After the player has both received the National Pokédex from Professor Oak and completed Baoba's second challenge, Baoba will eventually call the player to tell him or her, that he has come up with a new idea for the Safari Zone. After receiving this call, the player will then be able to place one block at a time of any of 5 categories of blocks inside of the Safari Zone, up to 30 blocks in one area at a time. Baoba will continue to call the player another three times after this giving the player 6 new blocks each time. There are 24 blocks in total, three of which are favored by Pokémon that live in grasslands, three which are favored by Pokémon that live in forests, three which are favored by Pokémon that live in rocky areas, three which are favored by Pokémon that live by the water and twelve others. Blocks that are favored by certain Pokémon can be used to draw out Pokémon that are not normally found in each area.
Also, for Pokemon found with objects, and also other info about the Safari Zone, look here.