Question
Hi.. The leaves look strong initially but gradually they turn black and the stem of the leaf has no strength and falls on the rim of the pot and is almost cut off from the main stem. I assumed that the plant is over watered. But the soil is completely dry on the top.

Answer
This sounds like a soil problem.

Most growers of African violets use a soil less mix on one third each of peat moss, vermiculite and perlite.  The two last items makes the peat moss lighter so water has a chance to dampen the soil and roots but does not stay there.  It drains quickly.

So I would suggest either you buy perlite and vermiculite and add to your soil.

Peat moss is very dry and will not keep water until it has been moistened before use.  Before using your soil, add water and let it sit for a day or so before use.

Nancy