Mulch vs Topsoil vs Compost: Which to Use Where

You are at the garden center looking at pallets of mulch, topsoil and compost - all sold in similar bags, all priced about the same, all looking like dark brown crumbly stuff. They are not interchangeable. Use mulch where you need topsoil and your plants will starve. Use compost where you need mulch and you will grow a healthy crop of weeds. Here is what each one does and where it goes.

What is the difference between mulch, topsoil and compost?

• Mulch is a surface layer of wood chips, bark or shredded leaves. It sits on top of the soil to retain moisture, suppress weeds and regulate temperature. It is not a planting medium and most plants will not grow in pure mulch.
• Topsoil is the upper 6-12 inches of natural ground - dark, mineral-rich soil that holds plant roots. It is a structural material: use it to fill in, level out, build raised beds and establish lawns.
• Compost is decomposed organic matter (food scraps, yard waste, manure) that is high in nutrients and microbial life. It is an amendment, mixed into topsoil at 20-30% to feed plants. Pure compost is too rich for most plants.

When should I use mulch?

Mulch goes on top of soil around established plants, in landscape beds and around trees. The recommended depth is 2-3 inches - thick enough to suppress weeds and hold moisture but not so thick that water cannot penetrate or air cannot reach the roots. One common question: how many bags of mulch for 100 sq ft? At 3 inches deep, 100 sq ft needs about 13 bags of standard 2 cubic foot mulch.

Do not mulch directly against tree trunks - the "mulch volcano" you see in parking lots is bad practice and traps moisture against the bark, leading to rot and insect damage. Keep mulch 3-4 inches away from any tree trunk or plant stem.

For exact quantities, the mulch calculator handles bed dimensions, depth and bag size in one step.

When should I use topsoil?

Topsoil is the structural material you use whenever you need volume. Specifically:

• Building a raised bed: fill with topsoil mixed with 20-30% compost
• Establishing a new lawn: 4-6 inches of topsoil over compacted subgrade before seeding or sodding
• Top-dressing an existing lawn: a thin (1/4 inch) layer to fill low spots and improve soil
• Filling planting holes: backfill around new trees and shrubs with native topsoil rather than amended soil
• Leveling out low spots: use topsoil for any depth-of-fill task in the yard

Topsoil is sold by weight (tons) or volume (cubic yards) for bulk loads, and in 0.75 or 1 cubic foot bags for retail. The topsoil calculator translates between sq ft of area and tons or cubic yards by application.

When should I use compost?

Compost is the feeding layer. Use it as:

• Soil amendment: mix 1-2 inches into the top 6-8 inches of existing topsoil before planting a garden bed
• Raised bed mix: 20-30% compost mixed with 70-80% topsoil (never plant in pure compost - it is too rich and may burn roots)
• Top-dressing for vegetables: a 1/2 inch layer around established plants mid-season feeds without burning
• Container gardens: small amounts mixed with potting soil for nutrients

Pure compost is high in nitrogen and salts. Using it as a planting medium often kills seedlings. Always mix with topsoil or potting mix unless you are specifically top-dressing a heavy-feeding crop like tomatoes or corn.

Can I use compost as mulch?

You can, but you should not. If you are wondering why - compost decomposes too fast to function as long-term weed suppression, and its high nutrient content actually feeds weed seeds that blow in. Wood-based mulch lasts 2-3 years before breaking down. Compost lasts one season before it disappears into the soil.

The right combination is compost under mulch: amend your soil with compost in spring, then top with 2-3 inches of bark mulch. The compost feeds the soil from below while the mulch suppresses weeds from above. This is what professional landscapers do.

Quick reference: depth and bag count for 100 sq ft

• Mulch (3 in deep): 25 cubic feet = 13 bags of 2 cu ft = 0.93 cubic yards
• Topsoil for raised bed (12 in deep): 100 cubic feet = 134 bags of 0.75 cu ft = 3.7 cubic yards
• Topsoil for new lawn (5 in deep): 42 cubic feet = 56 bags = 1.55 cubic yards
• Compost amendment (2 in mixed in): 17 cubic feet = 9 bags of 2 cu ft = 0.63 cubic yards

Notice topsoil for a raised bed requires 4-5 times more material than mulch for the same surface area - this is the single most common budgeting mistake when planning a garden project. Order bulk delivery for anything over 1 cubic yard. Bagged is fine for small jobs but the cost-per-cubic-yard is 2-3x higher than bulk.