It was filled with concrete by Nathan Radley
I think it was Mr. Radley who blocked the hole of the tree with cements, telling Scout that the tree is dying.
Nathan Radley cemented up the hole so that Boo could not leave any more surprises.Nathan Radley, (Boo) Arthur Radley's older brother, sealed up the tree hole using cement, preventing Boo from putting anymore presents in the hole for the Finch children, hence there would be no more surprises.
Mr. Radley filled the tree with cement, therefore nothing could go into the hole (because it wasn't there anymore).
Nathan Radley tells the children he sealed the knothole with cement in order to prevent Boo Radley from communicating with them or giving them gifts. He likely did this to further isolate Boo and maintain control over his actions.
Mr. Nathan Radley filled the tree trunk with cement.
Boo Radley leaves the presents for Scout and Jem in the knothole of the tree near the Radley house.
Nathan Radley, Boo Radley's brother filled the knothole with cement. When Jem and Scout asked him why he was doing this he told them that his tree was dying and filling the knothole with cement would keep it from dying. Scout and Jem went and asked Atticus if he thought the tree looked like it was dying and he said it looked perfectly healthy.
Mr. Nathan Radley told the kids that the tree was dying because he wanted to stop Boo Radley from contacting the outside world.
Nathan Radley fills the hole in the tree where gifts are placed with cement to stop Boo Radley from communicating with Scout and Jem.
Nathan Radley fills the hole in the tree where gifts are placed with cement to keep Boo from communicating with the children.
Nathan Radley is likely trying to prevent Boo Radley from making contact with the outside world through the tree. By saying that the tree is sick and needs to be cut down, he can keep Boo isolated and prevent him from communicating with Jem and Scout. Nathan's actions contribute to Boo Radley's continued seclusion and mystery in the neighborhood.
Boo Radley was the one who was leaving gifts for Jem and Scout in the knot hole of the oak tree.