long term effect of soil solarization on soil properties and cauliflower vigor

11
Long term effect of soil solarization on soil properties and cauliflower vigor T. A. Sofi & A. K. Tewari & V. K. Razdan & V. K. Koul Received: 2 March 2013 / Accepted: 17 July 2013 # Springer Science+Business Media Dordrecht 2013 Abstract The effect of soil solarization on physical, chemical and biological properties of soil was studied, along with the response of cauliflower seedlings fol- lowing solarization. Nursery beds were covered with transparent polyethylene sheet and soil temperature and moisture were recorded. Soil samples were collect- ed five times for analysis. Three cauliflower nurseries were raised at 30-day intervals; germination was recorded 10 days after sowing and seedling length 30 days after sowing. The maximum temperature in solar- ized soil ranged from 40.247.2°C, with an increase of 5.2° to 9.9°C over non-solarized soil. There was a conservation of 5.48% moisture in solarized soil as compared with non-solarized. Solarization significant- ly increased electrical conductivity, organic carbon, nitrogen and potassium over pre-solarized soil. The mean pH, EC, Ca, Mg, N, P, K and C recorded in solarized soil was higher than in non-solarized. Soil solarization reduced the population of fungi from 25.68 x 10 4 to 4.8 x 10 4 , bacteria from 20.28 x 10 6 to 5.66 x 10 6 , actinomycetes from 31.60 x 10 5 to 4.40 x 10 5 , and reduction in population was recorded even after 90 days, when compared with non-solarized soil. Solarization effectively reduced (>97%) population of plant parasitic and free living nematodes. Cauliflower seedlings in solarized soil had a better vigor index than non-solarized soil. Present findings reveal that soil solarization could be exploited for nutrient manage- ment and soilborne pests control, with a better vigor index of vegetable nursery. Keywords Temperature . Moisture . Microbial population . Nematodes . Nutrients Introduction Cauliflower (Brassica oleracea var. botrytis L.) is an important vegetable crop grown in India, with the second position in its production in the world; it has a total acreage of 3,69,000 ha, an annual production of 67,45,000 mt, and the productivity 18.3 mt ha -1 (NHB 2011). However, the productivity is low as compared with Italy, USA and China. Cauliflower grown in sub- tropical areas of Jammu (India) is considered as the queen of winter vegetables. Poor soil physical condi- tion, nutrient deficiencies, pests, weeds and increased fertilizer demand are explanations for low crop pro- ductivity. These fundamental constraints undoubtedly Phytoparasitica DOI 10.1007/s12600-013-0331-z T. A. Sofi (*) Division of Plant Pathology, SKUAST-Kashmir, Srinagar, Jammu & Kashmir, India e-mail: [email protected] A. K. Tewari Department of Plant Pathology, G.B. Pant University of Agriculture and Technology, Pantnagar, U.S. Nagar, Uttaranchal, India V. K. Razdan Division of Plant Pathology, SKUAST-Jammu, Jammu, Jammu & Kashmir, India V. K. Koul Division of Entomology, SKUAST-Jammu, Jammu, Jammu & Kashmir, India

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Long term effect of soil solarization on soil propertiesand cauliflower vigor

T. A. Sofi & A. K. Tewari & V. K. Razdan & V. K. Koul

Received: 2 March 2013 /Accepted: 17 July 2013# Springer Science+Business Media Dordrecht 2013

Abstract The effect of soil solarization on physical,chemical and biological properties of soil was studied,along with the response of cauliflower seedlings fol-lowing solarization. Nursery beds were covered withtransparent polyethylene sheet and soil temperatureand moisture were recorded. Soil samples were collect-ed five times for analysis. Three cauliflower nurserieswere raised at 30-day intervals; germination wasrecorded 10 days after sowing and seedling length 30days after sowing. The maximum temperature in solar-ized soil ranged from 40.2–47.2°C, with an increase of5.2° to 9.9°C over non-solarized soil. There was aconservation of 5.48% moisture in solarized soil ascompared with non-solarized. Solarization significant-ly increased electrical conductivity, organic carbon,nitrogen and potassium over pre-solarized soil. Themean pH, EC, Ca, Mg, N, P, K and C recorded in

solarized soil was higher than in non-solarized. Soilsolarization reduced the population of fungi from25.68 x 104 to 4.8 x 104, bacteria from 20.28 x 106 to5.66 x 106, actinomycetes from 31.60 x 105 to 4.40 x105, and reduction in population was recorded evenafter 90 days, when compared with non-solarized soil.Solarization effectively reduced (>97%) population ofplant parasitic and free living nematodes. Cauliflowerseedlings in solarized soil had a better vigor index thannon-solarized soil. Present findings reveal that soilsolarization could be exploited for nutrient manage-ment and soilborne pests control, with a better vigorindex of vegetable nursery.

Keywords Temperature . Moisture .Microbialpopulation . Nematodes . Nutrients

Introduction

Cauliflower (Brassica oleracea var. botrytis L.) is animportant vegetable crop grown in India, with thesecond position in its production in the world; it has atotal acreage of 3,69,000 ha, an annual production of67,45,000 mt, and the productivity 18.3 mt ha-1 (NHB2011). However, the productivity is low as comparedwith Italy, USA and China. Cauliflower grown in sub-tropical areas of Jammu (India) is considered as thequeen of winter vegetables. Poor soil physical condi-tion, nutrient deficiencies, pests, weeds and increasedfertilizer demand are explanations for low crop pro-ductivity. These fundamental constraints undoubtedly

PhytoparasiticaDOI 10.1007/s12600-013-0331-z

T. A. Sofi (*)Division of Plant Pathology, SKUAST-Kashmir,Srinagar, Jammu & Kashmir, Indiae-mail: [email protected]

A. K. TewariDepartment of Plant Pathology, G.B. Pant University ofAgriculture and Technology,Pantnagar, U.S. Nagar, Uttaranchal, India

V. K. RazdanDivision of Plant Pathology, SKUAST-Jammu,Jammu, Jammu & Kashmir, India

V. K. KoulDivision of Entomology, SKUAST-Jammu,Jammu, Jammu & Kashmir, India

limit the effectiveness of other yield-enhancing tech-nologies. Few appropriate technologies are availablefor resource-poor farmers to address these problems.Soil solarization is one such modification in soil envi-ronment that has been tested for the management ofsoilborne pathogens and to enhance agricultural pro-ductivity (Matheron & Porchas 2010; Sofi et al. 2009).

Soil solarization improves soil structure and in-creases the availability of nitrogen (N) and other es-sential plant nutrients (Elmore et al. 1997), which leadsto increased plant growth and reduced fertilizer re-quirements. A number of soilborne pathogens, viz.,Pythium, Phytophthora, Rhizoctonia, Fusarium,Sclerotinia, Sclerotium and phytonematodes likeHelicotylenchus, Tylenchorhynchus, Tylenchus, andHoplolaimus are associated with soilborne diseases.Control of soilborne pathogens is often hampered bythe fact that both the inocula of pathogens and lethalagents applied to the soil are affected by the physical,chemical and biological factors of the soil environ-ment. Seed and soil treatment with fungicides is acommon practice for the management of soilbornediseases but these have not produced a desired longterm solution and are not eco-friendly. Seed treatmentwith fungicides often fails to achieve effective man-agement because a number of pathogens are associatedwith soilborne diseases. Many alternative methods formanaging the soilborne plant pathogens have beentried, among them soil solarization – advocated to bean inexpensive and non-hazardous method. Soil solar-ization involves trapping of solar heat through poly-ethylene covering to raise the soil temperature to thelevel where it becomes lethal to temperature-sensitiveor mesophilic soil microorganisms, the category towhich most of the plant pathogenic microorganismsbelong. The potential advantage of soil solarization isthat it is a non-chemical method which is not hazardousto the user and does not involve substances toxic to theenvironment, consumer, host plant or beneficial micro-organisms. As global concerns regarding environmen-tal quality grow along with the human population,concepts such as solarization and other uses of solarenergy in agriculture will likely become increasinglyimportant.

The Jammu sub-tropical conditions offer adequateopportunity for the soil and nursery management throughsoil solarization. The lack of information about soil so-larization for the management of physical, chemical andbiological properties of soil motivated this investigation

into the effectiveness of soil solarization on these prop-erties and on the growth of cauliflower seedlings.

Materials and methods

Soil solarization In the selected field with a knownhistory of nematode and other pathogen infestation,the soil surface was smoothed and leveled prior tomulching. The thoroughly prepared nursery beds werepre-irrigated to the level of field capacity and raised15 cm above ground level. Transparent polyethylenesheets 25 μm thick were then laid with the edgesanchored firmly by burying in trenches surroundingthe treated area. The sheets were laid in completecoverage, in a crust and trough manner (like a wave).Solarization was carried out for a period of 80 days.

Soil sampling

The selected location, with hot summers, is located at32.43°N latitude and 74.54°E longitude at an altitude of300 m AMSL. The temperature at times rises up to48.0°C. Rainfall occurs from July to September withan average of 1115.9 mm. Humidity is highest duringJuly to September. In winter (December to February),temperature remains between 13.5° and 25°C, and rain-fall is 150 mm. High temperatures (35–45°C) occurduring May–June. The inherent physico-chemical prop-erties of the soil are sandy loam in texture with pH 7.3,EC 0.03 dsm-1, organic carbon 0.37%, available nitro-gen 195 kg ha-1, phosphorus 15.3 kg ha-1 and potassium153 kg ha-1. In order to carry out physical, chemical andbiological analysis of the soil, samples were collectedfive times: just before solarization, immediately aftersolarization (i.e., after removal of polyethylene mulch),and the remaining three samples at subsequent 30-dayintervals, viz., 30, 60 and 90 days after solarization. Thesoil samples were drawn to a depth of 10 cm from eachplot with the help of a soil auger. For all samples fivecores were collected per replication, bulked to form acomposite sample, and brought to the laboratory inpolyethylene bags for study.

Soil temperature Soil thermometers were placed ata depth of 10 cm beneath the polyethylene sheetto record the soil temperature. Temperature of non-solarized soil was recorded in the same way. Daily

Phytoparasitica

maximum soil temperature was recorded and theaverage standard week’s temperature calculated.

Soil moisture Soil samples collected from the experi-mental plots just before solarization, just after solari-zation, and the non-solarized control were used fordetermination of soil moisture. This was done by gravi-metric method, wherein the samples were weighed andthen dried in an oven at 105°C until constant weightwas achieved. The dried samples were weighed and themoisture percentage was calculated by using the fol-lowing formula:

Weightof wet soil−weightof oven dry solid

Weightof ovendrysoil� 100

Chemical properties of soil Composite soil sampleswere oven-dried at 105°C for 24 h. Dry soil sampleswere then sieved (2 mm) and the fine soil was used forchemical analyses (pH, EC, carbon (soluble), NPK, Caand Mg). Soil acidity (pH) was measured in a 1:2 (w/v)soil-to-water mixture by a pHmeter. Electrical conduc-tivity (EC) was determined at 25°C in a 1:1 (w/v) soil-to-water mixture by conductivity meter and expressedas dsm-1. Total nitrogen (N) was determined by micro

Kjeldahl method (Bremner & Mulvaney 1982). Avail-able phosphorus (P) was determined with a spectro-photometer (Olsen & Sommers 1982), available potas-sium (K) by flame photometry (Knudsen et al. 1982),total organic carbon (%) by potassium dichromate wetdigestion method (Schnitzer 1982). Calcium and mag-nesium were evaluated by ammonium acetate method(Metson 1961).

Microbial population Dilution plate method wasfollowed to determine microbial population of fungi,bacteria and actinomycetes in soil. The media usedwere peptone dextrose rose bengal agar for fungi, soilextract agar for bacteria, and starch ammonium agar foractinomycetes. The soil samples were analyzed to de-termine the colony forming units (cfu) of the fungi,bacteria and actinomycetes per gram of soil. One gramof the soil was taken from each thoroughly mixed air-dried sample and suspended in 9 ml sterile distilledwater. Further dilutions were made by transferring 1 mlof suspension to 9 ml of sterilized distilled water untilthe final dilution of 10-6 was obtained. Dilutions of 10-4, 10-5 and 10-6 for fungi, actinomycetes, and bacteria,respectively, were poured into petri plates containingappropriate media and spread evenly by horizontaltilting. The plates were then incubated at 24 ± 2°C, 2

Fig. 1 Standard weeklytemperatures of solarizedand non-solarized soil (at10 cm depth)

Phytoparasitica

days for bacteria and 5 days for fungi and actinomy-cetes. Microbial colonies were counted with the aid ofa colony counter.

Nematode population Total nematode population in-cluding free-living nematodes and different types ofplant parasitic nematodes was determined by Cobb’ssieving and decanting (gravity) method followed bymodified Baermann’s funnel method. A soil sample of250 g was placed in a bucket and 3 l of water was addedto it. It was stirred until all clods were broken, thenstirring was stopped for 30–60 seconds. The water wasthen passed through a 20-mesh sieve and subsequentlythrough 100-, 200- and 325-mesh sieves. The residueon the mesh sieves was then washed with a gentlestream of water to remove fine soil particles. The sieveswere then inverted and washed into a beaker with50 ml of water. The washings of different mesh sieveswere collected in the beaker and kept still for about30 min to settle the nematodes at the bottom. Extrawater was gently poured out to concentrate the nema-todes. The suspension containing nematodes was thenfurther processed by modified Baermann’s funnelmethod, wherein a coarse mesh with tissue paperoverlapping it was placed over the small bowl. The

concentrated suspension of nematodes was pouredthrough tissue paper and kept undisturbed for 48 hunder the continuous touch of tissue paper. After48 h the water in the bowl was collected in thecounting dish and examined under stereobinocularmicroscope to count different types of nematodes.The nematode population was expressed as numberper 250 g of soil.

Field preparation for nursery After removing thepolyethylene sheets from solarized plots, 50-cm2 bedsof both solarized and non-solarized were prepared at50 cm apart from each other, and seeds (2250 per 50cm2) were sown for nursery raising. The seeds sown innon-solarized soil served as check.

Seedling growth In order to evaluate the effect of soilsolarization on growth of seedlings, seeds were sownin the beds just after solarization (var. ‘Agheni’), 30days after solarization (var. ‘Snowball-16’) and 60days after solarization (var. ‘Snowball-16’). The vigorindex was calculated according to the following equa-tion (Orchard 1977):

Seedling vigor index (SVI)=(seedling length (cm)×germination percentage)

Twenty seedlings were selected randomly from eachreplication and seedling length was recorded immedi-ately after carefully uprooting the seedlings from thenursery beds. The seedlings were uprooted 30 daysafter sowing. Germination was recorded by countingthe number of seedlings 10 days after sowing (when nofurther germination occurred) and germination per-centage was calculated as follows:

Germination% ¼ Number of seedlings emerged

Totalnumber of seeds sown� 100

Table 1 Effect of soil solarization on soil moisture (mean ±SEM)

Treatment Moisture (%)

Pre-solarized soil 13.17 ± 0.45

Solarized soil 12.25 ± 0.35

Non-solarized soil 6.76 ± 0.41

t- value: Pre-solarized vs solarized soil 3.27*

Solarized vs non-solarized soil 10.07*

* Significant (P=0.05)

Table 2 Effect of soil solarization on chemical properties of soil (mean ± SEM)

Treatment pH EC (dSm-1) C (%) N (kg ha-1) P (kg ha-1) K (kg ha-1) Ca (me l-1) Mg (me l-1)

Pre-solarized soil 7.31 (± 0.10) 0.03 (± 0.00) 0.37 (± 0.03) 195.3 (± 16.4) 15.33 (± 0.87) 153.33 (± 3.33) 11.30 (± 0.15) 0.90 (± 0.05)

Post-solarization 7.56 (± 0.03) 0.16 (± 0.01) 0.58 (± 0.01) 362.0 (± 8.50) 21.20 (± 2.51) 202.1 (± 4.04) 11.80 (± 0.26) 2.00 (± 0.55)

Increase overpre-solarized soil

0.25 0.13 0.21 166.70 5.87 48.77 0.50 1.10

t- value 2.3ns 8.51* 7.57* 14.69** 3.54ns 41.20** 1.64ns 2.19ns

ns = Non-significant; * Significant (P=0.05); ** Significant (P=0.01)

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Results and Discussion

Soil temperature A significant difference between thetemperatures of solarized and non-solarized soils wasobserved during the course of the investigation. Themaximum temperature (weekly mean) ranged from40.2° to 47.2°C in solarized soil, with an increase of5.2° to 9.9°C over non-solarized soil (Fig. 1). The abso-lute (weekly maximum) temperature recorded in solar-ized and non-solarized soils ranged from 41.0–49.8°Cand 34.7–43.5°C, respectively. The absolute temperatureincrease in solarized soil over non-solarized soil rangedfrom 5.7° to 10.7°C. These findings are close to those ofseveral workers who have advocated that increased tem-peratures in solarized soil were due to the trapping of thesolar energy by polyethylene sheets and preventing theheat loss caused by evaporation and convection, thuscreating a greenhouse effect (Ioannou 1999; Raj et al.1997). Gelsomino & Cacco (2006) recorded an averagesoil temperature of 55°C beneath the polyethylene film at8 cm depth compared with 35°C in non-solarized soil.

Since in the present study the highest temperature of49.8°C was recorded in solarized soil at a depth of 10cm, at lower depths the temperature would have beenmore than 49.8°C.Microorganisms and plant propagules,present beneath the pre-irrigated polyethylene mulch,start germination and multiplication, which causewarming of the microclimate beneath the polyethylenesheet. During this process a lot of CO2 is released due torespiration of microorganisms and germinating seeds,which accumulate under the mulch. The humidity underthe tarp increases due to evaporation of water. The CO2

and water vapors create a greenhouse effect under themulch. The soil temperature may increase due to thesefactors as well and rise up to a lethal level (higher than45°C as compared with the control in the present study)to kill the microorganisms.

Soil moisture Covering the soil with a polyethylenesheet resulted in prolific condensation on the innersurface of the sheet. After solarization, loss of moisturewas observed both in solarized as well as in non-

Table 3 Effect of soil solarization on chemical properties of soil at different intervals after solarization

Treatment Solarized Non-solarized

Days after solarization Days after solarization

Chemical properties 0 30 60 90 Mean 0 30 60 90 Mean

pH 7.56 7.47 7.48 7.40 7.47 7.30 7.23 7.20 7.16 7.22

EC (dSm-1) 0.16 0.15 0.11 0.10 0.13 0.03 0.03 0.02 0.03 0.02

Ca (me l-1) 11.80 11.55 11.40 10.90 11.41 11.40 11.25 11.00 10.10 10.93

Mg (me l-1) 2.00 1.95 1.60 1.70 1.81 1.10 1.75 1.20 1.25 1.32

N (kg ha-1) 362.0 356.0 298.0 272.0 322.0 195.0 192.0 189.0 176.0 188.0

P (kg ha-1) 21.20 25.00 26.10 25.30 24.40 18.10 22.00 22.20 23.00 21.32

K (kg ha-1) 202.10 200.50 200.00 197.20 199.95 150.00 143.00 144.50 143.30 144.95

C (%) 0.58 0.57 0.57 0.53 0.56 0.33 0.41 0.48 0.42 0.41

C.D. (P = 0.05) pH EC Ca Mg N P K C

Treatment 0.09 0.02 0.25 0.34 7.25 1.79 4.31 0.02

Treatment × days NS NS NS NS 14.50 NS NS 0.04

Table 4 Effect of soil solariza-tion on the population (cfu =colony forming units) of fungi,bacteria and actinomycetes(mean ± SEM)

** Significant (P=0.01)

Treatment Microbial population (cfu / g of soil)

Fungi (x 104) Bacteria (x106) Actinomycetes (x 105)

Pre-solarized soil 25.68 ( ± 1.22 ) 20.78 ( ± 0.79 ) 31.60 ( ± 1.31 )

Post-solarization 4.80 ( ± 0.07 ) 5.66 ( ± 0.26 ) 4.40 ( ± 0.31 )

t- value 17.84** 15.93** 16.89**

Phytoparasitica

solarized soil. The initial soil moisture content of13.17% (before solarization) was reduced to 12.25%and 6.76% in solarized and non-solarized soil, respec-tively (Table 1), resulting in the conservation of 5.49%more moisture in solarized soil as compared with non-solarized soil. A reduction of only 0.92% moisture insolarized soil was observed as compared with the ini-tial (before solarization) moisture content. Conserva-tion of soil moisture by solarization is in agreementwith the findings of Stapleton et al. (1987) and Rao &Krishnappa (1995). The prolific condensation on theinner surface of polyethylene sheets during solarizationand prevention of loss caused by evaporation, could be

the reasons for enhanced conservation of moisture insolarized as compared with non-solarized soil.

Chemical properties of soil There was a significantincrease in electrical conductivity (0.13 dSm-1), organ-ic carbon (0.21%), nitrogen (166.70 kg ha-1) and po-tassium (48.77 kg ha-1) in solarized soil over pre-solarized soil; however, a non-significant increase in pH(0.25), phosphorus (5.87 kg ha-1), calcium (0.50 me l-1)and magnesium (1.1 me l-1) was observed (Table 2).Themean pH (7.47), EC (0.13 dsm-1) , Ca (11.41me l-1),Mg (1.81 me l-1), N (322.0 kg ha-1), P (24.4 kg ha-1), K(199.95 kg ha-1) and C (0.56%) observed in solarized

Table 5 Effect of soil solarization on microbial population (cfu = colony forming units) of fungi, bacteria and actinomycetes at differentintervals after solarization

Treatment Microbial population (cfu / g of soil)

Fungi (x 104) Bacteria (x106) Actinomycetes (x 105)

Days after solarization Days after solarization Days after solarization

0 30 60 90 Mean 0 30 60 90 Mean 0 30 60 90 Mean

Solarized soil 4.80(1.75)z

5.20(1.82)

5.92(1.93)

6.84(2.05)

5.69(1.89)

5.66(1.89)

7.10(2.09)

8.86(2.28)

8.62(2.26)

7.56(2.13)

4.40(1.67)

5.98(1.93)

8.52(2.25)

10.44(2.43)

7.33(2.07)

Non-solarizedsoil

19.02(2.99)

21.98(3.13)

24.98(3.25)

28.68(3.39)

23.66(3.19)

20.16(3.05)

21.42(3.10)

22.66(3.16)

25.72(3.28)

22.49(3.15)

26.82(3.32)

29.42(3.41)

31.42(3.47)

31.70(3.48)

29.84(3.42)

Mean 11.91(2.37)

13.59(2.47)

15.45(2.59)

17.76(2.72)

12.91(2.47)

14.26(2.59)

15.76(2.72)

17.17(2.77)

15.61(2.50)

17.70(2.67)

19.97(2.86)

21.07(2.96)

C. D. (P=0.05) Fungi Bacteria Actinomycetes

Treatment 0.03 0.05 0.07

Days 0.04 0.07 0.10

Treatment xDays

0.06 0.10 0.14

z Values in parenthesis are log transformed

Table 6 Effect of soil solarization on nematode population (mean ± SEM)

Treatment Nematode population (no./ 250 g of soil )

Plant parasitic nematodes Free-living nematodes Total no. of nematodes

Helicotylenchus Tylenchorhynchus Tylenchus Hoplolaimus

Pre-solarized soil 1026 ± 106 1130 ± 116 1439 ± 148 619 ± 61 1436 ± 146 5650 ± 578

Post-solarization 30.0 ± 2.0 31.0 ± 2.0 37.0 ± 3.0 17.7 ± 1.2 39.0 ± 3.0 152.0 ± 12.0

Percent reduction 97.07 97.25 97.42 97.14 97.28 97.30

t-value 20.54** 21.94** 19.20** 23.94** 22.73** 21.24**

** Significant (P=0.01)

Phytoparasitica

Tab

le7

Effectof

soilsolarizatio

non

nematod

epo

pulatio

natdifferentintervalsaftersolarizatio

n

Treatment

Microbialp

opulation

(No./2

50gof

soil)

Helicotylenchus

Tylenchorhynchus

Tylenchus

Daysaftersolarizatio

nDaysaftersolarizatio

nDaysaftersolarizatio

n

030

6090

Mean

030

6090

Mean

030

6090

Mean

Solarized

soil

30.0

(3.42)

z44

.0(3.79)

239.3

(5.48)

625.3

(6.42)

234.65

(4.78)

27.7

(3.35)

46.0

(3.84)

247.7

(5.51)

654.0

(6.47)

243.85

(4.79)

37.3

(3.63)

55.7

(4.02)

301.7

(5.71)

815.0

(6.69)

302.4

(5.01)

Non

-solar-

ized

Soil

1068

.0(6.97)

982.3

(6.88)

923.3

(6.82)

941.7

(6.84)

978.8

(6.88)

1121

.0(7.01)

1086

.0(6.98)

974.0

(6.87)

991.7

(6.89)

1043

.1(6.94)

1346

.7(7.20)

1292

.3(7.16)

1180

.7(7.07)

1230

.0(7.11)

1262

.4(7.03)

Mean

549.0

(5.19)

513.1

(5.34)

581.3

(6.15)

783.5

(6.63)

574.3

(5.18)

566.0

(5.41)

610.8

(6.19)

822.8

(6.68)

692.0

(5.42)

674.0

(5.59)

741.2

(6.39)

1022

.5(6.90)

Hop

lolaim

us

Free-liv

ingnem

atod

esTotal

no.

nem

atod

es

Daysaftersolarizatio

nDaysaftersolarizatio

nDaysaftersolarizatio

n

030

6090

Mean

030

6090

Mean

030

6090

Mean

Solarized

soil

17.7

(2.92)

24.0

(3.20)

140.3

(4.95)

372.3

(5.90)

138.5

(4.24)

39.3

(3.69)

58.7

(4.08)

309.7

(5.73)

818.0

(6.69)

306.4

(5.05)

152.0

(5.02)

228.3

(5.42)

1238

.7(7.12)

3284

.7(8.08)

1225

.9(6.41)

Non

-solar-

ized

Soil

565.0

(6.33)

569.7

(6.34)

541.7

(6.29)

558.3

(6.32)

558.6

(6.32)

1513

.3(7.31)

1241

.7(7.12)

1163

.0(7.05)

1228

.3(7.11)

1286

.5(7.15)

5614

.0(8.62)

5172

.0(8.54)

4782

.7(8.47)

4950

.0(8.50)

5129

.6(8.53)

Mean

194.3

296.8

341.0

465.3

776.3

650.2

736.3

1023

.128

83.0

2700

.130

10.7

4117

.3(4.62)

(4.77)

(5.62)

(6.11)

(5.50)

(5.60)

(6.39)

(6.90)

(6.82)

(6.98)

(7.79)

(8.29)

C.D

.(P=

0.05

)Helicotylenchus

Tylencho

rhynchus

Tylenchu

sHop

lolaimus

Free-liv

ing

nema-

todes

Totalno

.of

nema-

todes

Treatment

0.08

0.08

0.09

0.09

0.08

0.08

Days

0.12

0.11

0.12

0.13

0.11

0.11

Treatment

xDays

0.17

0.16

0.18

0.18

0.16

0.16

zValuesin

parenthesisarelogtransformed

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soil was significantly higher than non-solarized soil.Effect of soil solarization on chemical properties of soilat different time intervals revealed that after solarizationno significant difference was observed in pH, EC, Ca,Mg, P and K at 0, 30, 60 and 90 days after solarization(Table 3). EC (0.16 dSm-1), pH (7.56), Ca (11.8 me l-1),Mg (2.0 me l-1), N (362.0 kg ha-1), K (202.1 kg ha-1) andC (0.58%) were recorded at 0 days after solarization anddecreased with an increase in days after solarization.However, phosphorus slightly increased up to 60 daysafter solarization and thereafter decreased. In solarizedsoil the nitrogen level at 0 and 30 days after solarizationwas statistically at par and significantly higher than ni-trogen at 60 and 90 days after solarization, thus showinga trend to decrease from 30 to 90 days after solarization.The carbon level in solarized soil – (0.58%), (0.57%) and(0.57%) – observed at 0, 30 and 60 days after solariza-tion, respectively, was statistically at par but significantlyhigher than at 90 days after solarization (0.53%). Theseresults are in conformity with the results of severalworkers (Chauhan et al. 1988; Lazarovits et al. 1991).Stapleton et al. (1985) found that soil solarization in-creased concentrations of N, P, Ca, Mg and electricalconductivity. Sharma & Sharma (2002a) reported in-creased EC, organic Ca, N, K and decreased P aftersolarization. Overman & Jones (1986) reported an in-crease in soil pH by soil solarization. Gamliel & Katan(1991) recorded an increase in K+ in solarized soil.Sharma & Sharma (2002b) reported an increased organiccarbon in solarized soil. No significant change in pH, EC,Ca,Mg, P and Kwas observed among different days, i.e.,0, 30, 60 and 90 days after solarization. The pH, EC, Ca,Mg, N, P, K and C observed in solarized soil at 0, 30, 60and 90 days after solarization was higher than that ob-served in non-solarized soil, thus showing a long-termeffect of soil solarization of at least 90 days. A study inCalifornia showed that nitrogen concentration in the top15 cm soil depth increased 26–177 kg ha-1 (Katan 1987).Solarized soils commonly undergo an increase in solu-

ble substances that can be detected as a rise in theelectrical conductivity reported in our study. Thischange can be attributed to an increase in the rate ofdecomposition of organic matter at high temperaturesand as the mesophilic organisms are killed and degradedduring solarization, thereby liberating soluble sub-stances into the soil.

Microbial population Soil solarization was found to behighly effective in reducing the population of fungi, bac-teria and actinomycetes. The pre-solarized microbial pop-ulation was reduced from 25.68×104 to 4.80×104 (fungi),20.78×106 to 5.66×106 (bacteria) and 31.60×105 to4.40×105 (actinomycetes) per gram of soil after solariza-tion (Table 4). In solarized soil the lowest population offungi (4.8×104 g-1 soil), bacteria (5.66×106 g-1 soil) andactinomycetes (4.4×105 g-1 soil) was recorded at 0 daysafter solarization while the highest population of fungi(6.84×104 g-1 soil), bacteria (8.86×106 g-1 soil) and actino-mycetes (10.44×105 g-1 soil) was recorded at 90 days aftersolarization. The observations clearly revealed that thepopulation of fungi, bacteria and actinomycetes increasedsignificantly from 0 to 90 days after solarization, but it wassignificantly reduced when compared with non-solarizedsoil at different intervals (Table 5). Decrease in the micro-bial population has also been reported by several workers(Ashrafi et al. 2010; Wadi 1999). Sharma & Sharma(2002b) have reported that high temperature under a25-μm-thick mulch had a lethal effect on the fungal,bacterial and actinomycetes population under irrigated soilconditions. The observations in the present study revealthat the populations of fungi, bacteria and actinomyceteswere significantly increased from 0 to 90 days after solar-ization, though they were significantly reducedwhen com-pared with non-solarized soil at different intervals. Thesefindings are almost similar to the findings of severalworkers (Chaube & Singh 1991; Tjamos & Paplomatas1988). Direct hydrothermal effect is probably the majormechanism for inactivation of microbial propagules as a

Fig. 2 Effect of soil solari-zation on germination ofcauliflower seeds

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consequence of raised soil temperatures and has a mostpronounced lethal effect on a broad spectrum of soil or-ganisms. The significant reduction in the microbial popu-lation in solarized soil over non-solarized soil after 90 daysof solarization can be attributed to an initial drastic reduc-tion of microbial inoculum during solarization and limitedre-infestation by the microorganisms. There was a slowincrease of themicrobial population from 0 to 90 days aftersolarization. The observations of this experiment are inaccordance with ecology and ecosystems where popula-tion and communities interact in the food chain.

Nematode population The nematodes identified in bothsolarized aswell as non-solarized soilswereHelicotylenchus,Tylenchorhynchus,Tylenchus,Hoplolaimus and free-living.The data (Table 6) reveal that soil solarization washighly effective in reducing the initial total nematodepopulation of 5650 in pre-solarized soil to 152 aftersolarization. The reduction of the nematode populationin solarized soil was more than 97%. The maximumreduction in nematode population was observed inTylenchus (97.42%), followed by free-living (97.28%),Tylenchorhynchus (97.25%), Hoplolaimus (97.14%)and Helicotylenchus (97.07%) in solarized soil overpre-solarized soil. At 0 days after solarization the num-ber of nematodes was 30.0 (Helicotylenchus), 27.7(Tylenchorhynchus), 37.3 (Tylenchus), 17.7 (Hoplolaimus)

and 39.3 (free-living) in solarized soil as against 1068.0(Helicotylenchus), 1121.0 (Tylenchorhynchus), 1346.7(Tylenchus), 565.0 (Hoplolaimus) and 1513.3 (free-living) in non-solarized soil. In solarized soil thelowest number of total nematodes (152.0) wasrecorded at 0 days after solarization, while the highestnumber of total nematodes (3284.7) was recorded at90 days after solarization, thus showing an increasingpopulation from 0 to 90 days after solarization. How-ever, solarization significantly reduced the total nem-atode population even 90 days after solarization whencompared with non-solarized soil (Table 7). Thesefindings are in conformity with those of Barbercheck& Broembsen (1986), Lazarovits et al. (1991) andKamra & Gaur (1998). The reduction in the numberof nematodes and slow recovery after solarizationmay be due to sublethal heating of the nematodes inthe soil profile, resulting in reduced potential, lowersubsequent reproduction or egg hatching, and possi-bly induced bio-control.

Increased growth response The seed germination per-centage, seedling length and seedling vigor index insolarized soil was better than in non-solarized soil. Thepooled data show that the seed germination percentage insolarized soil was 76.12% compared with 58.78% innon-solarized soil (Fig. 2). Seedling length (Fig. 3) and

Fig. 4 Effect of soil solari-zation on seedling vigor in-dex of cauliflower

Fig. 3 Effect of soil solari-zation on seedling length ofcauliflower

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seedling vigor index (Fig. 4) were, respectively, 33.34 cmand 2537.84 in solarized soil compared with 24.33 cmand 1430.11 in non-solarized soil. The observations re-garding the seedling length among the three nurserieswere non-significant, indicating the effect of soil solari-zation up to the third nursery, i.e., up to 90 days aftersolarization. This is in close proximity to the results ofStapleton et al. (1985) and Matheron & Porchas (2010).Raj & Kapoor (1993) and Raj et al. (1997) recorded anincrease in length of tomato, cauliflower and other veg-etable seedlings grown in solarized soil over non-solarized soil. Katan (1995) believes that secretion ofmore amino acids by the roots of plants grown in solar-ized soil enhances plant growth. These reports corre-spond to a phenomenon known as increased growthresponse (IGR) that has been attributed to several mech-anisms, including increases in nutrient levels in the soil,stimulation of beneficial organisms and control of path-ogens. In the present study increased nutritional status,decreasedmicrobial, nematode andweed populationmaybe responsible for the enhanced germination, increasedseedling length and finally better seedling vigor index.Solarization can induce IGR also by enhancing biocon-trol processes and by reduction in soilborne pathogens(Le Bihan et al. 1997; Sofi et al. 2009). Sofi et al. (2009)reported a synergistic interaction between soil solariza-tion and the application of biocontrol agents that in-creased number of cauliflower seedlings and improvedthe seedling vigor index.

It is recommended that the field be solarized just beforea crop is sown so that it can benefit from the treatment inthe rotation cycle. Then, the land is left for three or foursucceeding crops and then solarized again prior to plantingthe same crop. The method is simple, safe, and effective,leaves no toxic residues, and can be easily used on a smallor large scale. Soil solarization has the long-term benefitsof increased nutritional status, and decreased microbial,nematode and weed population of the soil, which finallyleads to better plant vigor.

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