- Email: [email protected]
Soil heating by solarization inside plastic greenhouse in Santa Maria, Rio Grande do Sul, Brazil
AGRICULTURAL AND FOREST METEOROLOGY Agricultural and Forest Meteorology 82 (I 996) 73-82
Soil heating by solarization inside plastic greenhouse in Santa Maria, Rio Grande do Sul, Brazil N.A. Streck Depurtamento
*,
F.M. Schneider, G.A. Buriol
de Fitotecnia, Centro de Ciencias Rurais, Uniuersidade Federal de Santu Mariu, Cumpus-Cum&i, 97119-900, Suntu Muriu, Rio Grande do Sul, Brazil
Received 4 August 1995; accepted 19 January 1996
Abstract Solar heating of soil by mulching with transparent polyethylene (known as soil solarization) is a physical method of soil disinfestation. The increase in soil temperature by solarization inside a plastic greenhouse in Santa Maria, Rio Grande do Sul State, Brazil, was measured. It was estimated the time which soil temperature exceeded some thermal levels in solarized soil. During the 1992/93 and 1993/94 summers, experiments were carried out in a 10 m X 2.5 m plastic greenhouse. Soil temperature was measured at 2 cm, 5 cm, 10 cm, and 20 cm depths in one solarized and uncovered plot. An increase of 10°C or more was observed in solarized soil at 2 cm and 5 cm depths in comparison to the bare soil. Soil temperature exceeded 50°C or more several days during 1993/94 up to the 10 cm depth. The estimated exposure time at several thermal levels in solarized soil allow to conclude that solarization presents potential in the region.
1. Introduction
Heat treatment of soil has been used for many years to control soilbome plant pathogens in greenhouse plant culture (Pullman et al., 1981; Katan and Devay, 1991). However, steam heat treatment of field soil is not generally economically practical. Mulching with transparent polyethylene (PE) is commonly used to increase the temperature of agricultural soils. Katan et al. (1976) showed that during the hot summer season in Israel, PE mulching of moistened soil prior to planting can raise its temperature enough to control soilbome pathogens as well as weeds. This method, also referred
* Corresponding author. 0168-1923/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. F’II SO168-1923(96)02339-S
N.A. Streck et al./Agricultural
74
and Forest Meteorolqy
82 (1996) 73-82
to as soil solarization, is now praticed in many hot countries (Katan, 1981; Katan et al., 1987; Schneider et al., 1993). Advantages of this method are the low cost, simplicity, and the fact that it is nonhazardous. Although the thermal death of plant pathogens is obtained with temperature above 5O”C, studies have shown that temperatures below 45°C are considered to be ‘sublethal’ and can kill soilborne pathogens if maintained for long periods (Bigelow, 1921; Munnecke et al., 1976; Pullman et al., 1981). Pullman et al. (1981) developed thermal death curves of several plant pathogenic fungi which can be used as indicators for evaluating the control of soilbome pathogens in solarized soil. The primary effect of solarization is the increase in soil temperature. Transparent mulches are more effective in increasing soil temperature than opaque mulches (straw, paper, black and colored plastics), due to a relatively large net radiation at the soil surface and an increase in soil heat flux (Rosenberg, 1974; Liakatas et al., 1986; Schneider et al., 1993). Results of several experiments have demonstrated that the use of solarization has increased the soil temperature by 3°C to 18°C over bare soil at several depths (Katan et al., 1987; Schneider et al., 1993). Consequently, temperatures between WC and 50°C have been frequently reported in solarized soil in several regions (Katan et al., 1987; Katan and Devay, 1991; Schneider et al., 1993). The effectiveness of the soil solarization method depends on the solar radiation availability during the treatment and the thermal properties of the soil. Thus, solarization should be tested in the region where it will be used. This method has been used in regions with high air temperatures and solar radiation availability. In cooler periods or regions, however, soil solarization is not effective in the open field, but it can be envisaged in closed greenhouses (Mahrer et al., 1987). The technique of soil solarization is not well known in Brazil, which is a country with several climatic regions. In South Brazil the four climatic seasons are well defined while other regions of the country are typically tropical and equatorial. In a previous paper, preliminary results of the first experiment of soil solarization in open field carried out in the Central Region of the Rio Grande do Sul State were reported (Streck et al., 1993). The purpose of the current study was to quantify the increase in soil temperature by solarization inside a plastic greenhouse in Santa Maria (Central Region of the Rio Grande do Sul State, South Brazil). In an attempt to analyse the solarization potential of the region, the time which soil temperature exceeded some thermal levels in solarized soil was estimated.
2. Theory The general equation which describes temperature (T) in a homogeneous, isotropic soil at a given time (r) and depth (z> is given by Fourier’s law: aT
2
_+ at
where D is the thermal diffusivity.
inert, and
N.A. Streck et al./Agricultural
and Forest Meteorology
82 (1996) 73-82
7.5
The model assumptions include: 1. Soil temperature at the surface is sinusoidal: To,,, = T + T, . sin( of)
(2)
where T is the mean soil temperature at the surface, T, is the maximum temperature amplitude at the surface, o is the angular velocity of the earth (7.27 X lo-’ rad s- ‘1; 2. The soil is a homogeneous medium, with constant density and moisture in profile; 3. Energy transport in soil is only by conduction; 4. Thermal condutivity is constant in space and time. The second boundary condition that validate Eq. (1) is: T(PJ)
=
T
(3)
The classical solution of Eq. (1) subject given by (Decico, 1974): TC~.Q
=
to the boundary
conditions
(2) and (3) is
(4)
Tzj + To. exp-‘(_)“z.sin[-.i-((~)“2.T)]
The maximum amplitude of the temperature can be calculated by using:
wave at a depth different
from zero
@orto)
To.+o=To.exp The thermal 1979):
-z(&)“*
diffusivity
(5)
of the soil can be calculated
by (Decico,
1974; Schneider,
2
(6)
where ATi = Tmax,i- T( Zi); ATj = Tmax,j- T( soil temperature at i and j depths (z).
zJ).
Tmax,i and Tmax,j are the maximum
3. Material and methods The experiment was carried out in a 10 m X 25 m nonheated greenhouse covered with low density transparent polyethylene (PE), located at the Experimental Field of the Crop Production Department of the Federal University of Santa Maria, Rio Grande do Sul State, Brazil (29”43’ S latitude, 53”48’ W longitude, and 95 m altitude). Individual plots of 6 m X 4 m were covered with 75 p,rn thickness PE sheets, from 12 December 1992 until 07 March 1993 and from 28 December 1993 until 21 February 1994. Uncovered plots of the same size were used as controls (bare soil). The texture of the greenhouse soil is loam, with 36% sand, 38% silt, and 26% clay. The soil was plowed and disced before the mulching in order to keep soil in good tillage. Soil was moistened
N.A. Streck et al./Agriculturul
76
and Forest Meteorology
82 (1996) 73-82
(field capacity) to a depth of 50 cm two days before mulching on the two experimental periods as recommended (Katan, 1981; Katan and Devay, 1991). Two tensiometers placed at 20 cm depth monitored soil moisture in one covered and uncovered plot during the experiment 1992/93. Tensiometers readings were taken every 2 days at 9 h, local standard time (LST). No additional water was applied in the covered plots during solarization. The uncovered plots (controls) were irrigated by sprinkling to keep the soil moisture near that of the mulched plots. Herbicides were used to control weeds in uncovered plots. The greenhouse was kept open during the two experimental periods, The soil temperature was measured daily throughout the two experimental periods by the use of mercury-in-glass geothermometers (0.2”C resolution). Geothermometers were buried at 2 cm, 5 cm, 10 cm, and 20 cm depths in one solarized and uncovered plot. The measurements were taken at 9 h, 15 h 30 min, 16 h, 18 h, and 21 h (LST). During summer months, the minimum soil temperature at 20 cm depth occured near to 9 h (LST) and the maximum soil temperature at 2 cm, 5 cm, 10 cm, and 20 cm depths occured near the afternoon measurements above mentioned, respectively (Schneider, 1979). Thus, the mean soil temperature at 20 cm depth (T,,) was calculated by: T*, = (7.W + T*,tJ/2
(7)
where T9,, and T2,,, are the minimum and maximum soil temperature at 20 cm depth, respectively. According to Decico (1974) and Schneider (1979) if the mean soil temperature at a depth is known, then it can be assumed as being the mean soil temperature of the soil profile (r). On 11 February 1993, a clear day, soil temperature was recorded between 5h30 min and 21 h every hour. Near the time of minimum and maximum temperature occurrence on each soil depth the reading interval was reduced to 15 min. The thermal diffusivity of the solarized soil was calculated on 11 February 1993 using Eq. (6). The temperature amplitude at the surface was estimated by Eq. (5) using the amplitude of wave temperature at 20 cm depth. As the soil temperature waves follow a sinusoidal model, regardless of the depth, the amplitude of the soil temperature wave at each depth (Tz> was estimated by: T, = T,,,,:
- %I
(8)
Thus, Eq. (2) can be written: T,,, = 7 + T, . sin( wt)
(9)
The daily temperature wave of solarized soil was estimated by Eqs. (4) and (9). The agreement between observed values of maximum temperature in solarized soil and values calculated by Eqs. (4) and (9) was tested by linear regression analysis. This analysis was made with daily values on the four afternoon measurements per day and each depth. Using the equation that better estimates the soil temperature, it was estimated (a) the number of days which the soil temperature exceeded 50°C by 1 hour or more at 2 cm and 5 cm depths, and (b) the daily average time which soil temperature exceeded some thermal levels at 5 cm, 10 cm, and 20 cm depths. Thermal levels were stablished on the basis of those reported by Pullman et al. (1981).
N.A. Streck et al./Agricultural
and Forest Meteorology
82 (1996) 73-82
77
4. Results and discussion The soil moisture was similar in covered and bare soil throughout the experimental period of 1992/93. Tensiometers readings were always lower than 100 hPa (field capacity) in both treatments. Therefore, soil moisture was ideal for solarization method and to reach the steady conditions which the temperature-model assumes. The maximum temperature was always higher in solarized soil than bare soil during the two experimental periods, regardless of the depth. On average, maximum temperature of solarized soil was 11.9, 10.8, 9.8, and 8.6”C higher than bare soil at 2 cm, 5 cm, 10 cm, and 20 cm depth, respectively, in the experiment 1992/93. In the experiment 1993/94, average values of maximum temperature of solarized soil was 12.2, 12.0, and 9.1”C higher than bare soil at 5 cm, 10 cm, and 20 cm depth, respectively (Table 1). The temperature differences between solarized and bare soil were greater on clear days, when the incoming solar radiation was high. Smaller differences were recorded on cloudy days. Thermal storage in the soil from past clear days has contributed to temperature differences on cloudy days as reported previously by Liakatas et al. (1986). Maximum temperature in both solarized and bare soil decreased with increasing soil depth. Thus, the efficiency of the PE in soil heating decreases with increasing soil depth. The increase in soil temperature by PE during the daytime is due to a decrease in sensible and latent heat fluxes, and thereby it increases the amount of heat available for soil heating. This hypothesis has been proved by many reports (Mahrer, 1979; Avissar et al., 1986; Liakatas et al., 1986). Furthermore, field observations have shown that immediately after the laying of a PE sheet on wet soil, water condenses on its inner surface. As a result of the formation of water droplets, the transmissivity of polyethylene to terrestrial radiation is reduced, while its transmissivity to solar radiation is almost unaffected. Consequently, soil heating is also increased due to an increase in greenhouse effect (Mahrer, 1979; Avissar et al., 1986; Liakatas et al., 1986; Schneider et al., 1993). Maximum temperature reached larger absolute values in solarized soil during experiment 1993/94 than 1992/93 (Table 1). Many cloudy days were observed during
Table I Average and absolute values of maximum Maria, RS, Brazil Treatment
Bare
in solarized and bare soil of a plastic greenhouse.
Depth
Maximum
(cm)
Average 1993/94
1992/93
1993/94
2 5 10 20
43.8 42.0 38.3 35.3
44.6 42.2 36.5
54.4 50.2 46.0 41.2
> 55.5 55.4 50.2 42.4
2 5 10 20
31.9 31.2 28.5 26.7
33.4 32.4 30.2 27.4
40.4 37.6 33.6 29.7
38.0 37.0 33.6 30.6
1992/93 Solarized
temperature
temperature
(“Cl Absolute
Santa
N.A. Streck et al./Agriculturul
and Forest Meteorology
EXPERIMENT
82 (1996) 73-82
1993/94
---.~_ ‘-m7L~
40
42
45 Thermal
47
50
level PC)
52
;s
37
40
42
45 Thermal
47
50
52
55
level (“C)
Fig. 1. Frequency of days in which the observed temperature exceeded some thermal levels in solarized (a) and bare (b) soil of a plastic greenhouse. Santa Maria, RS, Brazil.
January 1993 in contrast with January 1994. Consequently, soil heating was high in summer 93/94. Fig. 1 shows a cumulative frequency in which the temperature exceeded some thermal levels in both solarized and bare soil in the two experiments. In general, temperatures exceeded 50°C several days up to 10 cm depth while temperature in bare soil did not reach 41°C at the measured depths. The estimated value of thermal diffusivity of the solarized soil on 11 February 1993 was 6.78 X 10e7 m2 s- ‘. This val ue was used in Eq. (4). The agreement between observed values of maximum soil temperature and those estimated by Eqs. (4) and (9) showed that maximum temperature in solarized soil was better estimated by Eq. (9) (linear coefficient near zero, angular coefficient near 1, and coefficient of determination near 1). Similar results were reported by Schneider (1979) in the same soil used in this study. By using Eq. (41, maximum amplitude of the soil temperature wave at the surface (7’,> is estimated from maximum amplitude of temperature at 20 cm depth. But, the maximum amplitude of temperature in solarized soil was higher than bare soil, particularly at 2 cm and 5 cm depths. The maximum amplitude of temperature of solarized soil on 11 February 1993 was 15.1, 10.9, 6.4, and 2.9”C, while the maximum amplitude of temperature of bare soil was 8.8, 7.2, 4.6, and 2.2”C at 2 cm, 5 cm, 10 cm, and 20 cm depths, respectively. Thus, Eq. (4) underestimated soil temperatures near to the oc-
N.A. Streck et al. /Agricultural Table 2 Number of days in which the estimated temperature of a plastic greenhouse (Santa Maria, RS, Brazil) Period Experiment 1992 / 93 21 Dee to 31 Dee 1992 01 Jan to 10 Jan 1993 11 Jan to20Jan 1993 21Janto3lJa.n 1993 01 Feb to 10 Feb 1993 1 I Feb to 20 Feb 1993 21 Feb to 28 Feb 1993 Experiment 1993/94 01 Jan to 10 Jan 1994 11 Janto20Jan 1994 21 Jan to 31 Jan 1994 01 Feb to 10 Feb 1994 1I Feb to 20 Feb 1994
und Forest Meteorology 82 (1996) 73-82
exceeded 50°C by
ND>50 2cm
I h or more (ND > 50) in solar&d
79
soil
5cm
9 6 0
curence time of extremes values (minimum and maximum). Consequently, Eq. (9) was used to estimate the data in Table 2 and Fig. 2. During January 1994 it was estimated that one can expected up to 4 days per a ten-days period with temperatures exceeding 50°C at 5 cm depth of solarized soil (Table 2). Fig. 2 shows the various time-temperature combinations estimated in solarized soil at 5 cm, 10 cm, and 20 cm depths. It was not possible to estimate the temperature wave at 2 cm depth because the maximum temperature exceeded the scale of geothermometers (55°C). Pullman et al. (1981) showed that there are a linear relationship between temperature and the logarithm of time required to kill 90% of the propagules (LD,,) of several plant pathogenic fungi. Exposure times varied from 10 to 70 min at 50°C 50 to 360 min at 47°C 2 to 14 h at 45”C, and 14 to 46 h at 42°C. Analysing Fig. 2, estimated soil temperature exceeded 42°C during 0.5 h day- ’ at 20 cm depth, 1.4-6.7 h day- ’ at 10 cm depth, and 1.8-8.1 h day-’ at 5 cm depth, while temperature exceeded 45°C during 0.9-5.2 and 1.5-3.6 h day-’ at 5 cm and 10 cm depths, respectively. The highest soil heating was particularly observed during January in the two experiments. February is a marginal month for solarization in Santa Maria. As the soil heating was relatively low at 20 cm depth, a long solarization period is required. However, soil treatment by solarization can be reduced because the course and pattern of heating during solarization treatment are different from those usually established for heat mortality curves under laboratory controlled conditions. In solarized soil, propagules of soilbome pathogens are exposed to sublethal and lethal temperatures during daytime alternated with lower temperatures during night, which contrasts with heating at a constant temperature, the usual procedure under controlled conditions (Katan and Devay, 1991). In addition, it should be considered that the soil environment is different from the culture medium where pathogens are grown in the laboratory.
80
N.A. Srreck et ul./Apiculrurul
EXPERIMENT
37
40
42 Thermal
37
46
Thermal
47
82 11996) 73-82
EXPERIMENT
1992/93
60
4b
42
Thermal
level PC)
42
40
und Forest Meteorology
46
47
60
37
40
level PC)
45
47
6b
47
60
level PC)
42 Thermal
1993/94
46 level PC)
(c) 12 10 -7 P8 3s 24 s2 0 3;
a,2
4’0
Thermal Period:
1. 21
Dee
46
47
37
40
42 Thermal
level PC)
46
47
31
Dee
1. 01
Jan
-
10 Jr,”
Jan Jan
2.11
.lan
-
20
3.
21
Jan
-
31
Jan
4.
01
Feb
-
10
Feb
5.
11 Feb
-
01 Jan 11 JB”
4.
21
Jan
5.
01
Feb
6.
11 Feb
-
20
Feb
7.
21
-
26
Feb
-
31 10
Period:
60
level PC)
- 10 - 20
2. 3.
Feb
6’0
-
20
Jan Jan Feb Feb
Fig. 2. Daily average time (AT) which estimated temperature exceeded some thermal levels in solarized soil of a plastic greenhouse. (a) 5 cm depth, (b) IO cm depth, and (c) 20 cm depth. Santa Maria, RS, Brazil.
NA. Streck et al./Agricultural
und Forest Meteorology
82 (1996) 73-82
81
According to Katan et al. (1976) and Pullman et al. (19811, these factors and possibly others can explain reductions in soilborne plant pathogenic fungi where soil temperature did not exceed 41°C. The mechanisms of heat inactivation in fungi by sublethal thermal exposure are very complex (Katan and Devay, 1991). Observed and estimated temperatures in solarized soil obtained in this study are similar to those reported in other regions (Katan et al., 1987; Schneider et al., 1993). In Israel, where the method was conceived and developed, temperatures of solarized soil at 5 cm and 20 cm depths were 45-55°C and 39-45°C (Katan et al., 1976; Katan, 1981). In California, temperatures of tarped soils were either similar or higher, reaching 60°C at the 5 cm depth (Pullman et al., 1981). Consequently, our results suggest that soil solarization is a potentially useful practice for commercial vegetable production in the Central Region of the Rio Grande do Sul State, Brazil, particularly inside plastic greenhouses. The exposure time at several thermal levels estimated in solarized soil are similar to the reported by Pullman et al. (1981) to kill soilbome pathogens under laboratory conditions. Thus, it can be concluded that Santa Maria presents potential to employ the solarization method in plastic greenhouse. Solarization should be avoided during February because soil heating was low in our two experiments. Other studies are needed to determine the best month for solarization and to verify its effectiveness on soilbome pathogens in field conditions.
Acknowledgements This research was supported by Funda$io de Amparo a Pesquisa do Estado do Rio Grande do Sul and Conselho National de Desenvolvimento Cientifico e Tecnolbgico.
References Avissar, R., Mahrer, Y., Margulies, L. and Katan, J., 1986. Field aging of transparent polyethylene mulches. I: Photometric properties. Soil Sci. Sot. Am. J., 50: 202-205. Bigelow, W.D., 1921. The logarithmic nature of thermal death time curves. J. Infect. Dis., 28: 528-536. Decico, A., 1974. A determina@o das propriedades t&micas do solo em condi@es de campo. Piracicaba, SP, Brazil, 1974. 78 p. Thesis of Livre-Doc&ncia, Escola Superior de Agricultura “Luis de Queir6s”. Katan, J., Greenberger, A., Alon, H. and Grinstein, A., 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilbome pathogens. Phytopahology, 66: 683-688. Katan, J., 1981. Solar heating (solarization) of soil for control of soilbome patogens. Annu. Rev. Phytopathol., 19: 21 l-236. Katan, J., Grinstein, A., Greenberger, A., Yarden, 0. and Devay, J.E. 1987. The first decade (1976-1986) of soil solarization (solar heating): A cronological bibliography. Phytoparasitica, 15: 229-255. Katan, J. and Devay, J.E., 1991. Soil Solarization. CRC Press, Boca Raton, FL, 267 pp. Liakatas, A., Clark, J.A. and Monteith, J.L.. 1986. Measurements of the heat balance under plastic mulches. I: Radiation balance and soil heat flux. Agric. For. Meteorol., 36: 227-239. Mahrer, Y., 1979. Prediction of soil temperature of a soil mulched with transparent polyethylene. J. Appl. Meteorol., 18: 1263- 1267. Mahrer, Y., Avissar, R., Naot, 0. and Katan, J., 1987. Intensified soil solarization with closed greenhouse: numerical and experimental studies. Agric. For. Meteorol., 41: 325-330.
82
N.A. Streck et al./Agriculrurd
and Forest Meteorology
82 (1996) 73-82
Munnecke, D.E., Wilbur, W. and Darley, E.F., 1976. Effect of heating or drying on Armillaria mellru or 66: 1363-1368. Trichoderma uiridae and the relation to survival of A. mellea in soil. Phytopathology, Pullman, G.S.. Devay, J.E. and Garber, R.H., 1981. Soil solarization and thermal death: a logarithmic relantionship between time and temperature for four soilbome pathogens. Phytopathology, 71: 959-964. Rosenberg, N.J., 1974. Microclimate: the Biological Environment. John Wiley & Sons, New York, 315 pp. Schneider, F.M., 1979. Comportamento e propriedades t&micas do solo Santa Maria. Piracicaba, SP, Brazil, 1979. 77 p. Dissertation (Mestrado em Agrometeorologia), Escola Superior de Agricultura “Luis de Queiros”. Schneider, F.M., Streck, N.A. and Buriol, G.A.. 1993. Moditica@es fisicas causadas pela solariza$Zo do solo. Rev. Bras. Agrometeorol., I: 149-157. Streck, N.A., Schneider, F.M. and Buriol, G.A., 1993. Soil temperature modifications caused by solarization in nurseries. Ciencia Rural, 23: 385-386.
Soil heating by solarization inside plastic greenhouse in Santa Maria, Rio Grande do Sul, Brazil N.A. Streck Depurtamento
*,
F.M. Schneider, G.A. Buriol
de Fitotecnia, Centro de Ciencias Rurais, Uniuersidade Federal de Santu Mariu, Cumpus-Cum&i, 97119-900, Suntu Muriu, Rio Grande do Sul, Brazil
Received 4 August 1995; accepted 19 January 1996
Abstract Solar heating of soil by mulching with transparent polyethylene (known as soil solarization) is a physical method of soil disinfestation. The increase in soil temperature by solarization inside a plastic greenhouse in Santa Maria, Rio Grande do Sul State, Brazil, was measured. It was estimated the time which soil temperature exceeded some thermal levels in solarized soil. During the 1992/93 and 1993/94 summers, experiments were carried out in a 10 m X 2.5 m plastic greenhouse. Soil temperature was measured at 2 cm, 5 cm, 10 cm, and 20 cm depths in one solarized and uncovered plot. An increase of 10°C or more was observed in solarized soil at 2 cm and 5 cm depths in comparison to the bare soil. Soil temperature exceeded 50°C or more several days during 1993/94 up to the 10 cm depth. The estimated exposure time at several thermal levels in solarized soil allow to conclude that solarization presents potential in the region.
1. Introduction
Heat treatment of soil has been used for many years to control soilbome plant pathogens in greenhouse plant culture (Pullman et al., 1981; Katan and Devay, 1991). However, steam heat treatment of field soil is not generally economically practical. Mulching with transparent polyethylene (PE) is commonly used to increase the temperature of agricultural soils. Katan et al. (1976) showed that during the hot summer season in Israel, PE mulching of moistened soil prior to planting can raise its temperature enough to control soilbome pathogens as well as weeds. This method, also referred
* Corresponding author. 0168-1923/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. F’II SO168-1923(96)02339-S
N.A. Streck et al./Agricultural
74
and Forest Meteorolqy
82 (1996) 73-82
to as soil solarization, is now praticed in many hot countries (Katan, 1981; Katan et al., 1987; Schneider et al., 1993). Advantages of this method are the low cost, simplicity, and the fact that it is nonhazardous. Although the thermal death of plant pathogens is obtained with temperature above 5O”C, studies have shown that temperatures below 45°C are considered to be ‘sublethal’ and can kill soilborne pathogens if maintained for long periods (Bigelow, 1921; Munnecke et al., 1976; Pullman et al., 1981). Pullman et al. (1981) developed thermal death curves of several plant pathogenic fungi which can be used as indicators for evaluating the control of soilbome pathogens in solarized soil. The primary effect of solarization is the increase in soil temperature. Transparent mulches are more effective in increasing soil temperature than opaque mulches (straw, paper, black and colored plastics), due to a relatively large net radiation at the soil surface and an increase in soil heat flux (Rosenberg, 1974; Liakatas et al., 1986; Schneider et al., 1993). Results of several experiments have demonstrated that the use of solarization has increased the soil temperature by 3°C to 18°C over bare soil at several depths (Katan et al., 1987; Schneider et al., 1993). Consequently, temperatures between WC and 50°C have been frequently reported in solarized soil in several regions (Katan et al., 1987; Katan and Devay, 1991; Schneider et al., 1993). The effectiveness of the soil solarization method depends on the solar radiation availability during the treatment and the thermal properties of the soil. Thus, solarization should be tested in the region where it will be used. This method has been used in regions with high air temperatures and solar radiation availability. In cooler periods or regions, however, soil solarization is not effective in the open field, but it can be envisaged in closed greenhouses (Mahrer et al., 1987). The technique of soil solarization is not well known in Brazil, which is a country with several climatic regions. In South Brazil the four climatic seasons are well defined while other regions of the country are typically tropical and equatorial. In a previous paper, preliminary results of the first experiment of soil solarization in open field carried out in the Central Region of the Rio Grande do Sul State were reported (Streck et al., 1993). The purpose of the current study was to quantify the increase in soil temperature by solarization inside a plastic greenhouse in Santa Maria (Central Region of the Rio Grande do Sul State, South Brazil). In an attempt to analyse the solarization potential of the region, the time which soil temperature exceeded some thermal levels in solarized soil was estimated.
2. Theory The general equation which describes temperature (T) in a homogeneous, isotropic soil at a given time (r) and depth (z> is given by Fourier’s law: aT
2
_+ at
where D is the thermal diffusivity.
inert, and
N.A. Streck et al./Agricultural
and Forest Meteorology
82 (1996) 73-82
7.5
The model assumptions include: 1. Soil temperature at the surface is sinusoidal: To,,, = T + T, . sin( of)
(2)
where T is the mean soil temperature at the surface, T, is the maximum temperature amplitude at the surface, o is the angular velocity of the earth (7.27 X lo-’ rad s- ‘1; 2. The soil is a homogeneous medium, with constant density and moisture in profile; 3. Energy transport in soil is only by conduction; 4. Thermal condutivity is constant in space and time. The second boundary condition that validate Eq. (1) is: T(PJ)
=
T
(3)
The classical solution of Eq. (1) subject given by (Decico, 1974): TC~.Q
=
to the boundary
conditions
(2) and (3) is
(4)
Tzj + To. exp-‘(_)“z.sin[-.i-((~)“2.T)]
The maximum amplitude of the temperature can be calculated by using:
wave at a depth different
from zero
@orto)
To.+o=To.exp The thermal 1979):
-z(&)“*
diffusivity
(5)
of the soil can be calculated
by (Decico,
1974; Schneider,
2
(6)
where ATi = Tmax,i- T( Zi); ATj = Tmax,j- T( soil temperature at i and j depths (z).
zJ).
Tmax,i and Tmax,j are the maximum
3. Material and methods The experiment was carried out in a 10 m X 25 m nonheated greenhouse covered with low density transparent polyethylene (PE), located at the Experimental Field of the Crop Production Department of the Federal University of Santa Maria, Rio Grande do Sul State, Brazil (29”43’ S latitude, 53”48’ W longitude, and 95 m altitude). Individual plots of 6 m X 4 m were covered with 75 p,rn thickness PE sheets, from 12 December 1992 until 07 March 1993 and from 28 December 1993 until 21 February 1994. Uncovered plots of the same size were used as controls (bare soil). The texture of the greenhouse soil is loam, with 36% sand, 38% silt, and 26% clay. The soil was plowed and disced before the mulching in order to keep soil in good tillage. Soil was moistened
N.A. Streck et al./Agriculturul
76
and Forest Meteorology
82 (1996) 73-82
(field capacity) to a depth of 50 cm two days before mulching on the two experimental periods as recommended (Katan, 1981; Katan and Devay, 1991). Two tensiometers placed at 20 cm depth monitored soil moisture in one covered and uncovered plot during the experiment 1992/93. Tensiometers readings were taken every 2 days at 9 h, local standard time (LST). No additional water was applied in the covered plots during solarization. The uncovered plots (controls) were irrigated by sprinkling to keep the soil moisture near that of the mulched plots. Herbicides were used to control weeds in uncovered plots. The greenhouse was kept open during the two experimental periods, The soil temperature was measured daily throughout the two experimental periods by the use of mercury-in-glass geothermometers (0.2”C resolution). Geothermometers were buried at 2 cm, 5 cm, 10 cm, and 20 cm depths in one solarized and uncovered plot. The measurements were taken at 9 h, 15 h 30 min, 16 h, 18 h, and 21 h (LST). During summer months, the minimum soil temperature at 20 cm depth occured near to 9 h (LST) and the maximum soil temperature at 2 cm, 5 cm, 10 cm, and 20 cm depths occured near the afternoon measurements above mentioned, respectively (Schneider, 1979). Thus, the mean soil temperature at 20 cm depth (T,,) was calculated by: T*, = (7.W + T*,tJ/2
(7)
where T9,, and T2,,, are the minimum and maximum soil temperature at 20 cm depth, respectively. According to Decico (1974) and Schneider (1979) if the mean soil temperature at a depth is known, then it can be assumed as being the mean soil temperature of the soil profile (r). On 11 February 1993, a clear day, soil temperature was recorded between 5h30 min and 21 h every hour. Near the time of minimum and maximum temperature occurrence on each soil depth the reading interval was reduced to 15 min. The thermal diffusivity of the solarized soil was calculated on 11 February 1993 using Eq. (6). The temperature amplitude at the surface was estimated by Eq. (5) using the amplitude of wave temperature at 20 cm depth. As the soil temperature waves follow a sinusoidal model, regardless of the depth, the amplitude of the soil temperature wave at each depth (Tz> was estimated by: T, = T,,,,:
- %I
(8)
Thus, Eq. (2) can be written: T,,, = 7 + T, . sin( wt)
(9)
The daily temperature wave of solarized soil was estimated by Eqs. (4) and (9). The agreement between observed values of maximum temperature in solarized soil and values calculated by Eqs. (4) and (9) was tested by linear regression analysis. This analysis was made with daily values on the four afternoon measurements per day and each depth. Using the equation that better estimates the soil temperature, it was estimated (a) the number of days which the soil temperature exceeded 50°C by 1 hour or more at 2 cm and 5 cm depths, and (b) the daily average time which soil temperature exceeded some thermal levels at 5 cm, 10 cm, and 20 cm depths. Thermal levels were stablished on the basis of those reported by Pullman et al. (1981).
N.A. Streck et al./Agricultural
and Forest Meteorology
82 (1996) 73-82
77
4. Results and discussion The soil moisture was similar in covered and bare soil throughout the experimental period of 1992/93. Tensiometers readings were always lower than 100 hPa (field capacity) in both treatments. Therefore, soil moisture was ideal for solarization method and to reach the steady conditions which the temperature-model assumes. The maximum temperature was always higher in solarized soil than bare soil during the two experimental periods, regardless of the depth. On average, maximum temperature of solarized soil was 11.9, 10.8, 9.8, and 8.6”C higher than bare soil at 2 cm, 5 cm, 10 cm, and 20 cm depth, respectively, in the experiment 1992/93. In the experiment 1993/94, average values of maximum temperature of solarized soil was 12.2, 12.0, and 9.1”C higher than bare soil at 5 cm, 10 cm, and 20 cm depth, respectively (Table 1). The temperature differences between solarized and bare soil were greater on clear days, when the incoming solar radiation was high. Smaller differences were recorded on cloudy days. Thermal storage in the soil from past clear days has contributed to temperature differences on cloudy days as reported previously by Liakatas et al. (1986). Maximum temperature in both solarized and bare soil decreased with increasing soil depth. Thus, the efficiency of the PE in soil heating decreases with increasing soil depth. The increase in soil temperature by PE during the daytime is due to a decrease in sensible and latent heat fluxes, and thereby it increases the amount of heat available for soil heating. This hypothesis has been proved by many reports (Mahrer, 1979; Avissar et al., 1986; Liakatas et al., 1986). Furthermore, field observations have shown that immediately after the laying of a PE sheet on wet soil, water condenses on its inner surface. As a result of the formation of water droplets, the transmissivity of polyethylene to terrestrial radiation is reduced, while its transmissivity to solar radiation is almost unaffected. Consequently, soil heating is also increased due to an increase in greenhouse effect (Mahrer, 1979; Avissar et al., 1986; Liakatas et al., 1986; Schneider et al., 1993). Maximum temperature reached larger absolute values in solarized soil during experiment 1993/94 than 1992/93 (Table 1). Many cloudy days were observed during
Table I Average and absolute values of maximum Maria, RS, Brazil Treatment
Bare
in solarized and bare soil of a plastic greenhouse.
Depth
Maximum
(cm)
Average 1993/94
1992/93
1993/94
2 5 10 20
43.8 42.0 38.3 35.3
44.6 42.2 36.5
54.4 50.2 46.0 41.2
> 55.5 55.4 50.2 42.4
2 5 10 20
31.9 31.2 28.5 26.7
33.4 32.4 30.2 27.4
40.4 37.6 33.6 29.7
38.0 37.0 33.6 30.6
1992/93 Solarized
temperature
temperature
(“Cl Absolute
Santa
N.A. Streck et al./Agriculturul
and Forest Meteorology
EXPERIMENT
82 (1996) 73-82
1993/94
---.~_ ‘-m7L~
40
42
45 Thermal
47
50
level PC)
52
;s
37
40
42
45 Thermal
47
50
52
55
level (“C)
Fig. 1. Frequency of days in which the observed temperature exceeded some thermal levels in solarized (a) and bare (b) soil of a plastic greenhouse. Santa Maria, RS, Brazil.
January 1993 in contrast with January 1994. Consequently, soil heating was high in summer 93/94. Fig. 1 shows a cumulative frequency in which the temperature exceeded some thermal levels in both solarized and bare soil in the two experiments. In general, temperatures exceeded 50°C several days up to 10 cm depth while temperature in bare soil did not reach 41°C at the measured depths. The estimated value of thermal diffusivity of the solarized soil on 11 February 1993 was 6.78 X 10e7 m2 s- ‘. This val ue was used in Eq. (4). The agreement between observed values of maximum soil temperature and those estimated by Eqs. (4) and (9) showed that maximum temperature in solarized soil was better estimated by Eq. (9) (linear coefficient near zero, angular coefficient near 1, and coefficient of determination near 1). Similar results were reported by Schneider (1979) in the same soil used in this study. By using Eq. (41, maximum amplitude of the soil temperature wave at the surface (7’,> is estimated from maximum amplitude of temperature at 20 cm depth. But, the maximum amplitude of temperature in solarized soil was higher than bare soil, particularly at 2 cm and 5 cm depths. The maximum amplitude of temperature of solarized soil on 11 February 1993 was 15.1, 10.9, 6.4, and 2.9”C, while the maximum amplitude of temperature of bare soil was 8.8, 7.2, 4.6, and 2.2”C at 2 cm, 5 cm, 10 cm, and 20 cm depths, respectively. Thus, Eq. (4) underestimated soil temperatures near to the oc-
N.A. Streck et al. /Agricultural Table 2 Number of days in which the estimated temperature of a plastic greenhouse (Santa Maria, RS, Brazil) Period Experiment 1992 / 93 21 Dee to 31 Dee 1992 01 Jan to 10 Jan 1993 11 Jan to20Jan 1993 21Janto3lJa.n 1993 01 Feb to 10 Feb 1993 1 I Feb to 20 Feb 1993 21 Feb to 28 Feb 1993 Experiment 1993/94 01 Jan to 10 Jan 1994 11 Janto20Jan 1994 21 Jan to 31 Jan 1994 01 Feb to 10 Feb 1994 1I Feb to 20 Feb 1994
und Forest Meteorology 82 (1996) 73-82
exceeded 50°C by
ND>50 2cm
I h or more (ND > 50) in solar&d
79
soil
5cm
9 6 0
curence time of extremes values (minimum and maximum). Consequently, Eq. (9) was used to estimate the data in Table 2 and Fig. 2. During January 1994 it was estimated that one can expected up to 4 days per a ten-days period with temperatures exceeding 50°C at 5 cm depth of solarized soil (Table 2). Fig. 2 shows the various time-temperature combinations estimated in solarized soil at 5 cm, 10 cm, and 20 cm depths. It was not possible to estimate the temperature wave at 2 cm depth because the maximum temperature exceeded the scale of geothermometers (55°C). Pullman et al. (1981) showed that there are a linear relationship between temperature and the logarithm of time required to kill 90% of the propagules (LD,,) of several plant pathogenic fungi. Exposure times varied from 10 to 70 min at 50°C 50 to 360 min at 47°C 2 to 14 h at 45”C, and 14 to 46 h at 42°C. Analysing Fig. 2, estimated soil temperature exceeded 42°C during 0.5 h day- ’ at 20 cm depth, 1.4-6.7 h day- ’ at 10 cm depth, and 1.8-8.1 h day-’ at 5 cm depth, while temperature exceeded 45°C during 0.9-5.2 and 1.5-3.6 h day-’ at 5 cm and 10 cm depths, respectively. The highest soil heating was particularly observed during January in the two experiments. February is a marginal month for solarization in Santa Maria. As the soil heating was relatively low at 20 cm depth, a long solarization period is required. However, soil treatment by solarization can be reduced because the course and pattern of heating during solarization treatment are different from those usually established for heat mortality curves under laboratory controlled conditions. In solarized soil, propagules of soilbome pathogens are exposed to sublethal and lethal temperatures during daytime alternated with lower temperatures during night, which contrasts with heating at a constant temperature, the usual procedure under controlled conditions (Katan and Devay, 1991). In addition, it should be considered that the soil environment is different from the culture medium where pathogens are grown in the laboratory.
80
N.A. Srreck et ul./Apiculrurul
EXPERIMENT
37
40
42 Thermal
37
46
Thermal
47
82 11996) 73-82
EXPERIMENT
1992/93
60
4b
42
Thermal
level PC)
42
40
und Forest Meteorology
46
47
60
37
40
level PC)
45
47
6b
47
60
level PC)
42 Thermal
1993/94
46 level PC)
(c) 12 10 -7 P8 3s 24 s2 0 3;
a,2
4’0
Thermal Period:
1. 21
Dee
46
47
37
40
42 Thermal
level PC)
46
47
31
Dee
1. 01
Jan
-
10 Jr,”
Jan Jan
2.11
.lan
-
20
3.
21
Jan
-
31
Jan
4.
01
Feb
-
10
Feb
5.
11 Feb
-
01 Jan 11 JB”
4.
21
Jan
5.
01
Feb
6.
11 Feb
-
20
Feb
7.
21
-
26
Feb
-
31 10
Period:
60
level PC)
- 10 - 20
2. 3.
Feb
6’0
-
20
Jan Jan Feb Feb
Fig. 2. Daily average time (AT) which estimated temperature exceeded some thermal levels in solarized soil of a plastic greenhouse. (a) 5 cm depth, (b) IO cm depth, and (c) 20 cm depth. Santa Maria, RS, Brazil.
NA. Streck et al./Agricultural
und Forest Meteorology
82 (1996) 73-82
81
According to Katan et al. (1976) and Pullman et al. (19811, these factors and possibly others can explain reductions in soilborne plant pathogenic fungi where soil temperature did not exceed 41°C. The mechanisms of heat inactivation in fungi by sublethal thermal exposure are very complex (Katan and Devay, 1991). Observed and estimated temperatures in solarized soil obtained in this study are similar to those reported in other regions (Katan et al., 1987; Schneider et al., 1993). In Israel, where the method was conceived and developed, temperatures of solarized soil at 5 cm and 20 cm depths were 45-55°C and 39-45°C (Katan et al., 1976; Katan, 1981). In California, temperatures of tarped soils were either similar or higher, reaching 60°C at the 5 cm depth (Pullman et al., 1981). Consequently, our results suggest that soil solarization is a potentially useful practice for commercial vegetable production in the Central Region of the Rio Grande do Sul State, Brazil, particularly inside plastic greenhouses. The exposure time at several thermal levels estimated in solarized soil are similar to the reported by Pullman et al. (1981) to kill soilbome pathogens under laboratory conditions. Thus, it can be concluded that Santa Maria presents potential to employ the solarization method in plastic greenhouse. Solarization should be avoided during February because soil heating was low in our two experiments. Other studies are needed to determine the best month for solarization and to verify its effectiveness on soilbome pathogens in field conditions.
Acknowledgements This research was supported by Funda$io de Amparo a Pesquisa do Estado do Rio Grande do Sul and Conselho National de Desenvolvimento Cientifico e Tecnolbgico.
References Avissar, R., Mahrer, Y., Margulies, L. and Katan, J., 1986. Field aging of transparent polyethylene mulches. I: Photometric properties. Soil Sci. Sot. Am. J., 50: 202-205. Bigelow, W.D., 1921. The logarithmic nature of thermal death time curves. J. Infect. Dis., 28: 528-536. Decico, A., 1974. A determina@o das propriedades t&micas do solo em condi@es de campo. Piracicaba, SP, Brazil, 1974. 78 p. Thesis of Livre-Doc&ncia, Escola Superior de Agricultura “Luis de Queir6s”. Katan, J., Greenberger, A., Alon, H. and Grinstein, A., 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilbome pathogens. Phytopahology, 66: 683-688. Katan, J., 1981. Solar heating (solarization) of soil for control of soilbome patogens. Annu. Rev. Phytopathol., 19: 21 l-236. Katan, J., Grinstein, A., Greenberger, A., Yarden, 0. and Devay, J.E. 1987. The first decade (1976-1986) of soil solarization (solar heating): A cronological bibliography. Phytoparasitica, 15: 229-255. Katan, J. and Devay, J.E., 1991. Soil Solarization. CRC Press, Boca Raton, FL, 267 pp. Liakatas, A., Clark, J.A. and Monteith, J.L.. 1986. Measurements of the heat balance under plastic mulches. I: Radiation balance and soil heat flux. Agric. For. Meteorol., 36: 227-239. Mahrer, Y., 1979. Prediction of soil temperature of a soil mulched with transparent polyethylene. J. Appl. Meteorol., 18: 1263- 1267. Mahrer, Y., Avissar, R., Naot, 0. and Katan, J., 1987. Intensified soil solarization with closed greenhouse: numerical and experimental studies. Agric. For. Meteorol., 41: 325-330.
82
N.A. Streck et al./Agriculrurd
and Forest Meteorology
82 (1996) 73-82
Munnecke, D.E., Wilbur, W. and Darley, E.F., 1976. Effect of heating or drying on Armillaria mellru or 66: 1363-1368. Trichoderma uiridae and the relation to survival of A. mellea in soil. Phytopathology, Pullman, G.S.. Devay, J.E. and Garber, R.H., 1981. Soil solarization and thermal death: a logarithmic relantionship between time and temperature for four soilbome pathogens. Phytopathology, 71: 959-964. Rosenberg, N.J., 1974. Microclimate: the Biological Environment. John Wiley & Sons, New York, 315 pp. Schneider, F.M., 1979. Comportamento e propriedades t&micas do solo Santa Maria. Piracicaba, SP, Brazil, 1979. 77 p. Dissertation (Mestrado em Agrometeorologia), Escola Superior de Agricultura “Luis de Queiros”. Schneider, F.M., Streck, N.A. and Buriol, G.A.. 1993. Moditica@es fisicas causadas pela solariza$Zo do solo. Rev. Bras. Agrometeorol., I: 149-157. Streck, N.A., Schneider, F.M. and Buriol, G.A., 1993. Soil temperature modifications caused by solarization in nurseries. Ciencia Rural, 23: 385-386.