Compost vs Fill Dirt

Compost

Decomposed organic matter used as a soil amendment to improve soil structure, fertility, and water retention. Quality compost is dark, crumbly, and earthy-smelling. It adds beneficial microorganisms and nutrients that support healthy plant growth and improve poor soils.

Fill Dirt

Subsoil material free of organic matter, used for grading, filling, and raising elevation. Fill dirt is a lower-grade soil that lacks the nutrients of topsoil, making it ideal for structural and non-growing applications where settling must be minimized.

Compost

• ✓ Soil amendment
• ✓ Garden beds
• ✓ Lawn top-dressing
• ✓ Raised beds
• ✓ Tree and shrub planting
• ✓ Seed starting mix

Fill Dirt

• ✓ Grading
• ✓ Filling depressions
• ✓ Raising elevation
• ✓ Backfill
• ✓ Construction site prep
• ✓ Under slabs

Compost

• + Dramatically improves soil structure and fertility
• + Increases water retention in sandy soils
• + Improves drainage in clay soils
• + Adds beneficial microorganisms
• + Reduces need for chemical fertilizers

• − Quality and composition vary widely between sources
• − Can be odorous when fresh
• − May contain weed seeds if not fully composted
• − Too rich for direct planting — must be mixed with soil
• − Settles significantly after application

Fill Dirt

• + Very affordable — often cheapest material available
• + Compacts firmly for structural support
• + Does not decompose or settle like organic soil
• + Widely available from construction sites
• + Ideal base under gravel or concrete

• − Not suitable for growing plants
• − Must be free of debris and contaminants
• − May contain clay that drains poorly
• − Needs compaction in layers for stability
• − Quality varies widely between sources

Compost

Fine, crumbly texture when fully decomposed. Dark brown to black color. Should smell earthy, not sour or ammonia-like. Screened compost has been filtered to remove large uncomposted pieces. Mushroom compost is a byproduct of mushroom farming and is particularly nutrient-rich.

Fill Dirt

Fine to coarse soil particles, free of organic matter like roots, grass, or leaves. Color ranges from tan to reddish-brown depending on local geology. "Clean fill" means free of debris, rocks larger than 3", and contaminants.