While outdoor soil may be heavy with clay, too sandy, low in organic matter, or on top of caliche, soil in a raised bed can be composed of whatever the gardener desires. Gardening with raised beds gives the gardener ultimate control over the soil mix in the beds.
Filling a raised bed with bagged potting soil is an option. However, using potting soil, especially the premium, is going to be very expensive. Very cheap potting soil often contains lots of sand and other undesirable elements, and is very low in organic matter. It will need to be amended. This defeats the purpose of creating the best soil mix in the first place.
Square Foot Gardening Mix
Mel Bartholomew, creator of the square foot gardening method, uses three components in his raised bed soil. Mel's soil is 1/3 compost, 1/3 peat, and 1/3 vermiculite. Plants can be grown in pure compost, and it's especially good for starting seedlings. Peat and vermiculite provide excellent moisture holding capacity.
Economical Soil Mix
Utilizing backyard soil can cut down on initial costs of soil mix. Soil can be mixed with 1/3 natural soil, 1/3 peat, 1/3 compost. Another possible recipe is 1/4 natural soil, 1/4 peat, 1/4 aged manure, 1/4 compost.
Good Ingredients for a Soil Mix
- Compost - adds vital nutrients, microbes, and makes for great soil texture. Make your own compost or purchase it from a garden center. Make sure the bagged compost is high quality compost. Take a bag home and inspect it. It should be loose, light and not stink. It should be made from recycled yard waste, animal manures, or processed sewage. Many municipalities offer free or reduced cost compost made from yard waste.
- Peat - has excellent moisture-holding properties and is slightly acidic.
- Coir - is a good substitute for peat. It is made from coconut hulls and is arguably a renewable resource, whereas peat is not being sustainably harvested.
- manure - most manures will need to be composted some before added to a soil mix. This is because they are high in nitrogen and need to break down some first. Otherwise the manure can burn plants, just like adding too much chemical fertilizer.
- mulch - mulch has some moisture holding properties, helps keep soil loose, and will break down over time. Mulch is a nice soil additive because local varieties are usually inexpensive. Mulches can be wood or bark mulches, and the finer ones work well in a soil mix. Don't forget to add some over the top of the soil after planting.
The above ingredients can be mixed and matched, along with yard soil if desired, to create a nice soil mix for raised beds. Keep the organic matter (manure and/or compost) high, and plants will be vigorous and healthy.
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