All decent substrate for bonsai has water retentive properties and junipers very much like water, just not like that of bald cypress. If you included only non-water retaining substrates in your mix, you'd be watering like a mad man. But junipers adaptable, to a degree.
The size of the substrate is arguably just as much an issue as the substate itself. The larger the particle size equals less water-retention capacity and more aeration capacity.
Based off this study:
Their findings:
Organic Substrates
- Coconut Coir: 129g @ 14.57% loss
- Compost: 26g @ 31.58% loss
- Pine Bark: 22g @ 33.33% loss
- Commercial Peat Moss: 6g @ 40% loss
Inorganic Substrates
- Diatomaceous Earth: 37g @ 30.19% loss
- Akadama: 22g @ 26.67% loss
- Kanuma Pumice: 22g @ 29.03% loss
- LECA (Turface): 30g @ 36.17% loss
- Vermiculite: 28g @ 41.67% loss
- Perlite: 14g @ 36.36% loss
- Sand: 12g @ 67.56% loss
- Lava Rock (Scoria): 4g @ 76.47% loss
- Expanded Shale: 2g @ 84.62% loss
- Granite Chips: 1g @ 94.44% loss
24-hour Retention Rankings
Organic Substrates:
- Coconut Coir
- Compost
- Pine Bark
- Commercial Peat Moss
Inorganic Substrates:
- Diatomaceous Earth
- Akadama
- Kanuma Pumice
- LECA (Turface)
- Vermiculite
- Perlite
- Sand
- Lava Rock (Scoria)
- Expanded Shale
- Granite Chips
Akadama, lava rock, and pumice (2:1:1 ratio) is commonly suggested as the 'gold standard' for juniper and other bonsai.
For the sake of this study, Akadama is one of the most water retentive of the substrates next to Napa 8822 (diatomaceous earth) -- and so is pumice. With a 2:1:1 of akadama, lava rock, and pumice, with water retention at ~ 61% water retention with ~17g weight.
Another, popular, cheap method is sifting Napa 8822 or Floor Dry, mixed with half perlite and 10 percent pine bark. 1:1:0.10 ratio, equaling ~ 67% water retention with ~ 25g weight.
The amount of water retained by both are acceptable. So long as it is a fast-draining substrate that retains water for the roots to stay moist and to allow air in for roots to breathe, it's really the same. The only thing that would change is frequency in watering and esthetics.
Though, the confounding variable in this study is the particle sizes not being measured, only its weight was. Common substrate mixes are not mixed together at different ratios and then measured. Also, the measurements are taken in only one environment and for only one day out of the year -- not to mention it didn't show any bonsai being grown successfully in the substrates.
But if the particle size is increased, the water retention and air capacity are then altered. Fine particles can clog the soil, drowning or suffocating roots, while too large particles might lead to insufficient water retention and roots drying out. A balance between water retention and aeration is essential and an even finer tweaking can help with watering frequency.
Roots need to breathe, they need moisture, and the soil needs to be fast draining for both those things. There are too many mixes to go over, but pine bark and Napa 8822 don't retain too much water for a juniper to grow in. Too many people have been successful with it and not by chance. Perhaps the ratio or particle sizes are to blame if someone has had a bad experience with this mix with their junipers -- among many other confounding variables, e.g. tree health/genetics, environmental stimuli, fertilizers frequency and quantities, watering habits, human error and neglect.
Akadama, lava, and pumice has been used for some time -- tried and true -- yet more expensive.
As requested, a cost-effective alternative, a mix of perlite, pine bark, and calcined clay (Napa 8822) can be used to great effect -- also tried and true, as many have resorted to this mix over recent years.
In the end, experiment for yourself and make adjustments as needed.
They have 'soil wars' here. It's usually a hot topic.
