edited by Dr. Frank J. Dainello, and produced by Extension Horticulture, Texas Cooperative Extension,
The Texas A&M University System, College Station, Texas.
appeared in the “Journal of Vegetable Crops Production,” Vol. 6(1) 2000,
published by The Haworth Press, Inc.
The use of plastic mulch has become an important cultural practice in the commercial production of fruits and vegetables in many regions of the world.Plastic mulches offer many advantages to growers, including earlier and higher overall yields, reduced evaporation, fewer weed problems, reduced fertilizer leaching, reduced soil compaction, elimination of root pruning, cleaner produce, more effective fumigation, and ability to double/triple crop.
The major disadvantages of plastic mulch are initial cost, removal, and disposal. At present, most mulch is placed in landfills. Recycling and incineration have been proposed, and are used in some area.
Commercial machines are available to life and loosen the mulch, but many growers still collect the mulch by hand after loosening and lifting it with machines. There are machines available to roll up the loosened mulch. The difficulty with mechanizing the rolling up of the mulch is that the speed of the roller must vary to compensate for the increasing diameter of the spool, as the mulch builds up on it, and to compensate for changes in tract ground speed. One machine uses a friction drive to address this problem. Another machine uses a hydraulic drive with a manually adjusted flow-control valve to vary speed. One grower-built machine used a hydraulic drive with a manually adjusted flow-control valve manipulated by an extra operator riding on the rear of the machine. However, there is a need for a reliable, fully automatic spool speed-control system to allow automated collection of plastic mulch. There is also a need for a better means of disposing of the plastic once it is lifted from the field.
Scientists at Louisiana State University have developed a machine for automatic collection of mulch which senses the torque on the pick-up spool, and automatically adjusts speed to compensate for changes in ground speed and changes in spool diameter. The objective of this study was to equip the automated machine with LP gas burners, to allow field incineration of the plastic during the lifting process, thus potentially eliminating the need to haul the plastic out of the field for later disposal.
The mulch collector was modified to test the concept of incineration of plastic on the machine during the lifting process. The spool on which the plastic was rolled was removed and replaced by two rollers, which pulled the plastic up and over the machine. The mulch was lifted loose from the ground in a separate operation before operating the incinerator. The lower roller was powered by a hydraulic motor in the same manner as the spool on the original machine; and the drive speed was automatically adjusted, as on the original machine. This roller was made of steel and was 76 mm (3 inches) in diameter. Stainless steel wire 1.3 mm (.050 inches) in diameter was wound around this roll in a double helix (diamond) pattern to provide a more aggressive grip on the plastic surface. The wire was replaced with 13 mm (0.5 inches) hemp rope during development. The upper roller was made of sponge rubber over steel and was 60 mm (2.38 inches) in diameter (TEW model R-15, TEW Manufacturing Co., Penfield, NY). Both rollers were 1.2 m (48 inches) long. The bearings for the upper roller were mounted in slots so that the upper roller could move upward, away from the lower roller, to allow trash or wadded plastic to pass through the rollers. It was held down by coil springs. A lever allowed an operator to easily open the rollers to feed the plastic at the start of the field, or whenever the plastic would tear.
The other major modification to the original machine was the addition of four LP gas burners to the rear of the machine under the frame. The burners (or the type used for flamed weed control) were rated at 147 kw (500,000 BTU/h) each (model CPM-001, Thermal Weed Control Systems, Inc., Neillsville, WI). The burners were mounted under and behind the rollers, aimed horizontally to the rear. They were placed 23 cm (9 inches) apart. Galvanized steel sheet was used to form a chute to guide the plastic from the rollers down past the burners. A plywood cover over the chute forced the plastic into the flames. The tips of the burners were approximately 15 cm (6 inches) from the plastic as the plastic fell past them.
The prototype machine was tested in a strawberry production field after the crop plants were killed with herbicide (paraquat) and shredded, and in a field where no crop had been grown. A standard mulch lifter was used to loosen the mulch on the bed surface, with the soil either removed or at least loosened and shattered. The prototype machine was then used to lift and incinerate the plastic. The tests were conducted at the Hammond Research Station, Hammond LA, in June 1999. Both manual pickup and operation of the prototype were evaluated. Time studies, fuel consumption, and percentage of plastic removal were measured on four replications of each method.
Two persons were needed to operate the incinerator machine, as with the original mulch collector. One person drove the tractor and the other walked alongside, re-threading the plastic into the rollers at the beginning of each row and whenever the plastic tore. The second person had to avoid the rear of the machine because of the flames in that area. A linkage was provided to facilitate lifting the upper roller from the side to insert the plastic. The mulch would not feed due to soil left on the mulch from the lifter and due to deterioration and weakening of the plastic from exposure to sunlight, so it was necessary to manually lift the plastic ahead of the prototype incinerating machine to remove all soil and, thus, reduce the tension needed to pull the mulch through the machine. The pickup rollers on the incinerator were less aggressive than the spool on the collecting machine, and were not able to lift the weight of plastic and soil. The plastic tended to slip through the rollers. An improved preliminary mulch lifter or more aggressive rollers on the incinerating machine might have eliminated this problem.
Even though preliminary work indicated that the plastic mulch was easily destroyed by flame, the mulch proved to be more difficult to incinerate than expected. A ground speed of less than 1.5 km/h (1 mile/h) was used to provide a relatively long dwell time for the plastic in the flame. Even with a speed that slow, the plastic tended to melt into globules rather than being consumed in the flame. The plastic would have had to be retained in the flame for a longer time to thoroughly burn. Most of the plastic was reduced to small solidified globules, some of which continued to burn after dropping to the soil. These globules, while small, may prove to be a residual problem in the field, since they will be less subject to deterioration by ultraviolet light than unburned plastic. No attempt was made to collect the globules on the machines; since the globules were burning as they fell, they were partially molten and would stick to any collection device. The results of the field trial are summarized in Table 1. The mass of the globules manually collected in the test plots was approximately equal to the initial mass of the plastic mulch.
The mulch incinerator prototype did not adequately incinerate the plastic mulch. The plastic mulch was merely melted into globules, not consumed by burning Since the globules are much thicker than the original mulch, they will likely break down more slowly in he soil. The labor require to operate the prototype was excessive.
Based on the performance data for the mulch incinerator tested in this study, considerable research is still needed to develop an effective mulch removal system. Field burning, where permitted, and landfill disposal still are the only means of disposing of used mulch.
Table 1. Performance data for mulch incinerator test.
MANUAL REMOVAL
FIELD INCINERATOR
- Man-hours/ha (@ 75% field efficiency, 2 people) = 9.1 (3.7 man-hours/a)
- Material removed = virtually 100%
- Man-hours/ha (@75% field efficiency, 2 people) = 47.6 (19.3 man-hours/a) (Includes time to manually lift plastic to remove excess soil)
- Fuel consumption = 335 l/ha (35.8 gal/a)
- Material removed = approximately 0%
