Soils For Pots and Raised Beds

Plants That Produce

SOILS FOR POTS AND RAISED BEDS

By Richard Frost

Plants feed primarily on minerals and expect minerals and rocks in the soil. Compost has a very low percentage of minerals by weight or volume. Further, most of the plants we grow in our gardens do not naturally occur in media of pure compost, so filling a raised bed with only compost is not an ideal way to garden. Here is the basic soil mix I recommend for container gardening and raised beds:

Quantity Substance

2 Fine-grained Composted Greens

2 Sphagnum Peat Moss

1 Horticultural Sand

1 Perlite #3

½ Worm Castings

A few exceptions: for succulents and plants from sandy soils use 2 parts horticultural sand and only 1 part peat moss. If you are making soil for rooting cuttings, then eliminate the worm castings and use equal parts of the others. For long-term plantings of perennials such as boxed trees, substitute Coir for the peat moss and substitute 3/16” to 1/4” Scoria or Rock Pumice (not lightweight pumice) for Perlite. These materials are much more resilient over the long-term.

If you want to make this mix on your own, then below are some more details about the ingredients and where to obtain them in the San Diego area. Agri-Service Inc. at the El Corazon facility in Oceanside is gearing up to manufacture it in bulk for yours truly, so check with them for availability later this year.

Fine-grained Composted Greens. Compost from greens provides textures that are beneficial to plant roots and a home for many organisms that are symbiotic with plants. The best composted greens I’ve ever encountered are from the El Corazon Greenery Facility in Oceanside. It is fine-grained, well composted, and OMRI-certified. It is free to Oceanside city residents and only a $1 per trash barrel to everyone else.

Sphagnum Peat Moss. This peat moss comes from Canada and has none of the environmental issues associated with mosses harvested elsewhere. It is acidic in a way that is good for most plants and has water retention properties to complement the other ingredients of the soil. You can obtain it inexpensively in compressed 3.8 cu. ft. bags from Crop Production Services in San Marcos.

Coir. Coir is compressed coconut fiber. Structurally it lasts much longer than peat and is a good choice for long-term plantings. It is also pH neutral, so check the soil pH after transplanting and feeding to see if an adjustment is needed.

Horticultural Sand. Horticultural sand consists of the “fines” that pass through all the screens in the process of crushing granite gravels, including the final 3/16^th inch slits. It contains pieces that are from about 1/8^th inch in diameter all the way down to rock dust. Here in San Diego County it is available from A1-Soils.

Perlite #3. Perlite is made from a mineral sand by heating it into foam. It provides both porosity and moisture retention in the form of humid air pockets. The largest size (#3) is appropriate for containers and raised beds for vegetables, herbs, and annuals in general. The least expensive source in the nation is right here in Escondido: Aztec Perlite Company.

Worm Castings. More accurately called worm poop – but a less agreeable name for some. It contains a plethora of beneficial organisms including worm eggs and mycorrhizae. The highest quality worm castings in San Diego County are available from Julie Harski (julieharski@aol.com) at local farmers’ markets.

SDHS member Richard Frost is a certified edible gardening nut. For copies of past articles and more information, please see www.PlantsThatProduce.com.

Reprinted with permission from August 2010 "Let's Talk Plants," the newsletter of the San Diego Horticultural Society, www.sdhortsoc.org