Please explain if both pots have drainage holes? What really are the advantages of the below ground method especially in the tropics. I understand they would be more stable but above ground pots can be staked off & trees acting as wind breakers planted around the area. This method to my mind doesn't prevent rootboundness it allows dwarfing of the tree by containerising the football as does above container planting. Again irrigation can be applied via drip irrigation similarly effectively & efficiently mulching to minimize . waterloss . Hmmm it's to me kinda overkill but maybe I am missing something.
usda zone 5b grower here. Main reason for pot in pot growing system is reducing season extremes. Most of commercially available plastic containers are black so in hot summer they might warm up to 55-60C which usually decimates roots on south facing side. In winter, it protects roots from freezing, so no overwintering structures are necessary which is quite time saving to be honest. And a nice bonus is plants staying upright and not getting blown over.
Soilless is industry speak for "potting mix"...every grower uses a different "recipe" (either commercial or made on-site)....it usually is a base mix of bark mulch or peat moss, sand, perlite and then some slow-release fertilizer pellets. The mix I make for use in my small nursery at home is hardwood bark mulch, digested manure solids, finished compost, and some osmocote and amendments; sulfur (to counteract the manure solids pH), gypsum, iron, blood meal, bone meal.
A lot of growers use a copper spray-lined pot that kills the sensitive root tips as they reach the pot, and some are switching to root bags or other air pruning pots.
