Best way of Planting seedlings?

Hi , When planting a seedling, I first dig a hole. The topsoil at my site is only about less than 3 cm deep, while the seedling's root system is approximately 8 cm long. In this situation, would it be beneficial to place the soil from the bottom of the hole on the surface and the topsoil at the bottom so that the roots are initially surrounded by the more fertile soil? Or is it still better to keep the soil layers in their original order? What are the scientific reasons behind the recommended approach?
Im in the amazon jungle.

Cheers

R.

Keep the layers in order honestly. The topsoil has microbial life and organic matter that feeds the roots as they grow out, and flipping it buries that activity where it can go anaerobic. With only 3cm of topsoil though, I would focus on building it up over time rather than rearranging what little you have. Mulch around your seedlings after planting and let the worms do the mixing for you. Over a season or two the topsoil layer gets noticeably deeper on its own.

Usually I try not to damage the root of the seedling as much as possible and plant them going straight down, and spread out as much as possible. It will be inevitable that the soil will mix slightly when you do this. Different plant roots have different purposes. For example I've heard that tomato roots at the surface are mainly feeder roots - they mostly absorb nutrients, wheras the deeper roots mainly absorb water. So mulching round a tomato will do more good than putting compost deep in the hole. Saying that, organic material also can act as a sponge so maybe beneficial at deeper levels...

It probably also is climate and soil dependant. If your soil is clay and you generally have planty of rain the answer might be different to someone with sandy soil and an arid environment.

Nature doesn't transplant, but they might mix horizons (pigs rooting) or mulch on top.

Ronaldo, if you can make or buy biochar ( https://permies.com/f/190/biochar ), I would be tempted to mix a bit of that into the soil you're putting back into the hole. It might help lighten the soil and support any microbes that find their way deeper.