21377

University of California Vegetable Research and Information Center

Page 1

Soil Solarization

A Nonpesticidal Method for Controlling

Diseases, Nematodes, and Weeds

UNIVERSITY OF CALIFORNIA

DIVISION OF AGRICULTURE AND NATURAL RESOURSES

PUBLICATION 21377

University of California Vegetable Research and Information Center

Page 2

Authors:

CLYDE L. ELMORE

Extension Weed Scientist

Vegetable Crops Department

Weed Science Program

University of California, Davis

JAMES J. STAPLETON

University of California Integrated Pest Management Plant Pathologist

Kearney Agricultural Center, Parlier

CARL E. BELL

University of California Cooperative Extension Farm Advisor

Imperial County

JAMES E. DEVAY

Plant Pathology Department, Professor Emeritus

University of California, Davis

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e-mail inquiries to anrpubs@ucdavis.edu

Publication 21377

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Cover: Solarization of beds for organic vegetables. Weeds are controlled in the beds by solarization and in the furrows by

cultivation.

University of California Vegetable Research and Information Center

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SOIL SOLARIZATION

A Nonpesticidal Method for Controlling

Diseases, Nematodes, and Weeds

How to Solarize Soil, 4

Plastic Sheeting, 7

Results of Solarization, 9

Factors that Limit Effectiveness of Solarization, 13

Combining Solarization with Other Control Methods, 14

Economics of Solarization, 15

Bibliography, 16

Soilborne pests can be controlled in

vegetable and fruit crops by preplant

application of pesticides, including the

fumigants methyl bromide, chloropicrin, and

metam sodium. The use of these materials,

however, is often undesirable due to their

toxicity to animals and people, their residual

toxicity in plants and soils, the complexity

of soil treatment, and their high cost.

Furthermore, restrictions on the use of

soil-applied pesticides seem imminent as

existing environmental legislation is

implemented. As a result, there has been an

increased emphasis on reduced-pesticide or

nonpesticidal control methods.

Soil solarization is a nonpesticidal

method of controlling soilborne pests by

placing plastic sheets on moist soil during

periods of high ambient temperature. The

plastic sheets allow the sun's radiant energy

to be trapped in the soil, heating the upper

levels. Solarization during the hot summer

months can increase soil temperature to

levels that kill many disease-causing

organisms (pathogens), nematodes, and

weed seed and seedlings. It leaves no toxic

residues and can be easily used on a small or

large scale. Soil solarization also improves

soil structure and increases the availability

of nitrogen (N) and other essential plant

nutrients.

Solarization is a simple, safe, and

effective method that has been used with

field, vegetable, and flower crops and in

orchards, vineyards, greenhouses, gardens,

University of California Vegetable Research and Information Center

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and landscapes in California for over 12

years. It can be combined with organic soil

amendments or reduced rates of pesticide

application for greater effectiveness. Large

increases in plant growth, harvestable yield,

and crop quality often occur in solarized soil

and may continue for more than one

growing season. The potential for using soil

solarization to control diseases and pests in

the warmer areas of California is excellent.

This publication is a brief introduction to

soil solarization. For further information,

consult your local Cooperative Extension

Farm Advisor and the references listed in

the bibliography at the end of this

publication.

HOW TO SOLARIZE SOIL

Soil Preparation

Solarization is most effective when the

plastic sheeting (tarp) is laid as close as

possible to a smooth soil surface.

Preparation of the soil begins by disking,

rototilling, or turning the soil by hand to

break up clods and then smoothing the soil

surface. Remove any large rocks, weeds, or

any other objects or debris that will raise or

puncture the plastic.

Laying the Plastic

Plastic sheets may be laid by hand (see

figure 1) or machine (see figure 2). The

open edges of the plastic sheeting should be

anchored to the soil by bury-

ing the edges in a shallow trench around the

treated area. Plastic is laid either in complete

coverage, where the entire field or area to be

planted is treated, or strip coverage, where

only beds or selected portions of the field

are treated.

Complete coverage. In complete

coverage, plastic sheeting is laid down to

form a continuous surface over the entire

field or area to be planted. The edges of the

sheets may be joined with an ultraviolet

(UV)-resistant glue or anchored by laying

adjacent strips of plastic and burying both

edges in soil (see figure 3). Anchoring the

edges in the soil may be more cost effective

initially than gluing the edges together but

may also result in untreated soil being close

to subsequently planted crops. The ends of

the sheets should be held in place by burying

them in the soil. If beds are formed after

complete coverage, care must be taken to

avoid deep tillage that could bring untreated

soil to the surface. Complete coverage is

recommended if the soil is heavily infested

with pathogens, nematodes, or perennial

weeds, since there is less chance of

reinfestation by soil being moved to the

plants through cultivation or furrow-applied

irrigation water.

Strip coverage. In strip coverage,

plastic is applied in strips over preformed

beds (see figure 4). Strips should be a

minimum of 30 inches (75 cm) wide; beds

up to 5 feet (1.5 m) wide are preferred

because several crop rows can be planted

Figure 1. Applying clear 2 mil polyethylene

tarps by hand Tarp is anchored to the soil by

burying the edges.

Figure 2. Applying polyethylene tarps on 42-inch

(105-cm) beds with a mechanical tarp layer.

University of California Vegetable Research and Information Center

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per bed. In some cases, strip coverage may

be more practical and economical than

complete coverage because less plastic is

needed and it is not necessary to join the

edges of the plastic sheets together. Strip

coverage effectively kills most pests and

eliminates the need for deep cultivation after

solarization. It is especially effective against

weeds, since the furrows are cultivated.

With strip coverage, however, longterm

control of soil pathogens and nematodes

may be lost because pests in the untreated

soil in the rows between the strips can

contaminate and reinfest treated areas.

Irrigation

Wet soil conducts heat better than dry soil

and makes soil organisms more vulnerable

to heat. The soil under the plastic sheets

must be saturated to at least 70 percent of

field capacity in the upper layers and moist

to depths of 24 inches (60 cm) for soil

solarization to be effective.

Soil may be irrigated either before or

after the plastic sheets are laid. If the soil is

irrigated beforehand, the plastic must be

applied as soon as possible to avoid water

loss; if heavy machinery is used to lay the

plastic, however, the soil must be dry

enough to avoid compaction. If the soil is to

be irrigated after the plastic is laid, one or

more hose or pipe outlets may be installed

under one end of the plastic; drip lines may

Figure 3. Wide polyethylene strips laid together

with soil covering both edges. The soil on top of

the polyethylene is untreated and can reinfest

treated soil after tarp removal

be installed before the plastic is laid (see

figure 5); or irrigation water may be run

underneath the plastic in furrows or in the

tracks made by tractor wheels if the plastic

sheets were applied by machine. Fields

treated by strip coverage can be irrigated by

drip lines on or in the bed.

The soil does not usually need to be

irrigated again during solarization, although

if the soil is very light and sandy, or if the

soil moisture is less than 50 percent of field

capacity, it may be necessary to irrigate a

second time. This will cool the soil, but

because of the increased moisture the final

temperatures will be greater.

Duration of Treatment

The plastic sheets should be left in place for

4 to 6 weeks to allow the soil to heat to the

Figure 4. polyethylene applied by machine on 30-inch

(76cm) beds. Strips narrower than these will not be as

effective for pest control across the top of the bed.

Figure 5. Drip irrigation lines providing moisture

under polyethylene during solarization of planting

beds

University of California Vegetable Research and Information Center

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greatest depth possible. To control the most

resistant species, leave the plastic in place

for 6 weeks. Experience has shown that

there is little or no need to take the tem-

perature of the soil. The greatest concern is

to solarize the soil during a period of high

solar radiation with little wind or cloud

cover. Soil in the Central Valley can be

solarized for 4 weeks any time from late

May to September. In coastal areas the best

time may be August to September or May to

June, transitional periods when fog or wind

may be at a minimum.

Removal of the Plastic and Planting

After solarization is complete, the plastic

may be removed before planting. Or, the

plastic may be left on the soil as a mulch for

the following crop by transplanting plants

through the plastic. Clear plastic may be

painted white or silver to cool the soil and

repel flying insect pests in the following

crop. A disadvantage of leaving the plastic

on the soil is that it may degrade and be

difficult to clean up in the spring.

Treated soil can be planted immediately to a

fall or winter crop or left fallow without the

plastic until the next growing season. If the

soil must be cultivated for planting, the

cultivation must be shallow-less than 2

inches (5 cm)-to avoid moving viable weed

seed to the surface.

Greenhouses

Solarization in greenhouses produces

significantly higher soil temperatures than

solarization in fields or gardens and can

therefore be more effective in cooler

weather. Greenhouse solarization is exten-

sively used in southern Europe and Japan to

control diseases of strawberries, tomatoes,

eggplants, cucumbers, and other intensively

managed crops.

The soil surface inside the greenhouse

should be leveled and irrigated before being

covered with plastic sheeting. Choose a time

of year with maximum solar radiation. To

maximize the transmission of light it may be

advisable to wash the roof of the greenhouse

before treatment. Once plastic is applied, the

greenhouse should be tightly closed for 4 or

more weeks to contain the heat.

Containerized Planting Media and Seedbeds

Soil solarization has been shown to be

effective for disinfesting containerized soil

and soil in cold frames (see figures 6 and 7).

Soil temperatures should be monitored

closely in this planting media to assure that

temperatures are high enough to control

pests. Materials can be solarized either in

bags or flats covered with transparent plastic

or in layers 3 to 9 inches (7.5-22.5 cm) wide

sandwiched between two sheets of plastic.

In warmer areas of California, soil inside

black plastic sleeves can reach

Figure 6. Containerized soil in tightly closed

polytheylene tunnel being solarized bfore

planting

Figure 7. Effect of solarization on citrus

nematode in containerized soil using plastic

planting sleeves, solarization, or solatirization and

plastic tunnel. All methods reduced the number of

citrus nematodes compared to the untreated soil.

University of California Vegetable Research and Information Center

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158°F (70°C) during solarization, equivalent

to target temperatures for soil disinfestation

by aerated steam. At these temperatures, soil

is effectively solarized within 1 week. A

double layer of plastic can increase soil

temperatures by up to 50°F.

Soil temperatures can be monitored using

simple soil thermometers inserted 4 to 6

inches (10-15 cm) into the soil mix or by

using thermocouples and a digital reading

logger. Temperatures can be monitored at

different locations, but the duration should

be lengthened to raise the temperature at the

coolest location to the desired level.

Orchards and Vineyards

Soil solarization is most effective before or

during establishment of new orchards or

vineyards. It has been successfully used on a

large scale to reduce Verticillium wilt

symptoms in young pistachio orchards in

California (see figure 8) and has also been

successfully used in vineyards and in

avocado, stone fruit, citrus, and olive

orchards in the state.

In the orchard or vineyard, clear plastic is

either laid by hand around the bases of

individual trees or vines and connected to

strips laid between the rows or laid in

anchored strips and glued along the tree

rows. For best results, begin solarization as

soon as trees are planted. Partial shading by

young trees does not prevent soil heating,

nor does soil solarization appear to bother

most young trees during treatment.

However, solarizing certain species of trees,

such as herbaceous perennials, avocado, and

young Prunus trees, with clear plastic may

result in plant damage, especially when trees

are young. (The Prunus trees were killed by

clear plastic but not by black film.)

In addition to killing soilborne pests,

solarization of orchards and vineyards can

greatly reduce the amount of water needed

for irrigation and increase the growth,

flowering, and/or fruit set of the trees. In

large commercial orchards, the cost of

postplant solarization should be compared to

the benefits before making a treatment

decision. Experience has shown that pests

that are not eradicated by solarization may

recolonize roots and soil, and pathogens and

nematodes may survive in roots remaining

in the soil. Periodic retreatment may be

necessary.

PLASTIC SHEETING

Clear vs. Colored Plastic

Transparent or clear plastic is most effective

for solarization. Black plastic, often used for

mulching, does not heat the soil as well as

clear plastic. It can be used for solarization

but its main effect is reducing weed growth.

In areas where solarization is ineffective

because of low solar radiation or a heavy

infestation of weeds, black plastic may

combine some solarization benefit with

residual weed control. It can also be used for

solarizing existing crops, for example, by

disinfesting soil while establishing

permanent tree or vine crops (see color plate

1).

Since soil temperatures are lower

with black plastic, the treatment time must

be lengthened for best results. Other colors

of plastic, such as green or brown, which

Figure 8. Postplant soil solarization with clear

polytheylene for control of Verticillium wilt in a

pistachio orchard.

University of California Vegetable Research and Information Center

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allow some heating of the soil but not to the

degree of clear plastic, require longer

treatment times. These other colors of

plastic give so much less effective

solarization that they should probably only

be used as mulch.

Types of Plastic

The thinner the plastic, the greater the

heating will be. Polyethylene (PE) plastic 1

mil (0.001 inch [0.025 mm]) thick is

efficient and economical but not very resis-

tant to tearing by wind or puncture by

animals. Users in windy areas should

consider plastic sheets that are 1.5 to 2 mils

(0.038-0.050 mm) thick. If holes or tears do

occur in the plastic they should be patched

with clear patching tape. Users are

encouraged to select plastic sheeting

containing ITV inhibiting additives that

prevent sheets from becoming brittle and

difficult to remove from the field and extend

the life of the plastic. Plastic sheets laid by

hand can often be used more than once for

solarization, although if the plastic is dirty or

dusty reuse is less effective.

Polyethylene sheets may be modified

by an additive that enables them to absorb

infrared (IR) radiation and improve their

capacity to retain heat. Although these are

available, they have not proven to be very

effective. Colored plastic films are available

that absorb light in the photosynthetic range

to inhibit growth of weeds and at the same

time heat the soil. These can be used for

solarization but generally do not heat soil as

well as transparent films.

There has been considerable interest

in the development of high-density or

"impermeable" plastic sheeting to better

contain fumigant chemicals in soil. These

plastics may also improve the effects of

solarization by sealing in more heat and

volatile compounds. However, these plastics

are under development at this time, and

information on benefits, sources, and prices

is not currently available. Experimental

work has also been done using a sprayable

polymer as a replacement for plastic

sheeting. Such a material would be easy to

apply and less expensive to use, but to date

suitable chemicals have not been found.

The use of a double layer of plastic

with air space between the layers mimics the

greenhouse effect and raises soil

temperatures from 2° to 10°F higher than

that obtained with a single layer. Using a

double layer requires additional preparation

time and expense but it may make soil

solarization more feasible in areas with

cooler climates.

Availability

For smal