Manicured lawns lend green hand to Washington farmland

Washington State University researchers rescue nitrogen benefits from discarded yard waste

By Cara Hungerford

June 2, 2005: Every year Americans throw away roughly 28 million tons of yard waste, amounting to nearly 13 percent of all solid waste collected.[1] While nearly half of this material is recycled, the other half--a perfectly compostable mixture of grass clippings, leaves, branches and other unwanted plant material--ends up lying at the bottom of a dump somewhere. This less-than-ideal situation led Washington State University researchers to wonder, was a valuable (and free) source of nitrogen taking up space in already overcrowded landfills? As the researchers noted in the introduction to their paper, "Yard Trimmings as a Source of Nitrogen for Crop Production," “Yard trimmings can provide nutrients and organic matter to benefit crop production and soil quality.” In addition, removing trimmings would alleviate the pressure on composting facilities that often operate above capacity during the spring which, according to the researchers, can “cause odor problems.”[2]

To prove the possibility of their relationship, the researchers took to the fields, setting up a three-year study that tested the ability of yard trimmings to provide the nitrogen needed to grow corn in a silage corn-winter triticale rotation. They set up five test plots: three received incremental amounts of yard materials (22, 44 and 66 mg/ha of dry material), one received inorganic N and the final was a no-N control. Each treatment was replicated four times. The trimmings used in the experiments were collected from a local composting facility that received them via a combination of curbside pick-ups and landscape company drop-offs. Before the trimmings were applied to the field, the recycling facility screened and ground the material and allowed it to aerate in windrows for three to five days. The fields were planted within three days of the yard material application and no additional fertilizers were applied to the fields.

Researchers found that at all application levels, corn grown with yard trimmings yielded as much or more than corn grown with inorganic N. At the highest level of dry material added (66 mg/ha ), the yard trimmings out performed the chemical inputs. In addition, researchers found that with the higher trimming applications the N effects carried over to the triticale seeding, resulting in higher yields in the spring. Results also showed that organic matter and K levels were greater in fields receiving yard trimmings.

While immature composts have been known to contain phytotoxic compounds that can delay germination or kill seedlings, the researchers experienced no such problems with the yard trimmings.

The only measure in which the yard trimmings seemed slightly inferior to the inorganic N source related to levels of apparent N recovery (ANR). Researchers found ANR levels were lower in soils fertilized with yard trimmings than in those where a chemical N source was used. Researchers were not surprised by these results since only a portion of the N from organic matter becomes available for plant uptake. Although the levels were lower than those for the fertilized field, the data showed the trimmings still supplied a substantial amount of available N to the crops.

In the end, slightly lower ANR levels have been outweighed by higher yields, lower inputs and community responsibility. Washington growers have already begun to use yard trimmings to grow a variety of plants from rhubarb to cabbage. It will be interesting to see in years to come if this ambitious relationship does indeed succeed at lowering waste levels, reducing stress on composting facilities and providing farmers with a valuable organic source for soil nutrients.

1. U.S. Environmental Protection Agency, 2000. Characterization of municipal and solid waste in the United States. 1999 update. EPA530-F-00-024. Office of Solid Waste and Emergency Response. EPA, Washington, DC.
2. Bary, A.I., Cogger, C.G., Myhre, E.A.. 2004. Yard Trimmings as a Source of Nitrogen for Crop Production. Compost Science and Utilization. Vol.12, No.1, pp. 11-17.