Such pourous screening also allows for needed air circulation as well. Once your seedlings have grown to about a half inch to an inch you might want to consider transplanting them to a new container with fresh soil. Let the soil first dry out a little. It is best that the soil is not fully wet, but also not fully dry, at which point it might harden up and encase the roots, causing the young rootlets to tear off when removing from the soil. With most clumping desert cacti, such as Ariocarpus and Lophophora, a deep container that does not extend more that an inch from the cacti is recommended.
Many species have their best growth in a relatively rootbound environment.
Once the seedlings are transplanted to a new container and are healthy and on thier way to maturity it is time to start testing out their natural sunlight capabilies. As with all cacti a sudden move from shade or partial sun to full sun is not recommended. Such a sudden shift in environments can cause the plant to get unsightly sunburn marks. It is best to slowly move them into more sunlight over the period of a few weeks, taking care to watch for browning over. Immediately move the plant back into more shade if this occurs.