Weed control in vegetables by soil solarization

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Barakat E. Abu-Irmaileh
Faculty of Agriculture
University of Jordan, Amman, Jordan

Abstract

Soil solarization using black (BPE) and clear polyethylene mulches (CPE), .08 and .06 mm thick, respectively, was tested for effectiveness of weed control. Solarization for six weeks reduced weed growth and enhanced crop yields. However, further mulching with BPE after solarization with either BPE or CPE gave the best results. Not all weeds were controlled with solarization. Some weed species were completely controlled and some weed species seemed to be enhanced by solarization. Solarization without further mulching was not better than the standard farmer practice in reducing weed growth or in increasing crop yield. Weed removal was required after the middle of the growing season. Soil disturbance after solarization reduced the weed control effect. Crops grew best in plots after solarization with BPE, if they were planted in the same mulch after it was perforated.

Introduction

Soil disinfestation by heat, steam or hot water are old and well-known practices for controlling soil pests (18). It was first reported in Germany in 1888, and was commercially used in the USA in 1893 (5). Raising the temperature of soils to 60°C by aerated steam for 30 minutes has been a standard recommendation for controlling soil pests (8). Trapping solar energy for the purpose of elevating soil temperature to control soil pests is also an old venture. Fifty years ago, Grooshevoy (13) obtained effective control of soil pathogenic organisms by trapping solar energy under cold frames subjected to direct sunlight prior to planting. Top soil temperature was raised to 40° - 60°C to a depth of 10 cm.

In Jordan and elsewhere, various studies have been conducted to evaluate the effectiveness of trapping solar energy by polyethylene mulching of moist soils, during periods of highest air temperature and bright sunlight, to increase soil temperature sufficiently to kill soil pests (1, 2, 3, 6, 16, 20). An increase of soil temperature by 10° to 19°C under clear polyethlene mulches (CPE) over that of the non-mulched soil was claimed (11, 19, 21). Clear polyethlene mulches, 0.03 mm and one year old CPE mulch 0.08 mm, were more effective for the control of soilborne fungi than either yellow or black mulches (4).

Soil solarization for weed control has been shown by many researchers. Weed responses to solarization have varied. Annual weeds have been effectively controlled by soil solarization with CPE or BPE mulches. A 90 percent reduction in total weed emergence was obtained, but the emergence of Cyperus rotundus L. (11) and Digitaria sanguinalis (L.) Scop. (7) seemed to be enhanced. Conyza and Malva spp. were relatively resistant (14). Perennial weeds have responded differently depending upon specie. Field bindweed, Convolvulus arvensis L., emerged in plots solarized with BPE. C. rotundus survived 80°C for 30 minutes, while the rhizomes of Cynodon dactylon (L.) Pers., and Sorghum halepense (L.) Pers., were sensitive (19). Soil solarization by CPE mulch up to six weeks reduced weed emergence (14, 21), and controlled the Egyptian broomrape (15). Weed seeds were killed faster in plots solarized with CPE than in those solarized with BPE mulches (21). Dormant weed seeds (11), and seeds buried at deeper layers (14, 19, 21) escaped the solarization effect. The number of seeds killed and depths to which they were killed varied with the species (21), and with the solarization period. Solarization for one to four weeks reduced weed emergence by 64 to 98 percent for the growing season, respectively (11). Weed control was more effective from solarization of wet soils than dry soils. A single irrigation prior to plastic placement produced similar controls as repeated watering (12,14), and better control than mulching dry soils (14).

In Iraq, soil temperatures under BPE and CPE mulches reached 45.9° and 49.7°C at 10 cm depth, respectively, compared to 33.2°C in the non-mulched soil. If the mulches were raised 5 cm above soil surface, the temperatures were raised to 46.8° and 52.9°C, respectively (2).

CPE mulch deteriorated within six weeks of solarization, while BPE mulches had a longer life span which extended throughout the growing season (9). BPE mulches could be considered more economical and more practical than CPE, if BPE proved to be effective as a solarization mulch against soil pests, and if BPE is perforated after solarization to be used as mulch for the rest of the season.

In this research, soil solarization with CPE and BPE mulches was tested for weed control effectiveness in naturally infested fields of some vegetable crops.

Materials and Methods

Solarization trials were done in naturally infested fields in the central Jordan Valley. The soil type is sandy with 55 percent sand, 25 percent silt, and 20 percent clay. The soil pH ranges from 7.7 to 8.7 and Ec ranged from 0.8 to 1.2 nm ohms cm^-1. Soil analysis indicated a range of 0.13 to 0.24 percent N. 40 to 200 ppm P. and 388 to 700 ppm K. The soil was prepared by tilling to form a fine-textured seedbed starting early July each year. The field was irrigated, ploughed, Ievelled, then furrowed at appropriate distances to suit the crops in the trial. The furrows were irrigated prior to the solarization treatments. After the solarization period was terminated, crops were planted with minimal soil disturbance Planting holes were driven in the soil by a hand chisel or wooden stick. The hole depth was 20 cm for the transplants, and 5 to 7 cm for planting the crop seeds. Fertilizers were not added. Irrigation was applied whenever needed by sprinklers to give uniform coverage over the experimental area. Damaged mulches were mended or replaced whenever needed during the solarization period. Weed species present and their dry weights were recorded from a quadrat, 50 cm x 50 cm, thrown three times randomly in each plot. Crop yield was taken from a 4 m length in the middle furrow of each plot, and was transformed to t/ha^-1. The data of all trials were statistically analysed, and the standard errors of the means were calculated.

Field trials. 1986 - 1987. - Five alternating strips, 4.5 m x 30 m, covered with CPE mulch, 0.06 mm thick, and uncovered were rayed out on 15 July, 1986. Each strip constituted three 1.5 m x 30 m furrows. The mulches were removed on 30 August. Tomato (Lycopersicon esculentum Mill. cv. 'Claudia RAF') transplants were planted on 15 September 1986. Five treatments, in plots 9 m x 7.5 m, were included. Each treatment consisted of six furrows, three solarized adjacent to three non-solarized furrows, in order to allow closer comparison of the solarization effect. The treatments were: (1) no hoeing; (2) surface hoeing, in which the weeds were removed by scratching the soil surface with a hand hoe, one month after planting; (3) deep hoeing, which involved weed removal from around the plants and turning the soil from the non-planted side of the furrow over the bare stems of tomato plants, one and two months after planting (farmers' practice); (4) mulching with perforated BPE mulch; and (5) hand weeding monthly.

The treatments were arranged in a randomized complete block design with four replications.

The weed species in the solarized - no hoeing and in the non-solarized check, and their dry weights were recorded on 15 April 1987. The final weed dry weight for each treatment was also recorded when the trial was terminated on 15 June 1987.

Field trails. 1987 - 1988. - Squash (Cucurbita pepo L. var pepo c.f. 'Regina Fl') and tomato were planted in furrows 120 cm wide on 20 September 1987. Each treatment consisted of three 7 m-long furrows. Solarization treatments started 21 July 1987, and lasted six weeks. Six treatments were rayed out in a randomized complete block design with four replications. The treatments were: (1) farmers' practice in open filed cultivation with two hoeings and turning soil over the crop plants from the non-planted side of the furrow, thus forming new furrows; (2) planting in perforated mulch (BPE mulching); (3) planting after solarization with CPE (SOL/CPE); (4) planting after solarization with CPE in perforated BPE mulch (SOL/CPE + BPE mulching); (5) solarization with BPE followed by planting in the same BPE mulch after it was perforated at the recommended distances, immediately after the solarization period was terminated (SOL/BPE + BPE mulching); and (6) check.

Weeds were removed from all treatments on 15 April 1988, and their dry weights were determined. The major weed species was determined by those weighing more than 100 g/m^-2 or, if the sample weighed less than 100 g/m^-2, then the weed with the greatest weight.

Field trials. 1988 - 1989. - The same treatments as in trials of the previous season, were established in 1988-1989.

Weeds were removed from the check plots on 30 April 1989. The treatments were rayed in 4.5 m x 7.5 m plots with three furrows, in a randomized complete block design with four replications.

Since frost occurred for two to three days in the first and third weeks of January 1989, dry weights of the crop plants in each plot were determined instead of yield. Five plants were pulled out from the middle furrow of each plot on 15 February 1989. The dry weight per plant was determined. Weed species present and their oven dry weights were determined on 30 April 1989. The weeds were harvested from 0.25 m² quadrats in the check, farmer, and SOL/CPE treatments. The weeds were collected, and identified from 4 m length of the middle furrow in the BPE, SOL/CPE + BPE and SOL/BPE + BPE. Weed dry weights were transformed to g/m^-2.

Results and Discussion

Soil solarization of wet soil for a peiod of six weeks during July and August in the Jordan Valley reduced weed development, weed growth and improved crop yield, thus, confirming the previous results obtained by other researchers. However, solarization with CPE without further mulching with BPE, was not enough in most cases to suppress weed growth throughout the growing season (Tables 1, 2, 3, 4). Weed growth continued in the solarized plots, but at a lower rate than those in the non-solarized plots (Tables 1, 2, 3). This result is not in full accordance with the previous finding by Horowitz, Roger & Herlinger (14) who reported that solarization with CPE for two to four weeks gave good weed control, which was still appreciable after one year. In this research we found that in field situations, the growth of most weed species was suppressed by solarization but not completely eliminated.

The heating effect of solarization diminishes with soil depth (14, 21 and many others). Solar heating mostly affects the top soil layer, where heat sensitive imbibed weed seeds would be affected (19).

Weed emergence after solarization is a function of the weed tolerance to solar heating effect, the depth at which the seeds are localized, and the ability of the germinated seeds to emerge from that layer. Seeds of weed species that were able to germinate and emerge from deeper layers would grow in the solarized plots. Some annual weed species and Orobanche aegyptiaca were completely controlled by solarization. Other weed species seemed to be enchanced, and emerged only in the solarized plots (Table 4). This effect was probably due to germination enhancement by the warming effect of soil layers in which the seeds of the tolerant weed species were present. Such results were obtained by other researchers (7). However, the sensitivity of the seeds to heating effect varies with the weed species (21).

Emerging weeds in the solarized plots grew at densities that required further removal or further BPE mulching. Soil disturbance by shallow or deep hoeing enhanced further weed emergence, but improved tomato yield (Table 1). Soil covering on the bare tomato stem was found to enhance the production of adventituous roots, and enhanced tomato growth and yield (17). Surface hoeing and deep hoeing is used to break the crusted top layer of the soil and improve water infiltration, and thus improve soil moisture content. In some of the tests BPE mulching after solarization significantly reduced weeds and improved crop yield over the same treatment in the non-solarized soil (Table 1). This could not be explained by the effect on weed growth, since weeds were reduced to the same level in both cases. This could result from improved availability of nutrients and improved nutrient uptake from solarization (10). Squash and tomato responded to solarization treatments in the same manner.

Solarization with BPE or CPE followed by mulching with BE were the best treatments for reducing weed growth and for improving yields. Since solarization with CPE involves extra cost and effort, it is advisable to solarize with BPE mulch, retain it in place and perforate it at the proper distances for crop planting to be used as mulch for the rest of the growing season.

Acknowledgement

This research was sponsored by a grant from the University of Jordan.

References

1. Abdel-Maksound, M. 1984. Solarization, mechanical and chemical weed control in garlic. Egyptian Journal of Horticulture 11:85-92.

2. Al-Hassani, N., I. D. Al-Mafragi, and L. Ahmed. 1985. Developing the technique of soil solarization to control weeds. In: Proceedings of the First Symposium on the Solar Energy Applications in Agriculture. pp. 183-197. Scientific Research Council-Solar Energy Research Center. Baghdad.

3. Al-Kalaielah, R. A. 1981. Effect of soil solarization using different thicknesses of transparent polyethylene on cucumber grown in plastic houses in the Jordan Valley. MSc. thesis. 118 pages. University of Jordan, Amman.

4. Al-Raddad, A. M. 1979. Soil disinfestation by plastic tarping. MSc. thesis. 95 pages. University of Jordan, Amman.

5. Baker, K. F. 1962. Principles of heat treatment of soil and planting material. Aust. J. Agr. Sci. 28:118-126.

6. Barakat, R. M. 1987. Comparative effect of different colors of polyethylene tarping on soilborne pathogens. MSc. thesis. 82 pages. University of Jordan, Amman.

7. Braun, M. 1987. Solarization for sanitation - possibilities and limitations, based on experiments in southern Germany and Sudan. Gesunde Pflanzen (Germany, F. R.) 39: 301-309.

8. Brazelton, R. W. 1968. Sterilizing soil mixes with aerated steam. p. 35. In: Hartman, H. T. and D. E. Kester. 1975. Plant propagation principles and practices. 3rd ed. Prentice-Hall, Inc. New Jersy.

9. Brighton, C. A. 1972. Degradation and disposability of plastics. Fifth International Colloquium. Budapest. Plastics in Agriculture 1:203-208.

10. Chen, Y. and J. Katan. 1980. Effect of solar heating of soils by transparent polyethylene mulching on their chemcial properties. Soil Sci. 130: 271277.

11. Egley, G. H. 1983. Weed seed and seedling reduction by soil solarization with transparent polyethylene sheets. Weed Sci.31: 404-409.

12. Grinstein, A., D. Orion, A. Greenberger, and J. Katan. 1979. Solar heating of the soil for the control of Verticillium dahliae and Pratylenchus thornei. pp. 431-438. In: Soil-borne Plant Pathogens. B. Schippers and W. Gams (eds.) Academic Press, London.

13. Grooshevoy, S. E. 1939. Disinfestation of seed-bed soil in cold frames by solar energy. The A. I. Mikoyan Pan-Soviet Sci. Res. Inst. Tob. and Indian Tob. Ind. (VITIM). Krasnodar, Publ. 137. pp. 51-56. 1939. (English summary). In: the Review of Applied Mycology 18:635. 1939.

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

15. 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.

16. Katan, J., A. Greenberger, H. Alon, and A. Grinstein. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilborne pathogens. Phytopathol. 66: 683-688.

17. Morsi, M. A. and A. I. Al-Merabe. 1960. Vegetable crops. Vol. 2. Vegetable crop production. 2nd ed. The Anglo-Egyptian Library, Cairo. 715 pp. (In Arabic).

18. Newhall, A. G. 1955. Soil disinfestation of soil by heat, hot water, flooding and fumigation. Bot. Rev. 21: 189-233.

19. Rubin, B. and J. Benjamin. 1984. Solar heating of the soil: involvement of environmental factors in the weed control process. Weed Sci. 32:138142.

20. Saurerborn, J. and M. C. Saxena. 1987. Effect of soil solarization on Orobanche spp. infestation and other pests in faba beans and lentil. p. 733744. In: Parasitic flowering plants. H. Weber and W. Forstreuter (eds). Marburg. F.R. Germany.

21. Standifer, L. C., P. W. Wilson, and R. Porche-Sorbet. 1984. Effects of solarization on soil weed populations. Weed Sci. 32: 569-573.

Table 1. Treatment effect on tomato yield and total weed dry weight with solarization (SOL) and without solarization (NSOL) during 1986-1987

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Table 2. Treatment effect on crop yield and weed dry weight (wdwt) in 1987-1988 season

Table 3. Treatment effect on weed dry weight (wdwt), all m², and dry weight of the crop plants (plt.dwt) during the 1988-1989 season

Table 4. List of weed species present in Trial B during 1988-1989 season¹

Weed species in solarized and non-solarized plots:

Amaranthus blitoides, A. retrolflexus, Ammi majus,
Anthemis plestina, Astragalus hamousus, Beta vulgaris, Bromus tectorum, L., Centaurea pallescens, Chenopodium murale*, Cynodon dactylon*, Eruca sativa *, Erucaria hispanica, Hordeum liporinum, Lactuca serriola, Lolium temulentum, Malva parviflora*, Medicago polymorpha, Poterium spinosa*, Phalaris brachystachys*, Papaver rhoeas, Polygonum aviculare, Rumex crispus, Solanum nigrum*, Spergula fallax*, Sisymbrium irio Trigonella moabitica, Urtica urens L.

Weed species in non-solarized plots only:

Anagallis arvensis, Avena sterilis, Aizoon hispanicum L., Cichorium pumilum, Centaurea pallescens Del., Hirschfeldia incana, Inula graveolens, O. aegyptiaca*, Senecio vernalis, Sinapis alba L., S. arvensis L.

Weed species in solarized plots only:

Astragalus cruciatus Link., Biscutella didyma L., Convolvulus arvensis L., Crepis aspera L., Melilotus indicus (L.,) All., Prosopis farcta (Banks et Sol.) Macbride, Senecio vernalis Waldst. & Kit., Vicia narbonensis L.

1 Species marked with * were identified as major weeds.

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