Why did the tips of the leaves of the plant turn black, precisely at the tip of each finger, and gradually began from the pointed tip of the leaf as if it were dying
Hmmm... don't even want to give us a photo let alone a diary? That would be of enormous help to us as we try to help you... but what you're describing is most likely nute burn... can't give you any advice beyond that as we don't have any information on what you're giving them in what amount...
Please start a diary and come back to ask you question... you'll not only get advice as to what the problem is but suggestions as to what to do about it.
Good luck.