Why was cement put in the hole in the tree in To Kill A Mockingbird?
Their contact with Boo Radley continues into the school year. Before the previous summer, Scout and Jem had discovered gum and Indian head pennies in a knot-holed tree by the Radley house. Now more objects begin to appear in the knothole, including replicas of Scout and Jem carved in soap. They decide to leave a note for whoever is leaving the objects, but before they can, Nathan Radley fills the hole with cement, upsetting Jem. (Enotes)
Nathan Radly did not want Boo to associate with Jem and Scout. Boo had been locked away and hidden from society for so long that the rumors about Boo in town had become somewhat of a legend. When Scout and Jem are making "contact" with Boo, his father feels it is best that the contact is cut off. Nathan fills the hole in the tree that acted as the portal for Scout, Jem and Boo's communication thereby severing any ability for Boo to have contact with the outside world. Trinkets cannot be left in the hole of a tree that is now filled with cement, so now a relationship or dialogue cannot be begun now that there is hole.
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.