5. Effect of soil solarization on weeds

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Use of solarization for weed control
Effect of soil solarization on the yield of food legumes and on pest control
Weed control in vegetables by soil solarization

Use of solarization for weed control

Clyde L. Elmore
Department of Botany, University of California, Davis, CA 95616, USA

The use of soil solarization for weed control is in the proven, but experimental stage. We know how to make it work in the field. It has been used in the field, but there are still obstacles to its use. Today I want to share with you some of the "pros" and "cons" of its infant steps. Yes, we have passed through the crawling stage and are at least walking.

Polyethylene sheeting for weed control has been widely used in agricultural and urban areas. Black polyethylene has been the dominant material where it is used as a barrier mulch. This mulch excludes light, thus prohibits photosynthesis and kills young annual weeds or even many perennial weeds if the mulch remains for a long enough period. In the early 1970s, research in Japan (5) demonstrated that a green polyethylene mulch that would absorb light at 450 nm wavelength, reduced weed growth, though not as effectively as black polyethylene mulch. Some of these coloured mulches are still used in greenhouses and in the field, not for soil solarization, but as a barrier for weed control.

In 1976, Katan et al. (6) described the use of clear polyethylene for soil solarization (soil pasteurization) to control soil pathogens and weeds. Subsequent studies for weed control have utilized this same application technology. Various synthetic mulches have been evaluated; however, polyethylene seems to have characteristics of transmittance and durability, for greatest effectiveness.

Soil solarization for weed control is both visually dramatic and highly effective when properly conducted. Though restricted to a time interval in summer during periods of high radiation, it can effectively control species that would be a problem in the subsequent fall or spring crops. Horowitz et al. (4) found in Israel that a two to four week solarization period effectively controlled annual weeds that was appreciable after one year. Rubin and Benjamin (9) reported similar results from solarization for four to five weeks. In the field, solarization has been best adapted for the control of weeds in cool seasons, and for fall seeded crops such as onions, garlic, carrots, broccoli or other brassica crops, and lettuce. In greenhouses or in the more temperate to tropical regions, solarization can be used before planting of the spring-planted crops such as tomatoes, peppers, squash, and cucurbits. Also in the field it can be used to preplant strawberries. Other crops that have been evaluated include broad beans, potatoes, orchard trees, and vineyards. In some studies there has been no need for additional weed control measures such as hand weeding or cultivation in the annual crop following solarization. Weed control has not been effective into the second or third years following treatment, though weeds are reduced. If beds are not disturbed following treatment, weed control can be effective for two crops (2).

The greatest difficulty in consistently achieving good weed control has been when the operator (farmer) cultivates after removing the mulch, thus bringing untreated soil (and seed) to the surface. Thus, much of the mulching for weed control has been on pre-formed beds, rather than flat soil and then bedding following treatment. Standifer (10) used full-tarped beds and achieved control of annual sedges and many other annual species. Rubin and Benjamin (9) used flat solarized soil, but did not form beds, and achieved control. Elmore (unpublished data) followed solarization by cultivating to 3 cm or 6 cm compared to an uncultivated area. This test was conducted from June to July or from August to September. No difference was found in germination of weeds at the 0 and 3 cm cultivations; however, a greater number of weeds germinated after a 6 cm cultivation than at the uncultivated site. The difference was greater between cultivation depths in the August to September treatment than the more optimum June solarization period. Hejazi et al. (unpublished data) cultivated at 0, 5 and 20 cm following solarization. They found there were no differences in control of annual grass species. A numerical, but not a statistical increase in germination of Portulaca oleracea and Malva parviflora occurred at 20 cm cultivation compared to the 5 cm depth. Under one- to two-week solarization experiments, Egley (3) found increased weed emergence following a 5 cm cultivation in solarized plots. Thus, it would seem that shallow cultivation would not be detrimental if the solarization was conducted under optimum conditions.

In bed solarization culture, it has been noted by the author that more uniform control is achieved when the beds are formed in a north/south direction as compared to an east/west direction. There is always a cool north side on the east/west bed. Mahrer and Katan (7) found that a 80 cm or more, bed-top width solarization was required to get a uniform temperature and control in the centre of the beds where crops were to be planted.

Weed Species Controlled

Winter annual weeds that germinate during short days and cool temperatures have been effectively controlled. (Table 1). These species seem to be very temperature sensitive and require small increments of temperature increases to achieve control. Weeds that germinate predominantly in the longer day length, warmer period of the year (summer annuals) are not as susceptible (Table 2). Most species, however, are controlled. In some areas of marginal solar radiation, windy or cloudy conditions, etc., some of these summer annuals may escape. Predominant species that escape include Melilotus sp., Medicago sp. and to some lesser extent Portulaca sp. (8). Results from the literature may not give the true picture of control of some of these summer annual weeds. If solarization conditions were marginal, species would be listed as "not controlled". Results on the perennial species Cynodon dactylon (bermudagrass) and Sorghum halapense (Johnsongrass) have varied (Table 3). In tests by the author in California, both species have been controlled. Convolvulus arvensis (field bindweed) has been controlled during the period of solarization and suppressed for two to three weeks afterward before regrowth occurred. Cyperus esculentus (yellow nutsedge) was only reduced by approximately 40 percent (Hejazi, et al, unpublished data) and Cyperus rotundus (purple nutsedge) was partially controlled or increased following solarization.

The control of seed or seedlings must take into account the seed germination depth, and both the temperature and duration of a temperature. Control of many small seeded weeds such as annual bluegrass and common groundsel is more effective than large seeded weeds such as wild oat since these large seeds can germinate from depths below which thermal killing occurs.

Seed size and germination depth, however, is probably not the primary protection mechanism for weed seeds. Seeds from various weed species vary in their sensitivity to temperature (thermal death point). Common groundsel (Senecio vulgaris) (Figure 1) for example is so temperature sensitive that even four hours at 40°C will reduce germination and eight hours will kill all imbibed seeds. Common purslane (P. oleracea), however, can be controlled with either high temperature for a short duration or lower temperature for a longer duration. Treating moist Portulaca oleracea (Figure 2) seeds at 45°C for up to eight hours did not reduce germination. At 50°C, germination was reduced by 16 percent at four hours of treatment or 43 percent at eight hours. At 60°C all germination was stopped when treated for four and eight hours with 90 percent reduction at 2 hours. Barnyard grass and lambsquarters were more temperature sensitive than P. oleracea, but less sensitive than S. vulgaris Other weeds such as Melilotus sp. and Medicago sp. seem to be very heat tolerant.

Summary

Soil solarization has been effective for the control of many weed species in various parts of the world. The treatment must be applied within the correct weather, timing and application perimeters to make the process work. However, when these factors are observed, then control is achieved. Solarization will not be an effective method of weed control in all crops or in some microclimates where it would normally be effective. This lack of control is most frequently observed under less than optimum application and weather conditions; however, it can be affected by weed species.

Treatment during periods of high radiation followed by good crop management practices can make the treatment effective and preserve the control for longer periods. Cool season crops will be most impacted; however, some summer crops can also be improved through solarization because of the decreased weed pressure in the crop seedling or new transplant stage.

References

1. Abdel-Rahim, M.F., M.M., Satour, S.A. El-Eraki, H.R. Elwakil, A. Grinstein, Y. Chen, and K. Katan. 1987. Soil solarization for controlling soil borne pathogens and weeds in furrow-irrigated Egyptian soils. In: Proc. 7th Control of the Mediterranean Phytopathological Union. p. 76-77.

2. Bell, C. E. and C. L. Elmore. 1983. Soil solarization as a weed control method in fall planted cantaloupes. Granada, Spain. Proc. Western Society Weed Science. 36:174-177.

3. Egley, G. H. 1983. Weed seed and seedling reductions by soil solarization with transparent polyethylene sheets. Weed Science 31:404409.

4. Horowitz, M., Y. Roger, and G. Herlinger. 1983. Solarization for weed control. Weed Science 31:170-179.

5. Inada, K. 1973. Photo-selective plastic film for mulch. J. Agri. Res. Quar. 7:252-256.

6. Katan, J., A. Greenburger, H. Alon, and A. Grinstein. 1976. Solar heating by polyethylene mulching for control of diseases caused by soil-borne pathogens. Phytopathology 66(5):683-688.

7. Mahrer, Y. and J. Katan. 1981. Spatial soil temperature regime under transparent polyethylene mulch: numerical and experimental studies. Soil Science. 131(2):82-87.

8. Porter, I. J. and P. R. Merriman. 1983. Effects of solarization of soil on nematode and fungal pathogens at two sites in Victoria. Soil Biology. Biochem. 15(1):39-44.

9. Rubin, B. and A. Benjamin. 1984. Solar heating of the soil: involvement of environmental factors in the weed control process. Weed Science 32:138-142.

10. Standifer, L. C. 1984. Effects of solarization on soil weed seed populations. Weed Science 32:569-573.

Table 1. Susceptibility of Winter Annual Weeds to Soil Solarization

S = Sensitive- controlled in all studies.
MS = Moderately sensitive = normally controlled, but may remain because of large seeds that may be deep in soil.
MR = Moderately resistant.
R = Resistant = poorly controlled.

Au = Australia
Ca = California, USA
Is = Israel
La = Louisiana, USA

Table 2. Susceptibility of Summer Annual Weeds to Soil Solarization

S = Sensitive = controlled in all studies.
MS = Moderately sensitive = normally controlled, but may remain because of large seeds that may be deep in soil.
MR = Moderately resistant.
R = Resistant = poorly controlled.

Au = Australia
Ca = California, USA
Eg = Egypt
Is = Israel
La = Louisiana, USA
Mi = Mississippi, USA

Table 3. Susceptibility of Perennial Weed Species to Soil Solarization

S = Sensitive = controlled in all studies.
MS = Moderately sensitive = normally controlled, but may remain because of large seeds that may be deep in soil.
MR = Moderately resistant.
R = Resistant = poorly controlled.

Ca = California, USA
Eg = Egypt
Fl = Florida, USA
Is = Israel
Te = Texas, USA
Mi = Mississippi, USA

Figure 1. Thermal death of Senecio vulgariscommon groundsel seeds at five

Figure 2. Thermal death of Portulaca oleraceacommon purslane seeds at five

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