When do I start dosing fertilizer into a newly setup planted tank?

The short answer is that you should start dosing water column fertiliser as soon as you have plants in the new tank and no later.

But why? Doesn't my aquasoil release nutrients in the first few weeks?

Some people think that because they have a rich aquasoil tank, they can skip adding fertiliser for a while because the new soil will leach nutrients in the beginning. Many aquasoils are enriched with ammonia, which acts as a long-term source of nitrogen to feed rooted plants. However, most aquasoils don't leach all the required nutrients at a sufficient rate. Many aquasoils can leach ammonia/nitrates but may lack sufficient potassium/magnesium. If you do not dose from the start, you are betting that your tap water will have low levels of the nutrients needed for newly planted tanks to get through the adaptation period. Plants need a full range of nutrients to grow optimally; it is not helpful to have an excess of N due to ammonia from the soil, but a deficiency of potassium or magnesium.

Water column feeding of non-rooted plants

New plants do not have an established root system, which limits their ability to take up nutrients from the soil. However, aquatic plants can also take up nutrients through their leaves, so the presence of nutrients in the water column gives them easy access to nutrients before their root zone is developed. When plants are adapting to a new environment, it is important to ease the transition stress. Providing them with plenty of CO2 and adequate nutrients is an important step in preventing melting and speeding up adaptation to the new environment.