Denver is one of several communities to mandate the use of compost in disturbed soil and new landscaping as a means of improving the ability of soil to conserve and manage water. As water stress and pressures have become increasing concerns in western US states, simple, cost-effective solutions such as requiring a minimum amount of organic matter to improve soil properties have become more common.
Colorado’s heavy clay soils increase stormwater runoff, and therefore help provide the impetus to create rules to improve the water-holding capacity of soil. The need to conserve water primarily drove the Denver Board of Water Commissioners to begin its Soil Amendment Program. The program incorporated a requirement into the Denver Water utility’s Operating Rules for adding soil amendments to newly installed landscaping projects, effective 2008.
Denver’s Soil Amendment Rule
Under Operating Rule No. 14.02.4, soil preparation via the use of four cubic yards of compost per 1,000 square feet is mandatory for the installation of new landscaping materials on an owner’s premises. The owner or occupants of the licensed property are responsible for compliance. Properties exempt from amending the soil include those with less than 300 square feet of amendable soil on their property, and those whose outdoor water use is only for planter boxes or non-permeable surfaces.
14.02.4 Soil Amendment for Irrigation of Turf at Newly Licensed Premises. Proof of proper soil preparation is required before installation of plant material. Penalties may apply if soil amendment is not completed prior to the installation of plant material. Proper soil amendment is the equivalent of adding approved compost at a rate of four cubic yards per 1,000 square feet of permeable area, incorporated (roto tilled) to a depth of six inches.
Adding soil amendments to landscaping is promoted by Denver Water as a best management practice, particularly because it is a one-time investment that builds water conservation into a yard’s design. Making water conservation a routine habit can help keep drought at bay, as suggested by the utility’s Conservation Plan. A Denver Water survey of one neighborhood found that water-wise remodeling of one’s yard, whether xeriscaping or raised beds, could significantly reduce the 12 gallons per square foot of irrigated area that are used annually for the typical Denver residential landscape, while also remaining aesthetically pleasing.
Other Colorado Programs and Policies
While there are no soil amendment regulations at the state level, soil amendment requirements are nothing unusual in the state of Colorado. According to Bob Yost, Chair of the Rocky Mountains Organics Council and Chief Technical Officer of A1 Organics: “They have had a notable positive impact beyond soil benefits, e.g. economic income which is realized through reduction of water consumption and less need for fertilizers.” Indeed, adding one percent of organic matter to the top six inches of soil can increase the soil’s water retention capacity to 27,000 gallons of water per acre, according to USDA’s Natural Resource Conservation Service.