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Geoderma 117 (2003) 283–295

Volcanic materials as mulches for water conservation

M. Tejedor*, C. Jimenez, F. Dıaz

Departamento de Edafologıa y Geologıa, Universidad de La Laguna, Tenerife, Spain

Abstract

Arid regions are characterised by a limited rainfall, a circumstance that acts as a barrier to dryland farming. Lanzarote in the

Canary Islands (Spain) is one of the most arid regions in Europe, with less than 150 mm annual rainfall and potential

evapotranspiration in excess of 2000 mm. A traditional farming system developed on the island has led to a diversified and

productive form of agriculture that uses no irrigation. The system utilizes a layer of 10–20 cm of tephra, covering the natural

soil. The tephra acts as mulch and is highly effective for soil water conservation.

In this work, we present the results of a 3-year monitoring of soil moisture content in three plots covered with two types of

basaltic tephra with different grain sizes. The results are compared to those obtained on adjacent plots that were not covered

with the mulch. Sampling was conducted once a month every 10 cm, up to a depth of 1 m. Compared to the noncovered soils,

the tephra-covered soils managed to retain eight times more water in the surface layer during the driest months. At a depth of 1

m, twice the amount of water was retained in the tephra-covered plots. This positive effect was explained by the tephra’s

physical properties, particularly its low water retention capacity and high porosity, enhancing water infiltration and reducing

evaporation rates. Differences observed in the water conservation depending on the mulch type was explained largely by grain

size. The pyroclastic materials proved to be very effective for soil water conservation under arid conditions.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Soil moisture; Mulching; Tephra; Water conservation; Canary Islands

1. Introduction eral cases nowadays, they still survive in some

Arid regions, where rainfall is extremely scarce,

irregular and torrential, are characterised by low

biological productivity. Dryland farming is often

difficult, if not impossible. In view of its scarcity,

water is a precious resource and conservation tech-

niques are therefore vital. Advisable in such regions

are water harvesting and soil use techniques that

reconcile conservation with satisfactory agricultural

output. Some such techniques have existed since

time immemorial and, although abandoned in sev-

0016-7061/$ - see front matter D 2003 Elsevier Science B.V. All rights re

doi:10.1016/S0016-7061(03)00129-0

* Corresponding author. Fax: +34-922-318311.

E-mail address: martesa@ull.es (M. Tejedor).

parts of the world. An example of such a technique

is the use of mulch or surface layers of inorganic

materials.

The role played by such coverings in water con-

servation is an aspect that has attracted much attention

in the literature on mulching. Most of this literature

has been based on laboratory experiments (Benoit and

Kirkham, 1963; Corey and Kemper, 1968; Unger,

1971; Modaihsh et al., 1985). Much rarer are articles

based on actual fieldwork, particularly when no irri-

gation is used.

In arid and semi-arid regions, inorganic mulches

are deemed effective for soil water conservation,

although the degree of effectiveness varies greatly

served.

M. Tejedor et al. / Geoderma 117 (2003) 283–295284

depending on the characteristics of the mulch, partic-

ularly thickness, type of material and grain size. Some

authors attribute the increased moisture content to the

reduction in evaporation (Benoit and Kirkham, 1963;

Modaihsh et al., 1985; Groenevelt et al., 1989; Kamar,

1994) whereas others point to the improvement pro-

duced in terms of permeability, with increased infil-

tration velocity, reduced runoff and, consequently, a

reduction in soil loss (Goor and Barney, 1976; Poesen

et al., 1990; Valentin and Casenave, 1992; Kamar,

1994). There is general agreement that surface mulch

is more effective than mulch used sub-surface (Groe-

nevelt et al., 1989; Unger, 1971) and also that the

more soil surface covered, the less water loss. How-

ever, there is no general consensus as to the adequate

thickness of the mulch. Modaihsh et al. (1985) and

Kemper et al. (1994) find that a layer of mulch of 5–6

cm is more effective than similar mulch 1–2 cm thick.

Perez (1991) arrived at a similar conclusion with

layers of 20–25 cm, compared to 10–15 cm. How-

ever, other authors conclude that thicknesses above 5

cm are not beneficial, a circumstance they put down to

the mulch’s water retention capacity and its intercep-

tion of the scarce rainfall (Perez, 2000). Inorganic

materials used as mulch should, therefore, possess low

water retention capacity. The most widely used are

gravel and sands, with volcanic materials much less

common. Using gravel and sand mulches, Corey and

Kemper (1968), noted that the former was more

effective. Groenevelt et al. (1989) compared volcanic

ash, zeolite and sand mulches, and found the sand to

be the most effective. Regarding grain size, there is a

general agreement that in order to produce a buffer

effect the mulch layer has to be coarser than the soil

underneath. Perez (1998) considers that it is not

merely a matter of grain diameter but also porosity:

capillary flow is interrupted if the soil pores are

smaller than the mulch pores.

Lanzarote, in the Canary Islands, is an arid region

with less than 150 mm annual rainfall. Since the 18th

century, it has been home to a number of soil and water

conservation techniques which have permitted a pro-

ductive and diversified system of dryland farming. The

most widespread of these involves the use of layers of

tephra on the soil.

Bibliographical references to the use of volcanic

materials as mulch are rare. Othieno (1980), who

experimented for over a year with crushed volcanic

rock sieved to 1.25 cm, in a zone with rainfall of

around 700 mm, noted an increase of 16% in soil

moisture content in the uppermost centimetres of the

soil, but this difference fell to just 1% at 30–40 cm.

Doolittle (1998), in a geographical work containing

virtually no data, compared maize production using

layers of pyroclastic mulch of 25, 50 and 75 mm

were used, and concluded, surprisingly, that 25 mm

was the most effective. Fernandez and Tejedor

(1987), in an earlier study lasting 6 months (which

did not include the driest period) and limited to the

uppermost layer of soil, found that moisture content

was 15% higher in the mulched soils. Chesworth et

al. (1994) conducted a study during the crop season

in Ethiopia, in conditions of annual rainfall of 755–

723 mm, potential evapotranspiration of around 1000

mm and ustic water regime. Using mulch consisting

of basaltic ash and pumice, with grain size of 2–20

mm and layers of between 3 and 6 cm, they found

that water content at 40 cm in the mulched soils was

10% larger than in the unmulched soils. Moreover,

the basaltic ash mulch with a thickness of 6 cm was

the most effective. A recent work examines the

influence of the modified soil moisture regime of

mulched soils on the classification of such soils

(Tejedor et al., 2002).

The objective of the present article is to outline the

results of 3 years’ monitoring of moisture content in

soils in three tephra-covered agrosystems. The results

are compared to those obtained in adjacent uncovered

soils and we endeavour to explain the mechanisms at

play in the mulch system and discuss why it proves

effective in increasing soil moisture.

2. Materials and methods

2.1. Site description

The study was conducted in Lanzarote, an island

in the Canaries, Spain, situated 110 km of the west-

ern coast of Africa, at 29j north latitude and 13j37Vwest longitude. It has a land area of 862 km2.

Lanzarote is considered to be one of the most arid

parts of the European Union, with the following

climate features: annual rainfall below 150 mm,

torrential rain which falls very heavily but briefly,

high relative air humidity (around 75% on average),

M. Tejedor et al. / Geoderma 117 (2003) 283–295 285

high potential evapotranspiration (in excess of 2000

mm in an evaporimetric tank), annual average tem-

perature of around 20jC, with large fluctuations

between day and night, and strong trade winds.

Monthly and annual rainfall data at the study sites

are listed in Table 1.

Like the other Canary Islands, Lanzarote is of

volcanic origin. Its geological history is well docu-

mented (Fuster et al., 1968; Abdel Monem et al.,

1971; Carracedo et al., 1992). Of all the islands,

Lanzarote has suffered the most prolonged recent

eruptions: six consecutive years of permanent erup-

tion between 1730 and 1736 covered the island with

basaltic outcrops and pyroclasts, with olivine-augites

predominant. Most of soils of the Lanzarote volcanic

ecosystem are characterized by low contents of short-

range-ordered materials but large amounts of carbo-

nates, leading to rather compact soil structures, with

bulk density values larger than 0.9 g cm� 3. These soil

properties are attributed to the hyperthermic and aridic

temperature and moisture regimes occurring in Lan-

zarote. The most common natural soils have therefore

been classified as Aridisols and Entisols (Soil Survey

Staff, 1999), while those covered by tephra, given that

their water regime has been modified, have been

Table 1

Monthly and annual precipitation (mm) for the study area

Period/year J F M A M

Site 1

P (mm) (1982–2000) 20.2 14.2 14.8 3.3 1.3

Standard deviation 18.0 23.3 14.5 6.1 1.7

P (mm) 1998 31.1 10.3 6.2 0.0 0.0

P (mm) 1999 35.4 1.6 4.9 0.0 0.0

P (mm) 2000 19.6 4.3 0.0 8.4 0.0

Site 2

P (mm) (1982–2000) 24.3 13.0 30.6 9.6 2.4

Standard deviation 24.9 15.8 43.5 8.6 3.1

P (mm) 1998 56.9 13.6 7.5 2.3 0.0

P (mm) 1999 52.6 4.4 14.9 0.7 0.0

P (mm) 2000 13.0 4.5 0.0 10.5 3.2

Site 3

P (mm) (1982–2000) 25.3 12.6 16.8 2.7 1.7

Standard deviation 30.2 21.9 14.5 4.0 4.8

P (mm) 1998 44.4 9.2 4.0 0.0 0.0

P (mm) 1999 49.0 2.4 14.4 1.8 0.0

P (mm) 2000 11.2 2.8 0.0 6.7 1.9

included in the Inceptisol order, as proposed by

Tejedor et al. (2002).

The study was conducted in soils of three land

use representative fields of the island. These fields,

referred as sites 1, 2 and 3, were located in different

parts of the island. Each site was chosen to have

adjacent mulched plot (10–12 cm of tephra) and

unmulched plot. The plots had an extension about

2000 m2. The tephra was placed some 30 years ago

on all plots, although in site 3 it was renovated a

decade ago. During the years preceding our study,

the mulched plots were used in similar fashion,

mainly onion crops. No cultivation took place during

the study period. Natural vegetation, in accordance

with the aridity, was very sparse and limited to

ephemeral grasses and scattered xerophytic shrubs.

The crops grown around our study area, in the some

soil type, were onions, potatoes, pumpkins, beans

and lentils.

2.2. Methods

Soil samples for gravimetric moisture content

determination were taken monthly using an auger,

every 10 cm to a depth of 1 m, from April–May 1998

J J A S O N D Total/

mean

0.3 0.0 0.0 1.7 11.9 19.5 32.4 124.1

0.8 0.0 0.0 2.4 14.8 23.7 34.5 11.7

2.8 0.0 0.0 1.3 0.0 0.0 56.1 107.8

0.0 0.0 0.0 1.6 41.2 12.4 22.4 119.5

0.0 0.0 0.0 0.0 2.3 0.0 30.8 65.4

1.0 0.1 0.0 4.0 15.8 20.1 46.5 164.5

1.9 0.3 0.1 5.0 24.0 25.5 39.6 16.0

0.5 0.0 0.0 0.4 0.8 0.0 42.7 124.7

0.0 0.0 0.5 6.6 32.2 32.3 34.8 179.0

0.0 0.0 0.0 2.2 9.0 2.3 32.5 77.2

0.0 0.0 0.0 0.8 13.5 21.7 33.8 144.4

0.0 0.0 0.0 2.0 20.1 22.3 31.4 12.6

0.0 0.0 0.0 0.0 0.0 0.0 52.0 109.6

0.0 0.0 0.0 2.7 49.5 14.2 28.0 162.0

0.0 0.0 0.0 0.0 1.9 3.5 35.2 63.2

M. Tejedor et al. / Geoderma 117 (2003) 283–295286

until March–April 2001. Samples were taken every

month from both the mulched soils and the adjacent

unmulched soils. Spatial variability was assessed to

test if the differences in moisture content between the

mulched and unmulched soils were significant. This

was done using a further sampling at different points

(n between 3 and 6) in each of the plots and also in the

adjacent unmulched soils. Two nonparametric statis-

tical tests for related data were used (Wilcoxon’s test

and sign test).

Grain-size distribution of the tephra was per-

formed by mechanical sieving. Solid density and

pore-size distribution of the mulch were measured

using mercury porosimetry. Soil bulk density was

measured by obtaining undisturbed cored samples,

at field moisture conditions, with three replicates.

Total porosity was determined from bulk and solid

densities. Mulch and soil water retention curves

were recorded using the pressure plate apparatus

method (U.S. Salinity Laboratory Staff, 1954).

Particle-size distribution (particles < 2 mm) was

determined after samples were dispersed in Na

hexametaphosphate solution and shaken on a hori-

zontal reciprocating shaker for 12 h using the

densimetric method (Day, 1965). Water infiltration

rates were measured using a double ring infiltrom-

eter (Bouwer, 1986); with three replications. Runoff

was estimated in the laboratory using a raindrop

simulator with the following characteristics: area

600 cm2, soil height 5 cm, mulch height 5 cm,

slope 6.67%, average rain intensity 78 mm h� 1. A

clayey-textured soil (Haplocambids from the same

Fig. 1. Grain size distributio

valley as the soil of site 1) was used, along with

three mulches with different granulometry : fine

(78% < 3.2 mm), medium (47% < 3.2 mm) and

coarse (19% < 3.2 mm).

3. Results and discussion

3.1. Properties of the tephra

Particle-size distributions of the tephra mulches are

shown in Fig. 1. Tephra mulches from sites 1 and 2

were both characterized by a predominance of diam-

eter range of 1.0–3.2 mm. In contrast, the tephra

mulch was coarser in site 3, with a maximum of

particle diameters located between 4 and 6.3 mm.

Bulk density was 0.95, 0.91 and 0.79 g cm� 3 and

porosity 0.63, 0.64 and 0.68 cm3 cm� 3 for the tephra

in sites 1, 2 and 3 respectively. Pore-size distribution

(Fig. 2) differed slightly also, larger pores predom-

inating in the site 3 tephra. Water retention curves

show that the amount of retained water was very low

for the tephra at different suctions. This differed

considerably from the underlying soils, as it can be

seen in Fig. 3, where the soil from site 1 (0–30 cm

layer) has been taken as an example. The curves also

show the predominance of macropores in the tephras,

which favour water transmission. From the above

data, it can be concluded that the selected tephra are

highly water-permeable and, in a non-saturated state,

water conductivity can be assumed to be extremely

low.

n of the tephra mulch.

Fig. 2. Pore size distribution of the tephra mulch.

M. Tejedor et al. / Geoderma 117 (2003) 283–295 287

3.2. Main soil characteristics

The main characteristics of the soils of sites 1 and 3

(both in mulched and unmulched plots) are listed in

Tables 2 and 3; whereas the main soil characteristics

for site 2, very similar to those for site 3, can be found

in Tejedor et al. (2002). The unmulched soils of the

three sites were classified as Haplocambids (Soil

Survey Staff, 1999) because they do not present andic

properties. The main differences were that the

unmulched soil of site 1 was more clayey and carbo-

nated than its sites 2 and 3 soil counterparts (mean

clay content was 63%, 45% and 45%, and carbonate

Fig. 3. Soil (site 1) and tephra mo

content was 18%, 10% and 11%, for sites 1, 2 and 3,

respectively: Tables 2 and 3). Aggregate size distri-

bution in a dry sample by sieving was approximately

the same in the three soils, with aggregates of 1–0.1

mm predominant.

The mulched and unmulched soils presented com-

mon characteristics: alkaline reaction (pH>8), large

amounts of carbonates (12–17%), low organic matter

content ( < 0.5 g kg� 1), electrical conductivity values

in saturated paste below 4 dS m� 1, predominance of

calcium in the exchange complex and ESP values

below 15% (Table 2). Texture was generally fine,

predominantly clayey, although differences were

isture characteristic curves.

Table 2

Site 1: main soil characteristics

Depth pH CaCO3 ECsa Ca2 + Mg2 + K+ Na+ CECb ESPc Clay Silt Sand Bulk

(cm) (H2O) (g kg�1soil) (dS m�

1)

(cmol kg� 1 soil)(%)

(g kg� 1 soil) density

g cm� 3

With mulchd

0–3 8.7 150 0.68 16.2 7.1 4.0 0.7 28.0 2.5 565 322 113 1.03

3–10 8.7 181 0.79 18.0 8.71 3.0 0.5 30.3 1.7 611 340 49 1.16

10–20 8.6 175 0.83 17.3 10.2 4.2 0.8 32.7 2.4 607 352 41 1.07

20–30 8.6 173 0.98 16.6 11.8 3.2 0.9 32.5 2.8 651 294 55 1.12

30–40 8.6 188 1.10 13.0 12.0 3.6 1.3 30.0 4.3 639 289 72 1.10

40–50 8.7 192 1.22 13.2 11.8 4.0 1.9 30.9 6.1 617 303 80 1.19

50–60 8.6 182 1.48 13.3 11.8 3.1 2.3 30.5 7.5 621 308 71 1.25

60–70 8.7 172 1.47 12.0 11.8 3.4 2.2 29.4 7.5 631 307 61 1.22

70–80 8.7 168 1.42 13.8 13.7 3.4 3.0 33.9 8.8 625 315 59 1.20

80–90 8.7 160 1.55 12.4 13.0 3.5 3.7 32.6 11.3 629 324 48 1.23

90–100 8.8 167 1.70 10.0 12.1 3.7 4.5 30.3 14.9 577 344 79 1.29

Without mulch

0–3 8.9 173 1.58 14.3 10.9 4.2 1.3 30.7 4.2 565 385 50 1.13

3–10 8.9 173 1.77 11.1 11.1 4.6 5.3 32.1 16.5 637 313 49 1.16

10–20 8.6 173 3.65 15.8 11.1 4.2 3.6 34.7 10.4 657 296 46 1.19

20–30 8.7 176 3.03 14.9 11.2 4.5 4.2 34.8 12.1 622 324 54 1.15

30–40 8.9 183 2.16 15.1 10.9 3.9 3.4 33.3 10.2 646 286 68 1.18

40–50 9.0 172 1.68 11.5 11.0 3.9 4.7 31.1 15.1 630 312 58 1.13

50–60 9.0 178 1.37 13.3 11.5 4.3 4.5 33.6 13.4 637 306 57 1.20

60–70 9.1 171 1.15 12.2 11.7 3.8 4.4 32.1 13.7 643 300 57 1.12

70–80 9.1 176 1.05 14.3 11.8 3.2 3.8 33.1 11.5 656 278 66 1.19

80–90 9.1 186 1.37 9.4 11.8 4.0 4.7 29.9 15.7 631 303 66 1.10

90–100 9.1 172 1.39 11.4 11.9 4.0 5.0 32.3 15.5 631 317 52 1.14

a ECs is electrical conductivity in saturated paste.b CEC is cation exchange capacity.c ESP is exchangeable sodium percentage.d Depth ‘‘0’’ corresponds to the limit between the original soil and the mulch layer.

M. Tejedor et al. / Geoderma 117 (2003) 283–295288

observed in the upper layers of the tephra-covered

soils, which presented greater sand content, particu-

larly in plot 3. This was explained by the mixture of

soil and tephra particles, which was more pronounced

in this plot, on which the old covering was replaced 10

years ago.

3.3. Infiltration and runoff

Field measurements of infiltration velocity, of the

mulched and unmulched soil, produced the following

findings: basic velocity was always greater (approx-

imately double) in the mulched soils, and took longer

to attain than in the unmulched soils; initial velocity

was also much higher in the mulched soils (around 2.5

times greater) (Fig. 4A); in accordance with the

criteria of FAO (1963), the infiltration velocity in

the mulched soils was moderately fast, and moderate

to moderately slow in the unmulched soils. These

results were accounted for partly by the greater per-

meability of the mulched soils and partly also by the

protection afforded by the tephra covering, which

prevents direct action by the rain on the soil. In the

noncovered soils, the impact of raindrops on the soil

surface can lead to aggregates disruption, pore filling

and formation of a surface sealing crust that consid-

erably reduces water infiltration (LeBissonais and

Singer, 1992; Valentin, 1991).

Experiments with rainfall simulation in the labora-

tory, using tephra of three grain-sizes, showed that, in

the unmulched soils, runoff started only 3 min after the

beginning of the rain, whereas, in their mulched soil

counterparts, the runoff time threshold occurred

between 26 and 31 min after the beginning of the rain,

Table 3

Site 3: main soil characteristics

Depth pH CaCO3 ECsa Ca2 + Mg2 + K+ Na+ CECb ESPc Clay Silt Sand Bulk

(cm) (H2O) (g kg� 1

soil) (dS m� 1

)(cmol kg� 1 soil)

(%)(g kg� 1 soil) density

(g cm� 3)

With mulchd

0–3 8.9 105 0.94 14.4 7.7 4.9 0.8 27.8 2.9 456 345 199 1.11

3–10 9.0 90 0.72 16.1 6.1 3.5 0.7 26.4 2.7 466 319 216 1.12

10–20 9.0 120 0.96 19.6 7.3 2.4 1.3 31.9 4.1 519 340 144 1.14

20–30 8.7 132 2.06 18.8 9.0 2.6 2.3 32.7 7.0 544 423 33 1.00

30–40 8.6 125 2.93 17.9 9.5 2.0 2.4 31.8 7.5 541 450 9 1.01

40–50 8.6 120 3.00 18.3 9.9 2.3 3.4 33.9 10.0 555 430 15 0.98

50–60 8.6 124 2.96 19.2 9.5 2.6 2.2 33.5 6.6 541 447 12 0.99

60–70 8.7 125 2.83 14.4 9.8 3.3 4.6 32.1 14.3 506 476 17 1.00

70–80 8.8 126 2.67 16.8 10.3 2.9 3.0 33.0 9.1 455 520 25 1.03

80–90 8.9 121 2.47 15.3 10.9 2.9 4.1 33.2 12.3 407 538 55 1.02

90–100 8.9 115 2.27 16.4 11.1 3.4 3.6 34.5 10.4 406 521 73 1.01

Without mulch

0–3 8.9 142 0.62 19.1 8.8 4.6 0.6 33.1 1.8 368 615 17 1.10

3–10 9.1 141 0.91 16.1 9.6 3.3 1.8 30.8 5.8 453 540 7 1.11

10–20 9.1 135 1.38 15.5 9.1 2.6 2.5 29.7 8.4 467 526 7 1.18

20–30 8.8 128 2.82 17.5 9.3 2.9 2.5 32.2 7.8 472 522 6 1.20

30–40 8.8 129 2.85 18.0 9.6 2.5 2.4 34.6 6.9 492 502 5 1.20

40–50 8.7 121 2.99 18.4 10.1 2.8 2.9 34.2 8.5 519 474 7 1.13

50–60 8.7 125 2.80 19.0 9.6 2.8 2.5 33.9 7.4 508 482 70 1.21

60–70 8.8 125 2.54 16.9 10.0 3.2 2.6 32.7 8.0 442 543 15 1.09

70–80 8.9 113 2.21 17.0 9.6 3.4 3.5 33.5 10.4 397 562 41 1.11

80–90 9.0 100 2.14 17.3 10.3 3.2 3.4 34.1 9.9 357 556 87 1.20

90–100 8.8 117 2.42 16.5 11.0 3.6 3.8 34.9 10.9 497 467 37 1.15

a ECs is electrical conductivity in saturated paste.b CEC is cation exchange capacity.c ESP is exchangeable sodium percentage.d Depth ‘‘0’’ corresponds to the limit between the original soil and the mulch layer.

M. Tejedor et al. / Geoderma 117 (2003) 283–295 289

depending on the tephra grain size. Fig. 4B gives the

results of the rainfall simulation using a mulch of

medium grain size and a soil taken from the same

valley as that of site 1. The experimental results lead to

the same conclusion as that reached in the field,

namely, that the tephra covering exerts a protective

effect on the soil, reducing the impact of raindrops,

preventing sealing crust formation and reducing run-

off.

3.4. Soil moisture content dynamics

The results of the soil moisture measurements for

the mulched and unmulched soils are shown for the

various soil depths and different sampling dates in

Figs. 5 and 6 for sites 1 and 3, respectively. These

results are as follows.

First, soil moisture vertical variability was low,

with coefficients of variation of the means up to a

depth of 1 m as follows: 0.7%, 2.3% and 1.3% in the

mulched soils of sites 1, 2 and 3, respectively; 0.9%,

1.0% and 0.6% in the unmulched plots, respectively.

Second, as already mentioned, soil moisture verti-

cal distribution was very similar in both sites 1 (Fig.

5) and 2 (Tejedor et al., 2002), despite the difference

in the clay percentage. This can be attributed to the

fact that the site 1 soils were richer in clays but

received less annual precipitation (Table 1) than their

site 2 soil counterparts.

Third, taking the moisture distribution throughout

the mulched and unmulched soils separately, differ-

ences were very often found between site 3, on one

hand, and sites 1 and 2, on the other hand (Figs. 5 and 6).

In the latter two sites, a high degree of regularity was

Fig. 4. (A) Infiltration rate of the soil (site 3) with and without tephra mulch. (B) Runoff in simulated rainfall experiment.

M. Tejedor et al. / Geoderma 117 (2003) 283–295290

observed year-round in the soil moisture vertical

distributions recorded in the mulched soils, with only

slightly lower soil moisture values in the first 10–20

cm of the soil profile. Similarly, in their unmulched

soil counterparts, soil moisture vertical distributions

became more regular below 40 cm depth whereas

moisture content was markedly uneven in the first

30–40 cm. In contrast, site 3 soils were characterized

by uneven distribution of moisture content which was

found up to 30 cm in both the mulched and

unmulched soils (Fig. 6). Furthermore, throughout

the year, and at all depths down to 1 m, moisture

content of the mulched soils exceeded the mean

wilting point in sites 1 and 2 whereas it was below

or close to the wilting point in site 3 (Figs. 5 and 6).

The exception for diverging behaviours between sites

1–2 soils and site 3 soils concerned their unmulched

soils where soil moisture values were always below or

close (in the surface layer occasionally) to the mean

wilting points (Figs. 5 and 6). The different behaviour

of the soils between site 3 and sites 1 and 2 was not so

much due to the difference between mulched and

unmulched, although this was important, as already

noted. Rather, it was in the much lower values of soil

moisture recorded in site 3, in this case in the mulched

soil, as shown in Figs. 7 and 8 where equivalent soil

water height of the 0–100-cm soil profile was com-

puted as a water balance index. The mean equivalent

water height over 3 years was 366 and 324 mm in

plots 1 and 2 mulched soils whereas it was only 251

mm in their plot 3 mulched soil counterpart (Figs. 7

and 8).

Fourth, in all three sites, differences of volumetric

water content were observed, throughout the year and

at all depths, between the mulched soils and their

unmulched soil counterparts, as follows. The most

Fig. 5. Mean seasonal volumetric moisture content in mulched and unmulched soil for site 1 and corresponding wilting point values (average

1998–2000).

Fig. 6. Mean seasonal volumetric moisture content in mulched and unmulched soil for site 3 and corresponding wilting point values (average

1998–2000).

M. Tejedor et al. / Geoderma 117 (2003) 283–295 291

Fig. 7. Rainfall and equivalent soil water depth to 100 cm (mm) in mulched and unmulched soil for site 1 and corresponding wilting point

values.

M. Tejedor et al. / Geoderma 117 (2003) 283–295292

pronounced discrepancies were observed in both sites

1 and 2, particularly in the uppermost soil layers and

during the driest months (Fig. 5). Further down,

differences were less marked, although, at 1 m depth,

13% more water was recorded in the mulched soil

compared to its unmulched soil counterpart (Fig. 5).

In site 3, with the same clay percentage as the soil of

site 2 in the profile overall, the differences were less

pronounced, particularly at 10–20 cm (Fig. 6), where

there is a mixture of soil and tephra. In sites 1 and 2,

the differences in moisture content between mulched

and unmulched soils were significant at the 99%

probability level for all the months and depths studied

(up to 1 m). In the case of site 3, the differences were

also significant ( p = 0.01) below 20 cm, whereas

between 0 and 20 cm, in the mixed tephra–soil layer,

significance level was only 95% and at times non-

significant.

Finally, it was worth noting that despite the con-

siderably lower rainfall in the third year, the figures

for moisture content in the three mulched soils were

still high.

The results obtained show that coverings of pyro-

clastic materials on soils can be very effective for soil

water conservation and also that there were differ-

ences between the studied plots (between site 3 and

sites 1 and 2). We will now account for these results

and will discuss on the underlying processes which

should explain the tephra mulch efficiency and the

discrepancies observed between the studied sites, as

follows.

The degree of effectiveness observed greatly excee-

ded the range given in the literature for inorganic

materials (Corey and Kemper, 1968; Othieno, 1980;

Chesworth et al., 1994) and was attributed to the

characteristics of the studied tephra and their mulching

effects. The basaltic tephra used had a very low water

retention capacity, a larger grain size than that of the

adjacent soils, a large porosity, large water infiltration

capacity and hydraulic conductivity when wet but low

hydraulic conductivity when dry, a low heat capacity

and a poor thermal conductivity (Gonzalez et al.,

1964). All these properties make it an excellent mulch.

Hence, full advantage is taken of the scarce rainfall,

which is usually very intense. Conversely, on the

noncovered soils nearby the frequent presence of a

sealing crust reduces infiltration and contributes to

water loss through run-off. Once the heavy rainfall

periods end, the considerable evapotranspiration leads

to rapid water loss in the tephra layer, the pores

Fig. 8. Rainfall and equivalent soil water depth to 100 cm (mm) in mulched and unmulched soil for site 3 and corresponding wilting point

values.

M. Tejedor et al. / Geoderma 117 (2003) 283–295 293

desaturate and the upward capillary flow in the soil is

reduced, resulting in less water being lost through

evaporation.

The difference between the mulched plots in terms

of soil water conservation effectiveness (which was

greater in site 1 and 2 soils than in site 3 soils) would,

mainly due to a combination of different precipitation,

tephra and soil properties, leading to different water

balance qualitatively identified by the soil moisture

vertical distribution and its dynamics:

– the specific coarse grain size of the tephra used in

site 3, and its greater porosity which enhances

vapor flux to the atmosphere. We are also carrying

out complementary experiments in 18 experimental

plots measuring 4.5� 5.5 m. The same soil (clayey,

brought from a nearby lowland plain) was covered

with volcanic materials of different types and

thicknesses (Tejedor et al., 2000). Among the

mulch used are tephras with grain size and

thickness similar to those used in sites 1, 2 and 3

of this paper. The results are the same as those

obtained in the soils described in the paper, namely,

water content in the soils covered in coarse mulch

is considerably lower than that in the soils covered

with less coarse tephra. Indeed, what we have

shown scientifically is something already known by

local farmers, who for years have opted for the less

coarse tephra, avoiding using the coarser varieties.

– although sites 2 and 3 soils had the same texture in

the overall profiles, the rainfall input was different,

with more precipitation in site 2 than in site 3,

– although sites 1 and 3 had similar rainfall patterns,

they differ by their soil clayey reservoirs, site 1

soils being richer in clays than their site 3 soil

counterparts.

Because we lacked crucial information such as

daily temperatures, it was impossible to establish a

quantitative water balance which can be written by the

following equation:

Stþ tV� St ¼ P � ETP� I

where St + tV is the water stock (in mm water height as

it is reported in Figs. 7 and 8) at time t+ tV, St is the

M. Tejedor et al. / Geoderma 117 (2003) 283–295294

water stock at time t, St + tV� St is the difference of

water stocks between two campaign of soil water

measurements, that is to say between times t + tVand t, P is rainfall (mm water height) between times

t+ tV and t, ETP is the evaporation and evapotranspi-

ration (also in mm water height), I is the water loss in

the system. The calculation of ETP, in accordance

with the classical Thornthwaite system, requires,

among other values, temperature. The weather sta-

tions near the three studied sites were for rainfall only

and not for recording temperature However, research

is in progress with experiments in columns for con-

trolling evaporation.

Another additional factor that could probably con-

tribute to the lower efficiency in site 3 was the 10-

year-old replacement of the mulch layer in this sys-

tem, as a result of which the top few centimetres of

soil became mixed with tephra particles, thus reducing

the insulation effect.

4. Conclusion

The 3-year monitoring of soil moisture content in

three plots covered with two types of basaltic tephra

with different grain sizes leads to the following

conclusions.

1. This first quantitative study explains the great

efficiency of the tephra cover to increase the water

stock in soils. This allows irrigation-free agricul-

ture in extremely arid environments.

2. The effectiveness of the tephra cover is very

dependent of its grain size distribution.

3. A combination of the tephra cover, different

precipitation and soil properties could explain the

differences in the hydric behaviour observed

between the three plots studied. Farming associated

to soil mulching is crucial to the island’s economy.

In summary, the results outlined in this article lead

us to conclude that a layer of basaltic tephra placed on

the surface of soils has a great efficiency for soil water

conservation in arid regions. The climatic conditions

in which this study was conducted are much more arid

than those reported in the literature. The mulch

efficiency is such that water is available to the plants,

meaning that irrigation-free agriculture is possible.

This form of farming is crucial to the island’s econ-

omy. Furthermore, it aids rainwater infiltration,

reduces erosive processes and is also a soil conserva-

tion technique. It requires little by way of energy or

chemical inputs, little or nor labour and what tillage is

performed does not interfere with the maintenance of

the mulch. It is clearly a sustainable agriculture model.

Acknowledgements

This research was funded by the Spanish

Government (Ministerio de Educacion y Cultura,

PB96-1028, y Ministerio de Ciencia y Tecnologıa,

BTE2000-0824).

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