I recently moved to a new house that had what looked like a fair amount of chemicals dumped on the land around it. What do you recommend as a base soil for raised beds for organic gardening if you do not want to trust what you have?
The first thing you might consider doing is having your soil tested. There are various labs that can test for toxins as well as for soil type and nutrient deficits. Here is a link to a WSU site, which lists the labs and what they do. These labs primarily serve agriculture, so you might consider getting a soil test from a lab that specializes in home gardens, such as University of Massachusetts, Amherst. Also, if you suspect heavy metals in the soil, there is a link from Seattle-King County Public Health to labs that will test for toxins.
In her book, Backyard Bounty (New Society Publishers, 2011), British Columbia gardening expert Linda Gilkeson says that for your raised bed (or any situation where there was either no native soil, or where you are replacing the existing soil) you should “buy the best soil available, and mix in a generous amount of compost, leaf mold (well-rotted leaves) and other organic matter as you fill (1 or 2 parts compost to 9 parts soil).”
If you purchase topsoil, make sure to find out what the composition of the topsoil is. Linda Chalker-Scott’s factsheet from Washington State University Extension indicates that the ideal percentage of organic matter [abbreviated as OM] in topsoil used for gardens is 5% by weight.
Excerpt from the WSU blog discussion of this topic:
“If you’re purchasing topsoil, check out what you will be getting before it’s delivered. Ask the seller what the topsoil contains and also ask for the producer’s test data regarding pH, salt level, nutrient levels, OM content, and texture. If they don’t have that data available, you may want to consider taking a sample and have it tested yourself. Also, find out if the soil has been screened to remove rocks. Before you’re stuck with unsuitable topsoil, know exactly what you’re getting.
Garden Hint: Topsoil is usually sold by the cubic yard. One cubic yard of soil will cover about 50 square feet to a depth of four to six inches.”