February we took on the task of trying to compare 15 different composts which we had purchased from 3 local outlets.  We had decided to carry out this comparison because of the increasing number of compost brands and sub brands that are now available to the public. As the ‘peat-free’ revolution has taken hold so many more companies have entered this space, offering a really bewildering choice. Together with many claims about the superior performance of their product !   The commercial nursery operators are now mostly using peat reduced mixes containing mostly fine bark and wood fibre products , some with coco fibre in the mix. The providers have worked hard to produce different grades, so that now the nurseries can even get products which will fill small module trays without the bridging problem. Nurseries can order bespoke mixes which suit their own plant species being potted on, but only if they order a full truck load.  There has been a reduction in use of green waste compost in the commercial sector, as they are seen to be too variable.   Products for the peat free amateur market comprise all of these components, relying on green waste to a larger extent, together with various other components.

The purchased composts were tipped out in trays along a long table & examined for perceived quality and possible value for money.The assessors were Louise Dowle and Gary Bradbury .  We were pleased to see a big reduction in metal and plastic contamination which had been much more common 3 to 5 years ago. We concluded that there was not always a direct line between unit price and quality!  There was certainly a general trend for that , but with one or two disturbing exceptions. Our various conclusions included : That purchase should be made earlier than need & bags then opened to allow them to dry if necessary – the bags can absorb rain when stacked outside. That peat free products really do need other ingredients mixed with them to make them perform in an optimal way. Sterilized loam via John Innes products; horticultural sand and grit ; cocofibre & perlite are the main ingredients to use. For seed sowing, adding John Innes & Perlite is beneficial – the earlier the sowing in the year, the more Perlite in the mix. Then to top off the sown tray or pots with perlite or vermiculite.   Users who only need small amounts for the year could buy products with John Innes already added , but for most customers, buying them separately is much more cost effective. Peat free used in pots can need watering more often than the peat based composts . But the water will sit at the bottom , so overwatering should be avoided. The best way to judge this, is by weight of the pot after it was first watered – use that as the benchmark. One real problem with peat free composts, when used in containers or large pots for more than a few months – is shrinkage. This happens as the components continue to break down and compress. This must be anticipated – by using a higher proportion of John Innes, sand and /or grit in the mix. The extra weight also stops the pots being blown over in the next winter. We felt that the members found the session interesting and useful , helping to shed light on a really complex subject !