Land development including landscaping practices damage soil structure and function by removing or compacting topsoil. These practices can impact water resources by decreasing infiltration, increasing erosion, impairing fish habitat, and increasing the need for permanent stormwater management.
These practices also create chemically dependent landscapes which are difficult and expensive to maintain and contribute to polluted runoff. Soil compaction also reduces the water retention capacity of soil which requires additional irrigation and increased public water supply demand. This fact sheet provides guidance on soil amendment practices and implementation of soil amendment standards and ordinances.
Compost, an organic material, absorbs and infiltrates rainwater, reduces flooding and soil erosion and filters out pollutants typically associated with stormwater runoff. Compost also stores water and nutrients for plants to use during drought conditions, promoting healthy plants and better looking lawns that require less irrigation, pesticides and fertilizers. In addition, healthy amended soils that require less irrigation reduce municipal water demand.
Programs developed to provide volume control through soil amendments may include MS4 standards and/or ordinances. Soil amendment guidelines as well as guidelines for standards and ordinance development are identified below. The program is ultimately dependent upon several factors including the MS4’s available resources, extent of development and/or redevelopment opportunities, and character of its soil and stormwater runoff.
Awareness campaigns inform the public, public employees, businesses, property owners, and elected officials of the negative effects of soil compaction and the benefits of soil amendments. Efforts can also contribute to generating acceptance of a new ordinance and encouraging individuals and organizations to implement soil amendments on a voluntary basis.
Brochures Develop informative brochures, and guidance for specific audiences such as developers, businesses, homeowners and local development permitting authorities.
Signage at MS4 installations Locate signage at parks and government buildings identifying compost-amended sites and the associated functions and benefits.
Workshops and seminars Workshops and seminars can be used to provide the technical assistance that developers, city staff and consultants will need in order to meet a new soil amendment ordinance.
Design variants are summarized below to provide guidance appropriate for implementing soil amendments within various site constraints and conditions. A good design approach will likely apply a combination of techniques at a single site based on the local conditions. There are soil and compost calculator worksheets in the Additional Resources section.
General guidance Unless soils are native and can be left undisturbed, the following guidance applies to techniques implemented:
Undisturbed native soil Areas of the site that do not need to be disturbed should be identified to protect areas of native vegetation. Fence off these areas to protect them from compaction during the construction phase.
Amend existing soil in-place Where the soil has been compacted or the organic layer (e.g. forest duff or upper soil horizon) removed, the simplest way to restore soil quality is to rototill compost into the existing soil. Apply a 2.5-inch deep layer of compost to the existing soil. Rototill compost into the soil to a depth of at least 8 inches. Tilling to this depth will require repeated passes with a large machine, such as a tractor-mounted or heavy rear-tine rototiller.