Effect of soil solarization on the yield of food legumes and on pest control

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K.H. Linke^1,2, M.C. Saxena¹, J. Sauerborn², and H. Masri¹

¹International Center for Agricultural Research in the Dry Areas (ICARDA), P.O. Box 5466, Aleppo, Syria.
²University of Hohenheim (380), P.O. Box 70 05 62, 7000 Stuttgart 70, F.R. Germany

Summary

Field experiments on soil solarization were conducted from 1985 to 1989 at the International Center for Agricultural Research in the Dry Areas, northern Syria. Solarization for 40 days of soil heavily infested with the parasitic weed Orobanche crenata increased seed yield in faba bean, lentil and pea by 331, 441 and 92 percent, respectively. The corresponding straw yield increases were 99,139 and 58 percent. Yield increase was mainly related to control of weeds, including Orobanche crenata, and higher availability of nitrogen and phosphorous in the soil. Under the conditions of no weed infestation, solarization increased the seed yield of pea by only 34 percent. Nematode population was decreased to a variable extent depending on the species, while the population of the insect Sitona lineatus was not affected. The Rhizobium population decreased radically immediately after the treatment, but it re-established quickly after the crop was sown. Nodulation was reduced in lentil, but not in faba bean.

Introduction

Solarization is a physical method for soil disinfestation in areas with high solar irradiation. Covering wet soil with clear polyethylene during the summer increases soil temperature by 10° to 15°C in the upper 15 cm soil layer. This temperature increase is considered LO be the main reason for various biological and physico chemical changes in the soil that affect plant growth (8). Crop yield increase by soil solarization was reported to be related to control of such pests as weeds, fungal pathogens, nematodes, bacteria and mites, and to higher availability of such soil nutrients as nitrogen, calcium and magnesium (1, 6, 7, 11, 12, 13, 15, 16). A major advantage of soil solarization over other soil disinfestation techniques is that it is safe to the user and the environment.

While considering solarization effects in legumes, another aspect of importance is the effect on the population of Rhizobium sp. because the performance of the legume crop is dependent on effectiveness of symbiotic nitrogen fixation in association with the Rhizobium sp. So far, only a few reports are available on the use of solarization for legume crops. Horowitz et al. (5) reported a better vegetative growth of field pea in solarized plots, mainly due to lower weed competition. Yield increases of 50 and 23 percent in pigeonpea and chickpea, respectively, were found at the International Crop Research Institute for the Semi-Arid Tropics, India, by Chauhan et al. (3) because of the control of Fusarium wilt, nematodes and weeds. At ICARDA, a substantial yield increase was recorded from control of O. crenata Forsk., and other weeds and nematodes in faba bean and lentil (10). However, Chauhan et al. (3) reported that the Rhizobium population after solarization was adversely affected. Similarly, Rhizobium meliloti was sensitive to temperatures around 40°C (2).

This paper presents results of solarization in a set of 10 experiments on faba bean (Vicia faba L.), lentil (Lens culinaris Med.) and pea (Pisum sativum L.) conducted at ICARDA in northern Syria.

Materials and Methods

Field experiments were conducted from 1985 to 1989 at ICARDA Research Station, 30 km south of Aleppo, Syria (Table 1). Soil type was cromic luvisol with 60 percent clay, 32 percent silt, 8 percent sand, 0.8 percent organic matter and a pH of 8.1 Solarization was conducted by covering tilled and irrigated soil with transparent polyethylene sheets (0.16 to 0.18 mm thickness) during the hottest period of the year (July to August). At 5 and 10 cm soil depth, the mean daily maximum temperature of uncovered soil was 38.9° and 33.8°C, and 49.56° and 44.9°C in covered soil, respectively, in the years from 1985 to 1988. Solarization was carried out for different periods of time in different experiments. However, only the results of the 40day solarization treatment in the first growing season are presented here.

Randomized block or split-plot designs (in cases where treatments other than solarization were also included) with four replications were used according to the need of the respective experiment. Unless otherwise stated, plot size was 3m x Sm. Crop seed material used included faba bean "ILB 1814", Ientil "ILL 4400" and field pea "Syrian Local". The seed rate was 20 seeds/m 22 in faba bean, 300 in lentil and 60 in pea. The soil was not cultivated between solarization and sowing.

Treatment effect, in terms of seed yield and total dry matter production, was evaluated after hand harvesting. The number of free living nematodes in the top 15 cm of soil was assessed by soil extraction according to Coolen (4). Sitona lineatus L. infestation was determined by examining 16 soil samples for the number of eggs/100g soil. Rhizobium population (most probable number counts) was determined in composite soil samples and the nodulation of crops was assessed using a scale from 1 to 5, where 1 = no nodules and 5 = good nodulation. No artificial inoculation was done in the experiments. The viability of O. crenata seed was measured by the tetrazolium chloride method. Soil samples from 0 to 15 cm depth were analyzed for ammonium and nitrate nitrogen, as well as the available phosphorous content (P-Olsen) during the growing season. Protein content of Ientil seed was determined using the Kjeldahl method.

Results

Crop yield. - A substantial increase in crop biomass production due to solarization was observed in each of the 10 experiments (Table 2). Averaged over crops and experiments, soil solarization doubled the biomass production. Mean seed yield increased by 313 percent and straw yield by 105 percent. As all the experiments except the one with pea (expt.10) were conducted on plots heavily infested with O. crenata, seed yield in the non-solarized plots was low due to a high infestation with the parasite. Without O. crenata infestation, seed and straw yield increase due to solarization was only 34 and 37 percent, respectively.

Harvest indexes were positively affected by soil solarization and increased from 19 to 31 percent over all the experiments (Table 2). The low harvest indexes in the check plots indicate the detrimental effect of Orobanche infestation on reproductive growth.

Soil nutrient status. - This aspect was studied in derail in the lentil experiment (expt. 6). Ammonium nitrogen and available phosphorous content in solarized soil measured at the beginning of March during the growing season were increased by 36.9 and 34.4 percent, respectively (Table 3). In contrast, the nitrate nitrogen content was 15 percent lower compared to non-treated plots. Growth of lentil in the solarized plots was extremely vigorous and analysis of the lentil seed at harvest demonstrated a 10.7 percent higher protein content of seed from solarized plots. With a weed yield increase from 255 to 1 143 kg accompanied by a protein content increase from 21.4 to 23.7 percent, the total seed protein production increased from 60.1 to 278.8 kg/ha due to solarization (Table 3).

Weed and Orobanche Control. - Most weeds, especially annual weeds, were well controlled by solarization. Weed cover, exemplified in experiment 3, was 75.7 to 84.6 percent lower after solarization (Figure 1). Assessed in terms of weed dry weight, solarized plots had 82.1 percent lower weed infestation then non-treated plots (Table 4).

Infestation with the parasitic weed O. crenata, as expressed by number and dry weight of the parasite, was considerably decreased after solarization (Table 5).

The evaluation of viability of Orobanche seed buried in soil indicated a complete kill of seed to a depth of 5 cm following solarization (Table 6). Seeds buried deeper (10 to 15 cm) were destroyed up to 99 percent. However, viability of the seed in the natural seed bank might be different from artificially buried seed. There was substantial decrease in the Orobanche seed bank.

Effect of Rhizobium and nodulation. - Most probable number (MPN) studies of solarized soil indicated that the native Rhizobium population was decreased considerably following solarization. However, by the time rapid crop growth occurred (i.e., four months after solarization, or two months after sowing of the crop), the Rhizobium population in the soil greatly recovered (Table 7).

The nodulation of faba bean at flowering stage was unaffected by solarization. In the case of lentil, however, nodulation was reduced due to solarization (Table 8).

Nematodes. - Control of free living nematodes under natural infestation varied according to the species (Figure 2). Complete control after 40 days of solarization was achieved for Tylenchorynchus sp. Helicotylenchus sp., Tylenchus spp., and Pratylenchus thornei were decreased by 91.4, and 87.1 and 95.7 percent, respectively. A 0.3 percent re-establishment of the P. thornei population occurred in the solarized plots of faba bean at flowering time (Table 9). After one year, however, the population of this nematode was fully re-established.

Sitona. - Sitona lineatus is one of the most important insect pests of lentil and faba bean because of the extent to which its larvae damage nodules of these crops. Hence, the effect of solarization on the number of eggs of Sitona present in the soil at early crop development was studied. The number of eggs/100 g soil ranged between 18 and 46 with no clear effect of solarization.

Discussion

Increased plant growth response after soil solarization is a frequently observed phenomenon. The main components of this effect may be seen in the control of soilborne pests and weeds and in the release of mineral nutrients from the soil. Additionally, an alteration of the soil microbiota to favour antagonists of plant pathogens as well as improved physicochemical conditions of the soil are considered to be important in this effect (3, 13, 14, 15).

An increase in crop biomass production and in seed yield under the conditions reported in the present study have to be attributed primarily to the control of the parasitic weed Orobanche, and secondly to control of other weeds, although some improvement in available soil nutrients also might be responsible. This is inferred from a lower increase in seed and straw yield after solarization in weed and Orobanche-free plots than in the weedy plots (Table 2). For example, in experiment 10 (where weeds were controlled mechanically), the increased growth following solarization was mainly due to the higher amount of available nutrients in soil, and here only a 1.3-fold increase of the seed yield was obtained as compared to a 4 or 5-fold increase in the other experiments, where weeds and Orobanche were present.

Weeds, when present in the field, strongly compete with the crop for light, water and mineral nutrients. The results of these experiments clearly show that this effective control was the main factor for crop yield increase after solarization. The mean percent dry weight reduction of non-parasitic weeds was higher (82.1 percent) than the reduction of Orobanche dry weight (72.3 percent), indicating that this parasitic weed is more difficult to control by solarization.

The nitrate concentration in solarized plots, assessed at the stage of rapid crop growth, was lower in solarized plots than in the control plots, perhaps because the more vigorous plants in solarized plots exhausted the available supply rapidly, although at the beginning of the cropping season the value might have been higher in the solarized soil, as has been observed by others (3, 16).

Harvest index was improved by solarization. This result is expected because the intensity of adverse effects of the pests otherwise controlled by solarization is accentuated as the crop enters reproductive growth in the non-solarized plots preventing the accumulation of dry matter info economic yield.

The Rhizobium population after solarization reached a level below 100 bacteria/g soil; crops under such conditions would need inoculation. However, Rhizobium population showed rapid re-establishment, so much so that in faba bean no difference in nodulation was observed. Only lentil experienced reduced nodulation due to solarization. The latter not withstanding, seed protein content and total protein production of lentil seed was considerably increased by solarization, indicating that increased nitrogen availability due to the soil nitrogen mineralization over, compensated any negative effect on symbiosis due to reduced Rhizobium population (Table 3). These results parallel those on chickpea and pigeonpea (5), where a slight reduction in the Rhizobium population and nodulation was found but with no effect on crop yield. Katan (6) investigated bean roots and mentioned that the Rhizobium population after solarization was sufficient to cause heavy nodulation. From these findings it is clear that no severe crop damage occurred from a short-term reduction in Rhizobium population.

The population of free living nematodes, as also reported by Sauerborn et al. (12), was reduced by 83 to 100 percent after solarization, but the effect varied in different nematode species. This outcome confirms earlier findings on related nematodes (9, 14). In the case of Pratylenchus thornei, a reestablishment of the population could be noticed during the growing season.

Sitona lineatus lays its eggs in the top layer of soil during December, and hatched larvae feed mainly on the root nodules. As the adults are mobile, no difference in the number of egg/g soil could be found due to solarization carried out in July/August. It would be interesting to investigate the effect of solarization on the survival of Sitona eggs; if they are killed, the overall population of Sitona adults would be reduced.

With regard to the economics, the profitability of soil solarization is determined by the costs of plastic, the labor to install and remove plastic and the price for the crop yield. Although the method might be rather expensive with food legumes when considering only one cropping season, it can be very profitable by taking into account the positive residual effect which was observed to persist after three cropping seasons (10).

The value of solarization with regard to its impact on environment and costs of application will improve with the development of biodegradable plastic which can be sprayed on the soil. Studies on this aspect are already underway in collaboration with the University of Hohenheim.

Acknowledgements

For the assistance in the most probable number count of Rhizobium and the determination of Sitona eggs in the soil samples, we are indebted to Dr. D. Beck and Dr. S. Weigand respectively. Thanks are due to the Deutsche Gesellschaft fuer Technische Zusammen-arbeit (GTZ), FRG, for financial support.

References

1. Braun, M., W. Koch, and M. Stiefvater. 1987. Solarization for soil sanitation. Possibilities and limitations. Gesunde Pflanzen 7:301-309 (in German).

2. Brockwell, J. and L. J. Phillips. 1965. Survival at high temperatures of Rhizobium meliloti inpeat inoculant on lucerne seed. Australian Journal of Science 27(11):332-333.

3. Chauhan, Y.S., Y. L. Nene, C. Johansen, M. P. Haware, N. P. Saxena, S. B. Sardar Singh, Sharma, K. L. Sahrawat, J. R. Burford, O. P. Rupela, J. V. D. K. Kumar Rao, and S. Sithanantham. 1988. Effects of soil solarization on pigeonpea and chickpea. Research Bulletin No. 11. Patancherum A.P. 502 324, India. International Crop Research Institute for the Semi-Arid Tropics.

4. Coolen, W.A. 1979. Methods for the extraction of Meloidogyne spp. and other nematodes from roots and soil. In: (F. Lamberti and C. E. Taylor eds.) Root-knot nematodes (Meloidogyne species), systematics, biology, and control. Academic Press, London, pp. 317-329.

5. Horowitz, M., Y. Regev, and G. Herzlinger. 1983. Solarization for weed control. Weed Science 31:110-179. ICARDA. 1986. Food Legume Improvement Program; Annual Report for 1986, p. 198.

6. Katan, J. 1981. Solar heating (solarization) of soil for control of soilborne pests. Annual Review of Phytopathology 19:211-236.

7. Katan, K. 1987. Soil solarization. In Chet, I (ed.) Innovative approaches IO plant disease control. Wiley-Inter Science publication, J.Wiley and Sons, New York, 77-105.

8. Katan, J., A. Greenberger, H. Alon, and A. Grinstein. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilborne pathogens. Phytopathology 76:683-688.

9. La Mondia, J.A. and B.B. Brodie. 1984. Control of Globodera rostochiensis by solar heat. Plant Disease 68(6):474-476.

10. Sauerborn, J., K. H. Linke, M. C. Saxena, and W. Koch. 1989. Solarization: a physical control method for weeds and parasitic plants (Orobanche spp.) in Mediterranean agriculture. Weed Research 29:391 -397.

11. Sauerborn, J. and M. C. Saxena. 1987. Effect of soil solarization on Orobanche spp. infestation and other pests in faba bean and lentil. In Weber, H. Chr. and W. Forstreuter. Parasitic flowering plants. Marburg, pp. 744-755.

12. Sauerborn, J., M.C. Saxena and H. Masri. 1990. Control of food legume nematodes by soil solarization in Syria. Arab Journal of Plant Protection (in press).

13. Stapleton, J.J. and JE. DeVay. 1982. Effect of soil solarization on populations of selected soil-borne microorganisms and growth of deciduous fruit seedlings. Phytopathology 72(3):323-326.

14. Stapleton, J.J. and J. E. DeVay. 1983. Response of phytoparasitic and free living nematodes to soil solarization and 1,3-Dichloropropene in California. Phytopathology 73(10):1429-1436.

15. Stapleton, J.J. and J.E. DeVay. 1986. Soil solarization: A nonchemical approach for management of plant pathogens and pests. Crop Protection 5(3): 190-199.

16. Stapleton, J.J., J. Quick, and J. E. DeVay. 1985. Soil solarization: Effect on soil properties, crop fertilization and plant growth. Soil Biology and Bio-chemistry 17:369-373.

Table 1. Details of the crops used and aspects studied in different experiments

1 H = high, M = medium, L = low, O = no Orobanche.
2 Measurements performed (+) or not performed (-).
3 R = Rhizobium, N = Nodulation.

Table 2. Effect of solarization on seed and straw yield and harvest index (Hl) of three food legumes

* Differences of seed and straw yields between solarized and untreated plots are significant at P = 99.9 %.

Table 3. Effect of solarization on phosphorous and nitrogen content in soil and on lentil seed protein yield

* Differences are significant at P = 95 %.

Table 4. Weed dry weight (excluding Orobanche) at crop harvest in solarized and non-solarized plots

* Differences are significant at P = 95 %.

Table 5. Dry weight of Orobanche crenata as affected by solarization

* Differences are significant at P = 95 %.

Table 6. Effect of solarization on the Orobanche seed bank (number of seeds/kg soil), viability of seed and the number of Orobanche shoots/m²

* Up to 15 cm soil depth.

Table 7. Rhizobium leguminosarum population as affected by solarization

Table 8. Nodulation at flowering in faba bean and lentil as affected by solarization for 40 days

* Score from 1 to 5 where 1 = no nodules, 5 = excellent nodulation.

Table 9. Influence of soil solarization on Pratylenchus thornei in faba bean at flowering stage (source: ICARDA, 1986)

Figure 1. Weed cover in faba bean as affected by solarization. Hand weeding was done on 11 January 1988. Emergence of Orobanche occurred during the latter part of March, 1988.

Figure 2. Number of free-living nematodes as affected by solarization.

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