You can NOT use ordinary dirt in your global buckets.
Why? Wicking! Key to the success of Global Buckets is the water wicking up from the reservoir up into the upper bucket where the plant's root soak up the water. Ordinary dirt or soil will not wick. You need a growing medium that wicks. Sphagnum peat and coir are excellent wicking materials.
Here in the USA you can buy bags of pre-mixed material which work well. It's often sold as "potting mix". The main ingredients should be listed as sphagnum peat and/or coir with the remainder as some combination of vermiculite and/or perlite. The following materials do NOT wick: screened topsoil, forest biproducts, bark, forest humus, sawdust or partially composted something. If the bag lists any of the preceding materials as the dominate material then do not buy it.
We prefer to make our own growing medium. Here are our recipes:
Recipe #1: Sphagnum Peat
70% Sphagnum Peat
Recipe #2: Sphagnum Peat and Coir
35% Sphagnum Peat
Recipe #3: Coir
Note: We're still testing the 70% Coir recipe.....
Pictured below is Recipe #2.
Perlite: very white material
Vermiculite: light grey
Sphagnum Peat & Coir: dark
We're very interested in Recipe # 3 which uses Coir and eliminates Sphagnum Peat. Why?
Coir, or coir pith, is a byproduct of the coconut industry. Until recently, coir was viewed as waste. It was burned or shoveled into massive piles which sat for decades. Recently, it was discovered that coir pith is an excellent substitute for Sphagnum Peat.
Why do we care? It takes 10,000 years for Sphagnum Peat to be created in wetlands and it's rapidly being depleted. Also, coir pith is widely available in many developing countries and regions, like India and central America...an area where Global Buckets could be utilized.