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MENDIP TIMES
The compost conundrum
IF, like me, your gardening efforts this year have been thwarted by disappointing results from bags of socalled potting With MARY compost then let me PAYNE MBE try to unravel the dilemma. Firstly, the word compost is applied to two totally different materials, thus instantly causing confusion. Compost for growing plants in pots or raising seedlings is a very different substance from the compost you can make from your own garden waste, but let’s start with potting composts. In the 1940s the John Innes Institute in Norwich researched the need for a stable, uniform material both for amateur and commercial growing. This resulted in the familiar John Innes Composts, J.1. no. 1, no. 2, no. 3, J.I. seed compost and J.I. ericaceous compost for acid loving plants. The numbers 1, 2 and 3 refer to the strength of fertiliser added to a basic mix of seven parts by volume of loam, three parts by volume moss peat and two parts by volume coarse grit. To this, lime was added to balance the acidity. John Innes composts are still around, but rarely resembling the original specification as the loam used is a variable product. Loam is the ideal proportions of sand, silt and clay particles for the perfect gardener’s soil, something not many of us have! It would appear that any topsoil will do these days. My ideal potting compost for many years was to mix 50% J.I. no.2 with 50% of a peat-based compost. The peat content of J.I. composts was specified as sphagnum moss peat and this also is now the crux of the problem. Peat itself is variable depending on what plant species it comes from. The most desirable is partially decayed sphagnum moss from waterlogged bogs, where the lack of oxygen prevents total decay. It is a uniform, sterile medium (once used in WW1 for dressing wounds) and ideal for growing. The other sort is sedge peat, from decayed sedges, which is much further decayed and less desirable as a growing media. (Found on the Somerset Levels). In the 1960s moss peat became PAGE 62 • MENDIP TIMES • OCTOBER 2021
virtually a universal growing media for commercial growers, valued for its uniformity and moisture holding capacity, but it lacks nutrients. This is easily solved by the addition of the major and trace elements required by plants as fertiliser. Peat-based composts soon appeared for amateurs e.g., Levington. We are now on a mission to drastically reduce the extraction of peat for composts although a significant amount is still burnt for heating! Peat bogs the world over are a highly desirable carbon sink and are being rewetted and re-seeded with sphagnum moss. It is essentially a renewable resource although painfully slow. The clamour to find a substitute for moss peat has led to the introduction of a variety of different materials with variable results. Composted Green Waste (CGW), from your green recycle bin, is an obvious replacement until we discover it is an infinitely variable product. I have experienced classic hormone weedkiller damage on tomato plants and nutrient deficiencies, to say nothing of the weed seedlings that have appeared from composts containing CGW and I now avoid it. Judging by comments I have heard; you too have had similar problems. Another debatable replacement for peat has been coir fibre (the husks of coconuts) imported from afar, including Sri Lanka. Is this a sound environmental practice? Composted bark is also used, and certainly finds more favour with me, although it too is variable depending on the tree species used. Wood fibre is also used, and often results in a crop of toadstools in your pots as it breaks down, but at least it is a useful product from waste wood offcuts. Certainly, the by-products of the timber industry are of great value in the garden e.g. wood chip for mulching, to retain moisture and control weed germination, and composted bark as a soil improver are popular. Melcourt Industries deal in a number of these products and produce a peat-free, composted green waste-free potting compost, SylvaGrow, but it does contain some coir. Extra slow-release fertiliser can be added when potting to feed your plants for the whole growing season. My ideal mix mentioned earlier of
Compost heap
50:50 J.I.2 and peat-based multi-purpose compost is no longer valid as the J.I. is so variable and my last bag resembled a bag of dark concrete, refusing point blank to leave the sack. This leaves us with the question. Can we use the compost we make in our compost heaps as a potting compost? Frankly, it is not a good idea, as the resulting compost may well contain disease spores, not killed by cool composting, and a huge amount of weed seeds. Well-made compost can be added for use with more mature plants, but I would certainly not encourage it for seed sowing or young plant production. Home-made compost is invaluable as a soil improver. If you are currently cursing the leaves that fall from the trees above your garden, then take heart. If you compost these in a separate compound, you will create magical stuff called “leaf mould”. Collect the leaves by mowing and the leaves will be chopped up and the grass clippings will add valuable nitrogen to speed the decay process and in 12-24 months you will have dark brown, crumbly, luscious leaf mould. This can be used as a soil improver or mulch or to bulk out a proprietary potting compost for pots and baskets, but it will need slow-release fertiliser adding to supply the essential nutrients required. Can you re-use old potting compost? I use it in pots for spring bulbs and as a soil improver, but never for potting seedlings or young plants or for another crop of tomatoes. But, beware of the dreaded vine weevil larvae.
For more details go to: www.queensgreencanopy.org
