Try a tonic for your soil
ONCE upon a time — or at least until the 1930s — the making of a successful plant compost was a secret held by the head gardener. Soil on its own is not very suitable for potting because it tends to cake in the pot, blocking water and air filtering down to the roots. Also, soil contains root-rotting fungi which is especially damaging to seedlings. Old-time gardeners used a range of materials to add to soil, such as well-rotted manure, leaf mould, rotted straw and garden compost. But there was great variation, hit-or-miss results and occasional crop failure.
These days, there are plenty of ready-touse versions available. But the choice can be bewildering. Multi-purpose, peat-free, John Innes added — these are some of the terms that describe composts used for raising seedlings and growing plants. This kind of compost comes in bags from the garden centre and it’s not garden compost, which is made by composting dead leaves and plant waste.
In the 1930s, in an effort to standardise growing composts, researchers at the John Innes Institute in England devised a series of standardised formulae: John Innes seed compost and John Innes potting composts No.1, No.2 and No.3. The seed compost has 2 parts sterilised loam, 1 part peat and 1 part sand, a nice open compost. The potting compost has the same contents but in the ratio 7:3:2. The composts have varied, increasing amounts of fertiliser, such as sulphate of potash, added per bucket. These composts were a great success because they achieved, for the first time, a fair degree of predictability. John Innes composts are still available. By the way, John Innes was an English philanthropist who funded the Horticultural Institute for research.
But loam, a good balanced natural topsoil, is subject to a wide degree of variability. To address this, the University of California carried out research that led to the UC compost formulae. These were based completely on peat, a more predictable substrate with good structure, and it is practically sterile. Modern, peat-based composts were based on this research. These included seed composts with fine peat, and coarser potting compost. To avoid having to buy two different bags, manufacturers came up with multi-purpose compost, good for seed sowing, potting, growing-on and for rooting cuttings.
The debate about peat usage, and the loss of bogland, brought consumer pressure to reduce peat, or remove it, and replace it with coconut fibre, or coir, green composted plant waste, or wood fibre. There are some composts that are peat-free, others are peat-reduced, using other organic waste materials. But the old problems of variability are still a challenge. Composted green waste can harbour plant pathogens.
‘‘Added John Innes’’ is something of a conundrum. Adding anything to the John Innes formula instantly changes it and it is no longer John Innes compost. However, the original formulae are not regulated, or patented. Adding some John Innes effectively means adding some loam, or soil of some sort, sand and peat. There is benefit in that because soil is a better chemical buffer than organic materials, releasing nutrients as the plants need them. The sand portion is beneficial in opening up the compost and lending some weight to a pot filled with compost.
So which compost to use? Because of root-rot pathogens, use a seed compost for seed-sowing. For all kinds of potting, use unsterilised good garden soil at 50pc and 50pc home-made garden compost or leafmould, sieved. Established plants resist pathogens quite well, and there is the bonus that unsterilised soil carries parasites that kill vine weevil grubs. If you have no garden soil or home-made compost available to you, or simply for convenience, use multi-purpose, peat-free, peat-reduced, or with John Innes added, according to your choice.