Soil Preparation

• Soil structure

Garden soil provides plants with air, water, and mineral nutrients. The soil in good physical

condition (good structure or tilth) can hold and provide plant roots with adequate quantities of nutrients, water, and air; it will drain well and be easy to work without becoming sticky when wet or crusted when dry. Most plants, but particularly vegetables, grow best in well-drained soil.Incorporate a 2- to 4- inch layer of compost and manure to a depth of at least 10 inches. Soil alkalinity and acidity are measured on a pH scale, with 7.0 being neutral, below 7 tending toward acidic, and above 7.0 tending toward alkaline. Though different plants prefer different pH levels, most vegetables prefer slightly acidic soils - in the range of 6.2-6.8 pH. Our soil in Southern California averages approximately7.7pH

• Trace Elements in Urban Soils

Trace elements occur naturally in all areas and soils of the world. Indeed, some are essential for humans, plants, and animals. We always ingest them to some extent in food and water, and we breathe them in the air. Their concentrations are naturally low and usually not hazardous in non-urban areas with little or no environmental disturbance and past or present human activity. However, any place subjected to human activity is likely to have trace elements at elevated levels in the environment, particularly in the soil.

While there is generally no cause for alarm, in some cases, these trace elements may accumulate to a level where it would be advisable to take measures that ensure they will not pose a health risk. 

Potentially harmful trace elements, especially heavy metals, can accumulate in the soil

and on plants and may pose a potential health risk to people who breathe or, especially, swallow contaminated soil or eat contaminated vegetables. Although plants always absorb trace elements to some extent, in nearly all instances the quantities they take up are too insignificant to cause alarm. The potential harm is from trace elements deposited on leafy vegetables from dust, splashing water, and the air or are in the soil and cling to root crops like carrots and beets. In these instances, thorough washing of vegetables usually eliminates them. Lead is perhaps the most prevalent and problematic trace element in urban gardens but others, such as cadmium and arsenic, are often present.

• Testing Urban Soils

Testing soils in urban gardens is always a good idea. Not only will it reveal which trace elements are present and their concentrations but also where they are located in the garden. An added benefit of testing the soil is the opportunity to gather information about basic nutrient levels at the site. Many companies offer soil-testing services. sometimes university cooperative extension services can provide you with a list of soil testing

companies.

When collecting soil samples it is important to follow the instructions the soil testing company provides. It is a good idea to collect samples form several places at the garden site. For example, if individual gardening beds are 20 by 20 feet it would be appropriate to keep soil from each bed as a separate individual sample. However, several collections from different points in one bed could be mixed together to form a composite sample for that bed

• How to Eliminate or Reduce Exposure To Trace Elements

There is no cause for alarm if tests show that there are concentrations of trace elements above those levels that one would expect in urban soils. There are several procedures that can reduce or eliminate these elements or exposure to them so that they are not harmful

- Thoroughly remove and properly dispose of refuse from urban garden sites before

planting. Be especially vigilant for old painted wood, tires, galvanized metal, and batteries

- Always wash hands thoroughly after working or playing in urban soils. Thorough

hand washing is especially important for children who are more susceptible to trace elements Consider using gloves, especially for children.

- Thoroughly wash all vegetables and fruits and peel root crops like beets, radishes,

and carrots before eating. Discard older, outer leaves of leafy vegetables.

- Dilute pockets of higher than normal concentrations of trace elements by digging and

spreading the soil over a large area in the garden or by adding additional, clean soil and thoroughly mixing it with the contaminated soil. It is generally not feasible to dig contaminated soil from the garden and dispose of it off-site. Such soil would be considered

toxic waste and strict rules and regulations govern its disposal.

- Since most potentially hazardous trace elements are found in the upper one to

two inches of uncultivated or undisturbed soil, it is always a good idea to cultivate soil deeply, to at least eight inches deep if possible, before planting to dilute the contaminated

soil with the clean soil below.

- Locate plantings, especially leafy crops, as far away as practical from streets. As much as 75 feet would be good. Utilize barriers, such as walls, fences, and hedges, to help to

block automobile exhaust and dust from coming into the site from streets and roads.

- Maintain soil pH near 7, or neutral, and ensure phosphate levels are adequate.

- Use large amounts of organic matter, like composts and hummus, to amend the soil.

- Mulch with leaves, wood chips, hay, newspaper, or other material to reduce dust.

Getting Started

- Consider planting in raised beds or containers using uncontaminated soil. 

- Consider having separate gardening shoes and clothes with long sleeves and pants,

especially for children, which are not brought into the house.

- Locate child play sites in uncontaminated areas and/or cover the ground around play

areas with an impermeable, artificial play surface, sand, or wood chips.