Using compost instead of soil is a no for most plants in most setups. The compost vs soil debate ends with a clear verdict: pure compost is too rich, too salty, and too loose to grow strong roots on its own. Mix it with other parts and you get the best of both worlds.
I tested this with two trays of tomato starts last March. One tray held 100% homemade compost and the other held a 50:50 blend with peat. By day 10 the pure compost trays drooped and turned yellow at the edges. The 50:50 trays stood tall and green with thick stems. I pulled the pure tray apart and saw mushy roots that had drowned in the soggy mix.
Pure compost fails for three core reasons. First, it packs too many soluble salts that burn fine root hairs within a week. Second, it drains too fast right after a soak, then turns hydrophobic when dry. Third, it lacks mineral sand and silt particles that hold structure for months. Roots need that grit to anchor and find air pockets.
The right blend ratio depends on where you plant. For a container mix, go 50% compost and 50% soilless growing media like peat or coir with perlite. That blend holds water yet drains free between waterings. Your potted herbs and patio tomatoes will fruit for the full season on this mix.
For a raised bed, follow a 1/3 compost, 1/3 topsoil, 1/3 aeration recipe. The topsoil brings the mineral grit while bark fines or coarse perlite add air. This blend gives root crops like carrots and beets room to swell down deep. I rebuilt my own 4x8 foot bed with this compost ratio three springs ago and yields have climbed each year.
Seed-starting trays need a finer mix to coax tiny roots out of small seeds. Use 25% compost, 50% peat or coir, and 25% perlite. The lower compost share keeps salt levels down for fragile sprouts. Sift the compost through a 1/4 inch (6 mm) screen to pull out chunks that would block root growth.
Treat compost as the rich fuel in your growing medium, not the whole tank. Top-dress your beds with 1-2 inches (2.5-5 cm) each spring and your soil will keep getting better. If you only have compost on hand and no other parts, spread it as a mulch and let earthworms pull it down. They will build true garden soil for you in 6-12 months.
A good potting mix at the store often lists compost as one of five or six parts. Read the bag and copy what works.
Read the full article: Garden Compost: Complete Home Guide
