Compost vs Lava Rock: Size, Cost, and Which to Use

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.

Lava Rock

Lava rock is a lightweight, porous volcanic stone commonly used in landscaping. Its rough texture helps retain moisture while allowing airflow to plant roots.

Compost

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

Lava Rock

• ✓ Landscaping
• ✓ Garden beds
• ✓ Decorative ground cover

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

Lava Rock

• + Very lightweight
• + Excellent moisture retention
• + Decorative appearance

• − Breaks down over time
• − Sharp edges
• − Can scatter easily

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.

Lava Rock

Typically 3/4"–1.5" sizes. Red or black.