Is standard bonsai soil OK for seedlings?

[QuantumSparky]

I have a few seedlings I've collected as practice material and I've potted them up in my standard organic mix (40:40:20 of pumice, clay, pine bark fines). It's been about 2 months and although the little guys look healthy, I've noticed zero growth. Perhaps this is normal but it made me think about my other trees which I've repotted - None of them have put on any growth despite looking completely healthy in that soil. On the other side, my nursery trees which are still in the original soil have all put on at least a small bit of growth. Most of my trees are on solid fertilizer at this point.

Is that standard type of organic bonsai mix OK for young trees that I'm trying to grow out? I can't find a good answer online anywhere. I can see the benefit to using that mix with established trees but my brain tells me that there must be a better alternative for beefing up young trees. Am I wrong? Aside from fertilizer, standard bonsai soil seems to be lacking in the nutrients I would expect young trees to require.

Ultimately, I'm trying to figure out if I should just continue to use my BonsaiJack organic mix for all new trees, or try to source some bulk pine bark fines and perlite (seems to be a popular mix for growing trees out) and use that instead. I bought about 7 gallons of organic mix for maybe 60 bucks, I feel like bulk components would give me a lot more soil to work with at a much lower cost.

 

[Bonsai Nut]

If the tree looks healthy, that should be your primary consideration.

If you dig up seedlings in June after their spring growth has hardened, there is a chance they may not push new growth that year, or that they are growing, but all the growth is happening beneath the surface of the soil as the tree replaces damaged roots. A lot depends on the species. Strong roots = strong foliage growth, weak roots = weak or no foliage growth... all other things considered equal.

Remember the job of soil = hold water for root uptake, allow oxygen exchange, provide structural anchoring and good root ramification, allow nutrient uptake. Depending on your tree species, where you are growing it (bonsai pot, versus nursery pot, versus flat, versus pond basket), piece size, environment, etc, you may get different results with different soils. Inorganic bonsai soil has a good reputation because it works so well in so many situations and is hard to beat, and it doesn't break down or change performance (much) over time. In almost all cases, an organic soil mix starts to decompose and compact from the moment you start to use it. Might not be a problem in the short term with a pond basket, or if you plan on repotting in a year, but if you use it in a bonsai pot for a couple of years you may have serious problems.

I will freely admit I use a 50%/50% blend of inorganic/organic in nursery pots or grow flats where I am just growing stuff as fast as I can. The organic particles I use are pine bark mini-nuggets, and I use expanded shale for the inorganic (based on where I live). However for anything in a bonsai pot I use inorganic bonsai soil - pumice, lava, akadama - even though it costs me a mint.

 

[Shibui]

Standard bonsai soil is fine for seedlings and fine for grow pots.
For deeper and larger pots the mix characteristics become less important and trees will grow in a wide range of soil mixtures. The shallower the pot the more critical the mix becomes.

Make sure you feed plants well for good growth.
if you have mixed your own brew there can be some deficiencies. Iron deficiency is common in soilless potting mixes and commercial mixes usually have slow release iron added to counteract that deficiency. Pine bark based mixes can also draw down nitrogen so it may be necessary to up fert rates until the mix stabilizes.
Also highly likely growth will be slow in seedlings transplanted during growing season as outlined by @Bonsai Nut above.
Definitely keep looking for answers but sometimes we just have to wait for the plants to do things in their own time.