Spring Seeding Guide - Soil
Soil mixes largely determine the health and staying power of starts. A mediocre planting mix creates struggle in your life – slower, smaller plants more susceptible to disease and predators, less able to ‘hold’ in pots, more susceptible to stress, requiring far more in the way of hand-holding such as additional feeding. A bad soil mix will stop you in your tracks – it is not unheard of for nurserypeople faced with evidence of a ‘bum batch’ of soil to throw all the plants out and begin again, many weeks into a seeding program. Boy, does that hurt.
Some gardening texts recommend germinating seedlings in sterile mediums such as perlite, vermiculite, sphagnum moss, sand or combinations thereof, which are devoid of nutrients necessary to support growth once the small seedling stage is passed. Instead, the plants are fed with liquid fertilizers (essentially hydroponically) until they are transplanted to other, richer mediums. Most gardeners avoid this approach and seed directly into a soil mix which contains nutrients sufficient to sustain growth without additional feeding.
Recipes for making soil mixes that happily support a wide variety of plants are plentiful, but do require a modicum of experience to fine-tune, not so much in terms of ensuring the right balance of nutrients is present (follow a formula) but rather in ensuring that the mixes are neither too light, nor too heavy – a critical factor in managing watering, particularly in the early weeks of the spring seeding season.
While storebought mixes provide a good basis for starting seedlings, they tend toward the nutrient-light and lack the ‘body’ to carry plants robustly past their initial phase. By all means use them, but consider adding amendments.
Read labels carefully on potting soils. Many of them were not designed for starting seed.
Most storebought mixes make a habit of proudly claiming that they are sterilized (to reduce the likelihood of disease). How surreal. Organic gardeners planting in sterilized soils? Adding healthy compost to your mix makes it more active, giving it and your plants greater life. Yes, your plants will require more management initially – the potential for unwanted ‘germs’ comes in with the wanted ones but, all told, you will raise plants that are happier and healthier.
Wormcastings are a highly concentrated form of goodness. Add them if you can. Fishmeal for nitrogen helps. Whatever amendments you add, do be sure to readjust the tilth of your mix by, for example, adding vermiculite and/or perlite to account for the increase in density. Too-dense mixes, watered, will suffocate your plants.
If you are using heat mats to germinate and grow up your seedlings, consider a sterilized medium until the seedlings are established and you can pot them up to an active mix. In non-sterile mixes the additional heat can encourage damping off although, admittedly, this has been only a very minor issue for me in the years I have been using bottom-heat for early-seeded heat-lovers. Sometimes I will ‘go light’ on compost in a mix I will be using on heat-mats.
By Nick Routledge for the Observer Allotment Blog