Yuma Ag and You: Urban Soils

Bobbi Stevenson-McDermott       10/25/15 publication

Ahh, it is not summer! My husband and I have decided that there are really only two seasons in Yuma, summer and not summer. You can tell it is not summer by the amount of outdoor activities occurring in neighborhoods all over town. Trees and bushes are being trimmed, lawns are being renovated with winter rye and the nurseries have hundreds of bedding flowers, vegetables, trees and cacti available for planting. The soils in your yard contribute to the success or failure of your planting efforts.

Soils in an urban area may share come properties with soils in farm fields or even other urban areas. There are large differences in soils as they occur naturally in farmed fields and these differences are changed when a area is converted to an urban area. Construction activities, compaction and surface sealing dramatically change soil properties and can sometimes result in reduced ability to perform the critical functions or activities of natural soil.

Characteristics of soil in any urban area depend on many things. They depend on how deep the site has been excavated during construction and if new materials were brought in and mixed with the original soil materials. The soil properties including permeability, compaction infiltration may be altered. They depend on the properties of the original natural soil and the past uses of the site, Many times topsoil is removed from the site prior to construction and may or may not be returned to the site. After excavation, subsoil may be place as fill over topsoil. Changing the order of the soil layers or mixing the topsoil and subsoil can alter soil properties. These variables make predicting soil behavior difficult in urban areas.

All soil is made up of air, water, numerous kinds of living and/or dead organisms (organic matter) and mineral matter, sand, silt and clay. Soils perform specific critical functions no matter where they are located and they perform more than one function at a time. Soils act like sponges, soaking up rainwater and limiting runoff. Soils act like faucets, storing and releasing water and air for plants and animals to use. Soils act like supermarkets providing valuable nutrients and air and water to plants and animals. Soils also store carbon and prevent its loss into the atmosphere. Soils act like strainers or filters, filtering and purifying water and air that flow through them. Soils buffer, degrade, immobilize, detoxify and trap pollutants, such as oil, pesticides, herbicides and heavy metals, and keep them form entering ground-water supplies. Soils store nutrients for future use by plants and animals above ground and by microbes within the soil.

Soil functions occur in spite of land use. Rain water must be dispersed or regulated in urban areas, and landscaping plant roots must have air available for growth. When areas are paved over, plans must be in place to handle rainwater. Buildings constructed on fill material must still be supported by the materials on the site.

An important task is convincing people living in the urban environment to consider soil information and data before urban projects begin. This information must be part of the planning process for all urban projects. As soil properties change because of construction or other disturbances, major changes occur in the capacity of the soil to function, as predicted by engineering properties.

Local resource information can be obtained from professional gardeners, garden clubs, Master Gardeners, landscape contractors, the Cooperative Extension Service and the Natural Resource Conservation Service. Remember, plan before you plant!