Introduction to Urban Soils

Unlike soils out in farm fields, urban soils often contain building rubble and contaminants. All of the Rock Island Community Garden sites are formerly where houses stood, so it is not uncommon to find pieces of glass, stone, or metal buried just below the surface. The ground can also be quite hard after being compacted during a demolition. As a result, there are some unique challenges to urban gardening. None of these challenges, however, are impossible to overcome.

Rubble

Most of the rubble in soil is left over from building foundations. It consists of broken concrete block, brick, and stone. Especially large pieces can damage tillers and other equipment, so caution is necessary. These pieces should be carefully removed and stacked in one spot, preferably at the back of the site. City crews will remove the rubble piles over time. Other materials, including glass and metal, should be handled the same way. Any other materials, including plastics and other trash, should be removed from the site and thrown away in the recycling or garbage.

Lead Contamination

All urban soils are contaminated by lead. The lead comes mostly from old paint and car exhaust. The vast majority of crops do not absorb lead into their edible parts. Accidentally consuming lead-contaminated soil itself is the main source of poisoning. While it may seem dangerous, there are a number of simple practices that can greatly reduce the risk posed by lead contamination in soil. 

• Limit consumption of root vegetables grown where lead concentrations are elevated.
• Thoroughly rinse crops with water. Remove outer leaves if hard to rinse properly.
• Use specially dedicated gloves, shoes, and clothes for gardening.
• Always wash hands after working in the garden.  

Research has been done to explore how lead in soils can be diluted or made less accessible. Some of those techniques include the following. 

• Plowing to move lead contamination deeper into the soil making it less accessible.
• Adding compost and manure to the soil to reduce lead concentration.
• Adding lime to keep soil pH near 7 so lead forms rock-like minerals that become hard to absorb. 
• Adding phosphate which binds with lead making it hard to absorb. 
• Covering bare soil with mulch or similar plant debris to keep down dust.

Poisoning from lead is very serious, but common sense practices like these can do a lot to help protect yourself and those around you.

Compost

How fertile the soil is on an urban site varies greatly. In some parts of a garden site, it may be very rich. In other parts, it may be very poor. This is mostly to do with in-fill material brought onto the site at the time of a building demolition. This fill material is usually of a lower quality than the original soil that surrounded the building. Soils can be improved, however. Adding compost is one of the best ways to improve soil. Compost is decaying organic material. It can include leaves, twigs, grass, and even food scraps like banana peels and apple cores. As these materials break down, they create a nutrient rich material which has the following uses.

• Adding nutrients to soil.
• Supporting soil organisms that help plants.
• Retaining moisture and reducing water run-off.
• Increasing soil pore space to help roots develop.

Compost can be made at home, but it can also be purchased at the store. When buying compost, look for a dark color and crumbly texture. It should have a mild odor. It should not be extremely wet or extremely dry, just a little damp.  

Other Fertilizers and Amendments

The Community Garden Program Policies prohibit the use of artificial chemical fertilizers. There are many naturally derived fertilizers and other amendments that can be used, however.

• Blood Meal: This adds nitrogen to the soil and can help to deter pest animals like moles and squirrels. It can also make soil more acidic by lowering soil pH.
• Bone Meal: This adds phosphorus to the soil, but only works when soils have a balanced pH level. When the pH is 7 or higher, bone meal is ineffective.
• Fish Emulsion: Made from ground up fish, this adds multiple different nutrients to the soil. Dilute it with water per directed and pour around the bases of plants.
• Fish Meal: A strong source of nitrogen, fish meal is a slow release fertilizer that is especially helpful for corn, eggplant, and leafy greens. Make sure to apply as directed on the package.
• Manure: Sourced primarily from cows, horses, and chickens, manure can help improve soil not just by adding nutrients but also increasing moisture retention. It is important not to use manure derived from humans, cats, or dogs due to the risk of parasite contamination. Fresh manure is too strong for plants, so it is important to use manure that has had time to age. It can also be blended with compost.
• Mulch: Made up of shredded wood or similar material, mulch does release some nutrients as it decomposes, but that is not its primary function. Mulch helps to protect root systems and retain moisture in soil. At the end of the season, it can be tilled back into the soil. Do not use non-organic mulch made of plastic, rubber, or other synthetic materials.
• Peat Moss: Used primarily to improve a soil’s ability to retain moisture and nutrients, peat moss is especially helpful when soil is sandy. It is incorporated when dry into the soil.
• Potash: This is a form of potassium that can be incorporated into the soil through tiling.
• Rock Phosphate: Best applied in early spring, this material helps to provide a slow release of phosphate into the soil over the course of the season. It is very similar to bone meal.
• Topsoil: While not a fertilizer or soil amendment, commercial topsoil can be used to fill in low spots on a site. It is an artificial mix of sand, rock, and organic materials meant to mimic high performing naturally occurring soils. When possible, topsoil should be blended with soil already present at a site.
• Wood Ash: This is a good source of potassium. It also serves as a pest control agent that can deter snails and slugs. Only untreated wood should be used for this and it should be applied in only small amounts. Ash may harm plants that prefer acidic soils as it raising soil pH.

Tilling and Plowing

Tilling breaks the soil into smaller pieces. It can be done using hand tools or using a machine like a rototiller. This is most often done at the beginning of the season and used to incorporate soil amendments like compost and increase the soil’s air and water capacity. This is different than plowing which goes deeper into the soil and flips it over. That is usually only necessary when the soil is especially compacted. While it is important to loosen soil in preparation for planting and to incorporate amendments, it is important not to do it more than is necessary. Frequent plowing and tilling can damage soil. In most cases, tilling once at the beginning of the season is sufficient. Urban soils may be especially difficult when first cultivated, but will become easier to work over time.