Composters offer a way to obtain Dirt by composting items. They can also be used to collect rain water.
Composting:
By right-clicking the Composter with a compostable item, it increases the level of compost in the Composter by 1. Once it's reached 8 levels of compost, or 8 compostable items, it will then start composting a Dirt block over the course of about 50 seconds. Once composted, you will see the compost change to dirt indicating it's finished and you can right-click it to obtain a dirt block.
Collecting Water:
When it rains, an empty Composter will begin filling with water. It will slowly fill up and, once full, change to a darker shade of blue to indicate that it's full. Once full, you can collect the water with an Iron Bucket or Clay Bucket. If there's anything above it blocking the rain, it will not fill up with water.
Pipes:
Composters can be used with Pipes. Pipes can insert compostable items into a Composter and Extraction Pipes can remove the Dirt block once it's composted. It's recommended to only insert items into the Composter one at a time, rather than in stacks of 16, since only 8 items can fit in a Composter. Once full, any items piped to the Composter will return down the Pipe in the direction they came from.
Compostable Items: