Here's a breakdown on some common container soil ingredients

All you need for a good potting soil are ample nutrients and good drainage. Certain materials affect fertility, others have an impact on drainage, and some do both. Knowing the benefits that certain ingredients provide will help you tweak your own blends and make knowledgeable substitutions when needed.

More Reading:
Potting Soil Recipes

Videos:
A Basic Potting Soil Recipe

Potting Soil Recipe for Annuals

Potting Soil Recipe for Cacti and Succulents

Potting Soil Recipe for Woody Plants and Perennials

The ingredients

Bagged potting soil
Compost
Composted chicken manure

Bagged potting soil is a good base. Not the same as a soilless container mix, a potting-soil blend contains a mixture of ingredients that improve fertility. Avoid brands that include synthetic fertilizers if you want to stay organic.

Compost is the key to growing organically. Compost adds high amounts of nutrients to a mix. Though you can buy compost in a bag from the store, you can make it for free in your own backyard with a little effort and patience.

Composted chicken manure has the highest natural-nutrient content available. Use it to amp up the fertility of your mix, but never use it raw: It must be composted or it will burn plant roots. Buy it by the bag at a garden center or feed store.

Garden soil
Rock dust
Peat moss

Garden soil is a ready way to start a recipe. Good dirt straight from the top few inches of your garden has trace nutrients and minerals that help balance a blend. Don’t use it by itself because it lacks the drainage required for containers.

Rock dust helps you mimic native soils. This powder made of pulverized stone contains trace nutrients and minerals from bedrock, the natural source material of all soils. Get it free at a local quarry, or buy a 5-pound bag from mail-order sources.

Peat moss holds moisture, adds nutrients, and increases soil acidity. A compressed bale, available from garden centers and agricultural suppliers, will last the average home gardener several seasons.

 
Worm castings
Calcined clay
Charcoal pellets

Worm castings are a natural source of fertility. What a worm leaves behind as it eats its way through the soil is high in nutrients and increases soil acidity. Buy a 25-pound bag at organic garden centers or through mail-order suppliers.

Calcined clay is another term for kitty litter. Kiln-fired, this material holds moisture and nutrients while creating air pockets for roots to absorb oxygen and release carbon dioxide. It is sold at grocery stores as “no lump” kitty litter.

Charcoal pellets decrease the odor of decomposition. The organic matter used in some recipes is often so rich that it can result in a smelly pot. Adding charcoal pellets, available at aquarium stores, absorbs the odor while improving drainage.

Coarse river sand
Expanded slate or shale
Perlite

Coarse river sand increases the surface area for roots to cling to. Adding air pockets so that roots can function efficiently, coarse river sand creates a free-draining mix. Buy a small 5-pound bag from bonsai plant growers.

Expanded shale or slate allows excess water to drain away. This natural rock is kiln-fired to form small pockets on its surface that hold air, moisture, and nutrients, making them available for plants. It is sold in bags as PermaTill or VoleBloc.

Perlite is synonymous with good drainage. That white stuff you see in store-bought blends is high-fired volcanic rock. Perlite lightens the soil by creating pockets for water to pass through and air to remain. Find it at any garden center or big-box store.

Playground sand
Pine bark
Rice hulls

Playground sand ensures a loose mix. The same bag of sand used in kids’ sandboxes can improve the drainage in your containers, helping your plants thrive. Available at any hardware store, a 50-pound bag typically costs less than five dollars.

Pine bark increases fertility as it decomposes. Large pieces create air pockets for water to pass through, and because it is organic, the bark releases nutrients as it decays. You can buy pine bark at any nursery, either partially composted or pure.

Rice hulls lighten the mix and add nutrients. The hulls of harvested rice increase drainage, provide organic matter as they decompose, and make good use of a plentiful by-product. Several sources are available online.

 

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