Saturation, Reduction, and Redox Morphology of Seasonally FloodedAlfisols in Taiwan

Zeng-Yei Hseu and Zueng-Sang Chen*

ABSTRACTAnthraquic conditions were defined in the soil taxonomy primarily

for rice paddy soils, but little data have been published on relationshipsamong saturation, reduction, and morphological features for suchsoils. We studied those parameters in three alternatively flooded andirrigated rice-growing Alfisols. Redoximorphic features were evaluatedusing morphological descriptions. Saturation was monitored at depthsof 50 and 100 cm using unlined boreholes and perforated wells.Reduction was evaluated by both measurements of redox potentialand reaction to a,a'-dipyridyl dye. The Siaoli soil (Typic Plinthaqualf)was saturated at 100 cm for virtually the entire year, and was saturatedat 50 cm for 44% of the year. Reducing conditions occurred at the50-cm depth. The Potu soil (Plinthaquic Paleudalf) was saturated at50-cm depth for 25% of the year, and at 100-cm depth nearly 50%of the year. Reduction varied with depth, but did not necessarilycorrespond to periods of saturation. The Luchu soil (Plinthic Paleudalf)was saturated at 50 cm for 36% of the year, and was not saturated at100-cm depth throughout the year. Reducing conditions were observedfrequently at 50 cm, but were never observed at 100 cm. All the soilpedons had redoximorphic features, and nodule size decreased in thesequence Potu > Siaoli > Luchu. All soils met the requirements foraquic conditions but the Luchu soil was marginally oxyaquic.

RCE-GROWING SOILS of Taiwan occupy about 60%(540000 ha) of the arable land (Chen, 1992). After

local reservoir construction in the 1950s, red soils andtheir alluvium on the Taoyuan Terrace in northern Tai-wan were developed for cropping rice. Seasonal floodingand draining cycles control saturation and reduction con-ditions in these soils. When a well-aerated soil is sub-merged, it quickly becomes anaerobic, and the redoxpotential drops sharply (Ponnamperuma, 1972). Previousstudies have indicated that reducing conditions in poorlydrained soils result in low chroma colors, and oxidizingconditions are responsible for concentrations of brightercolors in better drained soils (Quispel, 1947; Brinkman,1977; Evans and Franzmeier, 1986).

Redox depletions can be used as an indicator of poorlydrained soils (Ruhe et al., 1955; Vepraskas, 1992). Dan-iels et al. (1971) found that gray matrix colors generallyoccurred when the soil was saturated for 50% or moreof the year, but there were soils with some gray colorsthat were saturated for less than 50% of the year. Evansand Franzmeier (1986) described the correlation betweensaturation and soil color pattern for Alfisols in Indiana.Coventry and Williams (1984) indicated that there is astrong relationship between soil color and contemporarywater tables. Schwertmann and Fanning (1976) foundthe largest Fe and Mn concretions in the upper B horizonof the wetter, but not wettest soils, because periodic

Graduate Inst. of Agricultural Chemistry, National Taiwan Univ., 1 Sec.4, Roosevelt Rd. Taipei, Taiwan 106, ROC. Received 28 Apr. 1994.*Corresponding author.

Published in Soil Sci. Soc. Am. J. 60:941-949 (1996).

oxidation and drying appears to be essential for concre-tion formation.

McDaniel and Falen (1994) monitored the temporaland spatial patterns of episaturation in a Fragixeralflandscape of northern Idaho and related these patternsto soil, landscape, and climatic influences. Khan andFenton (1994) tested the application of redoximorphicfeatures as soil saturation indicators in the members ofa Mollisol catena.

All of the studies cited above dealt with natural condi-tions. Anthraquic conditions are those in which saturationand reduction are produced by human activity. Chen(1992) discussed the soil morphology of a drainage se-quence in Taiwan that experienced anthropogenic con-trols on soil drainage. These soils were emphasized inrecent revisions to the soil taxonomy (Soil Survey Staff,1992), along with the introduction and definition of re-doximorphic features, and epi- and endosaturation. Suchflooding and irrigation are essential to rice culture inmany regions. Redoximorphic features are defined foranthraquic soils just as they are for other soils withaquic conditions, and consist of redox depletions, redoxconcentrations, and reduced matrices. Examples of inter-pretations of redoximorphic features were illustrated byVepraskas (1992).

Redox potentials (Eh) can be a qualitative indicatorof Fe and Mn reduction and redoximorphic feature forma-tion in seasonally flooded soils (Cogger et al., 1992).At pH 7, Mn4+ and Fe3+ are reduced at potentials of220 and 120 mV for stirred soil suspensions, respectively(Sposito, 1989, p. 106-124). Soils may be consideredoxidized above 350 mV, moderately reduced between350 and 200 mV, and reduced below 200 mV withrespect to Fe at pH 7 (Patrick, 1980). The variabilityof Eh can be attributed to spatial heterogeneity of soiltexture, structure, organic matter content, aeration, andmoisture conditions (McKenzie and Erickson, 1954).Despite its high degree of spatial variability, Eh can beused to characterize soil reducing conditions (Meek andGrass, 1975). Platinum electrodes can record rapidchanges in soil redox status attributable to rising watertables (Megonigal et al., 1993).

In northern Taiwan, the red soils and alluvial soils onthe Taoyuan Terrace that are used intensively for lowlandrice culture include the Aqualfs and Udalfs. Soils onwhich controlled flooding has been imposed have anthra-quic conditions. Although anthraquic categories in thesoil taxonomy (Soil Survey Staff, 1992) were establishedto encompass these soils, anthraquic categories will re-main largely provisional until sufficient documentationis available. The objectives of this study were to: (i) assessthe impact of anthraquic conditions on the morphologyof three important rice culture soils; (ii) document thesaturation and reduction status of the flooded soils; and(iii) use recent amendments to the soil taxonomy to

941

942 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

:':<$::::5S:̂

120 121 122°

Luchu

°<*n:p30

1UJ20

Potu

Siaoli

Rock bed

1 2 3 4Distance(km)

Fig. 1. The location and elevation above sea level of three study sites in Taiwan.

describe and interpret aquic conditions and redoximor-phic features in these rice culture soils.

MATERIALS AND METHODSExperimental Plots

The study area is located on Taoyuan County in northwesternTaiwan, about 50 km southwest of Taipei City (Fig. 1). Threeexperimental plots (3 by 5 m) were selected for study alongthe Taoyuan Terrace over a distance of 4 km on the Siaoli(fine, mixed, hyperthermic Typic Plinthaqualf), Potu (fine,mixed, hyperthermic Plinthaquic Paleudalf), and Luchu (fine,mixed, hyperthermic Plinthic Paleudalf) soil series. The soilsdeveloped in an alluvial fan terrace that was derived fromQuarternary-aged materials that were approximately 5 m thick.Cobbles are overlain by finer alluvial material in the terrace,and water usually perches at the contact between these twolayers. The Taoyuan Terrace has a slope between 1 and 7%,going down gently from the eastern slopeland side to thewestern seashore. Average summer temperature is 27°C, andaverage winter temperature is 13°C. Annual rainfall in 1993was 1630 mm, and the average annual rainfall for the pastdecade (1984-1992) was 1640 mm. The distribution of monthlyrainfall for the last decade was similar to that of 1993, butthe rainfall of June in 1993 was much larger than that of Juneduring the last decade (Fig. 2). The soil temperature regime

of the study area is hyperthermic. Prior to 1950, the agriculturalland in the terrace was used for tea production. However,after an irrigation system and reservoir were constructed inthe 1950s, the soils of terrace were used for rice production.The rice growing season is approximately 110 d long and thesoils are flooded for most of the growing period. Rice isharvested twice per season. The rice-growing soils are fallowedin winter.

Saturation MonitoringTriplicate unlined boreholes were installed at four depths

(25, 50, 75, and 100 cm) in December 1992. Wells were 6 cmin diameter by 125 cm long poly vinyl chloride pipe perforated at2-cm intervals on the sides from the well bottom to the soilsurface (100-cm total). Wells were wrapped with 20-mesh(1-mm-diam. openings) nylon screen to prevent soil from fillingin the wells. The upper end of each well was covered with aPVC cap, and a small hole was drilled in the cap's side tofacilitate air entry. When determining water table depths, ifthe water table was < 25 cm, the 25-cm well data were usedto calculate the mean water table. As the water table depthincreased, the appropriate well data were used to calculate themean depth. The water levels reported here represent theapparent water table. This is the level at which water standsin a freshly dug, unlined borehole after adequate time forequilibrium in the surrounding soil (Adam and Ford, 1989).

HSEU AND CHEN: REDOX MORPHOLOGY OF SEASONALLY FLOODED ALFISOLS 943

800^ 7001 600~ 600| 4005 300

200100

02 3 4 5 6 7 8 9 10 11 12

MonthD: The average monthly rainfall from 1984 to 1992.•: The monthly rainfall during 1993

Fig. 2. The distributions of average monthly rainfall from 1984 to1992 and during 1993 at Shin-Wu meterological station near thestudy area.

Matric potential was estimated from the water content offield-moist soils. Field-moist soils were sampled by auger atthe depths of 25, 50, 75, and 100 cm. Samples were stored inplastic bags and their gravimetric water contents were measuredimmediately in the laboratory. Soil moisture release curves ofsamples at different depths were determined by pressure-plateapparatus (Klute, 1986). Water contents of the field-moist soilswere converted to matric potentials by those curves. Thismeasurement was done in triplicate.

Reduction ConditionsReduction was characterized either directly by measuring

redox potential, or indirectly in the field with a neutral (pH7) solution of a,a-dipyridyl dye, which identified the presenceof ferrous iron (Childs, 1981). Redox potential was measuredwith a platinum electrode and saturated calomel referenceelectrode on fresh soil cores collected at depths of 25, 50, 75,and 100 cm. The Eh values (in millivolts) were recorded whenreadings no longer drifted. The a,a'-dipyridyl solution wassprayed onto freshly broken surfaces of field-moist soils thatwere sampled at depths of 25, 50, 75, and 100 cm. The brokensurfaces where examined within 30 s of dye application todetermine the presence of Fe(II) as indicated by a red color

of the dye. Measurements of saturation and reduction condi-tions were made at 2-wk intervals from January to December1993.

Soil AnalysisA soil pit was excavated at each study plot in October 1993.

The morphological characteristics of three selected soils weredescribed and classified according to the soil taxonomy (SoilSurvey Staff, 1994). Soil samples were collected from eachhorizon of the profiles for physical and chemical analysis. ThepH of air-dried samples (<2 mm) was determined on a mixtureof soil/deionized water (1:1) and soil/0.01 M CaCl2 (1:1) byglass electrode (McLean, 1982). Bulk density was determinedby the core method (Blake and Hartge, 1986). Saturated hydrau-lic conductivity was measured by the constant-head methodwith core samples (Klute and Dirksen, 1986). Particle-sizeanalysis was determined by the pipette method (Gee andBauder, 1986). Organic C content was determined by theWalkley-Black wet oxidation method (Nelson and Sommers,1982). Exchangeable bases (K, Na, Ca, and Mg) and cation-exchange capacity were measured with the ammonium acetatemethod (pH 7.0) (Rhoades, 1982). Free Fe (Fed) was extractedby the dithionite-citrate-bicarbonate method and measured byatomic absorption spectrometry (Mehra and Jackson, 1960).

RESULTS AND DISCUSSIONSoil Properties and Classification

Soil profile description and selected physical and chem-ical properties are reported in Tables 1 and 2. The Siaolisoil is located in a depression. The Potu soil, which wasburied in the early 1980s, shows a bisequal profile. ThePotu soil at the 50-cm depth in the 2A horizon has ahigh bulk density (1.9 Mg/m3), low porosity (<28%),and low saturated hydraulic conductivity (1.66 cm/d).The high bulk density found in the 2A horizon at 43 to60-cm depth in the Potu soil resulted from frequenttillage. Plinthite occurred in all Btv horizons of three

Table 1. Profile descriptions of Siaoli, Potu, and Luchu pedons.Horizon

ApBABtvlBtv2Btv3

ApBw2A2Btvl2Btv22Btv3

ApABBtvlBtv2Btv3

Btv4

Depth

cm

0-3030-5555-7070-100MOO

0-2525-4343-6060-8585-110>110

0-2626-5050-7070-9090-120

>120t Redoximorphic feature

Matrix color

2.5Y 3/010YR 5/62.5YR 5/82.5YR 4/8

2.5Y 5/42.5Y 4/45Y4/110YR 5/82.5YR 3/62.5YR 4/6

2.5Y 4/210YR 5/42.5YR 5/62.5YR 4/62.5YR 4/4

2.5YR 4/4

abundance; M = many;

Redoximorphic features!

Siaoli pedon

DC 10YR 6/1 Fe depletions and 10YR 4/6 Fe massesMD 10YR 5/6 Fe masses and 10YR 6/1 clay depletionsCD 2.5YR 4/6 nodules and FF 10YR 2/1 Mn masses

Potu pedon

CF SYR 4/6 and 7.5YR 6/8 Fe massesCD SYR 5/6 Fe massesFF 10YR 2/1 Mn masses FP 2.5YR 4/6 nodulesCD 2.5Y 2/0 Mn pore linings and MD 2.5Y 6/4 Fe massesCD 2.5YR 4/8 nodules and FF 10 YR 7/1 Fe depletions

Luchu pedon

CD 7.5YR 4/6 Fe massesFF 2.5Y 6/4 Fe massesFF 2.5YR 4/8 nodulesCD 2.5Y 6/4 Fe masses, FF 2.5YR 4/8 nodules, and FF 10YR

7/1 Fe depletionsFD 2.5YR 4/8 nodules and FF 2.5Y 2/0 Mn masses

C = common; F = few. Redoximorphic feature contrast; F = faint; D

Texturei

CLSiCLSiCLSiCL

SiCLCLSiCCCC

CLSiCLSiCSiCSiC

C= distinct; P

Structure!

massive2vf&fabk2vf&fabk2vf&fabk

3f,m&cabk3vf,f&mabk3vf,f&cabk3f,m&cabk3vf&fabk3vf&fabk

2f&mabk2vf,f&mabk2vf&fabk2vf&fabk2vf&fabk

2vf&fabk

= prominent.t Texture class; CL = clay loam; SiCL = silty clay loam; SiC = silty clay; C = clay.§ Structure: 3 = strong, 2 = moderate, 1 = weak; vf •= very fine, f = fine, m = medium, c = coarse; abk = angular blocky.

944 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

Table 2. Selected physical and chemical properties of Sialoi, Potu, and Luchu pedons.

Horizon Depth

cm

pH(H20)

OrganicC

g/kg

Bulkdensity

Mg/m3

Sand

Texture

Silt1

Clay

% ————

Basesaturation CECt

cmoL/kg

Ferf

g/kgSiaoli pedon (Typic Plinthaqualfs)

ApBABtvlBtv2Btv3

0-3030-5555-7070-100MOO

5.36.06.26.46.4

24.65.85.85.04.6

1.31.61.71.6-

2018181917

Potu pedon (Plinthaquic

ApBw2A2Btvl2Btv22Btv3

ApABBtvlBtv2Btv3Btv4

0-2525-4343-6060-8585-110>110

0-2626-5050-7070-9090-120>120

5.94.95.15.55.65.5

5.35.45.75.75.95.9

16.916.114.27.95.65.2

17.011.77.85.45.53.9

1.51.51.91.61.51.5

Luchu pedon

1.51.51.61.71.6-

192116141312

(Plinthitic

242013171517

4946464545

Paleudalfs)474444333335

Paleudalfs)484947374339

3136363638

343540535453

283140464246

5453536059

889164846357

665954705350

9.27.57.57.59.0

6.95.96.96.67.48.4

7.26.66.26.58.08.3

42.449.338.443.860.2

35.431.837.356.556.252.7

35.046.144.745.855.048.7

t Cation-exchange capacity.t DCB-extracted Fe content.

pedons, but the least quantity of plinthite existed in theLuchu soil. The average soil pH of these three pedonsat 50 to 100 cm was approximately 5.7. We used thispH value to identify the Eh value that was diagnostic ofthe onset of Fe reduction from an Eh-pH phase diagram(225 mV) (Collins and Buol, 1970). The Fe(II)/Fe(III)transition is shown to occur at 225 mV in Fig. 3 to 5.Other Eh values used for this transition for horizonswith pHs >5.7 are noted in the text.

Saturation and Reduction ConditionsThe Siaoli Soil

The water table in the Siaoli soil was at or close tothe soil surface throughout the year (Fig. 3). However,it was a perched water table because the soil was saturatedfor shorter periods with depth. The soil was saturatedat 50 cm from mid-March to mid-August or about 44%of the year (Table 3, Fig. 3). The soil was saturated atthe 100-cm depth for only 24% of the year. Surfacewater hardly percolated to the 100-cm depth of the Siaolisoil because of the perching layer, and the 100-cm depthwas saturated only by shallow groundwater. Therefore,the duration of saturation at the 50-cm depth was longerthan that at the 100-cm depth in the Siaoli soil.

The Siaoli soil at the 50-cm depth was reduced through-out the year as shown by both the positive reactionto a,a'-dipyridyl dye and the Eh values, which wereconsistently <225 mV (Fig. 3). The presence of Fe(II)correlated well with the saturation conditions at the 50-cmdepth through the year (Fig. 3). At 100 cm, the Ehvalues were <100 mV for about 2 mo of the year, butthe Eh values were near the border of the estimated Fe(II)/Fe(III) phase line for most of the period of measurement.Tests for Fe(II) with the dye solution were negative, andwe concluded that the soil was not reduced at 100 cm.

According to the data of free Fe content in the Siaoli,Potu, and Luchu soils (Table 2), the average free Fecontent was more than 40 g/kg, so that lack of reducibleFe was probably not the reason the dye tests were negativefor Fe(II). This discrepancy between redox potential andthe dye test may be attributed to the following tworeasons: (i) spatial variability of Eh values and organicmatter distribution, and (ii) the pH values of Btv2 andBtv3 horizons were nearly 6.5, so that the redox potentialfor Fe reduction should be less than about 100 mV.

The Potu SoilThe water table fluctuation in the Potu soil was influ-

enced by cropping and flooding of the rice-growing soilfrom March to August, and by rainfall from Septemberthrough the following February (Fig. 4). When the watertable sharply dropped below 50 cm in February, the soilbecame unsaturated at the 50-cm depth, but the 100-cmdepth almost reached a saturated state. After rice wasplanted in March, the water table rose to the soil surface,and stayed at least 10 cm above the soil surface untilMay. The soil was saturated at both the 50- and 100-cmdepths. After rice was harvested in July, the water tablefell to below the 100-cm depth, and the soil at the 50-and 100-cm depths became unsaturated. During winterfallow on the Potu soil, there was no longer irrigationto produce the low seasonal water table during this dryseason in northwestern Taiwan (Fig. 2). In summary,the Potu soil was saturated at the 50-cm depth for 25%of the year and at the 100-cm depth for 50% of the year(Table 3).

During January and February, Eh values of the soilat both the 50- and 100-cm depths indicated the soil wasoxidized (>225 mV). After flooding in March, the Ehdropped immediately. The soil at the 50-cm depth re-

HSEU AND CHEN: REDOX MORPHOLOGY OF SEASONALLY FLOODED ALFISOLS

Siaoli site

945

ao

"S<u

.QCO

V_JtC

25 -

0 A

-25

-50

-75

-100

A A AA

A

N Positive reaction to a•, a ' dipyridyl dye.

I I I I I I

C5^JC

oc.

>£

'B'*^coa.oT!

D : 50 cm• : 100 cm

J F M A M J .7 A S 0 N PFig. 3. (a) Water table, (b) matric potential at 50 and 100 cm, and (c) redox potential (mV) at 50 and 100 cm for the Siaoli site.

mained reduced as shown by both the Eh values and thedye test, even though the water table dropped sharplyin June. The soil at the 100-cm depth in June showedsimilar trends compared with those soils at 50-cm depth,but had a longer duration of oxidizing conditions. Thesoil was reduced from March to August, because afterflooding, chemical and organic fertilizers facilitated soilreduction in the cropping season. The soil remainedreduced at the 50-cm depth in spite of unsaturated condi-tions that arose after irrigation was stopped. We speculatethat this may be due to very low diffusion rate of 62 inthe compact layer with high bulk density and low porosity(Megonigal et al., 1993). The soil had a redox potentialbelow 225 mV at the 50-cm depth for 35% of the year,and at the 100-cm depth for only 20% of the year (Table3). The low amount of decomposible organic matter and

rapid lateral flow of water resulted in less reduction atthe 100-cm depth of the Potu soil.

The Luchu SoilThe Luchu soil was flooded from February to May,

and from August to September (Fig. 5). Matric potentialsat the 50-cm depth were higher than those of the 100-cmdepth during most of the year (Fig. 5). The soil at the50-cm depth was saturated for 36% of the year, but thesoil at the 100-cm depth was never saturated (Table 3).When the Luchu soil was irrigated, a perched watertable developed between the 50- and 100-cm depths.

The Eh of soil at the 50-cm depth remained near 225mV from March to May, and from August to Octoberfor a total of 36% of the year (Table 3, Fig. 5). In

946 SOIL SCI. SOC. AM. J., VOL. 60, MAY-JUNE 1996

Potu site

so

0)

2~jcOB.

aS

Oa.

o•o<a

D : 50 cm• : 100 cm

O : 50 cm• : 100 cm

Fig. 4. (a) Water table, (b) matric potential at 50 and 100 cm, and (c) redox potential at 50 and 100 cm for the Puto site (asterisks on the watertable plot indicate positive reaction for a,o'-dipyridyl Fe(II) at 50 cm).

winter, the Eh rose to >400 mV at the 50-cm depth.The Eh values at the 100-cm depth indicated the soilwas oxidized throughout the year except for a few days.No Fe(II) was detected by the a,a'-dipyridyl solution ateither the 50- or 100-cm depth during 1993.

Redoximorphic FeaturesRedoximorphic features are formed by the processes ofreduction, translocation, and oxidation of Fe and Mnoxides. Vepraskas and Guertal (1992) reported that, assoils were saturated and reduced for longer periods, theamount of Fe depletions increased while the amount ofFe concentrations decreased. We found the Ap horizonof the Siaoli soil had a reduced matrix (2.5Y 3/0), andthe BA horizon had Fe depletions with Fe masses (Table1). The Fe depletions occurred along ped surfaces and

the Fe masses were found in the matrix of the Siaolisoil. In the other two soils, Fe concentrations occurredaround root channels. All soils contained Fe nodules inthe Btv horizons. These features resulted from alternatecycling of oxidation and reduction in the Btv horizons(Vepraskas and Wilding, 1983; Stolt et al., 1994). Ironmasses (chroma 6) lie parallel to some Fe depletions,and nodules (5-15-mm diam.) are common (2-20% ofexposed surface). Nowadays, these three soils have sea-sonal saturation and reduction conditions, however, theredoximorphic features that occurred in these three ped-ons are not relict because the boundaries between nodulesand matrix are diffuse, not distinct (Bouma, 1991).

The Ap horizon of the Potu soil had no redoximorphicfeatures (Table 1). Coatings of organic matter (chromaof 1) occurred in the 2A horizon of the Potu soil. Riceroots had spread horizontally along the boundary between

HSEU AND CHEN: REDOX MORPHOLOGY OF SEASONALLY FLOODED ALFISOLS 947

Luchu site

so

<u

CO

i,

a

fia.

*3c

oc.

eeS

c0)

oa.xoT3

25 -

D : 50 cm: 100 cm

Fig. 5. (a) Water table, (b) matric potential at 50 and 100 cm, and (c) redox potential at 50 and 100 cm for the Luchu site (there was no positivereaction for o,a' dipyridyl Fe(II) at 50 cm).

Bw and 2A horizons, but few roots penetrated the 2Ahorizon, which was highly compact. In the Btv horizonof the Potu soil, clay depletions form tongues that extendinto the horizon along ped surfaces. These are accompa-nied by Fe concentrations and Mn concentrations (Mnpore linings and coarse nodules [>15-mmdiam.]), whichare in the matrix (Vepraskas, 1992; Hseu and Chen,1994). The tongues of clay depletions of the argillichorizon in the Potu soil show the horizon is degrading(Ransom et al., 1987; Hseu and Chen, 1994).

The Luchu soil was the least influenced by surfaceirrigation and shallow groundwater and contains the leastredoximorphic features of the three soils. No Fe deple-tions (chroma <2) were found in either the Ap or BAhorizons. However, common distinct fine Fe and Mnnodules (<5-mm diam.), Fe masses, and Fe depletionswere found in the lower Btv horizons.

Color values of Fe and clay depletions in the eluvial

horizon of the Siaoli soil were higher than those of thePotu or the Luchu soils. The matrix chroma of the Siaolisoil, however, was less than those of the Potu or theLuchu soils. Overall, the Siaoli soil contained moreredoximorphic features in the upper parts of the pedon

Table 3. Duration of saturation and reduction conditions ex-pressed as percentages of the total number of observations,based on the monitoring period during 1993.

Duration of saturation or reductionSoil depth Saturation-reduction Siaoli soil Potu soil Luchu soil

SO

100

SaturationtReduction^SaturationtReduction^:

44602417

%25355020

363600

t Saturation conditions are matric potential at zero or positive.t Reduction conditions are Eh less than 225 mV.

948 SOIL SCI. SOC. AM. 3., VOL. 60, MAY-JUNE 1996

and various redox depletions and concentrations in thelower parts of the pedon. The largest nodules werefound in the moderately well-drained Potu soil. Theseobservations coincide with the results reported bySchwertmann and Fanning (1976).

Aquic ConditionsAquic conditions are identified by determining three

separate properties: (i) depth of saturation, (ii) occur-rence of reduction, and (iii) presence of redoximorphicfeatures (Soil Survey Staff, 1992). All three soils studiedhere had anthric saturation because saturation was in-duced for rice production in paddies that were perma-nently established for that purpose. All soils containedredoximorphic features. The occurrence of reductionwas confirmed by both Eh measurements and dye testsfor Fe(II) in the Siaoli and Potu soils. Occurrence ofFe(II), as determined by dye solution, matched well withthe soil saturation, Eh, and redoximorphic features inthese two soils. On the other hand, the Luchu soil wasreduced for short periods as shown by the Eh measure-ments, but no positive reaction to the a,a'-dipyridyl dyewas found. We feel this soil had aquic conditions, butthe short period of reduction suggests it is close to havingoxyaquic conditions. The redoximorphic features suggestthat Fe reduction does occur, but for short periods.

SUMMARYGrowing rice in soils on alluvial plains is very popular

in Taiwan. Surface water irrigation and shallow ground-water influence the soil morphology of these soils simulta-neously. The three soils studied had anthric saturation.Results indicated that the effect of irrigation water onpedogenesis with respect to saturation, reduction, andredoximorphic features was greater than that of the shal-low groundwater in the Siaoli and the Luchu soils. Onthe contrary, shallow groundwater was the main contribu-tor to saturation below 50 cm in the Potu soil.

The Btv horizon of the Potu soil (at about the 100-cmdepth) was saturated for 50% of the year, but littleorganic matter could be leached to that depth and fewroots penetrated below 50 cm. Therefore, the Btv horizonof the Potu soil was reduced for only 20% of the year.The 2A horizon of the Potu soil with very low porositywas reduced in spite of being saturated when irrigationwater was cut off, because of the slow diffusion of O2 intothe horizon. The results indicated that the redoximorphicfeatures here were not relict. Nodule size decreased asfollows: Potu > Siaoli > Luchu. The Btv horizons ofthe Potu soil were the most degraded of these three soils,with tonguing of redox depletions along ped surfacesowing to the frequent fluctuation of shallow groundwater.

This study demonstrates that moisture regimes in soilswith anthric saturation can be characterized in the sameway as in soils with either episaturation or endosatura-tion. The Siaoli and Potu soils met the requirements foraquic conditions in the soil taxonomy. We believe the

Luchu soil, with less reduction, is close to having oxy-aquic conditions.

ACKNOWLEDGMENTSThe authors thank Dr. Prof. M. J. Vepraskas, North Carolina

State University, Dr. Warren C. Lynn, soil scientist of USD A-NRCS Soil Survey Laboratory in Lincoln, NE, and Dr. Prof.Chris V. Evans, University of New Hampshire, for reviewingthis manuscript and making valuable comments. The authorsalso thank Mr. J.C. Liu, C.C. Tsai, and C.L. Chen for theirhelp on soil sampling and monitoring. The authors also thankthe Taoyuan District Agricultural Improvement Station, Tai-wan Provincial Government, for providing experimental plotsand field assistance.

NOTES 949

A.L. Page et al. (ed.) Methods of soil analysis. Part 2. 2nd ed.Agron. Monogr. 9. ASA and SSSA, Madison, WI.

Meek, B.D., and L.B. Grass. 1975. Redox potential in irrigateddesert soils as an indicator of aeration status. Soil Sci. Soc. Am.Proc. 39:870-875.

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