A recent question I had was what compost to use for seedlings. I know it is a bit early to think about sowing seedlings and transplanting them but it won’t be long. This person had used John Innes No 1 but found ‘slow growth and not very healthy’. They were worried about multipurpose compost being ‘too strong’ and wondered if there was a good fertiliser for seedlings.
To deal with John Innes compost first (I apologise for those not in the UK and Ireland who may not be aware of what this is). John Innes composts were developed in the 1930s to standardise composts. This advantage was then threatened when peat composts were developed which were also consistent and reliable. But we now garden in a time when peat is being phased out and there are so many different composts, all with different management requirements and delivering different results. John Innes composts range from ‘seed sowing’ to No 3, all basically the same but with different amounts of nutrients. The problem with them is that they are based on loam (topsoil) and because this varies from sandy to clay, the compost itself varies. These composts also usually contain some peat.
I have rarely bought a bag of John Innes compost recently that was anything like suitable for seedlings. It is usually wet and too heavy. But I do mix John Innes with multipurpose compost to ‘enrich’ the multipurpose or ‘open up’ the John Innes.
So, although seedsowing John Innes is fine for perennials and possibly beans and brassicas, I would not use it for most seeds.
For seeds and seedlings I use multipurpose compost. Because this varies a lot, I sometimes add perlite to improve drainage. For transplanting seedlings I also use multipurpose, although I mix in about a third of John Innes for seedlings that are slow to grow and will be in the trays for a long time (such as antirrhinums) because John Innes is richer in nutrients and retains nutrients longer.
The fertiliser levels in most multipurpose composts are low and will not damage young plants or prevent seed germination. There are some composts that boast higher fertiliser levels and as being suitable for tubs and baskets and these might need more careful use if you are worried about high nutrient levels.
Which brings us to fertilisers for seedlings. Almost all liquid fertilisers are suitable but because the multipurpose compost should have sufficient nutrients for growth for a month, I would not feed for a month after transplanting, so the plants can grow an effective root system. And then you can use your favourite fertiliser at half strength a few times till the plants are looking strong and then apply at full strength. I tend to alternate between fertilisers and include one that is based on, or contains, seaweed extract, but that is just me.