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.
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