Amending the Soil


Whether you're using existing soil, or importing soil to fill raised garden beds, soil amendments are a vital resource for ensuring a productive vegetable garden. Many neglected urban soils are compacted and low in organic matter. Soil amendments are designed to add organic material back into soil, reduce compaction, and improve soil life.

Compost cupped in a person's hands

In many urban areas, municipalities are taking organic waste products from urban areas, and finding a new life for them in community gardens. Not only does this turn waste into a resource, it also improves urban soil, and gets urban residents to consider where their waste products go and the impacts of waste on the ecology of a city.

Some common soil amendments are:

Biosolids

Biosolids are the digested, solid portion extracted from the wastewater treatment process. They may not sound pretty—but there’s nothing better for creating an amazing garden. In addition to being 100% recycled and full of macro- and micro-nutrients, biosolids work to build healthy soil like nothing else.

Biosolids are also held to stringent standards by the EPA and have been repeatedly shown to be a safe, effective way to build healthy soil. All municipalities are tasked with managing their own biosolids; the majority treat them to a level of cleanliness that’s safe for farmland (Class B), but not for the urban gardener. A small but growing number of cities, however, are investing in the technology to produce Class A biosolids—clean enough to distribute to those gardening in urban areas.

A good example of this is Tagro. The City of Tacoma, WA, invested in the late 1980s in a state-of-the-art treatment plant that creates the highest quality biosolids: Class A, or Exceptional Quality. Tagro, short for “Tacoma Grow,” has been available for gardening in Tacoma and surrounding Pierce County since 1991, and has a box of ribbons from the local Puyallup Fair to prove how well it works. Available throughout the Pacific Northwest, Tagro currently blends two different products: Tagro Mix and Tagro Potting soil.

For information about a project in Chicago that used biosolids, read here.

Composts

A man shoveling compost

Compost is organic material that has decomposed into a stable state that’s then available for adding to soil. Anything that was once alive can be composted; therefore different composts can vary in their properties. Some of the most common composts available to urban residents are made from the yard debris that is collected curb-side from residents. Read more in this compost guide (pdf) from the Washington Organic Recycling Council.

Some cities and counties around the country produce their own compost. In Tacoma, WA, and surrounding Pierce County, for example, the finished product of the composting process is called PREP (Pierce Recycled Earth Product) and is manufactured by Land Recovery, Inc. (LRI). In Seattle, WA, and surrounding King County, the finished product is called Cedar Grove (manufactured by Cedar Grove).

Manure

Manure can give your garden an incredible boost in nutrients. Make sure the manure you use has been well aged or composted. Check with local agriculture organizations to see if they have a list of local farms willing to share their manure.

Using amendments

You can add amendments to soil anytime, but the best times for working it into an existing garden are in the spring before planting, and in the fall when putting the garden to bed.

It’s not complicated! Generally two to three inches of biosolids/compost/manure is sufficient. Just spread it around the garden, and then either mix it into the soil with a shovel or use a small tiller.