Maple seedlings and soil mediums. (see pictures below)

Last spring I planted 24 newly sprouted green leaf Japanese maple seedlings (2-3cm). What was special about this planting is that I used 4 different growth mediums.
I used pure pine bark, pure chicken grit, pure red lava pebbles and pure akadema. I planted 3 seedlings to a three inch pot and used two pots of each medium to increase my sample size, meaning I decrease mistaking random effects for experimental results.
All the plants were placed in the same flat and received the same care all year; except that the watering requirements of each medium was different. The plants were individually watered with extra tap water accordingly. The plants were placed in a location sunny for half the day in spring; in summer the plants were moved to a shady location.

Chicken Grit. Survival - 0/6
The plants in the chicken grit died before the end of spring, via drying out. I was watering them once or twice a day, depending on my availability. It wasn’t enough.
Pine Bark. Survival - 5/6
The plants in pine bark hardly grew at all (all sized 4cm). They only needed watering twice, the whole year. I felt like feeding these plants with a liquid feed may have been drowning them. The root system of these plants wasn’t bad, but they were more stringy then bushy.
Akadema. Survival - 4/6
Some of these plants grew well, others didn’t (total sizes 3, 7, 15cm). In one pot, two seedlings died. The surviving seedling, in a pot by itself, was the smallest of the lot. In the other pot, one tree grew quite well, the other two grew only slightly better then the pine bark group. One thing to note is the leaves on all these trees was very rich red/green color, which suggested vitality to me. The roots on these plants were very robust and bushy, as yearling roots go.
Red Lava Pebbles. Survival - 5/6
Wow. All the plants of this group grew at least as well as the akadema group (total sizes 10-22cm). The roots are robust and bushy, the plants are tall.
Oddly, the maples from the akedama and lava pebbles groups still have their leaves from last year. It is odd because it is February now and we have been through two snow storms. I have no idea what leaf longevity indicates.

The seedlings that I drew the 24 from for the experiment from were growing in club mix bonsai soil. These seedlings were not in the same size pots, nor the same plant density, nor the same treatment conditions. However, one of these seedlings was as tall as the lava rock seedlings (total size 20cm), but most were the size of the middle sized akadema seedlings (total size 7cm).
I have repotted the 2 best growing trees of each of the three surviving soil mediums and planted them into 1 litre pots of the same soil mix they were in previously. I am interested in knowing what effect the soil medium will have on branch formation, root formation, girth size, and susceptibility/resistance to weather or disease. The rest of the seedlings I repotted in lava pebbles.
I have also begun another growth medium experiment with 8 mediums; lava pebbles; pine bark; akadema; chicken grit; sand; zeolite; club mix soil; and potting soil. These are set up the same way as last years experiment, with 3 seedlings, two pots, and identical treatment to all the plants. I will be careful to change the placement of the pots in the flat, in order that phototropism - light intensity affecting growth - does not skew my results.