10. Economics and limitations of soil solarization

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Cost of soil solarization
Current limitations to commerical uses of plastic mulches for soil solarization
Economic assessment of the long-term effects of the soil heating technology in Beni Suef Governorate

Cost of soil solarization

Clyde L. Elmore

Department of Botany, University of California, Davis, CA 95616

A pest control practice must be economically feasible for it to be used. Thus, it is beneficial to evaluate the practice on a cost/benefit analysis.

Few studies have been conducted comparing soil solarization to any "standard practice" of pest control. Most studies are done comparing soil solarization to untreated soil. When solarization is compared to untreated areas, yield increases are usually recorded (2, 3, 5, 6). There are many other factors that must be taken into consideration.

In this review I will try to give several examples of potential uses and evaluate some of the economic variables to consider for pest management.

Costs of solarization have been evaluated in three ways: 1) Utilizing an energy balance (4) technique. This method is the one that must eventually be used in the final analysis; 2) Comparing the cost of the treatment to the yield and/or quality gain from treatment; and 3) An evaluation of solarization to offset, or eliminate other pest control costs, or changes of crop culture.

There are at least seven areas where opportunities exist to use soil solarization economically. These include:

1) In a crop where there is no available pesticide due to lack of registration, availability, crop tolerance, hazard of application from a pesticide, or expense.

2) In a crop where there are pest problems that do not allow control by other means.

3) In circumstances where more than one pest problem (weeds, soil pathogens, etc.) can be solved with solarization.

4) Where a crop is grown without synthetic pesticides (organic).

5) Where solarization can change the cropping sequence or culture to increase yield on the same area or maintain yield on smaller areas.

6) In crops where early seedling vigour and rapid growth is an advantage.

7) Competition in markets - where "organic" foods are competing with conventionally produced pesticide-treated products.

The most complete analysis utilizing the second technique (yield/quality improvement) with various crops and crop cultures has been conducted by Basheer et al. (1) in an unpublished technical report in 1988. Most of these examples show the effect of yield increase due to reduced loss to soil pathogens or a nutrient response. These reports show a financial loss using solarization in a low value crop (broadbean) in two locations in Egypt over a 3year period. Yields were increased, but the yield increase alone did not offset the cost of solarization. There is no qualify evaluation differential given with this crop.

In tomatoes, six out of seven comparisons showed a benefit in net profit on this high value crop in two research stations in Egypt. Increases ranged from 1155 LE/ha to 4633 LE/ha above the variable costs. In these comparisons, however, solarization costs were about 60 percent to 65 percent of the total variable cost; thus, dramatically increasing the risk factor to the farmer. In the experiment with a loss, yield was increased 39 percent; however, it did not pay for the solarization costs.

In onion bulb experiments (4), reported losses were incurred in all experiments. Net income from locally utilized onion bulbs rather than export onions and lower yields in the experimental area versus other areas in Egypt attributed to a loss of income. It should be noted that there was no mention of a major pest problem involved; thus, though the yields in the non-treated areas were low, they, in most cases, had a minimum profit. To illustrate the effect of a devastating soil pathogen, Basheer et al. (1) made the following comparison. In a site heavily infested with a soil pathogen in Israel, there was a comparison of solarization; the fumigates ethylene dibromide and methyl bromide and an untreated area. The untreated area did not produce any yield.

The garlic in the methyl bromide yielded a high number of Grade B lower priced bulbs than either ethylene dibromide or solarized soil. The treatment that showed a positive income response was solarization. Part of this increase was due to the proportional increase in Grade A bulbs compared to other treatments.

The actual costs involved in solarization can, in some cases, be substituted for other methods of pest management. With weed control, this would include cultivation for weed control, hand hoeing, or pulling of weeds. These cultural practices may not be effective to reduce soil pathogens or nematodes. Actual costs include 1) cost of the polyethylene or other mulch material; 2) cost of application; 3) land preparation and irrigation (if different than normal seedbed preparation); 4) taking the land out of production for a period of time; and 5) environmental contamination (if the polyethylene cannot be recycled).

Using the first method of analysis (energy balance on input/output), Iacoponi et al. (4) evaluated the costs with Sclerotinia minor in central Italy on lettuce. In the short term, increased yields were obtained with increased profit. However, using a figure of about 30 000 MJ ha-1 of additional energy input from the polyethylene, they concluded that "the output/input balance shows a tendency to decrease in solarized lettuce crops" (from 0.27 to 0.24 and from 0.43 to 0.30, respectively, in lettuce cappucina and lettuce romana).

In an analysis on onions in southern Israel for pink rot control, it was found that solarization increased yields 125 percent. The yield increase of 31.9 T/ha was more than enough to cover the cost of solarization (8.4 T/ha required to offset costs).

In a second experiment on pink rot in onions, yield of Grade A bulbs were increased 82 percent and Grade B increased 28 percent. The increase was not only for quantity, but of qualify of bulbs which gave a profit. In an experiment on peanuts, however, for quality effects from a disease, it was found that though quality was improved, variable costs were greater than income.

In experiments in California, USA (Elmore, unpublished data), three areas of interest were subjected to economic analysis. First is the weed control effect and subsequent affect on broccoli yield. In these studies, field areas were solarized in July and August followed by polyethylene removal and direct seedling in September. A comparison was made to a standard trifluralin treatment, preplant incorporated. A weeded and unweeded control was included. Yields and value (dollar/A) (Table 1) show an increase due to solarization.Yields were significantly different because of the control of annual weeds and suppression of Convolvulus arevense (field bindweed). Also, no fertilizer was added, thus the nutrient response in the solarization blocks was also partly responsible for the yield increase. Similar yield increases have been observed in green onions and carrots.

Secondly, there is a possibility of solarization replacing a soil fumigant treatment. Experiments were conducted to evaluate the effect of solarization versus methyl bromide plus chloropicrin or metham preplant in strawberries. These tests were conducted in the cooler, coastal climate of California. Yields of strawberry were highest in methyl bromide treated areas (significantly higher than check plots).

Yields of strawberries in solarization treated areas were not significantly different from the untreated. In a field with low pest pressure, this would not be of concern, however, it could be detrimental to crop yield in heavy, soilpest pressure areas.

The third area of major economic contribution from solarization is to change the crop culture, and grow more crop on less area or increase total output. In seed onions one row of plants are planted on 75 cm beds. This is necessary so multiple cultivations can be used for weed control. During the crop season, two herbicides may be used, with applications of one of the materials used two times. Hand hoeing is also required for good weed control.

In a field study in California (Elmore, unpublished data), different planting arrangements were evaluated compared with the standard method of one row per 75 cm (single bed). Other combinations of planted rows and beds (Table 2) give different onion plant populations. These different planting schemes can only be possible if cultivation in the beds is not required. To evaluate the economic differences, the 75 cm wide bed with I row and the 150 cm bed with 6 rows were compared (Table 3). The primary pests al this site were weeds; however, there could have been an additional effect of a soil pathogen. Not only was it shown that more onions could be grown on less area by changing the management system, but it would be more profitable to grow multiple rows of onions on larger beds if the woods can be controlled. A major reason is that the fixed costs are better utilized. This technique may be possible on other crops as well.

To summarize the economic concerns of soil solarization, it is apparent that specific concerns must be met: 1) to be viable there is need for a pest to reduce yields under "normal" use situations; 2) solarization must be used on high value crops for a yield increase to be an economic advantage; 3) allowing a specific crop to be grown where under normal pest pressure it could not be grown or where no other treatment is available; 4) changing a crop culture to increase production; and 5) where there is an environmental concern with pesticides.

Another analysis that should be made is on non-pesticide treated or "organically grown" crops. This application could be used on crops that are grown without synthetically produced pesticides or in markets where there is a concern for potential pesticide residues in the food crop. Since the organic or "produced without pesticides" food is currently selling for a premium, a reduced yield is possible (Table 4) with a net income still higher (Case B) or Lower (Case A) than solarization, depending upon yield of the crop. Commodities from fields that were solarized could also be considered organic and may demand the same high prices as in Cases A and B then higher income is possible with solarization. Thus, it is currently feasible to produce some crops cheaper than when solarization is used, unless there is heavy pest pressure. When there is heavy pest pressure, reduced net income is likely. This is a hypothetical economic analysis.

Actual use costs for pest control with solarization in the Jordan Valley was supplied by a local farmer (Table 5). These costs indicate that the use of black plastic can give pest control, and in addition be used as a mulch to plant into for subsequent vegetable plantings. Transparent plastic costs were less than methyl bromide fumigation.

References

1. Basheer, A.M., D. Yaron, Y. Wijler, A. Auizohar, A. Dinar, M.F. Adel Rahim, M. Satour, J. Katan, and A. Grinstein. 1988 An economic analysis of soil disinfestation for controlling soil borne diseases. Unpublished Tri National Project Report (Egypt, Israel, USA).

2. Chauhan, Y.S., Y.L. Nene, C. Johansen, M.P. Haware, N.P. Saxena, Sardar Singh, S.B. Sharma, K.L. Sahrawat, J.R. Burford, O.P. Rupela, J.U.D.K. Kumar Rao, and S. Sithanantham. 1988. Effects of soil solarization on Pigeonpeas and Chickpea. ICRISAT Research Bulletin No. 11. 16 pp.

3. Grinstein, A., J. Katan, A. Abdul-Razik, O. Zeydan, and Y. Glad. 1979. Control of Sclerotium rolfsii and weeds in peanuts by solar heating of soil. Pl. Dis. Report. 63:1056-1059.

4. Iacoponi, L., M. Miele, A. Materazzi, G. Vannaccu, and E. Triolo. 1989. Abstr. Solarization in vegetable crop production. Effectiveness and Economic Analysis in Lettuce Crop Control. In: Intern. Sym. on New Applications of Solar Energy in Agriculture. Università degli Studi di Catania, Sicily, Italy.

5. Jacobsohn, R., A. Greenberger, J. Katan, M. Levi, and H. Alon. 1980. Control of Egyptian broomrape (Orobanche aegyptiaca) and other weeds by means of solar heating of the soil by polyethylene mulching. Weed Sci. 28:312-316.

6. Martyn, R.D., and T.K. Hartz. 1985. Soil solarization for the control of Fusarium wilt in watermelon. Texas Agri. Exp. Station Report - 1302. 13 pp.

Table 1. Effect of solarization on annual weed control, field bindweed suppression and broccoli yield - U.C. Davis, California

1 Field bindweed cover evaluated two weeks after plastic removal.

Table 2. Solarization and weeding costs ($/A)

1 Hand weeding the beds plus cultivation of furrows. DCPA applied at 10 Ibs/acre preplan! incorporated.
2 Hand weeding plus polyethylene.

Table 3. Partial accounting method of economic analysis of solarization in seed onion production, U.C. Davis

1 $/Acre

Table 4. Hypothetical economic analysis for an organically grown crop

1 Includes cultivation, pre-irrigation and cultivation, 1 or 2 times to reduce weeds.
2 Costs would change depending on amount of fixed costs needed to produce the crop.
3 Cost for land (neutral or purchase), equipment, etc.

* Case A - A major reduction in yield due to an organism (pathogen, nematode and/or weeds).
** Case B - Low population of pests but still with reduced yield.

Table 5. Jordan Valley - Cost of solarization with transparent and black polyethylene for solarization compared to methyl bromide fumigation - a farmer's cost (Jordanian Dinars)

1 Same costs involved for each treatment.
2 One-half of cost is for solarization since black polyethylene is also used for mulching (50 percent cost).

Current limitations to commerical uses of plastic mulches for soil solarization

James E. Brown¹, Clauzell Stevens², Victor A. Khan²

George J. Hochmuth¹, Walter E. Splittstoesser⁴

Darbie M. Granberry⁵ and Brandon C. Early¹

¹Department of Horticulture and Alabama Agricultural Experiment Station, Auburn University, AL 36849.

²George Washington Carver Agricultural Experiment Station, Tuskegee University, Tuskegee Institute, AL 36088.

³Vegetable Crops Department, University of Florida, Gainesville, FL 32611.

⁴Department of Horticulture, University of Illinois, Urbana IL 61801.

⁵Cooperative Extension Service, University of Georgia, Tifton, Georgia 31793.

Abstract

Perhaps a major limitation to commercial uses of plastic mulches for soil solarization is the disposal of the plastic film after use which presents an environmental pollution problem. Plastic film is usually removed from the field by one of the following ways: 1) discing, 2) burning, 3) physical removal and 4) removal and storage of plastic. The labour cost of removing the film at the end of the exposure period is high. Farmers in various regions have reported removal and/or disposal costs of more than $240 per ha. Burying the plastic with a disc harrow, although never desirable, can be used for transparent film after it has become brittle through the degrading action of UV radiation. Long-term degradable film requires many years to degrade, builds up in the field and interferes with future planting operations. Plastic film residue left over in the soil may clog harvesting machinery. Burning it in situ with a flame-thrower has proved to be too laborious. Also, plastic cannot be safely burned because it tends to give off toxic smoke. Environmental protection laws have made burning difficult or impossible in many areas. The third alternative would be to collect the film into piles, load it on trucks and then dispose of it where dumping is permitted. However, many landfills no longer permit dumping of agricultural plastics. Storing the used plastic is an alternative to disposal where space is available. However, most farms do not have sites available.

Introduction

Disposal of the plastic film after harvest presents an environmental pollution problem (20). From 200 to 500 kg of plastic film per ha may be used. In 1985, it was estimated by the NAPA Congress that approximately 52 million kg of plastic mulch film were used in the United States (7). Survey results showed that the plastic used on 31 000 ha of agricultural land was disposed of in public landfills (19). Lingle (13) reported that USA consumers in 1987 used 3 billion kg of plastics for packaging.

Mulch Removal and Disposal Problem. Carnell (1) outlined four methods for removal of plastic mulch namely: 1) discing 2) burning 3) physical removal 4) removal and 5) storage of the plastic mulch. The labour cost of removing the film at the end of the crop season is high. Farmers in various regions have reported removal and/or disposal costs of more than $240 per ha (15). Burying the plastic with a disc harrow, although never desirable, can be used for transparent film after it has become brittle through the degrading action of UV radiation. Non-degradable black film never becomes degraded, builds up in the field and, interferes with future planting operations. Plastic film residue left over in the soil may clog harvesting machinery (1, 7, 8, 12, 21).

The third alternative would be to collect the film info heaps, load on trucks and then dispose of it. Where dumping is permitted, tons of plastic debris accumulate in garbage sites or landfills, where it remains unchanged for years. Many landfills no longer permit the dumping of agricultural plastics (7). Storing the used plastic is an alternative to disposal where space is available and farmers cannot justify the use of valuable farm land for this purpose (12).

Biodegradable and Photodegradable Plastic Film. By the beginning of the 1970s mulching of vegetable and fruit crops was already widely practiced. The relatively low price of plastic materials did not encourage retrieval and recycling. However, because of the vast amounts of plastic involved, researchers began to plastic films which would self destruct by suitable chemical modifications (5).

One method of handling plastic removal and disposal is to have a plastic film which will degrade after harvesting. In the 1960s and 1970s, scientists started to investigate the possibility of using bio-photodegradation as a self-destructive disposal technique for plastic film (7, 11). Biodegradability alone has been found to be an interior method because most polymeric materials are resistant to bacterial attack. The chemical groups required for biodegradability frequently cause a significant reduction in the desired properties of the plastic material (6, 18).

The Introduction of starch filler into some plastic products has recently occurred (15, 16). So far, starch-based films containing sufficient starch to improve their biodegradability have inferior physical properties, initially or after soil contact, and their rate of degradability is difficult to predict (17).

Photodegradation of PE involves a photo-oxidizing action on the polymer chain. The main difficulty lies not in initiating photodegradation but in gelling it to occur after a suitable (and predictable) lapse of time (5). Early formulations of photo-degradable plastic were unpredictable and inefficient. In some cases, the plastic decomposed too early, too late, or unevenly. Farmers could not count on the plastic to protect their crops during the growing season. Sometimes the incompletely decomposed plastic had to be removed from the field prior to harvesting. At the 1983 NAPA Congress, it was reported that none of the photodegradable plastic mulches tested by Agway Inc. between 1973 and 1983 were commercially acceptable (3,14).

For the past several years, a new product called Plastigone^R used in Israel appears to provide a solution to the problem of time-controlled degradation (10). Plastigone^R is one of the better degradable plastics that has been evaluated to date (7). However, some limitations still occur. For example, a grower who normally does not apply herbicides beneath a black plastic mulch should consider the potential late season weed problems which may develop after the mulch has degraded. Also, it is necessary to disk up the buried undegraded edges at the end of the season and expose them to sunlight which is required for the degradation process to occur. Another limitation of degradable mulch is that double cropping is made difficult (8).

The Need for Recycling Plastic in Agriculture. Recycling is technically possible, but past efforts have not been economically viable. Surveys of plastic mulch disposal in the United States in 1987 and 1988 showed no successful recycling programmes (19). Cornwell (4) advocated plastic recycling and suggested that the following steps to attack the plastic waste problem: 1) develop a comprehensive educational anti-litter campaign, and 2) form a committee under the auspices of NAPA Congress with representation from extrusion plastic manufactures, resin suppliers, plastic processors, Federal Drug Administration and the Environmental Protective Agency to study the problem of plastic used in agricultural applications. He further suggested that monies should not be spent on developing photodegradable and biodegradable resins. He felt that efforts should be directed towards educational anti-litter programmes and the establishment of plastic recycling centres.

References

1. Carnell, D. 1978. Photodegradable plastic mulch in agriculture. Proc. Nat. Agr. Plastics Cong. 14:143-148.

2. Carnell, D. 1983. Plastic mulches in North America. Plasticulture Review, No. 59. 59:39-44.

3. Chu, C. C. and B. L. Matthews. 1984. Photodegradable plastic mulch in Central New York. HortScience. 19:497-498.

4. Cornwell, J. T. 1989. The recycling of plastics in agriculture. Proc. Nat. Agr. Plastics Cong. 21:60-64.5.

5. De Carsalade, B. 1986. Plastics and mulching of crops. Plasticulture 72:31 36.

6. Eggins, H. O. W., J. Mills, A. Holt, and G. Scott. 1971. Biodeterioration in biodegradation of synthetic polymers. pp. 267. In: Microbial Aspects of Pollution, Sykes & Skinner (eds.) Academic Press, New York, N.Y.

7. Ennis, R. S. 1987. Plastigone, a new time controlled photodegradable plastic mulch film. Proc. Natl. Agr. Plastics Cong. 20:83-90.

8. Failon, J. B. 1989. Effect of row covers on the degradation of plastigone (TM) photodegradable mulch. Proc. Natl. Agr. Plastics Congress 20:8390.

9. Garnaud, J. C. 1974. The intensification of horticultural crop production in the mediterranean basin by protected cultivation. FAO of the United Nations, Rome.

10. Gilead, D. 1985. Plastics that self destruct. Chemtech. pp. 299-301.

11. Hanras, J. C. 1979. Agricultural and horticultural uses for photodegradable polyethylene films. Plasticulture 41:43-58.

12. Johnson, G. Jr. 1989. Plastigone photodegradable film performance in California. Proc. Nat. Agr. Plastics Cong. 21:1-6.

13. Loy, J. B. and B. Bushenell. 1984. Comparison of polyethylene and spunbonded fabric row covers over muskmelon and summer squash. Proc. Nat. Agr. Plastics Cong. 19:63-66.

14. Matthews, B. L. and C. Chu. 1983. An overview of Agway's research on degradable plastic mulch. Proc. Nat. Agr. Plastics Cong. 17:71-76.

15. Otey, F. H. and R. P. Westhoff. 1980. Biodegradable starch based plastic films for agricultural application. Proc. Nat. Agr. Plastics Cong. 15:9093.

16. Otey, F. H. 1983. Starch based plastics and related production for agriculture. Proc. Nat. Agr. Plastics Cong. 17:77-81.

17. Otey, F. H. and R. P. Westhoff. 1984. Starch-based films: Preliminary diffusion evaluation. Ind. Eng. Chem. Prod. Res. Dev. 23:2.

18. Potts, J. E., R. A. Clendinning, W. B. Ackart, and W. D. Niegisch, 1973. Biodegradability of synthetic polymers. pp. 61-79. In: Polymers and ecological problems. James Guillet (cd.). New York: Plenum.

19. Schales, F. D. 1989. Survey results on plastic mulch use in the United States. Proc. Nat. Agr. Plastics Cong. 21:95-101.

20. Stall, W. M. and H. H. Bryan. 1981. Removal and disposal of plastic mulch in Florida. Proc. Nat. Agr. Plastics Cong. 16:133-141.

21. Tresemer, B. 1986. Agricultural plastics: responsibilities and possibilities. Proc. Nat. Agr. Plastics Cong. 19:10-13.

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