ORIGINAL PAPER

An artificial capillary barrier to improve root-zone conditionsfor horticultural crops: response of pepper, lettuce, melon,and tomato

Eviatar Ityel • Naftali Lazarovitch •

Moshe Silberbush • Alon Ben-Gal

Received: 14 November 2010 / Accepted: 6 March 2011 / Published online: 31 March 2011

� Springer-Verlag 2011

Abstract Capillary barriers (CBs) occur at the interface

between two soil layers having distinct differences in

hydraulic characteristics. In preliminary work without

growing crops, it was demonstrated that CBs implemented

in sandy soils increased hydrostatic volumetric water

content by 20–70%, depending on soil texture and depth of

barrier insertion. We hypothesized that the introduction of

an artificial CB at the lower root-zone boundary of horti-

cultural crops can increase yields as a result of increased

water content and uptake efficiency. The effects of intro-

duced CBs on soil water content, plant growth, and yields

of bell peppers (Capsicum annum L), lettuce (Lactuca

sativa L), tomatoes (Lycopersicon esculantum L.), and

melons (Cucumis melo L.) were studied in a desert envi-

ronment in southern Israel. Inclusion of a CB increased soil

water content by 60% and biomass and fruit yields by 25%

for pepper, and increased matric head and biomass yield by

80 and 36%, respectively, for lettuce. Neither tomatoes nor

melons reacted significantly to the presence of CBs, in

spite of increased soil moisture. Daily soil matric head

amplitude was reduced fivefold when lettuce was grown

with a CB. Spatial variability was highly reduced when a

CB was present. When peppers were grown with a CB, the

standard deviations of water content and biomass yield

were reduced by 40% relative to control.

Introduction

Stress in plants is induced when available root-zone water

is not sufficient to support the potential transpiration flux

(Tp). Water stress of magnitude and duration ranging from

mild and short to severe and long commonly reduces

uptake (Jones 1992). De Wit (1958) suggested a positive

linear correlation between relative transpiration and rela-

tive biomass production:

Ya

YP¼ Ta

TPð1Þ

where YP is potential yield under potential transpiration

(TP) and Ya is actual yield under actual transpiration (Ta). It

is thus clear that in order to maximize biomass production,

transpiration must be maximized. To satisfy this condition,

available water for uptake by roots must be sufficient to

satisfy water requirements dictated by TP.

Horticultural crops grown in arid zones are often sub-

jected to extreme weather conditions. High temperatures

coupled with low relative humidity result in high vapor

pressure deficit (VPD) that leads to extremely high tran-

spiration demand. Under such conditions, and to avoid

exposure to temporal water stress, water in the root zone

must be kept at a relatively high potential (Jones 1992).

Moreover, saline water is often the only available water for

irrigation in many desert regions around the world (Oron

et al. 2002; Ben-Gal et al. 2009) and as such, further

reduces water potential and introduces an additional root-

zone stress-causing factor. In order to minimize yield

Communicated by J. Ayars.

E. Ityel � N. Lazarovitch (&) � M. Silberbush

Wyler Department of Dryland Agriculture, French Associates

Institute for Agriculture and Biotechnology of Drylands,

Jacob Blaustein Institutes for Desert Research, Ben-Gurion

University of the Negev, Sede Boqer Campus,

84990 Beersheba, Israel

e-mail: lazarovi@bgu.ac.il

A. Ben-Gal

Department of Environmental Physics and Irrigation,

Agricultural Research Organization, Gilat Research Center,

Mobile Post Negev 2, 85280 Negev, Israel

123

Irrig Sci (2012) 30:293–301

DOI 10.1007/s00271-011-0281-5

losses caused by reductions in osmotic water potential

coupled with high transpiration demand, it has been sug-

gested that matric head should be kept at an optimal, high

potential (Feddes et al. 2001).

Soil and landscape variability at the field scale is a major

cause of spatial irregularity in crop yields (Kravchenko and

Bullock 2000; Li et al. 2001). High correlation between

soil texture and cotton yield variability was found by Terra

et al. (2006). It seems that sandy soils are rather more prone

to spatial variability of hydraulic properties than other soil

types, as small textural differences can lead to significant

hydraulic discrepancies (Vauclin et al. 1983). For example,

pearl millet (Pennisetum glaucum) cropped on sandy soils

in Niger State at the fringe of the Sahara desert was found

to suffer from very high yield variability within a very

short horizontal distance due to variations in soil spatial

physical properties (Geiger and Manu 1993). It has also

been shown (Warrick and Yates 1987) that high spatial

variability in the cropping plot eventually leads to reduced

average yield.

The Arava Valley, located in southern Israel, is a desert

area with an annual rainfall of only 0.03 m y-1 and

evaporation demand (Class A pan) of more than 3 m y-1.

Midday temperatures from August to September, when

horticultural crops are commonly planted, often exceed

40�C, while low temperatures can approach 0�C during

winter. Relative humidity is typically low and, coupled

with the high temperatures, contribute to very high VPD

values. The Arava Valley is the country’s major winter

producer of horticultural crops including bell pepper,

melon, and tomato exported to Europe, Russia, and North

America. Bell pepper (C. annum L) is the most economi-

cally important and widely cultivated crop and is typically

grown in protective structures covered with 25-mesh net-

ting. Daytime relative humidity in the net houses may

reach 20–50%, while typical values for outdoor relative

humidity are below 20%. Even with reduced radiation and

increased humidity compared to the outside, midday VPD

in the net house can exceed 4 kPa, resulting in very high

transpiration demand. Irrigation water in the region,

pumped from local aquifers, is saline with electrical con-

ductivity (EC) values ranging from 2 to 4 dS m-1 (Ben-Gal

et al. 2009). Due to water allocation policy and pumping

restrictions, both seasonal and daily water supply to

growers is limited, intensifying the need for the highest

irrigation efficiency possible.

Soils in the Arava region (Table 1) tend to contain a high

percentage of flint stones, a substantial (25%) silt fraction,

and typically have saturated hydraulic conductivity lower

than 5 9 10-6 m s-1. Crop production in these soils is

rarely practiced, mainly due to difficulties in mechanical

cultivation and their extremely high spatial variability. The

traditional solution for horticultural crop cultivation in the

area has been to create an arable root zone by overlaying the

local soil with a 0.4-m layer of imported sand. However,

due to resource depletion and environmental constraints,

local sand supply has become unavailable, and efforts have

been invested in finding alternative root-zone construction

(Russo et al. 2007; Ityel et al. 2011).

Capillary barriers (CBs) occur at the interface between

two soil layers having distinct differences in hydraulic

characteristics like a fine soil layer overlaying a coarse

textured soil (Kampf et al. 1998). For example, if a coarse

gravel layer (particle diameter [0.01 m) is located below a

fine grained soil layer, the matric head at the interface will

be 0 under hydrostatic state. In preliminary work without

growing crops (Ityel et al. 2011), it was demonstrated that

CBs implemented in sandy soils increased hydrostatic

volumetric water content by 20–70%, depending on soil

texture and depth of barrier insertion. Sandier soil and

shallower placement of the CBs resulted in higher water

content increase relative to controls.

We hypothesized that the increased matric head in the

root zone of horticultural crops generated by CBs would

lead to increased growth and yields. We further speculated

that the presence of a CB would increase crop homogeneity

by reducing the spatial variability of root-zone media

hydraulic properties.

Table 1 Parameters of soil and other root-zone format components

Material hr (m3 m-3) hs (m3 m-3) KS (m s-1) Bulk density

(kg m-3)

Stones

[/[ 0.02 m] (%)

Sand (%) Silt (%) Clay (%)

Arava Reg 0.045 0.355 5.2E-06 21 68 25 7

Yair sand 0.045 0.42 2.8E-05 1,520 93 3 4

Zohar sand 0.064 0.41 3.1E-05 1,570 87 8 5

Scoria (0–8) 0.22 0.58 2.8E-04 1,200

Gravel� 0 0.39 4.7E-03 1,300

Residual water content (hr), saturated water content (hs), saturated hydraulic conductivity (KS)

� After Bussiere (1999)

294 Irrig Sci (2012) 30:293–301

123

Materials and methods

Four horticultural crops (pepper, tomato, lettuce, and

melon) were grown with and without CBs installed at the

lower root-zone boundaries. Root-zone formats were

evaluated in experiments conducted at two sites in the

Arava Valley. Bell pepper (C. annum L, cv. ‘‘7187’’,

Zeraim, Israel) and tomato (L. Esculantum L. cv. ‘‘Mital’’,

Hazera, Israel) were tested at the ‘‘Yair’’ Experimental

Station located in the central part of the valley (30�460N,

35 140E; 128 m below sea level), while lettuce (L. sativa.

L, Cos, cv. ‘‘Yellow Leaf’’, Hazera, Israel) and melon (C.

melo, cv. ‘‘6003’’ Hazera, Israel) were studied at the

‘‘Zohar’’ Experimental Station located in the northern part

of the valley, just south of the Dead Sea (30�570N, 35�230E,

350 m below sea level). Selected soil properties are given

in Table 1. Comparisons were made in each experiment

between control root zones (Format A, Fig. 1), consisting

of a 0.1-m layer of volcanic scoria over sand, and root

zones including CBs (Format B, Fig. 1), with a 0.1-m layer

of volcanic scoria above 0.2 m of sand separated by

50-mesh net from a 0.05-m layer of gravel and enclosed in

an Agripal� (Agripal Black 100, Picplast, Israel) fabric.

Agripal� is a black, woven polypropylene fabric that is

water permeable and has bulk density of 100 kg m-3.

Irrigation in each experiment at ‘‘Yair’’ and ‘‘Zohar’’ sta-

tions was with the local saline water of 2.4 and 3.8 dS m-1,

respectively. Irrigation was applied in quantities relative to

crop maximal evapotranspiration (ETmax). A lysimeter

station consisting of two detached units filled with media

(Perlite 206, Deshanim, Israel) positioned inside the pro-

tective house served to determine ETmax. Each lysimeter

unit received 3 daily irrigations with the same water quality

used for irrigating the entire experiment. Water quantity in

each application was equal to the total daily ET measured

for the previous day. One lysimeter unit contained plants,

while the other unit was without plants, mulched with

polyethylene. Drainage water from each of the units was

measured daily and ETmax values were calculated by sub-

tracting the drainage volume of the mulched unit (irrigation

volume) from that of the unit with plants (irrigation vol-

ume-ET) and dividing the difference by the lysimeter’s

effective crop surface area (2.25 m2). In the pepper and

lettuce experiments, the two variables: water application

and root-zone format were fully factorial, and a split-plot

randomized blocks design was used. In the melon and the

tomato trials, where only one variable (root-zone format)

was tested, a randomized block design was applied.

Experiment 1: bell pepper

Pepper seedlings were transplanted at the end of August

2007 in a protective structure covered with 50-mesh

netting. Plants were grown in two rows per bed spaced

1.6 m apart and 0.4 m within the row to give density of 3.2

plants m-2. Experimental design was randomized split

block with 4 replicates. The root-zone treatments served as

main plots within which the irrigation treatments were

randomized. Each root-zone format was tested under 4

relative irrigation (I/ETmax) application rates of: 0.8, 1.2,

1.6, and 2.4 I/ETmax, where I stands for the actual irrigation

depth (L). Differential irrigation treatments were initiated

30 days after transplanting and reached seasonal quantities

of 0.61, 0.86, 1.06, and 1.52 m. Irrigation water was con-

sistently applied every 24 h via 2 drip laterals positioned at

the center of each bed, 0.2 m apart, with drippers spaced

every 0.2 m (Uniram� AS-17012, Netafim, Israel). Fertil-

ization was provided in the irrigation water at a rate of

70 ppm N by injecting a liquid fertilizer (N: P2O5: K2O:

4-2-6) plus micro elements (‘‘Mor’’: 4-2-6, Fertilizers and

Chemicals, Israel).

The root-zone sand layer positioned 0.1–0.3 m below

the surface was sampled twice during the growth season,

100 and 132 days after planting, and analyzed for gravi-

metric water content. A single soil core from 0.1 to 0.3 m

was taken from each plot at the end of the irrigation cycle,

Fig. 1 Root-zone components for implementing and evaluating

introduced artificial capillary barriers. a Format A control, (b) FormatB with a capillary barrier. 1 Surrounding soil, 2 lower (capillary

barrier) boundary, 3 material separating root-zone media from the

capillary barrier layer, 4 lower root-zone media, 5 upper boundary

mulch layer, 6 barrier separating root zone from surrounding soil, 7line of symmetry, and 8 drippers spaced on lateral

Irrig Sci (2012) 30:293–301 295

123

about 24 h following an irrigation event. Volumetric water

content was calculated by multiplying the gravimetric

values by the soil bulk density of 1,520 kg m-3. Ripe fruits

were harvested nine times along the season, between

December 20 and April 12, 2008. Three plants in each plot

were collected after the last harvest to determine fresh and

dry matter of the green fruits, leaves, and stems. Two-way

ANOVA statistical model was used to evaluate means

effects. Spatial variability was tested using a fit y/x model

in JMP 7 software (SAS Institute Inc. USA) using the

Bartlet test for unequal variances.

Experiment 2: tomato

Tomato seedlings were planted on February 10, 2008, in a

polyethylene-covered greenhouse. Plants were grown in

two rows per bed spaced 1.5 m apart and 0.75 m within the

row space to give a population of 1.8 plants m-2. The

greenhouse was cooled by a wet pad (cool pad, FY033007,

Dagan Automation, Israel) system for the last 80 of the

total 134 cropping days. Harvest commenced 90 days after

planting and was repeated 7 times over a period of 44 days.

On the 110th cropping day, plants were topped in order to

stop their epical growth. After the last harvest, three plants

from each plot were sampled to determine fresh and dry

matter production. Dry matter content was determined

upon samplings of harvested clusters 15%, removed leaves

8%, and canopy 13%. Irrigation and Fertilization were the

same as used for the pepper experiment, with an accumu-

lated water application of 0.67 m. The experiment was

conducted in 4 randomized blocks, and treatment effects

were tested with one-way ANOVA statistical analyses.

Experiment 3: lettuce

Lettuce seedlings were transplanted on January 23, 2005,

into a polyethylene-covered greenhouse. Irrigation treat-

ments of I/ETmax = 0.5, 1, 2, and 3 were tested in each

format. As water application increased so did the irrigation

frequency, in such a way that time interval between two

irrigations was in the range of 8–0.5 days (Fig. 2) where

higher application rates received higher irrigation fre-

quencies. After 39 days, total water application accumu-

lated to 0.025, 0.05, 0.1, and 0.15 m, respectively.

Fertilization was the same as used for the pepper and

tomato experiments. Plants were grown in 2 rows per bed

spaced 1.6 m apart and 0.3 m between plants within the

row to give a density of 4.4 plants m-2. Tensiometers

(Mottes, Israel) were positioned at the bed center, at 0.25 m

depth, midway between the two drip laterals in all eight

treatments to monitor matric head. The experiment was

conducted in 4 randomized blocks and replicated 4 times.

Design was split plot in which the water application

treatments were positioned in the main plots and the root-

zone treatments randomized within. Two-way ANOVA

statistical analysis was used to evaluate mean effects.

Experiment 4: melons

Melon seedlings were transplanted on Jan 4, 2007, into a

polyethylene-covered walk-in tunnel. Plants were grown in

a single row per bed spaced 1.6 m apart and 0.4 m within

the row to give a density of 1.5 plants m-2. Irrigation and

Fertilization setups were the same as in the tomato exper-

iments. Daily irrigations were applied at a single rate for

both formats and reached a seasonal quantity of 0.87 m.

Harvest commenced 60 days after planting, and lasted

60 days. Harvested fruits were classified as high-quality

(exportable) and low-quality fruits suffering from incom-

plete netting, non uniform shape, and brown spots. At the

end of the harvest period, three adjacent plants were cut at

ground level and separated for green fruits and foliage to

determine total biomass dry matter production (without

roots). The experiment was conducted in 4 complete ran-

domized blocks, and one-way ANOVA statistical analyses

were used to evaluate means effects.

Results

Crop response

Pepper plants responded to both water application rate and

root-zone format (Fig. 3). Positive response was achieved

in all yield parameters to the presence of the CB. Fruit

yield, fruit number, and biomass yield increased in Format

B (CB) relative to control by 25, 27, and 26%, respectively,

for all water application rates. As water application

increased from 0.8 to 1.6 I/ETmax, biomass and fruit yield

0

2

4

6

8

10

0 10 20 30 40

ET0.5

ET1

ET2

ET3

Time after planting (day)

Irri

gat

ion

inte

rval

(d

ay)

Fig. 2 Irrigation frequency for lettuce grown in Experiment 3 under

4 irrigation application rates of I/ETmax (irrigation relative to

maximum potential evapotranspiration) = 0.5, 1, 2, 3

296 Irrig Sci (2012) 30:293–301

123

increased by 40 and 35%, respectively, for Format A

(without CB). In Format B, biomass production increased

as irrigation increased from 0.8 to 1.2 I/ETmax but further

increases in irrigation resulted in only small, insignificant

increases in biomass. Fruit yield continuously increased as

water application increased in Format B, while no further

response was achieved in the control, Format A, as water

application increased from 1.6 to 2.4 I/ETmax. Post-harvest

quality parameters were affected neither by water quantity

nor by root-zone format (data not presented).

Tomato fruit (Fig. 4) and biomass (data not presented)

yields increased slightly due to the presence of a CB.

Accumulated fruit yield increased by 10% relative to

control but only at the last harvest did the difference

become significant. Lettuce plants responded to both water

application and root-zone format (Fig. 5). Biomass pro-

duction was 36% greater for Format B than in Format A,

for all water application rates. Response to water applica-

tion rates in the control was close to linear as yield

increased by 130% in response to increased water appli-

cation from 0.5 to 3 I/ETmax (Fig. 5a). When a CB was

present, maximal yield response to water application was

reached earlier, at 2.0 I/ETmax.

Melon seasonal fruit yield expressed in quantity and

quality parameters (Fig. 6a), and total biomass yield (data

not presented) were not affected by the presence of a CB,

while early-season yields were significantly higher for

Format B compared to the control, seasonal differences

were insignificant. Quality parameters expressed in

exportable yield and rind brown spots (Fig. 6b, c) indicated

higher quality for Format B only during early season

harvests.

Effects on water content and matric head

In Experiment 1 (pepper), root-zone water content was

highly influenced at all water application rates when a CB

was present, with an average water content increase of 60%

over the control (Fig. 3d). Applied water quantity had no

significant influence on soil water content measured 24 h

after the irrigation events, in both root-zone formats.

In Experiment 3 (lettuce), the presence of a CB

increased matric head measured at 0.25 m depth by

80% for all water application rates (Fig. 5b). In the control,

Irrigation (I/ETmax )

.8 1.2 1.6 2.0 2.4

Wat

er c

onte

nt (

m3

m-3

)

.05

.10

.15

.20

.25

.30

Rel

ativ

e fr

uit

yiel

d (Y

/Ym

ax)

.2

.4

.6

.8

1.0

Rel

ativ

e fr

uit

num

ber

(Y/Y

max

)

.2

.4

.6

.8

1.0

Rel

ativ

e bi

omas

s yi

eld

(Y/Y

max

)

.2

.4

.6

.8

1.0

Format AFormat B

a

b

c

d

Fig. 3 Peppers grown in Experiment 1 under 4 irrigation applications

rates. I/ETmax (irrigation relative to maximum potential evapotrans-

piration) = 0.8, 1.2, 1.6, 2.4, and in two root-zones formats: FormatA control, Format B including a capillary barrier. a Relative biomass

yield, [Yield (Y) relative to maximum yield (Ymax)], (b) relative fruit

yield, (c) relative fruit number, (d) mean (of two sampling dates)

volumetric water content

Time after planting (days)

90 100 110 120 130

Rel

ativ

e fr

uit

yiel

d (Y

/Ym

ax)

0.0

.2

.4

.6

.8

1.0 Format A

Format B

Fig. 4 Accumulated yield (Y) relative to maximum yield (Ymax) of

tomato in Experiment 2 grown in two root-zone formats: Format Acontrol and Format B with a capillary barrier

Irrig Sci (2012) 30:293–301 297

123

root-zone matric head increased by 40% as water appli-

cation increased from 0.5 to 3.0 I/ETmax. In spite of this,

root-zone matric head of the highest application rate in

Format A did not reach that of the lowest application rate

when a CB was included (Fig. 5b). In Format B, matric

head values were high and similar for all the irrigation

rates. Twenty-four-hour matric head amplitude in the

control treatment (Fig. 7a) was 0.4 m, while in the CB

format, a fivefold reduction in matric head fluctuations was

measured (Fig. 7b). Matric head values in Format A mat-

ched those of Format B only for short intervals, close to

the irrigation events, but were much lower at all other

times.

Effects on spatial variability

The effect of the CB on spatial variability was evaluated in

Experiment 1 with pepper plants by testing the standard

deviations (SD) of water content and biomass yield under

the hypothesis of unequal variances (Table 2). SD of both

soil water content and biomass yield decreased by 40%

relative to the control due to the presence of a CB.

Discussion

The extent of water content and matric head increases due

to CBs placed at the bottom of the root zone (Figs. 3d, 4b)

was in the range reported by Ityel et al. (2011) where tests

were carried out without plants. In the present study with

growing pepper plants, average root-zone water content

increased by 60% when a CB was present (Fig. 3d). This

augmented water content was responsible for average yield

increases of 26 and 36% for pepper and lettuce, respec-

tively (Figs. 3a, 5a). When no CB was present in the

pepper experiment, increasing water application rate from

Rel

ativ

e yi

eld

(Y/Y

max

)

.2

.4

.6

.8

1.0Format A

Format B

Irrigation (I/ETmax)

.5 1.0 1.5 2.0 2.5 3.0

Mat

ric

head

(cm

)

-80

-60

-40

-20

0

b

a

Fig. 5 Canopy yield (a) and 24 h mean matric head (b) of lettuce

grown in Experiment 3 under 4 irrigation applications rates: I/ETmax

(irrigation relative to maximum potential evapotranspiration) = 0.5,

1, 2, 3 in two root-zone formats: Format A control and Format B with

a capillary barrier. Yield (Y) relative to maximum yield (Ymax)

Rel

ativ

e fr

uit

yiel

d (Y

/Ym

ax)

.2

.4

.6

.8

1.0Format A

Format B

Rel

ativ

e ex

port

able

yi

eld

(Y/Y

max

)

.2

.4

.6

.8

1.0

Time after planting (days)

80 100 120 140

Rel

ativ

e br

own

frui

tnu

mbe

r (

Y/Y

max

)

.2

.4

.6

.8

1.0

a

b

c

Fig. 6 Melon yields and quality in Experiment 4 grown in two root-

zone formats: Format A control and Format B with a capillary barrier.

a Accumulated relative yield (actual yield Y relative to maximum

yield Ymax), (b) accumulated exportable relative yield, c accumulated

brown rind relative yield

298 Irrig Sci (2012) 30:293–301

123

0.8 to 2.4 I/ETmax did not have any impact on root-zone

water content 24 h after irrigation (Fig. 3d). In the lettuce

experiment, increasing water application rate from 0.5 to

3.0 I/ETmax (Fig. 5b) did cause a substantial 40% increase

in the matric head of the control. Those differences in

responses could be attributed to the fact that in the pepper

experiment all irrigation treatments received a single reg-

ular daily irrigation, while in the lettuce experiment,

irrigation frequency increased with application rate

(Fig. 2), thus apparently prolonging processes of water

redistribution and drainage.

In spite of the fact that no changes in root-zone water

content were found due to the application rate in the pepper

experiment control treatment, an increase in biomass yield

of up to 40% was found as water application range

increased from 0.8 to 2.4 I/ETmax (Fig. 3a). This illustrates

the need for cautiousness when crediting increased yields

to increases in matric head. Under the experimental con-

ditions where irrigation water salinity was high (i.e.,

EC = 2.4 dS m-1), it is expected that increasing water

application increased leaching. In spite of the fact that

drainage volume and salinity were not reported in this

study, increased leaching would be expected to improve

plant performance by reducing root-zone salinity (Ben-Gal

et al. 2008).

Maximal biomass and fruit yield in the control was

attained at an irrigation rate of 1.6 I/ETmax (Fig. 3a), while

similar yield was achieved in the CB treatment at 0.8

I/ETmax application rate. This corresponds to an approxi-

mate 100% improvement in applied water production effi-

ciency, which can be attributed to the presence of the CB.

Increasing water content in a sandy soil can have an

additional beneficial effect on water and nutrient avail-

ability since unsaturated hydraulic conductivity, water

flow, and solute transport from the soil bulk to the roots can

limit water and nutrient uptake in dry compared to wet soils

(Wallach et al. 1992; Ben-Gal and Dudley 2003; Segal

et al. 2006). Positive effects on plant growth due to CBs

located under the root zone are likely due to such increased

uptake. Similar influence on water content, hydraulic

conductivity, and uptake could presumably be achieved by

increasing irrigation application frequency (Ben-Gal and

Dudley 2003; Segal et al. 2006). However, in a study on

pepper plants grown in sandy loam soil (Assouline et al.

2006), pulse irrigation (10 irrigations per day) achieved

neither a profound impact on water content nor effects on

biomass or fruit yield. Reduced leaching efficiency may

also be a drawback of pulse irrigation when saline water is

used for irrigation (Meiri and Plaut 1985; Assouline et al.

2006).

In the case of lettuce, CB placement in the root zone

increased matric head by 80% compared to control and

consequentially yield increased by 36%. Efficiency of

irrigation was increased by 400% due to the CBs, as lettuce

plants in the control root zones achieved the same yield

when irrigated at a rate of 2.0 I/ETmax as that reached by

plants in root zones containing CBs receiving only 0.5

I/ETmax (Fig. 5a).

Soils that are highly heterogeneous are prone to prob-

lematic spatial variability in their hydraulic properties,

which can be subsequently reflected in crop production

Time (hour)

0 100 200 300

Mat

ric

hea

d (

cm)

-60

-40

-20

Mat

ric

hea

d (

cm)

-60

-40

-20

0a

b

Fig. 7 Matric head measured during Experiment 3 with tensiometers

inserted at -0.25 m depth midway between the two laterals. Data

presented were taken from irrigation treatment 2 I/ETmax (irrigation

relative to maximum potential evapotranspiration) for lettuce grown

in two root-zones formats: (a) Format A control, (b) Format B with a

capillary barrier

Table 2 Standard deviation values for biomass yield (kg m-2) and

volumetric water content (m3 m-3) for the two root-zone formats

Biomass

(kg m-2)

Water content

(m3 m-3)

Format A 0.34 0.04

Format B 0.20 0.02

Relative SD (format B/A) 0.59 0.56

Bartlett (probability P [ F) 0.05 0.02

Probability of F values equal or less than 0.05 are significantly

different

Irrig Sci (2012) 30:293–301 299

123

response (Kravchenko and Bullock 2000; Li et al. 2001;

Lazarovitch et al. 2006). Our findings demonstrated that

CBs at the bottom boundary of the root zone reduced

spatial variability significantly in the sandy soils repre-

sentative of the study area. This reduction in variability

could be attributed to the neutralization of the hydraulic

influences of the lower soil layers on the root-zone media.

In the pepper experiment, the introduction of a CB resulted

in a reduction of 40% in SD of measured water content and

in a subsequent 40% reduction in the biomass yield SD

(Table 2). The contribution of increased uniformity to

yields and irrigation efficiency in commercial horticulture

is expected to be quite substantial.

Results for melons and tomatoes in the current study

show that the introduction of CBs did not increase yields in

those cases. Crop response to water content is dependent on

plant type and potential transpiration demand. When VPD

is relatively low, root-zone water content and hydraulic

conductivity are not likely to limit actual transpiration, and

therefore, relative yield is not expected to be affected

(Eq. 1). In our case, the pepper plants grew under net

cover, while melons and tomatoes grew under plastic cover

with consequential higher relative humidity and lower

VPD. This could partly explain the apparent differences in

crop type response to the presence of CBs.

Crop production responses to water are highly influ-

enced by system feedback mechanisms including changes

in transpiration due to salinity (Shani et al. 2007). In the

present cases, where irrigation water salinity was high, the

dynamics between water application rate, plant uptake rate,

and salinity must be considered. Increasing water appli-

cation above 1.0 I/ETmax essentially affected plant growth

not only by providing more available water but also by

decreasing root-zone salinity. The higher the irrigation

water salinity and the more sensitive the crop, the higher

the expected crop response to irrigation quantity associated

with improved leaching. Irrigation water salinity was 2.4

and 3.8 dS m-1 for the pepper and lettuce, respectively.

Relative differences in salt tolerance of the two crops as

well as the difference in water quality could partly explain

the greater response to water application observed in the

control root zone for lettuce compared to pepper. In addi-

tion, part of the lettuce response to water application could

be attributed to the varied irrigation frequency unique to

that experiment.

Conclusions

The introduction of a CB to the lower root-zone boundary

has the potential to increase yields of horticultural crops

grown in sandy soils or other highly conductive porous

media. When saline water is used for irrigation, a CB is

expected to mitigate the salinity effect on water uptake and

plant production as a result of increased matric head. This

will not always be attained as other environmental and

biological factors interact and influence plant growth and

production. Increasing matric head in the overlaying root

zone is a direct effect of the CB in all cases. In this study,

we found subsequent increased yields when pepper and

lettuce were grown in a CB enhanced root zone, while no

yield response was found for tomato and melon crops.

Reduction in soil spatial variation when a CB is present is

expected to contribute to its attractiveness as an irrigation

management tool, as crop uniformity is significantly

enhanced.

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