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9. Soil solarization in pest management
Simulated and field effects of ammonia-based
fertilizers and soil solarization on pathogen survival, soil
fertility, and crop growth
The efficacy of nematicides, solar heating and
the fungus paecilomyces lilacinus in
controlling root-knot nematode Meloidogyne
javanica in Iraq
James J. Stapletonš, James E. DeVay˛, and Bert Lear˛
šUC/Statewide Integrated Pest Management Project, 733 County
Center III, Modesto, CA 95355, USA
˛Department of Plant Pathology, University of California, Davis,
CA 95616, USA
Abstract
Treatment of soil with organic and inorganic, ammonia-based fertilizers and/or soil solarization was evaluated with respect to fungicidal and/or nematicidal activity, and effect on plant growth. Soil concentrations of NH4N, NO3-N, P. K, Ca, and Mg were increased in non-amended soils heated in an incubator with a diurnal temperature regime similar to that encountered during field solarization in California. Commercial urea, ammonium phosphate, ammonium sulfate, or manure fertilizers were active against natural populations of Pythium ultimum and Meloidogyne incognita in soil. Field experiments in two soil types showed that addition of NH4-N-based fertilizers (305 kg N/ha) to soil sometimes reduced numbers of P. ultimum and Verticillium dahliae. Solarization in June-July (3.7 weeks duration) always reduced numbers of the monitored pathogens. Combining fertilizers with solarization of field soils sometimes reduced numbers of V. dahliae more than fertilization or solarization alone. Growth and yield of fresh marker tomato was stimulated more consistently by solarization than by NH4-N fertilization.
Introduction
Ammonia (NH3) has been widely reported to adversely affect survival or germination of certain soilborne fungi and nematodes (2, 3). Increased levels of available mineral nutrients, especially ammonium (NH4-N), and nitrate-nitrogen (NO3-N), often are found after soil solarization (1, 7). A previous study reported the increases to be roughly equivalent to a commercial, pre-plant fertilization dosage (7). The experiments described herein were designed to compare the biocidal effects of NH4-N-based fertilizers and/or solarization on certain soilborne fungi and nematodes, and to relate increases in NH4-N during solarization to the mode of action of soil disinfestation. A simple laboratory method of simulating the effects of N-fertilization and solarization was tested and used to aid in these studies.
Materials and Methods
Incubator simulation of soil solarization. - Two soils, silty clay (10 percent sand, 44 percent silt, 46 percent clay; 3 percent organic matter; pH 6.0), and a haploxeralf of loamy sand texture (86 percent sand, 12 percent silt, 2 percent clay; 0.5 percent organic matter; pH 7.5) were used. Bulked samples of each soil were brought to approximate field capacity. Four-hundred ml aliquots of each of the moistened soils were placed in polyethylene bags and tightly sealed with wire closures. Five bags were placed in an unlighted incubator connected to a 24-hour household appliance timer. The timer was adjusted (8 hours heat per 24 hours) to give a diurnal soil temperature curve with maximum of 45°C and minimum of 28°C, which is similar to that of field solarization in the San Joaquin Valley during June-August. Another five bags were left at room temperature (20-24°C), and a third set which were non-heated, non-moistened controls also was used. After four weeks incubation, aliquots of each of the three soil treatments were removed and assayed for changes in soluble NH4-and NO3-N, P. K, Ca, and Mg as previously described (7).
Incubator fertilizer/solarization simulation. - Fine sandy loam soil (66 percent sand. 23 percent silt, 11 percent clay; 1.3 percent organic matter; pH 6.7) naturally infested with 25-70 colony-forming units (cfu)/g of Pythium ultimum, and 1.1-5.6 x 103 second-stage juveniles/1 of Meloidogyne incognita was used. Sufficient urea, ammonium phosphate, or ammonium sulfate fertilizer was dissolved in water to bring 400 ml of soil to field capacity with 100, 200, 300, or 400 mg/l NH4-N when mixed. Composted chicken manure (3 280 mg NH4 -N/kg) was similarly incorporated at rates equivalent to 2 690 and 5 381 kg/ha. Water alone was used to treat control soil. Soils were handled and incubated in a similar manner as described above. However, soil in polyethylene bags was further sealed in 473 ml capacity glass jars to improve retention of the soil vapor phase. Also, soil was incubated for three days only, at a lower temperature regime of 41°C maximum and 22°C minimum, in order to more easily differentiate N vs. heating effects. Aliquots of soil were removed after treatment and air-dried for assay of P. ultimum on a selective agar medium. The degree of root galling caused by M. incognita also was assayed on tomato seedlings after soil treatment by ammonium phosphate and/or heating. Each experiment was done twice.
Field plots. - Two field plots were used; soils were fine sandy loam (pH 6.8) and Yolo loam (pH 7.1). Plots were designed with five - 3 x 4 m replications of each treatment. Soil probes connected IO recording thermographs were buried at 15 cm in solarized and non-covered control plots to monitor soil temperatures. Soil samples were taken prior to treatment to obtain background levels of NH4-N and NO3-N, total (Kjeldahl) N. and soil pH; and population densities of P. ultimum and Verticillium dahliae. Urea, ammonium phosphate, and ammonium sulfate fertilizers described above were surface-applied as granules; aqua-ammonia was injected with commercial, tractor-drawn equipment. All fertilizers were applied al 305 kg N/ha and incorporated with a pre-irrigation of cat 7.6 cm water applied within two hours after fertilizer application. Transparent, polyethylene film (0.025 mm thick) was randomly applied to half of the plots the following day, and left in place for 26 days. Two days before film removal, plots were sprinkler-irrigated to roughly equalize the moisture contents of the covered and non-covered plots. Two days after film removal, four randomly-sampled soil cores per replication were taken and composited for determination of soil N and changes in population densities of P. ultimum and V. dahliae on selective media.
Effect of treatments on tomato crop. - Experimental soils were aired for one week following treatment, then rototilled to a depth of cat 23 cm. Fifteen 'Early Pak 7' fresh-market tomato seedlings previously greenhouse-grown in sterile soil were transplanted into all replications in a 3 rows x 5 plant configuration. The crop was raised using sprinkler irrigation and mechanical wood control, and was harvested when the first tomatoes reached the mature green stage, due to a killing frost. Plant survival, fruit numbers and fresh weight, vegetative fresh weight, and disease incidence data were taken.
Results
Incubator simulation of soil solarization. Concentrations of soil NH4-N, NO3-N, P. K, Ca, and Mg following four-week incubation are shown in Table 1 and compared with reference values obtained after six weeks of field solarization (7). Simulated solarization generally resulted in increased concentrations of mineral nutrients similar to those seen after field solarization in the loamy sand and silty clay soils.
Incubator fertilizer/solarization simulation. - Addition of ammonium phosphate fertilizer to soil significantly reduced galling of tomato roots by M. incognita (Table 2). Composted chicken manure, but not ammonium phosphate reduced numbers of P. ultimum (Tables 2 and 3). Fungal numbers and galling index generally decreased with increasing fertilizer dosage. Soil heating reduced cfu of P. ultimum and nematode galling in all cases. No interaction between ammonium phosphate fertilization and heating was found by factorial analysis of variance (ANOVA).
Field soil temperature data. - The mean maximum temperature at 15 cm depth in the solarized plot was 42°C (9°C higher than non-covered control soil), and the mean minimum was 34°C (7°C higher than the control). Maximum temperatures al 15 cm depth in the fine sandy loam plot were 44°C and 36°C in solarized and control plots, respectively.
Effect of treatments on soil N and pH Ievels. - Pre- and post-treatment mean values for Kjeldahl-N, NH4-N,, N03-N, and soil pH arc shown in Table 4. No consistent differences in Kjeldahl-N values were found among treatments. Fertilizer application, with or without solarization, normally resulted in increased NH4- and N03-N Ievels, and lower soil pH. Fertilized soil which was then solarized normally had higher N values than non-solarized plots.
Effect of treatments on V. dahliae and P. ultimum. - Factorial ANOVA revealed significant activity of fertilizers against V. dahliae and P. ultimum in the loam soil, but not in the fine sandy loam (Table 5). Fungicidal activity of solarization was highly significant in both soils. A highly significant fertilizer-solarization interaction was found against V. dahliae in the loam soil.
Effect of treatments on field tomato growth. - There were no differences in number of surviving plants in either soil, and no symptoms of Verticillium wilt were observed. Factorial ANOVA showed both fertilizers and solarization to be highly significant in increasing all growth parameters in the loam soil; however, no fertilizer-solarization interactions were found (Table 6). No significant differences in tomato growth due to fertilizers were found in the m˛, sandy loam plot. Solarization, however, was highly significant in increasing numbers of fruit and fruit fresh weight.
Discussion
The results agree with previous reports (2, 3) indicating that addition of NH3-based fertilizers to soil reduces population densities of certain fungi and nematodes. Results from incubator studies showed that effects of nitrogenous amendments and solarization could be reasonably simulated by a simple procedure in the laboratory and greenhouse. Our simulation method required no special equipment other than a laboratory incubator and a timer device. However, a more flexible system may be necessary to test specific temperature thresholds.
Concentrations of NH4-N and N03-N during simulated solarization increased over the four-week incubation period. The extent of these increases generally corresponded with those seen after filed solarization. Previous reports (1, 5, 7) have discussed the liberation of nitrogenous compounds during solarization as a mode of action in increased plant growth response. The results of the present study also implicate the release of N in the lethal effect of solarization on certain soil microbiota.
Although increasing NH4-N concentrations gave better control of P. ultimum and M. incognita during simulated solarization, field results showed that the addition of nitrogen sources to the solarization process rarely increased the extent of control of P. ultimum and V. dahliae. Similar results were found after the combination of several soil pesticides with solarization (4, 6). This may be accounted for by release of sufficient NH3 during solarization for maximal biocidal effect or a comparatively minor role of NH3 in the disinfestation process. Since addition of N fertilizers to soil increases the possibility of groundwater contamination, further investigation should be done to determine quantitative and qualitative effects of N cycling during field solarization.
A complex mode of action has been postulated for soil disinfestation by solarization, including direct and indirect thermal effect, induced biological control, and changes in soil volatile components (5). The results presented here suggest that liberation of nitrogenous compounds during the solarization process, presumably in both vapor and liquid phases, is a component of the mode of action. Increased concentrations of reduced N compounds would then nitrify after termination of solarization to provide sufficient N03-N for increased crop growth.
Acknowledgements--We thank D. Brissonet, J. Broome, R. Duncan, J. Sibole, and R. J. Wakeman for technical assistance.
References
1. Chen, Y., and J. Katan. 1980. Effect of solar heating of soils by transparent polyethylene mulching on their chemical properties. Soil Science 130:271-277.
2. Chun, D., and J. L. Lockwood. 1985. Reduction of Pythium ultimum, Thielaviopsis basicola, and Macrophomina phaseolina populations in soil associated with ammonia generated from urea. Plant Disease 69:154158.
3. Rodriguez-Kabana, R. 1986. Organic and inorganic nitrogen amendments to soil as nematode suppressants. Journal of Nematology 129-135.
4. Stapleton, J. J. and J.E. DeVay. 1983. Response of phytoparasitic and free-living nematodes to soil solarization and 1,3-dichloropene in California. Phytopathology 73: 1429- 1436.
5. Stapleton, J. J. and J.E. DeVay. 1986. Soil solarization: a non-chemical approach for management of plant pathogens and pests. Crop Protection 5: 190-198.
6. Stapleton, J. J., B. Lear, and J. E. DeVay. 1987. Effect of combining soil solarization with certain nematicides on control of target and non-target organisms and plant growth. Annals of Applied Nematology (Journal of Nematology, Supplement) 1: 107- 112.
7. Stapleton, J. J., J. Quick, and J. E. DeVay. 1985. Soil solarization: effect on soil properties, crop fertilization, and plant growth. Soil Biology and Biochemistry 17:369-373.