Nowadays when building a home, all the top soil is removed and a few inches of soil is added again once everything is finished. This makes for a very nutrient poor soil.
Now how can you add good soil in an organic way?
Good fertile soil is created by decaying biological material. Most people think of animal dropping and dead plants/animals. And rightly so. But more important are all the roots of dead plants. Most plants take far more space beneath ground than above it. And when the plant is eaten the roots often remain – and die off. When the roots decay, they leave behind space in the soil that act as vessels for water and nutrients.
So to add fertile soil we want to simulate this process. To do this you have to add the rights plants to your garden and make them grow fast. I suggest you start with a solid grass that has deep roots. Permuga grass is famous for his deep roots – they can become 8 to 12 feet(!) The best type of grass or you is very dependent on the area you live in though (and the climate you have there).
You can also add clovers to the grass, they can grow long roots as well. And it helps the grass grow by decreasing the nitrogen level of the soil. Letting the grass grow a bit longer helps make the roots get longer.
Ideal would be to have an animal graze on the lawn now, but for most of you that is probably not an enticing option. Mawing the lawn will do the job of killing some of the grass (and clover) while the roots remain in the soil.
This might sound like this will only get you results with this in tweny plus years, but you will be surprised what can happen in one or two seasons. If done well, the ground will actually elevate visibly by a few centimeters per year. And that is all extremely fertile soil for years to come!