“Mr. Radley, ah – did you put cement in that hole in that tree down yonder?”
“Yes,” he said. “I filled it up.”
“Why’d you do it, sir?”
“Tree’s dying. You plug ’em with cement when they’re sick. You ought to know that, Jem.”
– Harper Lee
To Kill a Mockingbird, Chapter 7. When the Radley tree is filled with cement by Nathan, it symbolizes the end of Boo Radley’s attempts to communicate with Scout and Jem. He does this through the gifts he leaves in the tree for them. They represent the friendship he has towards the children. Nathan filled the knot hole with cement because he caught Boo leaving presents there for the children. Atticus told them the tree didn’t seem sick.