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Enriching your soil with compost and tilling it in is a good way to prep the soil for the transplant. Make sure to put down garden bed liner anywhere there will not be a plant or tree. This can help with weeds, but unfortunately, to have a weed free area will require both chemical means and diligence with manual labor. It is quite difficult to prevent all weeds in an area without both.

This article will help you in preparing the soil: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm

I usually see recommendations of topsoil mixed with compost or aged manure. Then about 3 inches of mulch on top.

Our research recommends a mix to suit what you will be planting:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-soil-raised-garden-beds.htm

You will take 8 meters and turn it into centimeters. That will be 800. Multiply that by 400 (4 meters) and multiply that by 4 centimeters and you get 1280000 cm cubed. This is 1280 Liters. divide that by 30 and that will give you just under 43 bags.

This article answers your questions:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

I would not store it in bags at all! The longer it sits in bag the more opportunity for infection there is.

In fact, a good way to re-use spent soil is to compost it! Just add it to your compost pile, or composter. This way, it has proper aeration and can be turned as it needs to be.

Re-using soil in this manner, becoming part of your compost, will leave you with a better product after composting.

Here are some articles that will help you to make your old soil usable again:

https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm

https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm