Container plants live or die by what's in the pot, and the biggest mistake happens before planting: filling containers with dense garden soil. Here's how to read the bags and build a mix that drains, breathes, and feeds.
Nora Finch
Soil that grows great plants in the ground performs terribly in a container. In the ground, water drains through metres of soil structure and worms keep it open; in a pot, the same soil compacts into a dense brick that waterlogs at the bottom and suffocates roots. It also imports weed seeds, fungus gnats, and disease into a small closed system with no ecosystem to balance them.
Potting mix - confusingly, usually containing no soil at all - is engineered for pot physics: a blend of organic matter (peat or coir) for moisture, plus perlite or bark for air pockets and drainage. The bag that says 'potting mix' or 'container mix' is the right one; 'garden soil', 'topsoil', and 'raised bed soil' are for the ground, whatever the picture on the bag shows.
Peat moss or coco coir is the base - it holds water and gives the mix body (coir is the renewable choice and rewets more easily when dried out). Perlite, the white popcorn-like bits, creates the air pockets; vermiculite holds extra moisture; composted bark adds chunky structure that keeps mixes open for years. A good general mix lists several of these, not just 'forest products'.
Two label items worth a glance: added fertilizer ('feeds for 3 months') means skip feeding for the first weeks, then start - it's a head start, not a year's supply; and wetting agents help dried-out mixes absorb water again. Very cheap mixes are cheap because they're mostly fine sawdust-grade filler that compacts within a season; mid-shelf is the value sweet spot.
General-purpose potting mix suits most things: herbs, vegetables, annual flowers, and the standard houseplant lineup. Cactus and succulent mix is the same idea with much more grit and faster drainage - right for succulents, rosemary, and anything that hates wet feet. Orchid 'mix' is mostly bark chunks, because orchid roots want air more than soil.
Two specials worth respecting: seed-starting mix is finer and sterile so damping-off fungus doesn't slaughter seedlings (worth using for trays, unnecessary afterward), and ericaceous (acid-loving) mix is the one blueberries, azaleas, and camellias actually require - they sulk and yellow in standard mixes. You can also upgrade a general mix yourself: an extra few handfuls of perlite per bag for houseplants, or a third compost mixed in for hungry vegetables like tomatoes.
Skip the gravel layer at the bottom - it doesn't improve drainage, it raises the waterlogged zone up into the roots (the drainage hole plus a good mix do the actual work; a single mesh scrap over the hole keeps soil in). Fill with mix lightly firmed, not packed: pressing hard crushes exactly the air structure you paid for.
Leave two or three centimeters between the soil surface and the pot rim as a watering well, so water can pool and soak instead of sheeting off the top. For very large planters, you can legitimately cheat the volume - upside-down nursery pots or chunky pruned branches in the bottom third save mix and weight for shallow-rooted plantings - but deep-rooted crops like tomatoes want the full depth honest.
Potting mix wears out - the organic matter breaks down, the structure collapses, and after a season or two it drains worse and feeds less. But wholesale annual replacement is overkill for most pots: topping up with a third fresh mix, scratched into the surface, plus regular liquid feeding keeps a big planter going for years. Houseplants want full fresh mix only when repotted, every couple of years.
Do replace mix entirely when a plant died of disease or root rot in it, when the surface stays crusted white with salts, or when water either races straight through or won't soak in at all. Spent-but-healthy mix isn't waste - dig it into garden beds or the compost, where it finishes its decomposition usefully. And store opened bags closed and dry; a soggy open bag on the patio over winter breeds gnats and loses structure.
Yes, if last year's plants were healthy: tip it out, break up roots, blend in a third fresh mix or compost, and feed regularly through the season since its nutrients are spent. Don't reuse mix from diseased or rotted plants - that goes to the garden beds.
Compost is decomposed organic matter - a rich amendment. Potting mix is an engineered growing medium built for drainage and air. Pure compost in a pot stays too dense and wet; the right move is blending compost into potting mix (up to about a third) for hungry crops.
The mix has likely shrunk away from the pot walls or turned water-repellent (peat does this when fully dried) - water races down the gap and out. Soak the whole pot in a basin for 20-30 minutes to rehydrate, and top up with fresh mix to close the gap.
For edibles, many gardeners prefer it - it relies on natural fertility rather than synthetic fertilizer prills. Structurally, quality matters more than the organic label: a well-made conventional mix beats a poorly-made organic one. Read the ingredient list over the badge.
