Nathan Radley, Boo's older brother
Nathan Radley tells Jem that he filled in the knothole with cement because the tree was dying and he wanted to stop it from spreading. In reality, he likely did it to prevent Boo Radley from continuing to communicate with the children.
Mr. Radley filled the knothole up with cement.
They can not because the knothole is filled with cement by Mr. Radley
That the knothole has been filled with cement. That's what was on Spark Notes.
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.
Nathan Radley said that when a tree is dying you fill is with cement to save it.
Mr. Radley cut off Boo's only link to the outside world by sealing the knothole in the tree with cement. Boo Finch used to communicate with Jem and Scout by leaving small gifts and trinkets in the knothole.
Boo Radley leaves gifts for the children in the knothole of the tree.
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.
Nathan Radley knows Boo has been leaving gifts there for the children so fills the hole in with cement. When the children enquire as to why he's doing this, he lies and says that the tree was dying and this would save it.
Jem is devastated when he discovers the knothole filled with cement. He understands that Nathan Radley had destroyed the only communication Boo had with other people.
He tells them the tree is dying.