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.
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.”
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.