Soil Science Society of America Journal 66:202-206 (2002)
© 2002 Soil Science Society of America
DIVISION S-5 - PEDOLOGY
Soil Moisture Regime Changes in Tephra-Mulched Soils
Implications for Soil Taxonomy
M. Tejedor*,
C. C. Jiménez and
F. Díaz
Dpto. Edafología y Geología, Universidad de La Laguna, Tenerife, Spain
* Corresponding author (martesa{at}ull.es)
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ABSTRACT
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Large areas of the Canarian archipelago are catalogued as aridic according to U.S. soil taxonomy. A traditional system of cultivation, based on the application of a mulch layer of tephra, is widely practiced. The objective of this work is to compare the classification of soils with and without mulch using U. S. soil taxonomy, and illustrate the problems in estimating the soil moisture regime on the basis of meteorological data. In this study the moisture regime of a natural soil and an adjacent soil mulched with a 12-cm layer of basaltic tephra was characterized on the basis of gravimetric moisture measurements, taken over a period of >1 yr, at 10-cm intervals to a depth of 1 m. The moisture regime of the soil was also estimated using meteorological data. Results obtained for the natural, nonmulched soil using measured values and those estimated from atmospheric data are very similar, and both indicate an aridic soil moisture regime. The difference with the soil covered with basaltic tephra is noteworthy. In the latter case, the soil remained moist throughout the year, representing a udic moisture regime. This noteworthy difference is reflected in the classification of the soils: Aridisols in the first case and Inceptisols in the second. A proposal is made to include mulching with tephra as a cultural practice along with irrigation and fallowing in the definition of the classes of soil moisture regimes in U.S. soil taxonomy. This would prevent the practice of mulching with tephra from changing the soil order placement in U.S. soil taxonomy.
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INTRODUCTION
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IN RECENT DECADES, and particularly since the Río de Janeiro Environmental Conference (Secretariat for the Convention to Combat Desertification [CCD], 1998), a certain degree of interest has arisen concerning traditional agricultural systems and how they function and are used. Although the use of rock fragments as mulch is relatively frequent and its effects have been described in numerous articles, many of them seem to report on laboratory studies rather than on actual field observations (Corey and Kemper, 1968; Modaihsh et al., 1985; Gale et al., 1993; Nachtergaele et al., 1998). The application of volcanic ash as mulch is more uncommon (Fernández-Caldas and Tejedor, 1987; Groenevelt et al., 1989) and its effects have not been discussed in relation to soil classification. This relationship is the main topic of this work.
The soil moisture regime is a significant diagnostic parameter in different classifications and, in particular, in U. S. soil taxonomy. This term "refers to the presence or absence either of ground water or of water held at a tension of less than 1500 kPa in the soil or in specific horizons during periods of the year" (Soil Survey Staff, 1999). Ultimately, the aim is to determine the water available for plants throughout the year.
The lack of direct measurements in the field that would allow for a correct assessment of the soil moisture regime has led to its estimation from meteorological data using different types of simulation models. The results if compared, for example, with the response of the vegetation, are acceptable in many cases, but are not so in others (Porta et al., 1999).
In this work, the moisture regime of a soil with a tephra mulch and that of an adjacent, nonmulched soil are determined on the basis of field measurements. The values obtained are compared with those estimated from climatic data. Likewise, this evaluation is also discussed in terms of soil classification.
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MATERIALS AND METHODS
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Study Location
The Canary Islands, which are volcanic in origin, are situated in the Atlantic Ocean between 27°38' and 29°25' N lat. and only 110-km distant from the northwest coast of Africa. A large area of this land is one of the driest zones in the European Union. General characteristics of the climate are: extremely low rainfall, with mean annual values frequently below 150 mm and irregular distribution; a mean annual air temperature around 20°C; strong, constant, trade winds throughout the year; a daily average of some 8 h of sunshine; high potential evapotranspiration (about 2000 mm annually); and high values of relative air humidity (73% daily average). A nonirrigated cultivation technique, locally known as arenado (or sand application) has been developed here, based on mulching with basaltic tephra. This traditional agricultural system attained maximum development after the 1730 to 1736 eruption on the island of Lanzarote where most soils were covered by basaltic tephra.
Field and Laboratory
The study was carried out in two adjacent plots, with an approximate area of 1000 m2 (unmulched soil) and 2000 m2 (mulched soil), situated on gentle (<2%) slope. Both plots have similar soils developed from Pliocene basalt (Cambids with thin ochric epipedon). One of the plots was covered 
25 yr ago with a layer of basaltic tephra that acts as a mulch. Figure 1
shows the grain-size distribution of the tephra in which fine gravel (49%) and coarse sand (34%) predominate. The natural vegetation, in accordance with the aridity, is sparse, and consists of ephemeral grasses and scattered xerophytic shrubs (Launaea arborescens). The natural vegetation has been maintained on both plots throughout the study period for comparison purposes. The period covered by this work was April 1998 to October 1999, although sampling will be continued.
The samples were taken once monthly, every 10 cm to a depth of 1 m, using a probe, and the moisture content was measured by gravimetry. The depth was taken, in both cases, from the original surface of the soil, in the case of the mulched soil, depth zero corresponded to the boundary between the surface of the original soil and the mulch layer. To estimate the moisture retained at a tension of 1500 kPa, a pressure-plate extraction was done with a porous ceramic plate (USDA-NRCS, 1996).
Table 1 shows the monthly and annual precipitation recorded over 42 yr at a station located in the study area. The data corresponding to the 2-yr study period are shown separately. Both years can be considered normal after its definition in Soil Taxonomy "as a year that has plus or minus one standard deviation of the long-term mean annual precipitation" (Soil Survey Staff, 1999). The monthly values also fall into the normal category. Figure 2
is based on meteorological data: precipitation, temperature, and potential evapotranspiration, with this last value calculated by the Thornthwaite system (Thornthwaite, 1948). This diagram clearly illustrates an aridic moisture regime.
Bulk samples were allowed to air-dry and then ground to pass through a 2-mm mesh sieve for laboratory analysis. Soil reaction was measured in a 1:2.5 soil/water suspension (Chapman and Pratt, 1982). Organic C was determined by oxidation with K(CrO4)2; the electrical conductivity was measured in saturated paste (USDA-NRCS, 1996). The cation-exchange capacity was determined after Bower et al. (1952). Exchangeable Na and K were extracted with a buffered neutral 1 M NH4OAc solution, and Ca and Mg by 1 M NaOAc at pH 8.2. Solution concentrations were determined by atomic absorption spectrophotometry. Particle-size analysis (particles <2 mm) was determined after samples were dispersed in Na hexametaphosphate solution and shaken on a horizontal reciprocating shaker for 12 h using the densimetric method (Day, 1965). Carbonate contents were determined according to Allison and Moodie (1965). Clay mineralogy was studied in nonoriented powdered samples, as well as oriented aggregates with Co x-rays, using a PW-1720 diffractometer (N.V. Philips' Gloeilampenfrabrieken-Eindhoven, The Netherlands.
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RESULTS AND DISCUSSION
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General Soil Characteristics
Both soils have several characteristics in common: low organic matter content, alkaline reaction, the presence of CaCO3 throughout the profile, and clayey textures (Tables 2 and 3). Illite and kaolinite are the predominant clay minerals. The soils have Ca as the predominant cation occupying the exchange complex. The only detectable difference between the two profiles was related to the Na content, both in the exchange complex and in soil solution. The exchangeable Na percentage decreased about 50% in the mulched soil; and the electrical conductivity (EC) values of the 1:1 extract also were much lower in the mulched soil (about one third). The authors observed in other sites that these differences in exchangeable Na and EC values between mulched and nonmulched soils increased with the age of the mulch (Tejedor et al., 1999).
Soil Moisture Regime
Although the moisture content values have been measured from the original surface to a depth of 1 m, only those corresponding to the control section are presented in Fig. 3 and 4
, which extends from 10 to 30 cm, considering the clayey particle-size class (Soil Survey Staff, 1999). At greater depths, the moisture contents are similar to those in the control section. The differences in moisture contents between the natural soil and that covered with tephra mulching ranged between 15 and 17% and were always statistically significant (Fig. 3 and 4). This result could be explained by the particular physical properties of the tephra, especially its high porosity. Rainwater infiltration is favored, and soil water loss by evaporation is considerably reduced because of the very low unsaturated hydraulic conductivity of the dry tephra. An in-depth study of these mechanisms is currently ongoing.
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Fig. 3. Total gravimetric field and 1500 kPa water contents in natural soil without mulch during the studied period.
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Fig. 4. Total gravimetric field and 1500 kPa water contents in soil with mulch during the studied period.
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On the basis of soil taxonomy definitions (Soil Survey Staff, 1999), soil moisture regimes were classified as follows:
Natural NonMulched Soil (with Field Data)
"Aridic" moisture regime. The control section, in normal years, is totally dry for more than 50% of the days when the soil temperature is higher than 5°C at a depth of 50 cm, and is totally or partially moist for <90 consecutive days when the soil temperature at a depth of 50 cm is above 8°C (the temperature is above 8°C all year) (Fig. 3).
Soil with Mulch (with Field Data)
"Udic" moisture regime. The control section is not dry in any part over 90 cumulative days (Fig. 4).
It is concluded from this study that the moisture regime estimated from climatic data is valid in the study area for the case of the natural soils and that it is far from reality in those soils with even a shallow mulch of fine gravel and coarse sand tephra. Similar results are expected whether the mulch layer has been placed by human or has formed naturally. This points to the need to carry out soil moisture measurements if the soils are to be correctly classified.
The udic regime, characteristic of soils in a humid climate with rainfall regularly distributed throughout the year, occurs under mulch in an arid climate zone defined with the most widely used climatic indices, such as desertic, hyperarid, etc. Local use of the soils reflects their ability to supply plant-available water; crops are grown on mulched soils but not on nonmulched soils.
Classification
The two soils considered (with and without mulching) present the same diagnostic horizons (Soil Survey Staff, 1999): ochric and cambic; short-range ordered minerals are not dominant, the Al-humus complexes that can characterize andic soil properties are not observed, nor do these soils present cracks that open periodically, accompanied by slickensides.
Their placement at the order level will depend on the moisture regime under consideration. Using climatic data to estimate soil moisture regime, both soils would be classified as Typic Haplocambids. Using field measurements data to estimate soil moisture regime, there are noteworthy differences between the situation occupied by each soil in the classification.
Natural Soil without Mulch (With an Aridic Moisture Regime)
The classification coincides with that defined earlier when estimating the moisture regime on the basis of climatic data.
Soil with Mulch (With an Udic Moisture Regime)
A first classification option, in accord with the current definitions in U.S. soil taxonomy, would be to include the soil in the order Inceptisol (Typic Eutrudepts).
We present the following issues for discussion: Which of these taxa (Aridisols, Inceptisols) best reflect actual conditions? When estimating the moisture regime from climatic data, soils, both with and without mulch, have been included in the Aridisols. However, as seen above, the field measurements and soil use reflect a very different situation. The natural, nonmulched soils correspond well to the existing concept of the order Aridisol, soils whose main limitation is the lack of available water for the majority of plants. This is not the case of the soils mulched with tephra, which have considerably more available water than is required for this order. Consequently, it would not be valid to consider them Aridisols although they are situated in an arid environment. The mulched soils fulfil the requirements of the Inceptisols, although they do not fit the concept of these soils in the sense that "they can form in almost any environment, except for an arid environment" (Soil Survey Staff, 1999). There is no doubt that the conditions of development of both soils were exactly the same before the modifications were introduced 25 yr ago, when one soil was mulched. The available data have led us to believe that new pedogenic processes may be taking place since the tephra mulch was added. For example, more leaching of soluble salts is suggested by the results shown in Table 3. A study performed over a longer period and during different seasons of the year will allow us to reach more definitive conclusions.
Another possible option for classification, probably more in accord with the philosophy of the system, is outlined below.
On the one hand, in the description of the sixth attributes of soil taxonomy it is stated that: "The differentiae keep an undisturbed soil and its cultivated or otherwise human-modified equivalents in the same taxon insofar as possible. Changes produced by a single or repeated plowing that mixes the surface soil to a depth of 18 to 25 cm (7 to 10 in), for example, have the least possible effect on the placement of a soil in soil taxonomy" (Soil Survey Staff, 1999). On the other hand, in the definition of the classes of soil moisture regimes it is assumed that "... the amount of stored moisture is not being increased by irrigation or fallowing. These cultural practices affect the soil moisture conditions as long as they are continued" (Soil Survey Staff, 1999).
In accord with these principles, the use of tephra mulching is a farming practice that increases soil moisture, as has been clearly shown. If this practice is used jointly with irrigation and fallowing in the definition of the classes of soil moisture regimes, the moisture regime of the mulched soil could be considered to be aridic and consequently the soils would be included in the order Aridisol with the nonmulched soils.
The layer of mulch represents a significant soil surface modification in regard to the use and management of the soil, and it should therefore be manifested at a phase level. Therefore, introducing mulching into the definition of soil moisture regimes would be justified. This proposal, intended as a contribution to improve U.S. soil taxonomy, would involve modification by mulch addition, with a minimum of disturbance. This proposal would prevent anthropic actions such as mulching with tephra from changing the soil order placement in soil taxonomy. Practical applications such as soil mapping would be favored.
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ACKNOWLEDGMENTS
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This research was funded by the Spanish Government (Dirección General de Educación Superior, Grant PB96-1028).
Received for publication April 24, 2000.
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REFERENCES
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ABSTRACT
INTRODUCTION
