22.4: Soil Reconditioning Strategies

Once you have an understanding of the specific problems associated with a particular urban property, decide on the most appropriate reconditioning (improvement) strategies (Table 22.3). Most make the decision to pursue mitigation (coping) versus removal strategies at this point. Using excavators and trucks to remove contaminated soil is an expensive and extreme option that may be required for highly contaminated sites, and regulations on excavated soil with contaminants make the whole process difficult as well as costly.

Again, improving the soil so that it is safe for food production and for the community will take time and could prove costly. Before you begin, you should have a plan in place that accounts for this time and cost.

Table 22.3 Typical Reconditioning Techniques for Degraded Soils

Technique	Physical	Chemical	Biological
Soil removal	X
Raking	X
Tillage and subsoiling	X
Drainage	X
Soil amendments and additives*	X	X	X
Recyclers			X
Cover crops			X
Mulch	X	X	X
*Examples can include manufactured additives to improve soil structure (physical), commercial fertilizers and composts.

Practices to improve urban soils fall into physical, chemical and biological categories, just as they do in any agricultural setting. In urban situations, the strategies outlined here should generally be considered and used in that order, from physical to chemical to biological.

Physical practices can provide immediate solutions to compaction, poor drainage or the presence of toxic materials in the soil, but they’re not necessarily easy. If contaminant levels are modest or concentrated near the surface, scraping only the top layer of soil and replacing it with quality topsoil might be more feasible. Additionally, a thin layer of contaminated topsoil can be diluted by using tillage or subsoiling to mix it with soil deeper down. This will also alleviate existing compaction problems. If compaction is the primary concern with an urban soil as opposed to contamination, removal is not a recommended approach but amelioration in place makes more sense. Other physical practices include removing old structures and trash, and raking the soil to either level it or to remove old construction debris and trash near the soil surface.

Figure 22.4. Huerta del Valle, a four-acre urban farm that serves low income communities in Ontario, CA, uses organic waste from a local food distributor to produce compost on site. Photo by Lance Cheung, USDA.

Depending on soil test results, you will probably need amendments to alter nutrient and mineral levels, or pH. Phosphorus binds to lead, making it less dangerous over time, so be sure to use phosphorus fertilizers if the soil tests indicate a deficiency. Mineral amendments, such as lime or dolomite, may help with poor drainage or to stabilize pH.

Compost, cover crops and other organic amendments are usually required before producing any crops to increase organic matter, improve soil structure and promote soil biological activity, and they should be used each growing season to maintain soil health. Like tillage, mixing in compost will further dilute toxic compounds. Also, organic matter binds some contaminants, making them less available to plants. Compost is readily available in urban areas, but be sure to use only high-quality compost from reliable sources and pay special attention to finding a supply that is itself free of contaminants and weeds. Local restaurants, cafes, arborists and municipal compost piles are common sources (Figure 22.4). The use of cover crops is discussed in Chapter 10, and compost is discussed in Chapter 13.

Mulches, including living mulches, can be used to suppress weeds and reduce erosion. When soil contamination is a concern, mulches have the added benefit of acting as a barrier that reduces contact with contaminated soil. They can also reduce the splashing of soil onto crops.

Maintaining Healthy Soils

Even after an urban farm or garden has been put into production, good soil management remains critical. Since most urban farms are under continuous, intense production during the growing season, soils can lose fertility quickly and need to be replenished. The best ways to maintain soil health in urban systems are the same as in rural ones. They are described in detail elsewhere in this book, including:

• Cover crops (Chapter 10)
• Crop rotation (Chapter 11)
• Composting (Chapter 13)

Rather than try to improve a property’s soil, many urban farmers and gardeners opt to build raised beds instead, filling them with a mix of imported topsoil and compost. Again, make sure the topsoil and compost you plan to use is free of toxic materials before buying it. Placing a layer of landscaping fabric on the soil surface before adding the new soil for the raised beds helps to limit roots reaching the original soil. A fabric barrier also lessens soil in the bed mixing over time with the buried soil deeper in the ground through biological activity.