It is well known that plastic
mulch film increases yields of many vegetables and fruits, especially
yields early in the season when prices tend to be higher, most likely
by inhibiting weed growth, increasing soil temperature and moisture
and reducing pest infestations. Recently, colored and reflective mulches
have been developed to suit needs of individual crops and locations.
As a result, most vegetables are grown on plastic (polyethylene) mulch
and use has reached =130 million kg-year-1 in the U.S. alone.
However, the removal and disposal of polyethylene mulch from the fields
after harvest is difficult and expensive (=$250/ha). Much of this
is improperly disposed of by burning or burying leading to environmental
and health problems. Disposal in landfills can be problematic due
to pesticide residues in plastic mulches. Recyling is difficult due
to the amount of dirt on the plastic and some degradation in polyethylene
molecular weight and strength. Also, residues of black plastic in
some harvested crops such as cotton or pulp wood make the finished
product unacceptable. Thus a biodegradable mulch which could be tilled
into the field and fully biodegrade would be desirable.
Tar-coated paper mulches began to be used in the late 1800’s,
long before polyethylene was available. Paper alone degrades too quickly
and loses most of its strength when wet so paper was coated to seal
out water and protect the paper from microbial action. Such mulches
were very effective in increasing the yields of a variety for fruits
and vegetables, though the use of tar now would probably be considered
environmentally unacceptable. Paper mulches were later replaced by
cheaper and tougher polyethylene.
Recently, paper mulches have begun to attract more attention since
they are biodegradable and are made from a renewable resource rather
than petroleum. Yields of tomatoes were similar for polyethylene coated
paper and polyethylene film but were somewhat lower for wax coated
paper. Recently work found that melon yields and soil temperatures
decreased in the order: black polyester coated paper = black polyethylene
> black paper = wax coated black paper > bare ground. Uncoated
paper degraded at the soil line within 4 to 5 weeks while the wax
coating extended the lifetime to > 6 weeks for some replications.
Paper coated with the biodegradable polyester was intact at the end
of the experiment. Similar yields of peppers and tomatoes were also
found for paper and polyethylene mulches. Paper soaked in soybean
oil was found to slow the degradation of the paper. The advantage
of using soybean or other vegetable oil as a coating is low cost and
wide availability.
Previous work has shown that coating kraft paper with polymerized
vegetable oil resins increases wet strength and decreases the rate
of biodegradation (measured by weight loss) in soil. Half lives of
the coated paper in soil inceased from 2 weeks for uncoated paper
to 4.5 to 12 weeks for resin coated papers, depending on coating type
and amount. It was postulated that the oil based coating serves as
a temporary barrier to water and microorganisms thus protecting the
underlying cellulose fiber network from degradation. In this Florida
Study, the performance of paper-polymerized oil mulches vesus a standard
black polyethylene control for raising watermelon at an experimental
farm in northern Florida was compared. The effects of coating type,
amount, method of polymerization and added color and preservative
on ease of field application visual degradation rates, weed growth,
and crop yields were studied.
Paper mulch application was considered acceptable with minimal tearing
although the speed used for the paper (3.2 km-h-1) was less than for
the plastic (4.8 km-h-1). Holes punched in the mulch by the mechanical
transplanter tended to tear an additional 2.5 to 5.0 cm. The paper
mulches laid tightly to the raised beds and resulted in acceptable
mulch beds.
The mulch used for the 2000 trial was brown but allowed some light
through the paper since no carbon black was used. Soil temperatures
as measured on 27 Mar. were similar for the coated paper and black
polyethylene mulch. Weeds grew under the nonfumigated plots of both
mulches and thus pushed the mulch up somewhat. Fumigation eliminated
weeds from these plots early in the season. The watermelon yields
were similar for the coated paper and polyethylene mulches. These
yields were lower than normal for the area due to severe crow damage.
The yields and average fruit weights of the fumigated plots tended
to be slightly higher, but these differences were not significant.
The soybean oil coated paper mulch showed initial signs of degradation
(rating 3) after 52 d and maintained its integrity until harvest (76d).
Unlike the 2000 study, the paper-oil mulches were applied wet and
allowed to cure in the field. The soybean and linseed oil coated paper
mulches and polyethylene performed similarly in terms of yield with
nearly 67,000 kg-ha-1. Average fruit weight for each mulch treatment
was also similar at 9 kg. There were significant differences in the
number of nutsedge plants between all paper mulches and the polyethylene
mulch. No sedge plants peirced any of the paper mulches by 18 Apr.
and only 11 plants/m2 were detected by 8 May. On those same dates,
however, 110 and 120 nutsedge plants/m2 , respectively, were found
in the polyethylene mulch. The nutsedge plants germinated under all
mulch treatments since no fumgant was used. In the case of polyethylene
mulch, the sedge was able to pierce the plastic and continue to grow
through it. In the paper mulch treatments, however, the sedge was
not able to pierce the paper and, as carbon black was added to absorb
light, further sedge growth was suppressed. The reason for this is
not understood, but may be due to high strength of individual cellulose
fibers which might be more resistant to rupture from the sharp tip
of the sedge shoot than the weaker polyethylene. The heavy nutsedge
populations in the plastic mulch plots made pulling the plastic mulch
for disposal at the end of the season very difficult.
Degradation ratings of the different mulches as a function of time
after application, for the first 54 d, only slight degradation occured
at the soil line of the buried tuck (rating < or = to 3). By this
time, watermelon vines had
covered the entire field and held the mulch in place so that further
mulch degradation which might cause detachment at the soil line was
not critical. Paper mulches containing linseed oil showed less degradation
(rating 2) after 54 d than mulches made with soybean oil (rating 3).
The slower rate of biodegradation of linseed than soybean oil in soil
has been observed previously and is thought to be due to the greater
number of double bonds and hence cross-linking density for linseed
oil. The addition of the preservative mix consisting of various organic
acids and aldehydes had no significant effect on degradation rates.
Since degradation seemed to occur at similar rates in the 2000 and
2001 studies, this suggests that it is not necessary to cure the paper-oil
before application and that sufficient air reaches the buried tuck
area to cause polymerization of the oil.
In this trial, coated paper mulches were wiped with a rubber blade
to minimize surface oiliness and zinc compounds were tested as preservatives.
There were no significant differences in early or total yield between
the coated paper and polyethylene mulches. Total marketable yield
was in the range of 55 to 85,000 kg-ha-1 , which is considered a good
yield for this area. No nutsedge pierced the paper mulches for the
first 60 d of the trial while a small number were able to grow through
the polyethylene mulch. Nutsedge populations were low in the plot
area. Similar to the 2001 study, nutsedge sprouted under the paper
mulches, but could not grow further.
Degradation of the coated paper mulches appeared more rapid that in
the previous two seasons, with some mulch detachment from the buried
tuck area occurring by 55 d (rating 4). This did not seem to be a
problem, however, as the plant vines and remaining buried tuck kept
the mulch in place. The more rapid degradation may have been due to
the smaller amounts of oils used for coating in the 2002 trial (=
80% add-on) versus 2000 and 2001 trial (= 100% add-on) or perhaps
some differences in weather. Previous work has shown that degradation
rates in soil increased as the amount of oil coating was decreased.
Paper coated with soybean and linseed oil degraded at similar rates
while paper coated with linseed oil containing 3.3% zinc oxide was
slower to degrade early in the season. Zinc oxide is known to have
moderate fungicidal activity.
In summary, vegetable oil coated paper mulches were applied using
the same equipment as for polyethylene with only occasional tearing.
Similar yields of watermelon were obtained for polymerized vegetable
oil coated kraft paper and polyethylene mulches over 3 years of trials
in Florida. The paper mulch blocked the growth of nutsedge while the
polyethylene did not. This should help lessen the requirement for
fumigation, especially methyl bromide which is currently being phased
out in the U.S. The coated paper mulches begin to degrade significantly
after 40 to 60 d, but this was long enough for one crop cycle. Coatings
containing linseed oil and ZnO tended to have slower degradation than
those made with soybean oil and no preservative.
Problems with the oil coated paper mulches include oil on farm equipment,
variability in degradation rates, lifetime of just one crop and higher
initial costs than polyethylene mulch. The amount of surface oil on
the surface of the mulch was minimized by wiping with a rubber blade
but some oil would still get on surfaces of application equipment.
This can be removed before it hardens with a cloth and alcohol. Alternatively,
coatings made from epoxidized soybean oil and a curing agent such
as citric acid can be rapidly thermally cured as the paper is being
rolled, thus eliminating the oiliness problem. There will probably
always be variation in mulch degradation rates due to change in soil
types, microorganism levels, temperature and rainfall. A margin for
error in which degradation begins in earnest after cropping is finished
would be desirable. There are certainly more effective fungicides
than ZnO, which could be used, perhaps only on the edges of the mulch
to further slow degradation. Alternatively, a more highly unsaturated,
slower degrading oil like linseed or tung could be used in the center.
Recent work on the degradation of ESO-CA coated paper has shown that
it would last for 20 weeks or two cropping seasons. Finally, the cost
of polyethylene mulch is typically $250 to $370/ha compared with a
cost of a polymerized oil-coated paper mulch of perhaps $610/ha or
more. This is offset by the elimination of the approximate $250/ha
of removal and disposal of polyethylene mulch. Thus, such a biodegradable
mulch might appeal more to smaller farmers growing high value crops
and in areas of the country where where disposal of plastic mulch
is especially difficult.
