Topsoil vs Garden Soil vs Compost
Topsoil is the general-purpose upper layer of soil used to build volume; compost is decomposed organic matter used to enrich soil, not as a standalone growing medium; and garden soil is topsoil already amended for planting. Use topsoil for the bulk, compost to feed it, and garden soil for ready-to-plant beds.
What each is for
Topsoil is cheap and used to fill, level, and build up beds and lawns. Compost is richer and lighter, added in smaller amounts to improve structure and feed plants. Garden soil is a blend sold ready for planting in beds.
Compost alone is too rich and drains too fast to grow most plants well, so it is mixed in rather than used by itself.
How to combine them
For a new bed, a common approach is mostly topsoil with about 25 to 30 percent compost mixed into the top layer. For an existing bed, top-dress with an inch of compost each year.
Use the topsoil and compost calculators to get the cubic yards for each, then mix to your ratio.
FAQ
Can I plant directly in compost?
Usually not. Pure compost is too rich and drains too quickly for most plants. Mix it into topsoil or garden soil instead, at roughly a quarter to a third by volume.
What is the difference between topsoil and garden soil?
Topsoil is plain soil for building volume; garden soil is topsoil already amended with compost and nutrients so it is ready to plant in.
How much compost should I mix with topsoil?
About 25 to 30 percent compost by volume worked into the topsoil for a new planting bed.