Seed compost is different from regular all purpose compost and having a blend that is right for seeds is ideal for getting them off to a good start. A seed has everything that it needs to get going already available to it so the one of the things that it doesn’t need is more nutrition. Because of this, as good seed compost has a low nutrient content, it also needs to hold moisture as a seed drying out will stop it germinating or worse if it’s already started will kill it. There is a lot more to seeds than I’m wanting to put here but suffice to say if you give a viable seed the right environment and the right conditions then it will start to grow. What happens after that is another thing altogether but there will be more to follow in that subject later.
Here is the basic composition that I’m using this year, it’s not complicated but there is some logic to making it this way. In reality once the seedlings have germinated they don’t spend long in here as they’re pricked out into modules or pots quite quickly. As I said before, it’s a low nutrient mix that is there as a medium for the seed and seedlings to get moisture and something to get their roots established.
Why is seed compost a low nutrient mix?
Seedlings don’t want to put on too much vegetative growth at the beginning which can make them weaker and more prone to problems like damping off. What they really want is something that is fairly water retentive to get their roots into and allow them to get some initial leaves up and start photosynthesising. They also are robust enough to withstand some minerals in rich composts so the magic ingredient here is the leaf mould. Traditionally peat has been used for the purpose of holding moisture in the seed compost, this is a really bad environmental issue and should’t be used at all. If you buy composts make sure that they are peat free.
What would I change?
The addition of vermiculite has been a cause of concern for me though, the leaf mould should retain enough water although this does help more. Last year I bought a large bag of it on a whim and I am slowly working my way though it. Vermiculite is a naturally occurring mineral that is mined throughout the world and some needs to be transported a long way, although it’s very light this is one concern. Back in the 1980’s there were also issues with the vermiculite getting contaminated with asbestos, as it was used prolifically in home insulation this became a huge problem. Fortunately this is no longer the case and you can be certain that the vermiculite you buy commercially is clean.
The other is that it requires exfoliation, this is quite a hight energy process or at least it was. A team from The University of Nottingham in 2009 won an environmental award for devising a microwave exfoliation process for vermiculite that hugely reduced this problem. But with anything that if pulled from the ground, processed and shipped elsewhere there is going to be an impact. Horticultural grit for example is heavy to transport and again uses a lot of energy to mine and process, it really is a minefield and one that needs consideration when walking through.
This is the finished product, it’s really light and actually quite soft to the touch. It holds water well especially on my heated propagator and it’s easy for the seedlings to get their roots established.
If you don’t make your own then make sure you get a decent peat free one from elsewhere, somewhere like these guys who are make some really great products.
Let me know if you have a special mix?