Place a block inside the Placer and it will place whatever block is inside it as long as there's an open space in front of it. Place a crop, seed, or sapling inside the Placer and the Placer will plant whatever is inside it on the block in front of it. The usual planting limitations apply, such as seeds requiring farmland or saplings requiring dirt or grass.
The Placer also works with Sieves
to automatically sift blocks. When using a Sieve with a Placer, the items will drop in front of the sieve:
Note: There is a limit of 30 placers per island.