Namaskaram!
A unique aspect of the Miyawaki method is that we don’t fetch saplings from a nursery and plant them straightaway. Instead, we grow them for two to two-and-a-half months. Doing so helps develop their root system. The reason is that the soil in which most of the nursery-grown saplings are held is like a block. If we fail to water the plants for two consecutive days, the soil becomes so hard that we have to literally break it. There is no point in faulting the plant nursery staff. When they have to plant and sell thousands of saplings a day, they can only do it with whatever soil is available.
Upon receiving the saplings, we have to make a potting mixture of cocopeat, cow-dung, rice husk and soil, taken in equal proportions. Cocopeat retains moisture in the soil. Rice husk prevents clump formation. Husk can be replaced with wood shavings from timber mills or furniture factories. Cow-dung is the fertilizer, and soil is the base material. A mixture of these ingredients will form a loose medium that will not harden. If watering gets delayed, the mix will only crumble and the chances of it turning hard are less.
Saplings from nurseries should be re-potted into this mixture after discarding the soil sticking to the roots. Or else, this soil will stay there as a lump, liquefying when watered and hardening otherwise. Grow bags can also be used. But their recycle value is less, and they are difficult to reuse. The plants might also get destroyed while being taken out of the bags. If pots are used, saplings can be quickly taken out without damage.
I recommend plastic pots, discarded plastic buckets or other such containers due to a number of reasons. Though plastic is best avoided, plastic containers can be reused multiple times. The next point is the cost component. Plastic pots are cheap. The ease of handling is the next positive factor. It is easy to turn the conical pots upside down and tap the plants out of them. If it is a matter of a plant or two, earthen pots can be used. But practical difficulties arise when thousands of saplings have to be potted and taken out. They will result in excess expenditure as well.
Next is the question of how to take the plants out of their pots after they have grown sufficiently. Many people pour water into the pots but it is not the right approach. When watered, the soil gets detached from the roots. Long back, little children used to play making mud pies. While making mud pies, soil is pressed into coconut shells and turned upside down. The soil comes out taking the shape of the shell. For such a thing not to happen with potted plants, remember not to water them on the previous day. To detach the plants, the pots should be held upside down and gently tapped on their base. The plants will come out easily, and are ready to be inserted into pits. In this way, the roots can be saved from damage. Also, the location change will not affect the plants strongly.
There is another bit of advice that Miyawaki gives about the care to be taken when saplings are taken to a different climate. Avoid shifting young plants growing in the shade to a sunny place all of a sudden. Instead, keep them in the pots, shift them to the new place and let them remain there for two weeks to permit acclimatization with the changed environment. According to Miyawaki, it is a good practice to replant the saplings after they get accustomed to the changes.