3. Physical effects of soil solarization

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Thermal inactivation of crop pests and pathogens and other sol changes caused by solarization
Physical aspects of soil solarization
Application of solar-heated water for soil solarization

Thermal inactivation of crop pests and pathogens and other sol changes caused by solarization

James J. Stapleton
Statewide Integrated Pest Management Project,. University of California, 733 County Center III, Modesto, California 95355, US

Abstract

Thermal inactivation curves have been determined for a number of plant pathogenic fungi, bacteria, weed seed, and nematodes. Although lethal dosages (time/temperature) vary by organism, generally temperatures above 45°C require only minutes of exposure to reach LD₉₀ levels. The effects of diurnal soil temperature fluctuations on organism survival, as opposed to constant temperature regimes, have not been fully elucidated. As with other forms of soil heating, solarization results in complex changes in soil physical, chemical, and biological properties. Availability of many mineral nutrients is increased following solarization, particularly those tied up in organic fraction, such as NH₄-N, NO₃-N, P. Ca, and Mg. These increases in nutrient availability are an additional advantage to solarization, and may provide the equivalent of a pre-plant fertilizer dosage. In addition, liberation of compounds such as NH₃, CO₂, and other volatiles may play a role in the soil disinfestation process. Solarization produces a partial biological vacuum in soil, allowing recolonization by organisms which are more competitive. These organisms often are those which are antagonistic to plant pathogens and pests, which tend to have more strictly defined growth requirements. Each of these factors contribute to the overall effect of solarization, The qualitative and quantitative changes in solarized soils are dependent upon variables such as soil type, organic material, extent of heating, amount of soil moisture during treatment, pathogen and pest species present, cropping history, and other components of the soil ecology.

Introduction

Solarization is a hydrothermal process for disinfestation of soil (10). It is currently accomplished by incubating soil under transparent or black plastic film (primarily polyethylene or polyvinyl chloride=PVC) during hot months with minimal cloud cover, thus producing a "greenhouse effect" which raises soil temperature to levels which are lethal or injurious to many plant pathogens and pests. Effects of solarization are usually maximal when treated soil is moist or wet, since moisture in soil conducts heat better than dry soil. In addition, soilborne propagules of pathogens and pests may imbibe water and/or increase metabolism, which renders them more susceptible to lethal or injurious treatment dosages (6, 10). Prior to the ready availability of pesticidal chemicals in the late 1940s, the use of heat for disinfestation of agricultural soils was widespread. The principles of soil disinfestation by heat, particularly by steaming are well characterized and were summarized by Baker (l). Solarization is a similar process, except that the temperatures encountered are generally lower, the time of exposure to heat is longer, and temperatures fluctuate diurnally (10). Although solarization presents itself as a very simple method, the mode of action is complex, and can be described by physical, chemical, and biological effects.

Physical Changes - Thermal Inactivation

The direct hydrothermal effect of solarization is probably the most important in disinfesting soil (6, 10). Lethal dosage is a function of amount of both temperature and time (6). A number of plant pathogenic fungi, nematodes, and weed seed have been tested for their susceptibility to solarization. Temperatures commonly reached under normal conditions of soil solarization during the hot months of the year are 35°- 60°C, depending on soil depth. In the San Joaquin Valley of California, summer solarization normally heats soil in the upper 15 cm to 45°C or higher. Most soilborne pathogens and pests can survive a few hours at most in this temperature range (5, 7), under experimental conditions. Thermal death curves for fungal pathogens was shown to be logarithmic in nature (7). Many soilborne plant pathogens and pests are adequately controlled by 4-8 weeks of solarization (6, 10). Solarization may also control minor pathogens which may be deleterious to plant health and growth. A good, inverse linear correlation was obtained (6) when plotting "total" numbers of fungi and bacteria against growth of walnut seedlings (Figure l). However, the success of solarization in any particular location depends upon many factors, including climate and weather conditions, soil type and properties, susceptibility of target organism(s), survival and recolonization by antagonistic microbiota, and other variables of the ecosystem (6, 10 11).

Given the fact that temperature of solarized soil is highest near the soil surface and becomes cooler with increasing depth, it is reasonable to assume that the dynamics of pathogen and pest control will behave in a like manner. Normally, this is indeed the case. The majority of target pathogens and pests which have been studied in solarized soil have been most adequately controlled in the upper 10-30 cm of soil (6, 10). In most cases this works well, since populations of target organisms are usually concentrated in the same soil depth range - the rooting zone of host plants. In a study of solarization of fruit tree nursery soils in the San Joaquin Valley of California (8), colony-forming units of Agrobacterium spp. were most significantly reduced by solarization in the 0-15 cm depth range. Significance of reduction by solarization declined in the 15-30 and 30-46 cm depth range (Figure 2). On the other hand, Stapleton and DeVay found the root-knot nematode Meloidogyne hapla to be greatly reduced after solarization to a soil depth of 90 cm (10). In cases where solarization reduces population levels of soilborne plant pathogens or pests which are limiting to plant growth to very low levels, benefits can persist for up to several growing seasons (6,14).

Sub-lethal doses of heat also may adversely affect populations of soilborne pathogens and pests. Weakening of propagules may result in effects such as reduced or delayed germination or egg hatch; reduction of growth, vigour, or infectibility; increased susceptibility to attack by hyperparasites or predators (biological control), or to action of chemical pesticides (6, 10). These phenomena would be manifested as long-term control and induced suppressiveness of soil. Not all plant pathogens are routinely controlled by solarization, however. Pathogenic fungi including Macrophomina phaseolina, and some Pythium and Fusarium spp.; as well as certain nematode and weed taxa are reported to be resistant to the treatment (6, 10).

Chemical Changes

One of the common results of soil heating is an increase in concentration of certain soluble mineral nutrients (1, 2, 12,13). When soil is heated, much of the resident microbiota is killed and degraded, thus liberating the mineral nutrients. When the soil is moist, the soluble nutrients enter the soil solution. During solarization, increased amounts of nitrogen (principally NH₄-N and/or NO3-N) usually are liberated (2, 12, 13). The relative concentrations of each species are dependent upon the reducing nature of the soil (a function of soil physical properties and moisture content), and the presence of nitrifying micro-organisms (primarily Nitrobacter and Nitrosomonas spp.) (4). Therefore, high temperature and soil moisture content, particularly in soils high in organic matter, during solarization may kill much of the soil microbiota (including nitrifying micro-organisms) and produce micro-aerobic conditions, favouring accumulation of NH₄-N. On the other hand, relatively low temperatures and/or moisture content, especially in soils with little organic material, would allow increased survival of soil biota and promote aerobic conditions. In this case, liberation of nitrogenous compounds would be minimal, and these would be rapidly nitrified and perhaps lost from the system. Wet soil covered by polyethylene film but protected from heating in the San Joaquin Valley of California had chemical properties which were the same as control soil (13).

In addition to nitrogen, other mineral nutrients in soil including extractable phosphorus (P), potassium (K⁺), and calcium + magnesium (Ca²⁺ + Mg²⁺) have sometimes been found in greater concentration after solarization (13). Results with concentrations of micro-elements such as Fe³⁺, Mn²⁺, Zn²⁺, and Cu², and physical parameters including electrical conductivity (EC), organic matter, and pH also have been negative or inconclusive.

In many cases, the increase in soluble mineral nutrients after solarization is sufficient for a pre-plant fertilizer dosage for many crops (Table 1), and addition of supplemental, pre-plant fertilizer could result in phytotoxicity. This should be determined, and calculated into cost-benefit analyses of solarization. Nitrate is easily leached from soil, however, so if a maximal fertilizer effect is desired, crops should be planted shortly after solarization.

In greenhouse experiments conducted with three field soils without major pathogens, increased growth responses were obtained in pots with solarized soil (first crop). However, when a second crop was planted into the pots, IGR was not observed when soils were not fertilized, or when fertilized with nitrogen only. However, when soils were fertilized with a complete nutrient solution, IGR was observed in all three soils (Table 2). This indicates that, in solarized soils without pests or pathogens which are limiting factors to plant growth, the nutrient effect of solarization can, by itself, be the factor responsible for observed IGR (13).

Biological Changes

Compared to other methods of using heat for soil disinfestation, such as aerated steam, temperatures achieved during soil solarization are mild (1, 10). Because of this, effects of solarization on soil microbiota are more selective. Thermophilic and thermotolerant fungi, bacteria, and actinomycetes may survive and even flourish under solarization (3, 8). Lethal effects of solarization are most pronounced on micro-organisms which are not good soil competitors (10). Many plant pathogens fall into this group, since they tend to have specialized physiological requirements which are more adapted to coexistence with the host plant (10).

The solarization treatment causes a partial biological vacuum in soil. The substrate in soil available for colonization after solarization tends to be utilized by the more competitive micro-organisms. The resulting population shift of soil microbiota thus tends to favour antagonists, including Bacillus spp., fluorescent pseudomonads, and thermotolerant fungi (3, 8), and may suppress pathogen recolonization. Long-term benefits of solarization resulting from control of major pathogens have been reported (6, 14). However, induced soil suppressiveness or long-term control implies that the major factor limiting plant growth or yield is a soilborne pathogen. If the limiting factor is an element of soil fertility or other physical condition, or a pathogen capable of recolonizing solarized soil, long-term control is not expected.

Augmentative release of supplemental biological control microorganisms into solarized soil may or may not increase efficacy of the treatment. Some studies have shown synergistic activity of biological control agents or pesticides used in conjunction with solarization (6). Other reports, however, indicated that the solarization process often is optimally efficient in itself, and that further benefits from supplementing with additional control measures may not normally be expected (1, 9, 10). Use of soil-applied pesticides in conjunction with solarization should be investigated prior to large-scale use, since the breakdown of synthetic materials which depend on microbial degradation may be either accelerated or retarded by solarization treatment (6, 11).

Soil solarization, like other methods of soil disinfestation, must be thoroughly understood and applied correctly in order to achieve maximum effect and benefit.

References

1. Baker, K.F. 1962. Principles of disinfestation of heat-treated soil and planting material. Journal of the Australian Institute of Agricultural Science 28:118-126.

2. Chen, Y., and J. Katan. 1980. Effect of solar heating of soil by transparent polyethylene mulching on their chemical properties. Soil Science 130:271-277.

3. Gamliel, A., E. Hadar, and J. Katan. 1989. Soil solarization to improve yield of gypsophila in monoculture systems. Acta Horticulturae 255:131-138.

4. Hasson, A.M., T. Hassaballah, R. Hussain, and L. Abbass. Effect of soil sterilization on nitrification in soil. Journal of Plant Nutrition 10:18051809.

5. Heald, C.M., and A.F. Robinson. 1987. Effects of soil solarization on Rotylenchulus reniformis in the lower Rio Grande Valley of Texas. Journal of Nematology 19:93-103.

6. Katan, J. 1987. Soil solarization. Pages 77-105 in: Innovative Approaches to Plant Disease Management, I. Chet, ed. John Wiley & Sons, New York.

7. Pullman, G.S., J.E. DeVay, and R.H. Garber. 1981. Soil solarization and thermal death: A logarithmic relationship between time and temperature for four soilborne plant pathogens. Phytopathology 71:959-904.

8. Stapleton, JJ. 1981. Population dynamics of soil-borne bacteria and fungi as influenced by soil solarization with emphasis on (UY) Agrobacterium spp. MS Thesis, University of California, Davis. 54 pages.

9. Stapleton, J. J. 1990. Soil solarization in tropical agriculture for pre- and post-plant applications. In: Proceedings of the First Internation Conference on Soil Solarization. (In Press).

10. Stapleton, JJ. and J.E. DeVay. 1986. Soil solarization: a non-chemical approach for management of plant pathogens and pests. Crop Protection 5:190-198.

11. Stapleton, J.J., B. Lear, and J.E. DeVay. 1987. Effect of combining soil solarization with certain nematicides on target and nontarget organisms and plant growth. Annals of Applied Nematology (Journal of Nematology, Supplement) 1: 107-112.

12. Stapleton, J.J., J.E. DeVay, and B. Lear. 1990. Simulated and field effects of ammonia-based fertilizers and soil solarization on pathogen control, soil fertility and crop growth. Proceedings of the First International Conference on Soil Solarization. (In Press).

13. 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.

14. Tjamos, E.C., and EJ. Paplomatas. 1988. Long-term effect of soil solarization in controlling verticillium wilt of globe artichokes in Greece. Plant Pathology 37:507-515.

Figure 1. Fresh weight of English walnut (Juglans regia) seedlings (g/tree) plotted against numbers of soil micro-organisms in solarized (m) and control (a) soil. Line indicates best fit by linear regression. (Stapleton and DeVay, unpublished data.)

Figure 2. Effect of soil solarization, by depth, on numbers of Agrobacteriumspp. at two field sites. Brackets ([,]) indicate data points not different at (P<0.05). Solid lines indicate solarized soil; broken lines indicate control soil. (From reference 8).

Table 1. Effect of soil solarization (6 weeks) on soluble ammonium- and nitrate-nitrogen in four field soils

* Value different from control at P<0.05. (From reference 13).

Table 2. Effect of soil solarization, cropping, and different fertilization regimes on radish and lettuce plants in the greenhouse. (From reference 13)

* Values different from control at (P<0.05) using Student's l-test.
+ Values followed by different letters are different at (P<0.05) using Duncan's Multiple-Range Test.

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