Compost looks like a flecked earthblock and is found in the Resources section of inventory. A compost block takes up a 1x1 block space and has full (albeit shovel-able) collision.
How to Obtain
Compost doesn't generate naturally in any world. It is a crafted resource that you make with a Composter or by planting flower bulbs in a purified world. Once the bulb grows a mature flower, it will turn into a block of compost. Compost also drops as loot from treasure chests, including mechanical chests. You can also trade for this item in a market world.
How to Use
Compost functions only in temperate and arctic biomes.
You can use it in purified temperate worlds to grow green grass, shrubs, and small trees. Such greenery is a crucial part of creating gardens and in giving developed worlds a less apocalyptic feel. To use compost, place it anywhere in a purified temperate world where the compost block has unobstructed sky above. Purified rain must be able to reach the block from the very top of the map. Grass usually appears with the first rain, but it can take several days for larger trees and shrubs to grow.
Compost can also be placed in arctic worlds to grow pine trees. If a block sprouts grass instead of a sapling, you can mine the grass for flax. With the next snowfall, that block will have a chance of growing a tree instead of grass. Arctic biomes do not need to have completed world machines for compost to function.
Finally, compost can be used instead of flower pots as a way to display flowers and trees that you have in inventory. First place the compost, then place the flower or tree directly above it.
Note that since compost can be shoveled, even in protected areas, you shouldn't use it to contain liquids or a pet/android if your build is on a world that other people can access.