Choosing Soil, Potting Mix and Containers

Potting soil is not garden soil

Plants in the ground can spread their roots wide and deep, and excess water drains away below. Pots are different. The volume of soil is small and the bottom is closed, so water can only escape through a few holes. Scoop in yard or hillside soil as is, and every watering packs it harder, while water pools at the bottom and roots can't breathe. Outdoor soil may also carry insect eggs, weed seeds and pathogens. For a container garden, starting with a bagged potting mix made for pots leads to far fewer failures.

Potting mix ingredients and what they do

Commercial potting mix blends several materials to balance drainage, water retention and air. The ingredient list on the back of the bag tells you what kind of mix it is. Lots of water-holding material means it dries slowly; lots of gritty material means it drains fast and dries quickly. Also check whether fertilizer is included. Mixes sold for starting seeds are fine-textured and low in nutrients, so if you grow large crops in them for long, you may need to add fertilizer.

• Peat moss and coir: hold water and nutrients
• Perlite: light white granules for air and drainage
• Vermiculite: holds on to water and nutrients
• Compost: adds nutrients and microbes

The gravel layer myth

There's a long-standing belief that a layer of gravel or broken pottery at the bottom of a pot improves drainage. But when a coarse layer sits under fine soil, water tends not to cross that boundary right away and stays in the soil above it, so the saturated zone actually moves upward. All it really does is shrink the amount of soil the roots can use. What controls drainage is the hole in the bottom and the nature of the soil itself. If you're worried about soil washing out of the hole, a single piece of mesh over it is enough.

Choosing pot size

Pick pots based on how big the crop will be when fully grown. Leafy greens like lettuce and scallions have shallow roots, so several can be spaced out in a long trough. Large fruiting crops like cherry tomatoes and peppers need plenty of depth and soil volume, usually one plant per pot. A small pot dries out fast, and in midsummer you may have to water several times a day. On the other hand, a young seedling in an oversized pot sits in wet soil for a long time, so plan for the plant's final size but adjust how much water you give.

• Leafy greens: several plants in a shallow, long pot
• Fruiting crops: one plant in a deep, roomy pot
• Herbs: easier to manage with one kind per pot
• Every pot: check for a drainage hole

Material and color

Plastic pots are light and cheap, and the soil dries slowly. They suit places like balconies where weight is a concern. Unglazed clay pots let water evaporate through their walls, so the soil dries faster and root temperatures stay lower. If you tend to water often, clay may suit you; if you can't water often, plastic may be better. Fabric pots breathe well but dry out very quickly. Color matters too. Black pots heat the soil a lot in summer sun and can stress roots, so in direct midsummer sun a lighter color is an advantage.

How to plant

Moistening the potting mix slightly beforehand helps water soak in evenly. Dry mix tends to shed water at first, leaving the surface wet and the inside dry. Fill the pot, dig a hole a little wider than the nursery pot, set the seedling in with its root ball intact, and press the surrounding soil down lightly by hand. Packing it hard squeezes out the air, so go gently. Fill only to two or three centimeters below the rim so water doesn't overflow. Right after planting, water thoroughly until it runs out the bottom.

Reusing and storing soil

After a season, soil has fewer nutrients, contains leftover roots and has broken-down particles that drain poorly. To reuse it, spread it out to dry while picking out old roots and insects, then mix in fresh potting mix and compost to restore it. Growing the same type of crop in the same pot year after year lets the diseases and pests that favor it build up, so rotate crops. It's safer not to reuse soil that grew a diseased plant. Store leftover soil and mix tied up in the shade and out of the rain. How to dispose of used soil varies by area, so check your local guidance.