Soul Sand is a block added by the Nether Ores mod. It can be found in the Nether, and is used to craft Soul Soil and Nether Wart Blocks. Soul Sand is a valuable resource, as it is the only way to obtain Soul Soil, which is necessary for growing Nether Warts.
To get Soul Sand in ATM9 To The Sky, you will need to travel to the Nether. You can do this by crafting a Nether Portal using Obsidian and Flint and Steel. Once you have entered the Nether, you will need to find a Soul Sand Valley. Soul Sand Valleys are large, open areas filled with Soul Sand.