Potting Soil: What It's For, What's In It, and What to Skip
Peat, coir, perlite, bark, compost — the five ingredients in most bagged mixes, and the ratios that matter. Plus when to skip the bag and mix your own.
Potting soil is not soil — it's a soilless mix of peat (or coir), perlite, bark, and compost. The job is to hold moisture, drain freely, and stay light enough that roots can breathe. Different plants want different ratios: cacti want gritty + lean; tropicals want moist + organic; vegetables want everything. Never use 'garden soil' or 'topsoil' in pots — it compacts and suffocates roots.
What's in the bag
Most general-purpose potting mixes are 50-60% peat moss (or coir, the sustainable alternative), 20-30% perlite (the white volcanic glass that aerates), 10-20% bark fines (adds structure), and 5-10% compost (nutrients + microbes). Some include a slow-release fertilizer; some add mycorrhizal fungi; some claim moisture-retention crystals (skippable).
Read the bag. 'Premium' or 'professional grade' usually means the right ingredients in real proportions. Cheap bagged mixes are often mostly bark with minimal peat — they dry out fast and don't hold nutrients.
Garden soil vs potting soil
The bagged 'garden soil' next to potting soil at the box store is for in-ground beds, NOT containers. It's heavy, holds water for too long in pots, and compacts into a brick within weeks of being potted. The only place garden soil belongs is amended into existing in-ground beds. The label often warns this but the warning gets ignored.
Plant-specific mixes
Cacti / succulents: 50% potting soil + 50% coarse sand or pumice. Drains within seconds of watering.
Tropical houseplants: standard potting mix + extra perlite (1 part perlite to 4 parts mix). Holds moisture without staying soggy.
Orchids: pure fir bark with charcoal — not standard potting mix at all. Orchid roots need air more than moisture.
Vegetables in containers: potting mix + 25% compost. Containers need more nutrient density than in-ground beds because there's nowhere else to draw from.
DIY potting mix
Standard mix: 3 parts coir (or peat), 1 part perlite, 1 part fine compost. For 30 gallons of potting mix from scratch costs $15-20 vs $40-60 for the equivalent bagged. Worth it if you're filling 10+ containers. Wet the coir bricks in a wheelbarrow before mixing — they expand 5-7x volume.
Reuse, refresh, replace
Potting mix degrades over 1-2 years. Perlite floats to the surface; peat decomposes; bark breaks down. By year 3, the mix has lost half its air pockets and roots struggle. Top up containers annually with fresh mix; fully replace every 2-3 years for serious plants. Old mix isn't garbage — work it into in-ground beds where it improves soil structure.