Why was cement put in the hole in the tree in To Kill A Mockingbird?

Nathan Radley filled the knothole in the tree. He claimed
it was dying. He is trying to save it. That is what Mr. Radley tells Scout and Jem in
their conversation with Mr. Radley about why he filled the knothole in the
tree.

Scout and Jem had a type of communication with Boo
Radley because of the knothole. Boo would fill the knothole with items that created an
interest for Scout and Jem. In return, they put thank you notes in the knothole. Now,
Scout and Jem feel a disconnection with Boo for he cannot fill the hole with special
items.

Scout cries about the knothole being filled. Atticus
thought the tree looked healthy.

Could it be that Mr.
Bradley is trying to stop Scout and Jem from communicating with Boo Radley? Mr. Radley
could be worried about the relationship that is developing between Scout, Jem and Boo.
He feels he has to put a stop to it.