Soil Development and Fertility Characteristics of a Volcanic Slope in Mindanao,
the Philippines
D. D. Poudel* and L. T. West
ABSTRACT
1984; Parfitt and Saigusa, 1985; Shoji et al., 1988; van
Wambeke, 1992; West et al., 1997). Nieuwenhuyse and
Breemen (1997) analyzed soil samples from eight pedons representing Costa Rican volcanic ash–derived soils.
Of all the pedons studied, five pedons were classified
as Andisols, one pedon as Entisols, and two pedons as
Oxisols. They assume that the older soils were originally
Andisols because of the similarities of their parent materials, lower bulk densities (≈0.9 g cm23), and the presence of short-range order materials. Yerima et al. (1987)
studied eleven Vertisol and associated Mollisol pedons
developed on volcanic ash parent materials in El Salvador. Except for one pedon, all pedons showed a decrease
in bulk density with depth and amorphous components
comprised ≈20%, suggesting a volcanic influence on soil
properties. Also, they reported a higher percentage of
amorphous components in pedons closer to areas of
volcanic activity.
Phosphorus is often the most yield-limiting element
for common crops grown in volcanic ash soils (Egawa,
1977; Otsuka et al., 1988; Martini and Luzuriaga, 1989;
van Wambeke, 1992; Sannyal et al., 1993). Volcanic
ash–derived soils, particularly Andisols, are well-known
for their high capacity for P fixation, especially under
acid conditions (Kamprath, 1973; Egawa, 1977; van
Wambeke, 1992; Soil Survey Staff, 1996). Higher P fixation in these soils is attributed to their amorphous mineral components, especially allophane and imogolite,
which are poorly crystalline materials and have variable
charge (Parfitt and Wilson, 1985; Parfitt and Kimble,
1989; Wada, K., 1989). The amounts of P fixation are
generally found in the increasing order: montmorillonite
, kaolinite , goethite , gibbsite , amorphous hydrated oxides of Fe and Al, suggesting that the more
crystalline the material, the smaller the fixation capacity
(Kamprath, 1973; Nanzyo et al., 1993a).
The amount of K in volcanic ash–derived soils tends
to decrease rapidly with the advance of weathering due
to leaching in the humid tropics (van Wambeke, 1992;
1993; Mongia and Bandyopadhyay, 1993; Shoji et al.,
1993), and crop yields are often limited by a low K
supply (Martini and Luzuriaga, 1989). Potassium availability also appears to be diminished by allophane
through K fixation (van Reeuwijk and Devilliers, 1968).
Volcanic ash–derived soils display variable CEC due
to their typical variable charge characteristics (i.e., pH
Thirteen pedons representing the mountains, the upper footslopes,
the lower footslopes, and the alluvial terraces of a volcanic slope in
Mindanao, the Philippines were studied to understand relationships
between the degree of soil development and fertility characteristics.
Soils in the upper and the lower footslopes were Oxisols as were soils
in the alluvial terraces, while those in the mountains were Ultisols and
Inceptisols. Presence of “amorphous components”, such as allophane
and imogolite, in all the pedons studied was indicated by a .9.4 soil
pH in NaF. Halloysite, gibbsite, goethite, hematite, and cristoballite
were more common minerals in the clay fraction. Surface layers of all
the pedons were slightly acidic and pH increased by depth. Phosphate
sorption maxima ranged from 6944 to 14208 mg P g21, and it was
closely associated with oxalate-extractable Al (Alo) and clay content.
Inceptisols had higher phosphate sorption maxima than Oxisols. Soil
samples representing the mountains showed the lowest level of both
the available K and the potential buffering capacity for K (PBCK),
while the upper footslopes had the highest level of available K. The
PBCK values were lower for Inceptisols than for Oxisols, and they
were found to be positively correlated with soil pH. There was a
large difference between the cation-exchange capacity (CEC) and the
effective cation-exchange capacity (ECEC), an indication of a large
pH-dependent charge. Mountain soils showed lower base saturation
than soils representing the upper footslopes, the lower footslopes,
and the alluvial terraces.
V
olcanic ash–derived soils are among the most
productive soils in the world (Miller and Donahue,
1992; Shoji et al., 1993). These soils support dense populations in areas such as central Java (Shoji et al., 1993).
Volcanic ash–derived soils are mostly associated with
mountain slopes (Wright, 1963; Dudal, 1964; Tan, 1965;
Buol, 1973; Frei, 1978; Buringh, 1979; Radcliffe and
Gillman, 1985), and they are often used for intensified
agricultural production systems such as highland commercial vegetable production in Taiwan, the Philippines,
Japan, Indonesia, and the Costa Rican Andes (Tan,
1984; Martini and Luzuriaga, 1989; Shoji et al., 1993;
Midmore and Poudel, 1996). These intensified agricultural systems are characterized by heavy use of agrochemicals and fertilizers, and often associated large soil
erosion and down-stream impacts (Shoji et al., 1993;
Midmore et al., 1996).
Not all volcanic ash–derived soils are Andisols. At a
point in their weathering, Andisols lose their unique
properties, and they change into other soil orders such
as Inceptisols, Alfisols, Entisols, Oxisols, Spodosols,
Vertisols, Mollisols, or Ultisols (Leamy et al., 1980; Tan,
Abbreviations: ARKe, degree of K availability; BS, base saturation;
CEC, cation-exchange capacity; d [subscripted], citrate-dithionite extractable; ECEC, effect cation-exchange capacity; masl, meters above
sea level; o [subscripted], oxalate extractable; PaM, P adsorption maxima; PBCK, buffering capacity for K; Q/I, quantity/intensity relationship; SANREM-CRSP, Sustainable Agriculture and Natural Resources Management Collaborative Research Support Program;
2DK8, measures of labile K.
D.D. Poudel, Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616; and L.T. West, Dep. of Crop and Soil Sciences,
Miller Plant Sciences Building Room 3111, Univ. of Georgia, Athens,
GA 30602. Received 24 July 1998. *Corresponding author (ddpoudel@
ucdavis.edu).
Published in Soil Sci. Soc. Am. J. 63:1258–1273 (1999).
1258
POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
dependent charge), which result from their particular
clay mineral and humus content (Radcliffe and Gillman,
1985; Nanzyo et al., 1993a). Surface charges on clay
particles are developed either through isomorphic substitution in 2:1 clay minerals or through the desorption
and adsorption of protons in 1:1 clay minerals and
“amorphous materials”, including allophane, imogolite,
and oxyhydroxides of Fe and Al (Sposito, 1984; Radcliffe and Gillman, 1985; McBride, 1994). The charge
associated with the protonation and deprotonation is
dependent on ion concentration and pH of the soil solution, while the charge developed through the isomorphic
substitution is unaffected by the concentration and pH
of electrolytes. Depending on parent material characteristics and degree of weathering, volcanic ash–derived
soils generally contain a mixture of variable and constant-charge colloids (Nanzyo et al., 1993a). As a result,
volcanic ash–derived soils show variable CEC and a
relatively smaller anion exchange capacity (Radcliffe
and Gillman, 1985; Nanzyo et al., 1993a). This variable
cation-exchange property of volcanic ash–derived soils
challenges many management activities including liming
and fertilization (Uehara and Gillman, 1981; Radcliffe
and Gillman, 1985; Tisdale et al., 1993).
An understanding of the relationships between the
degree of soil development and P fixation, K availability,
and exchange properties of volcanic ash–derived soils
will help in the design of appropriate soil management
strategies at the farm and policy level. These strategies,
if implemented, will help to maintain or enhance agricultural sustainability and environmental quality of the
area. Knowledge gained in one landscape can be transferred to other regions with similar soils and where basic
information on fertility characteristics is lacking.
This study was undertaken as a part of a larger project
of the Sustainable Agriculture and Natural Resources
Management Collaborative Research Support Program
(SANREM-CRSP) in the Manupali watershed, located
in the Bukidnon province of north-central Mindanao,
the Philippines. The objective was to identify relationships between soil development and fertility characteristics such as P fixation, K availability, CEC, and base
saturation (BS).
MATERIALS AND METHODS
Description of the Study Area
The Manupali watershed lies between 1248 479 to 1258 089
E and 78 579 to 88 089 N on the island of Mindanao in the
Philippines. This watershed has an area of ≈ 50 496 ha (Kanemasu et al., 1997), and lies on the slope of Mt. Kitanglad, an
extinct volcano. Parent materials for soils are thick deposits
of volcanic ejecta either deposited in place (volcanic cone) or
as colluvial and alluvial deposits (volcanic piedmont). Elevations in the watershed range from 320 to 2938 m above sea
level (masl), and mean annual precipitation (1958–1978) measured at the Pulangi River Irrigation Project in the nearby
town of Valencia is 2347 mm. Rainfall is not equally distributed
throughout the year, but there is no month with ,72 mm of
mean monthly rainfall. Air temperatures range from 20 to
348C in areas below 500 m, and 18 to 288C in areas above
500 m. Major crops grown are corn (Zea mays L.), sugarcane
1259
(Saccharum officinarum L.), and rice (Oryza sativa L.) at
the lower elevations, while tomato (Lycopersicon esculentum
Miller), potato (Solanum tuberosum L.), and leafy vegetables
are dominant field crops in the upper elevations. This watershed has four broad geomorphic units: the mountains (1400–
1900 masl), the upper footslopes (700–1400 masl), the lower
footslopes (370–700 masl), and the alluvial terraces (320–370
masl) (West et al., 1997). The mountains and the upper
footslopes are characterized by steep slopes and numerous
creeks, the lower footslopes are undulating to rolling, and the
alluvial terraces are nearly level.
The soil development gradient increases from the mountains to the lower footslopes, with the mountains having the
youngest, least developed soils. According to the USDA classification system (Soil Survey Staff, 1996), all soils sampled from
the upper and lower footslopes were dominantly Oxisols, as
were those sampled from alluvial terraces. Soils in the mountains classified as Inceptisols and Ultisols, because of a recent
ash cap deposited at the surface. However, subsoil properties
of soils in the mountains were similar to those of Oxisols
in other geomorphic units; however, Oxisols on the alluvial
terraces may reflect development in materials that were weathered in the uplands and then eroded, transported, and redeposited in the terraces rather than old landscapes.
Field Methods
Thirteen pedons (Table 1) representing the four geomorphic units (mountains, the upper footslopes, the lower
footslopes, and the alluvial terraces) were described and sampled by horizon from the faces of pits. The lower footslopes
unit was subdivided on the basis of topography into undulating
to rolling, gently sloping to undulating, and level to gently
sloping. At least one pedon was sampled from each of the
physiographic regions, including subdivisions of the lower
footslopes, to ensure that all soil conditions in the region were
represented. All pits had a 2 by 1.5 by 2 m (length by breadth by
depth) dimension, and 1-kg samples representing each horizon
were taken on the same day and were kept in plastic bags.
Soils were described by standard terminology (Soil Survey
Staff, 1994). Selected site characteristics for each pedon are
presented in Table 1.
Laboratory Methods
Samples from each horizon of all the pedons studied were
air dried and crushed to pass a 2-mm sieve. Particle-size distribution was determined by pipette and sieving (Kilmer and
Alexander, 1949). Soil pH was measured in 1:1 H2O, 1:1 KCl
(Soil Survey Staff, 1996), and in 1:50 M NaF solution (Fieldes
and Perrott, 1966). Total C was determined by dry combustion
(Tabatabai and Bremner, 1991). Exchangeable K, Ca, Mg,
Mn, and Na were extracted with 1 M NH4OAc at pH 7, and the
cations in the leachate were measured by atomic adsorption
spectrophotometry. The CEC was determined by saturation
with NH4OAc at pH 7 and pH 8.2 and subsequent replacement
of NH41 by KCl extraction. Extractable Al was measured in
1 M KCl extracts. Effective cation exchange capacity was
calculated as the sum of extractable cations plus KCl extractable Al. Ammonium oxalate–soluble Fe (Feo), Al (Alo), and
Si (Sio) were determined by leaching 1-g samples with 100 mL
of 0.2 M ammonium oxalate (pH 3) for a 4-h period in the
dark (Soil Survey Staff, 1996). Bulk density and moisture
content were determined by equilibrating soil samples at 0.033
MPa moisture tension (Soil Survey Staff, 1996), and water
content at 1.5 MPa was determined on ground air-dried samples (Soil Survey Staff, 1996). Pretreatment and fractionation
1260
SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
Table 1. Site characteristics for the pedon locations.
Pedon
Location
Latitude and longitude
Elevation
Geomorphic position
m asl†
Slope
Land use
Great group
%
Mountains
1
2
3
Bol-ogan
Tinusuhan
Kaatuan
884910.2″N 124855936.3″E
88492.8″N 12485199.5″E
883937.9″N 12580918.3″E
1500
1300
1270
back slope
summit
back slope
Upper Footslopes
53
8
15
abandoned cropland
forest land not grazed
forest land not grazed
Humitropept
Hapludult
Humitropept
4
5
6
7
Bol-ogan
Kibangay
Patag
Insibay
883941.1″N
882947.3″N
881942.3″N
882914.2″N
124855941.5″E
124853936.3″E
124857914.3″E
12581936.1″E
1410
1150
925
900
shoulder
shoulder
back slope
summit
Lower Footslopes
15
10
2
5
abandoned cropland
cropland
cropland
cropland
Kandiudox
Kandiudox
Kandiudox
Kandiudox
8
9
10
11
Subsob
Poblacion
Bantuano
Patpat
883934.9″N
880934.7″N
880934.3″N
882923.3″N
12584927.4″E
1258194.8″E
12584913.2″E
12586916.4″E
690
630
510
500
back slope
summit
back slope
back slope
Alluvial Terraces
12
1
8
5
cropland
cropland
other
forest land not grazed
Acrudox
Kandiudox
Hapludox
Hapludox
12
13
Katsing
Valbueco
880925″N 12585954.9″E
880950.2″N 12585958.8″E
370
340
summit
back slope
3
1
cropland
cropland
Acrudox
Kandiudox
† asl is above sea level.
of clays for x-ray analyses was done according to Jackson
(1979) and Whittig and Allardice (1986). Carbonate was removed by treating samples with 1 M NaOAc and organic
matter was removed by using 30% (w/v) H2O2. The samples
were extracted with 0.3 M Na-citrate/0.1 M Na-bicarbonate
solution and Na-dithionite following Mehra and Jackson
(1960). The citrate-dithionite-extractable Fe (Fed), Al (Ald),
and Si (Sid) were analyzed by atomic adsorption spectrophotometry. Once Fe removal was considered complete, the samples were centrifuged and the supernatant was kept for Fe
determination by atomic absorption. Fifty milliliters of deionized water and 2 mL of 1 M sodium carbonate were added
to the samples and were shaken overnight for clay dispersion.
Clay was separated by centrifuging tubes and decanting the
supernatant for several times. The filter-membrane technique
(Drever, 1973) was used to prepare oriented clays for x-ray
diffraction analyses. Saturation treatments were: Mg-saturated, air dry; Mg-saturated, glycol solvated; K-saturated, air
dry; and K-saturated, heated to 100, 300, and 5508C. These
slides were scanned from 3 to 328 2 u on an diffractometer
equipped with a theta-compensating slit and curved-crystal
monochromatic. The presence of halloysite was interpreted
by the presence of a 0.445-nm peak with one-half the intensity
of the 0.72-nm peak (Dixon, 1989).
Specific surface area measurement was done for the upper
two horizons of all the pedons studied as was phosphorus
adsorption isotherms determination. Specific surface area was
determined by N2 gas adsorption (Brunauer et al., 1938) on
a MICROMETRICS FLOWSORB II instrument (Micrometrics Instrument Corp., Norcross, GA). Phosphorus adsorption
isotherms were determined according to Fox and Kamprath
(1970). Three grams of soil samples were equilibrated in 30
mL of 0.01 M CaCl2 containing 10, 20, 35, 50, 100, and 1000
mg L21 P. After 10 d of equilibration, samples were centrifuged
at 14 500 g and phosphate was determined in supernatant
by the molybdate blue method (Murphy and Riley, 1962).
Phosphate that disappeared from the solution was considered
to have been sorbed by soils. The Uniform-Surface Langmuir
Equation (Barrow, 1978) was used in the determination of
relative P sorption capacities of soil samples (López-Piñeiro
and Garcı́a Navarro, 1997) in this study.
The technique of immediate quantity/intensity (Q/I) relationships for K proposed by Beckett (1964a, 1964b) was used
to determine K availability in samples from the surface layers
of all the pedons studied. In this study, 0.2-, 0.5-, 1.5-, 4.5-,
and 7-g samples were equilibrated with 50 mL of 0.002 M
solution with respect to CaCl2. An additional five samples
(7 g) were equilibrated with 50 mL of 0.002 M solution with
respect to CaCl2 and varying concentration of KCl (0.0002,
0.0005, 0.0008, 0.001, and 0.002 M ). All samples were equilibrated in a reciprocating shaker for 20 h at constant temperature. For extractable K determination, 5, 5, and 10 g of samples
were equilibrated with 50 mL of 1.0 M NH4OAc at pH 7, 20
mL of 0.05 M HCl and 0.01 M H2SO4, and 20 mL of distilled
H2O in a reciprocating shaker for 30, 5, and 30 min respectively. After centrifugation and filtration, the samples were
analyzed for their equilibrium concentrations of Ca21, Mg21,
K1, and Na1 by inductively coupled plasma-atomic emission spectrometry.
Correlation and linear regression analyses were done in
SAS (SAS Institute, 1994) to identify relationships between
fertility parameters such as Q/I measures, P adsorption maxima (PaM), and selected morphological features and chemical properties.
RESULTS AND DISCUSSION
Morphological Properties
Although the soil development gradient ranged from
Inceptisols in the mountains to Ultisols and Oxisols in
the lower elevations, their morphological characteristics
were surprisingly uniform, reflecting their similar degree
of weathering and the influence of ash parent materials.
Solum thickness ranged from 89 to .200 cm (Table 2).
The surface horizon of all pedons showed brown to dark
brown color, well-expressed fine to medium granular
structure and very friable to friable moist consistency.
The subsurface horizons showed brown to reddish
brown color, weak to moderate subangular blocky structure, and friable to very friable with slightly sticky and
slightly plastic moist consistency. Soils in all the geomorphic units were clayey (Table 2), when estimated by
1500 KPa water retention (Soil Survey Staff, 1996).
Mineralogical Properties
No distinct differences in clay mineral distribution
with depth were observed, and pedons from all geomorphic surfaces had similar mineral components. Halloysite and kaolinite comprised more than half of the clay
1261
POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
Table 2. Selected physical properties and NaF pH of all pedons.
Horizon
Depth
Sand
Silt
cm
Clay
Estimated
clay†
pH
(NaF)
%
Bulk density
33 KPa
Surface
area
g cm23
m2 g21
Water retention
33 KPa
1500 KPa
%
Mountains
Pedon 1
Ap
AB
Bw1
Bw2
Bw3
BC1
BC2
Pedon 2
A
BA
Bt1
Bt2
Bt3
BC1
BC2
BC3
BC4
C
Pedon 3
A1
A2
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon
Ap
B/A
Bw1
Bw2
Bw3
C1
C2
C3
Pedon
Ap
BA
Bt1
Bt2
Bt3
BC
Pedon
Ap1
Ap2
Bt1
Bt2
Bt3
BC
Pedon
Ap
BA
Bt1
Bt2
Bt3
Bt4
Bt5
0–11
11–25
25–46
46–75
75–132
132–178
178–200
53.9
71.2
68
60.5
52.7
30.5
26.5
36.6
26.2
24.2
27.5
39
40.2
45.1
9.5
2.6
7.8
12
8.3
29.3
28.4
66
68
64
56
46
60
63
11.06
11.49
11.24
11.12
11.08
10.44
10.18
0.64
0.64
1.01
1.04
1.07
1.09
1.14
50.97
52.41
0–14
14–24
24–53
53–71
71–98
98–110
110–123
123–134
134–149
149–200
28.8
35.3
2.7
3.5
4.1
11.7
10.2
18.5
16.3
16.2
50.9
25.4
32.2
42.3
39.1
46.9
51.4
47.3
49.9
45.4
20.3
39.3
65.1
54.2
56.8
41.4
38.4
34.2
33.8
38.4
89
87
90
93
10.68
11.26
10.62
10.81
10.38
10.49
10.62
10.51
10.52
10.55
,0.9
,0.9
0.62
1.09
1.02
1.1
1.06
1.03
0.92
0.89
40.19
73.25
0–12
12–28
28–67
67–110
110–143
143–170
170–190
52.5
65.9
72.9
57.3
48.1
40.5
37.3
34.6
29
14.3
8.7
11.4
10.6
11.2
12.9
5.1
12.8
34
40.5
48.9
51.5
11.43
11.58
10.96
10.68
10.60
10.59
10.78
Upper footslopes
,0.9
,0.9
1.05
0.94
0.91
0.91
0.9
41.03
60.97
0–9
9–22
22–41
41–66
66–89
89–116
116–123
123–190
12
10.7
7.7
5.1
5.6
22.7
42.3
40.3
30.2
27.2
22
21.2
21.7
27.8
28.5
31.9
57.8
62.1
70.3
73.7
72.7
49.5
29.2
27.8
72
73
73
84
91
63
59
60
10.14
10.47
10.50
10.49
10.62
10.49
11.00
10.98
1.1
0.96
1.19
1.14
1.14
1.08
0.97
0.94
0–13
13–37
37–79
79–130
130–171
171–200
16.1
14.2
33.8
26.7
15.3
46.4
27
18.8
18.7
23.9
25.3
18.3
56.9
67
47.5
49.4
59.4
35.3
63
66
61
72
87
79
10.13
10.34
10.51
10.67
10.44
10.85
0–15
15–26
26–42
42–71
71–94
94–122
12.7
11.9
10.2
10.7
13.9
35.1
33.8
29.6
26.6
22.5
26.1
27.9
53.5
58.5
63.2
66.8
60
37
63
67
68
75
70
61
0–16
16–43
43–77
77–106
106–131
131–160
160–200
8.9
2.1
17.1
29.6
48.9
59
57.9
24.2
10.8
15.3
12.1
11.7
10.5
7.9
66.9
87.1
67.6
58.3
39.4
30.5
34.2
70
78
78
78
82
81
92
92
92
86
80.4
40.8
42.1
43.4
52.4
46.3
26.4
27.3
25.4
22.2
18.5
24.1
25
84.6
40.9
51.3
41
43.3
47.2
64.5
70.1
35.7
34.8
36.1
37
40
36.8
40
36.9
34.5
40.9
38.4
57.6
63.9
60.3
61.6
32.4
26.1
22.7
37.4
43
45.8
44.7
48.87
50.79
35.9
43.4
32.6
39.5
38.5
39.5
45.3
58.2
28.9
29.3
29.3
33.7
36.2
25.1
23.5
24
1
0.96
1.06
1.1
1.16
1.02
52.55
65.17
38.8
46.1
38.2
42.8
38.9
52.6
25.2
26.5
24.5
28.7
34.7
31.5
10.03
10.02
10.21
10.43
10.45
10.58
1.1
1.03
1.03
1.06
1.06
0.98
52.02
56.28
33.5
38.9
42.2
46.2
42.6
41.1
25
26.6
27.1
29.8
28
24.3
10.00
10.33
10.71
10.46
10.67
10.67
10.64
0.93
0.86
0.84
0.86
0.92
1.0
1.04
71.76
82.31
39.2
55.7
58.4
57.9
56.1
46.8
43.1
27.8
31.1
31
31.3
32.6
32.4
36.7
81
65
57
94
4
5
6
7
Continued next page.
fraction of all horizons in all pedons. As indicated by a
low angle shoulder on the 0.7-nm peak and intense
0.445-nm peaks (Dixon, 1989), halloysite appears to be
the dominant kaolin mineral in most horizons. Only in
near-surface horizons of one pedon was kaolinite more
abundant than halloysite. Gibbsite, goethite, hematite,
and cristoballite were also identified in the clay fractions. In most horizons, amounts of these minerals were
low except for an abundance of gibbsite in upper horizons of pedons from the mountains. Soil pH in NaF
was .9.4 for all horizons (Table 2), suggesting that all
horizons had at least minor amounts of amorphous or
poorly crystalline components (Fieldes and Perrott,
1966). X-ray diffraction patterns suggested that the portion of halloysite relative to kaolinite was greater in
deep BC, CB, and C horizons than in shallower horizons
of pedons from the mountains. This trend suggests that
halloysite is converting to kaolinite as horizons become
more weathered. Lower horizons from mountain pedons also had higher Alo and Sio than overlying horizons
1262
SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
Table 2. Continued.
Horizon
Depth
Sand
Silt
cm
Clay
Estimated
clay†
pH
(NaF)
%
Bulk density
33 KPa
Surface
area
g cm23
m2 g21
Water retention
33 KPa
1500 KPa
%
Lower footslopes
Pedon
Ap
AB
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon
Ap
BA
Bw1
Bw2
BC
CB1
CB2
Pedon
Ap
BA
Bt1
Bt2
Bt3
BC1
BC2
Pedon
Ap
BA
Bt1
Bt2
Bt3
Bt4
BC
8
0–11
12–24
24–52
52–104
104–140
140–178
178–195
10
3.4
2.7
2.8
2.5
3.2
5.5
23.4
21.7
19.9
17.5
17.8
17
17.7
66.6
74.9
77.4
79.7
79.7
79.8
76.8
73
80
79
78
79
78
78
10.19
10.24
10.54
10.53
10.47
10.52
10.67
1.03
0.92
0.88
0.97
0.96
0.94
1.04
81.25
84.87
46.2
50.1
51.1
46.3
46.6
47.5
42.3
29.2
31.9
31.6
31.1
31.4
31
31.1
0–15
15–37
37–64
64–94
94–130
130–169
169–200
10.3
10.3
4.5
1.7
1.4
1.6
1.5
26.9
21.4
16.1
10.6
8.4
5.9
6.6
62.8
68.3
79.4
87.7
90.2
92.5
91.9
70
88
85
87
86
81
77
10.39
10.68
10.71
10.44
10.47
10.60
10.55
0.95
0.84
0.69
0.84
0.86
0.87
0.97
78.35
89.15
38.3
48.4
60.7
50.8
51.9
59.5
47.3
27.9
35.1
34
34.6
34.4
32.5
30.7
0–12
12–30
30–67
67–106
106–142
142–178
178–210
3.4
3.3
3.5
8.3
6
5.7
15.9
21.3
18.5
17.8
17.7
24
29.1
34.1
75.3
78.2
78.7
74
70
65.2
50
70
77
81
81
82
80
70
10.79
10.45
10.49
10.49
10.52
10.50
10.47
0.96
0.92
1.11
1.05
1.04
1.05
1.08
65.18
75.55
53.8
43.3
44
45.2
46.6
46.9
42.4
28.1
30.9
32.3
32.3
32.8
31.9
27.8
0–14
14–27
27–60
60–104
104–156
156–200
2.7
2.1
2.6
2.5
2.5
4.5
10.6
19.7
15.6
13.8
13.3
14.1
22.2
37.7
77.6
82.3
83.6
84.2
83.4
73.3
51.7
79
10.26
79
10.43
81
10.68
83
10.67
89
10.61
81
10.63
72
10.50
Alluvial terraces
0.99
1.07
1.1
1.01
0.98
1.03
–
61.52
73.21
42.4
42.5
45.6
45.9
50
47.2
–
31.5
31.7
32.5
33.3
35.4
32.4
28.8
0–13
13–32
32–66
66–112
112–160
5.8
2.3
1.9
1.8
1.3
25.6
13.7
12.5
9.4
9.7
68.6
84
85.6
88.8
89
75
84
84
86
87
10.59
10.77
10.54
10.55
10.53
1.11
0.99
1.09
1.12
1.02
68.43
81.79
30.6
48.4
40.1
34.9
41.6
30
33.4
33.6
34.3
34.6
0–11
11–22
22–47
47–69
69–113
113–140
9.3
11.2
7.6
6.8
9.6
17.6
31.7
30.5
25.5
23.9
25.3
30
59
58.3
66.9
69.3
65.1
52.4
68
67
72
74
76
78
10.04
10.43
10.61
10.30
10.53
10.69
0.9
1.06
1.16
1.2
1.18
1.06
51.49
56.25
58.8
43.5
41.1
40.8
41.1
43.5
27.2
26.7
28.6
29.7
30.3
31
9
10
11
Pedon 12
Ap
BA
Bt1
Bt2
Bt3
Pedon 13
Ap1
Ap2
Bt1
Bt2
Bt3
Bt4
† Estimated clay (%) 5 [(% water retained at 15 mPa 2 % organic C) 3 2.5], but not more than 100 (Soil Survey Staff, 1996).
(Table 3), a finding which supports the characterization
of these horizons as less weathered. Similar depth trends
were not observed in pedons from other geomorphic
units suggesting that transformation of amorphous and
disordered components had progressed beyond the
depth of sampling. Horizons from pedons in the mountains also had slightly higher pH values in NaF than
pedons from other geomorphic units (Table 2), suggesting a relatively greater abundance of amorphous
and poorly crystalline materials and implying a lower
degree of weathering in the mountain soils.
Physical Properties
Except for the mountains, all soil samples representing different geomorphic units showed more or less similar physical characteristics (Table 2). The upper two
horizons of all the mountain pedons showed a bulk
density of ,0.9 g cm23, while bulk density values for
the surface layers of the remaining pedons ranged from
0.9 to 1.11 g cm23, suggesting that the soils in the mountains are formed from more recent ash caps. Smaller
particle densities of allophane and imogolites are considered the factors responsible for this lower bulk density (Wada, 1989). The upper two horizons of the mountain pedons had low clay content as measured by pipette,
and ratios of clay to 1500 KPa water content suggests
that these horizons did not disperse with standard treatment. According to Nanzyo et al. (1993b), major factors
contributing to poor dispersion in volcanic ash–derived
soils include: noncrystalline materials acting as cementing agents, reaction of noncrystalline materials
with excess amounts of sodium hexametaphosphate, and
the differences in the point of zero charge of the inorganic colloids. Surface areas of the upper horizons
ranged from 40.2 to 81.3 m2 g21 with generally lower
values for the mountain pedons. However, surface area
measurements by N2 gas adsorption in volcanic ash–
POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
1263
Table 3. Selected chemical properties of the pedons studied.
pH
Horizon
H2O
KCl
Total
C
Fed†
Ald
Feo‡
Sio
Alo
%
Alo/Ald
Feo/Fed
Sio/Alo
PO4
retention§
%
Mountains
Pedon 1
Ap
AB
Bw1
Bw2
Bw3
BC1
BC2
Pedon 2
A
BA
Bt1
Bt2
Bt3
BC1
BC2
BC3
BC4
C
Pedon 3
A1
A2
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon 4
Ap
B/A
Bw1
Bw2
Bw3
C1
C2
C3
ROCK
Pedon 5
Ap
BA
Bt1
Bt2
Bt3
BC
Pedon 6
Ap1
Ap2
Bt1
Bt2
Bt3
BC
Pedon 7
Ap
BA
Bt1
Bt2
Bt3
Bt4
Bt5
4.4
4.5
5.1
5.5
6
5.9
6.1
4.3
4.5
4.7
4.7
4.8
4
3.9
8.19
8.42
2.85
0.87
0.37
0.14
0.09
4.8
4.9
5.2
4.5
4.6
4.2
4.2
2.3
2.5
1.7
1.1
1.2
0.6
0.4
1.76
1.95
1.19
0.34
0.57
0.36
0.32
0.43
0.34
0.44
0.52
1.06
0.12
0.09
2
2.06
1.53
1.18
1.97
0.41
0.31
0.87
0.82
0.90
1.07
1.64
0.68
0.78
0.37
0.40
0.23
0.08
0.12
0.09
0.08
0.22
0.17
0.29
0.44
0.54
0.29
0.29
96
97
95
88
92
62
55
5.6
5.4
5.7
5.7
5.7
5.7
5.6
5.6
5.5
5.4
4.2
4.3
4.2
4.1
4.2
4.1
4.3
4.3
4.3
4.4
13.71
3.57
0.74
0.44
0.26
0.18
0.12
0.09
0.09
0.07
6.2
6
5.5
5.6
5.1
4.5
4.7
4.9
5.2
4.8
2
1.4
0.7
0.5
0.5
0.5
0.4
0.5
0.5
0.4
2.61
1.13
0.61
0.59
0.64
0.5
0.7
0.72
0.68
0.69
0.19
0.15
0.09
0.09
0.11
0.1
0.11
0.11
0.11
0.11
1.88
1.07
0.54
0.44
0.4
0.45
0.45
0.44
0.43
0.4
0.94
0.76
0.77
0.88
0.80
0.90
1.13
0.88
0.86
1.00
0.42
0.19
0.11
0.11
0.13
0.11
0.15
0.15
0.13
0.14
0.10
0.14
0.17
0.20
0.28
0.22
0.24
0.25
0.26
0.28
98
92
70
69
69
69
70
69
74
69
4.2
4.4
5.4
5.5
5.8
5.7
6.3
4.3
4.6
5.2
5.4
5.5
5.2
5.2
10.38
6.94
1.63
0.65
0.28
0.26
0.37
5.8
6.2
9.2
10.2
10
6.3
5.8
2.4
2.6
2.1
1.5
1.3
1.2
0.9
1.66
1.64
0.62
0.52
0.51
0.54
0.54
0.69
0.63
0.30
0.35
0.39
0.45
0.60
0.23
0.22
0.09
0.11
0.17
0.21
0.22
0.07
0.09
0.19
0.19
0.24
0.19
0.20
94
95
84
91
91
89
85
4.7
4.7
5.4
5.8
5.6
5.4
5.4
5.6
6
4.3
4.3
4.8
4.6
4.7
4.8
4.8
4.8
5.3
5.29
5.1
1.48
0.42
0.22
0.31
0.45
0.34
–
4.4
4.4
5.3
3.8
3.6
4.5
4.4
3.8
–
1.1
1.1
1.1
0.6
0.5
0.7
1
0.8
–
0.91
0.91
0.28
0.3
0.31
0.38
0.4
0.39
0.24
0.04
0.04
0.04
0.06
0.07
0.09
0.4
0.53
3.2
0.57
0.6
0.41
0.32
0.3
0.37
0.98
1.15
6.16
0.52
0.55
0.37
0.53
0.60
0.53
0.98
1.44
–
0.21
0.21
0.05
0.08
0.09
0.08
0.09
0.10
–
0.07
0.07
0.10
0.19
0.23
0.24
0.41
0.46
0.52
77
77
78
66
65
67
85
81
–
4.4
5
5.6
5.4
5.7
5.6
4.1
4.9
5.4
4.8
4.7
4.8
4.03
1.73
0.8
0.36
0.25
0.4
4.6
5
5.5
5.2
4.8
5.2
1.2
1.2
1.3
1
0.7
1
0.52
0.46
0.31
0.26
0.31
0.46
0.04
0.06
0.05
0.09
0.08
0.26
0.54
0.48
0.34
0.36
0.3
0.75
0.45
0.40
0.26
0.36
0.43
0.75
0.11
0.09
0.06
0.05
0.06
0.09
0.07
0.13
0.15
0.25
0.27
0.35
67
79
80.5
74
69
85
4.5
4.3
4.6
5.3
5.6
5.7
4.2
4
4.5
–
–
–
3.46
3.18
1.83
0.95
0.68
0.5
4.4
4.6
4.7
4.8
5.2
5.6
1.1
1.1
1.1
1.1
1.4
1.3
0.56
0.56
0.6
0.63
0.42
0.37
0.05
0.06
0.08
0.11
0.09
0.13
0.51
0.5
0.38
0.38
0.32
0.37
0.46
0.45
0.35
0.35
0.23
0.28
0.13
0.12
0.13
0.13
0.08
0.07
0.10
0.12
0.21
0.29
0.28
0.35
63
69
72
73
79
77
4.5
5.5
6
6.1
5.9
5.7
5.7
4.4
5.2
5.6
5.4
5
4.9
4.9
3.53
1.49
0.76
0.53
0.39
0.33
0.29
6.4
8.1
7.7
6.6
7.7
6.6
7.9
1.3
1.3
1.3
1.2
1.2
1.2
1.1
0.5
0.42
0.43
0.45
0.53
0.6
0.72
0.06
0.06
0.08
0.08
0.09
0.1
0.11
0.46
0.41
0.43
0.44
0.43
0.44
0.49
0.35
0.32
0.33
0.37
0.36
0.37
0.45
0.08
0.05
0.06
0.07
0.07
0.09
0.09
0.13
0.15
0.19
0.18
0.21
0.23
0.22
69
77
83
85
86
87
86
1.36
0.12
1.35
0.15
0.81
0.12
1.1
0.1
1.65
0.12
1.34
0.1
1.29
0.11
Upper footslopes
Continued next page.
derived soils are often underestimated (Wilson et al.,
1996). Wilson et al. (1996) found that the surface areas
measured by N2 gas adsorption were two to seven times
lower than EGME (ethylene glycol mono-ethyl ether)
values, and these lower values from N2 gas adsorption
were explained by the inability of N2 gas to measure
internal mineral surfaces. Gallez et al. (1976) also reported similar differences in measuring surface areas
of basaltic-derived Nigerian soils and attributed these
differences to noncrystalline or imogolite-like minerals.
Water retention was high both at both 33 and 1500 KPa
tension (Table 2), and pedons in the mountain province
had higher water retention than pedons in other units.
Water retention at 1500 KPa of tension was .18.5%
for all the horizons. High water retention in these soils
is attributed to the presence of allophane and allophanelike materials. Allophane contributes to enhanced water
retention due to its fine particle size and hollow spheri-
1264
SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
Table 3. Continued.
pH
Horizon
H2O
KCl
Total
C
Fed†
Ald
Feo‡
Sio
Alo
%
Alo/Ald
Feo/Fed
Sio/Alo
PO4
retention§
%
Lower footslopes
Pedon
Ap
AB
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon
Ap
BA
Bw1
Bw2
BC
CB1
CB2
Pedon
Ap
BA
Bt1
Bt2
Bt3
BC1
BC2
Pedon
Ap
BA
Bt1
Bt2
Bt3
Bt4
BC
8
4.6
5.3
5.4
5.5
5.7
5.8
6
4.3
4.8
5.1
4.9
5.1
5.3
5.2
2.46
1.57
1.01
0.48
0.38
0.25
0.23
6.5
6.6
8.8
8.6
8.6
8.9
8.3
1.4
1.5
1.5
1.5
1.4
1.3
1.3
0.5
0.45
0.46
0.45
0.5
0.57
0.7
0.05
0.05
0.06
0.07
0.09
0.09
0.11
0.49
0.47
0.49
0.46
0.45
0.42
0.48
0.35
0.31
0.33
0.31
0.32
0.32
0.37
0.08
0.07
0.05
0.05
0.06
0.06
0.08
0.10
0.11
0.12
0.15
0.20
0.21
0.23
73
78
81
80
85
85
86
4.3
5.2
5.5
5.2
5.3
5.4
5.5
4.1
5
4.7
4.4
4.4
4.5
4.5
2.93
1.26
0.7
0.43
5.26
0.26
0.22
5.5
5.7
5.6
5.7
5.6
6.1
6.1
1.1
1.1
1
1
0.9
0.9
0.9
0.47
0.48
0.52
0.48
0.45
0.38
0.39
0.04
0.06
0.09
0.09
0.06
0.06
0.06
0.69
0.66
0.59
0.49
0.48
0.43
0.37
0.63
0.60
0.59
0.49
0.53
0.48
0.41
0.09
0.08
0.09
0.08
0.08
0.06
0.06
0.06
0.09
0.15
0.18
0.13
0.14
0.16
74
83
83
81
79
80
82
4.9
5.4
5.8
5.7
5.8
5.8
5.9
4.5
4.9
5.2
4.7
4.4
4.5
4.5
2.93
1.57
0.64
0.34
0.26
0.2
0.15
6.2
6.1
6.3
5.9
6.2
6.2
6
1
0.9
0.9
0.8
0.8
0.9
1.4
0.41
0.32
0.34
0.27
0.26
0.3
0.35
0.04
0.04
0.05
0.05
0.06
0.07
0.09
0.56
0.43
0.38
0.31
0.35
0.36
0.44
0.56
0.48
0.42
0.39
0.44
0.40
0.31
0.07
0.05
0.05
0.05
0.04
0.05
0.06
0.07
0.09
0.13
0.16
0.17
0.19
0.20
72
77
74
74
74
76
72
4.7
5.2
5.5
5.5
5.4
5.6
5.7
4.3
4.5
4.9
4.8
4.2
4.3
4.4
4.02
1.74
0.73
0.49
0.34
0.22
0.18
5.9
6
6
6.8
6.3
6.5
6.2
1.2
1.2
1
1
1
0.9
0.8
0.56
0.54
0.46
0.4
0.38
0.38
0.36
0.47
0.45
0.46
0.40
0.38
0.42
0.45
0.08
0.06
0.07
0.05
0.04
0.04
0.05
0.07
0.07
0.11
0.13
0.16
0.18
0.19
74
77
75
74
74
73
70
5.2
5.9
5.7
6.2
5.8
4.8
5.5
5.6
5.1
4.6
2.89
0.83
0.52
0.28
0.25
6.2
6.5
6.6
6.6
6.4
1.1
1
1
1
0.9
1.01
0.39
0.4
0.36
0.36
0.08
0.04
0.05
0.05
0.06
0.82
0.41
0.38
0.33
0.39
0.75
0.41
0.38
0.33
0.43
0.16
0.06
0.06
0.05
0.06
0.10
0.10
0.13
0.15
0.15
81
73
76
77
74
5.5
5.2
5.7
6.2
6.3
5.9
4.6
4.7
5.2
5.5
4.7
4.4
3.59
2.84
1.55
0.73
0.38
0.25
4.6
5.1
5.1
5
4.9
4.6
0.7
0.9
0.9
0.8
0.8
0.7
1.34
0.98
0.65
0.68
0.59
0.54
0.11
0.08
0.08
0.1
0.11
0.1
0.57
0.66
0.58
0.55
0.55
0.49
0.81
0.73
0.64
0.69
0.69
0.70
0.29
0.19
0.13
0.14
0.12
0.12
0.19
0.12
0.14
0.18
0.20
0.20
73
69
77
75
74
70
9
10
11
Pedon 12
Ap
BA
Bt1
Bt2
Bt3
Pedon 13
Ap1
Ap2
Bt1
Bt2
Bt3
Bt4
0.45
0.04
0.37
0.04
0.42
0.05
0.37
0.05
0.26
0.06
0.28
0.07
0.31
0.07
Alluvial terraces
† Subscripted d is citrate-dithionite-extractable.
‡ Subscripted o is oxalate extractable.
§ At 1000 mg L21 P equilibrating solution.
cal structure (Wada, 1989; Nanzyo et al., 1993b). High
water retention of volcanic ash–derived soils allows the
application of large amount of fertilizers without causing
serious burning damage to seed and roots (Martini and
Luzuriaga, 1989). On the basis of the physical characteristics of all these soils (Table 2), it can be safely stated
that these soils are deep; have very good tilth, high
water-holding capacity, and good aeration; and present
an excellent physical environment for crop growth.
Chemical Properties
Soil pH (H2O) values for these soil samples ranged
from 4.2 to 6.3, with an average of 5.4. Water pH values
were lower in surface than in subsurface horizons (Table
3). On average, KCl pH values were 0.74 units lower
than those in H2O, indicating a net negative charge for
all horizons (Ping et al., 1988). Both the pedons from
alluvial terraces (i.e., Pedon 12 and 13) had water pH
values between 5.2 to 6.3 with an average of 5.7, indicating that soils in the lower elevations are generally less
acidic, which is believed to be due to the incorporation
of less weathered material eroded from deep gullies
common at high elevations.
Total C in the surface horizons of mountain pedons
ranged from 8.19 to 13.21%, while total C in pedons at
lower elevations ranged from 2.46 to 5.29%. Total C
declined rapidly with depth in all the pedons (Table
3). Higher total C content in the mountain pedons is
attributed to the presence of recent deposition of volcanic ash as the parent material for near-surface horizons
(West et al., 1997). However, cooler temperatures, more
recent clearing for cultivation, and management differences such as shifting or longer fallow cultivation at
higher elevation (Poudel et al., 1998) also may have
contributed to these differences. There are several
POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
1265
Table 4. Regression coefficients, observed level of significance (P value), and number of points included in the equation C/Pa 5 a 1
m C, where, C is equilibrium P concentration and Pa is sorbed P per gram of soil, to compute P adsorption maxima (PaM 5 1/m )
in the Langmuir equation Pa 5 K(PaM)C/(1 1 KC).
Pedon
Horizon
Slope (m )
R2
Intercept (a )
P value
Points
PaM
mg P g21
Mountains
1
2
3
4
5
6
7
8
9
10
11
12
13
Ap
A
BA
A1
A2
0.000013
0.000307
0.000048
0.000071
0.000033
0.003816
0.002383
0.005414
0.002157
0.003611
Upper footslopes
0.02
0.17
0.16
0.88
0.15
0.8036
0.5899
0.5049
0.0053
0.5136
6
3
6
6
5
78 616
3 237
20 664
14 208
30 039
Ap
B/A
Ap
BA
Ap1
Ap2
Ap
BA
0.000114
0.000104
0.000136
0.000131
0.000144
0.000128
0.000129
0.000107
0.003629
0.005876
0.004638
0.00419
0.005626
0.005343
0.00498
0.005289
Lower footslopes
0.98
0.94
0.98
0.99
0.97
0.97
0.99
0.98
0.0013
0.0068
0.0015
0.0001
0.0003
0.0005
0.0004
0.0016
5
5
5
6
6
6
5
5
8 772
9 615
7 353
7 634
6 944
7 813
7 752
9 346
Ap
Ap
BA
Ap
BA
Ap
BA
0.000123
0.000123
0.000099
0.000119
0.000114
0.000116
0.000104
0.003659
0.003188
0.003614
0.005619
0.003768
0.004963
0.006016
Alluvial terraces
0.99
0.99
0.95
0.97
0.97
0.98
0.94
0.0001
0.0001
0.001
0.0003
0.0016
0.0002
0.0066
6
6
6
6
5
6
5
8 130
8 130
10 101
8 403
8 772
8 621
9 615
Ap
BA
Ap1
Ap2
0.000097
0.000125
0.00012
0.000123
0.00514
0.003293
0.004527
0.006984
0.89
0.99
0.99
0.88
0.0153
0.0001
0.0005
0.0059
5
6
5
6
10 309
8 000
8 333
8 130
hypotheses that have been proposed for protection of
organic matter from microbial decomposition in volcanic ash–derived soils, including formation of humic complexes and chelates with allophane and allophane-like
materials (Wada and Aomine, 1973), formation of
metal–humic acid complexes of Fe and Al (De Coninck,
1980; Tate and Theng, 1980), and physical and steric
blocking of accessibility to organic molecules (Sen,
1961). Because of slow decomposition of organic matter
in volcanic ash–derived soils, mineralization is slow
(Martini and Luzuriaga, 1989; Saito, 1990), and outside
N sources are necessary for agricultural production.
Saito (1990), cited by Shoji et al. (1993), compared the
N mineralization potential with the pool of total organic
N between Andisols and nonandic soils from northern
Japan. He reported an average of 3.5% mineralizable
N for Andic compared with 8.2% for nonandic soils.
He attributed lower mineralizable N values for Andisols
to the formation of Al–humus complexes and the reaction of proteinaceous constituents with humified organic
matter. Similarly, Martini and Luzuriaga (1989) conducted a greenhouse study on Costa Rican Andepts
with tomato as indicator plants. They reported 38%
lower crop yield in treatments without N application
compared with treatments with N application.
Phosphate-Fixing Capacity
Although all soil samples showed a high PO4 retention
at 1000 mg L21 P equilibrating solution, phosphate retention by the upper two layers of the mountain pedons
was higher than similar horizons in other pedons (Table
3). Phosphate retention by the surface layers of mountain pedons ranged from 94 to 98%, while that of other
pedons ranged between 63 and 81%. Similar high values
for PO4 retention in volcanic ash–derived soils have
been reported by other researchers (Ping et al., 1988;
Wilson et al., 1996; Johnson-Maynard et al., 1997).
The Uniform Surface Langmuir Equation (Barrow,
1978) could be fitted for the upper two horizons of all
pedons (Table 4). However, except for the A1 horizon
of Pedon 3, the equation fitted for horizons from pedons
in the mountains were statistically invalid, and these
horizons were omitted from subsequent analyses. Since
all the pedons on which the Langmuir equation could
be fitted had similar isotherms, only surface layers of
four pedons, representing the mountains (Pedon 3), the
upper footslopes (Pedon 4), the lower footslopes (Pedon
10), and the alluvial terraces (Pedon 13) are presented
(Fig. 1a, 1b, 1c, and 1d). The isotherm for the surface
layer of Pedon 3 represented the highest PaM value,
while rest represented medium level PaM values (Table
4). Indeed, the PaM for these soils (Table 4) were much
greater than derived from the two-surface Langmuir
model for Sri Lankan Alfisols (Morris et al., 1992), Mekong Delta (Vietnam) Entisols, Ultisols, and Inceptisols
(Quang et al., 1996), Vertisols of southeastern Spain
(López-Piñerro and Garcı́a Navarro, 1997), and selected
south and southeast Asian acidic soils (Sanyal et al.,
1993). In our study, the higher phosphate sorption maxima for the mountains compared with other geomorphic
units (Table 4) indicated a close association between the
degree of soil development and PO4-fixation capacity in
these volcanic ash–derived soils. Because lower values
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SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
of Alo/Ald and Feo/Fed reflect a higher degree of weathering (Sakurai et al., 1996) and soil development, and
mountain soils have higher Alo/Ald and Feo/Fed values
(Table 3) compared with other geomorphic units, it can
be safely stated that the inherent high P fixation capacities of volcanic ash–derived soils (Soil Survey Staff,
1996) decreases with the degree of ash weathering and
soil development.
Phosphate sorption maxima were positively corre-
lated with Fe and Al oxide content, total C, clay estimate, and 1500 KPa water retention (Table 5). Clay
estimate here refers to the percentage of clay estimated
by 1500 KPa water retention (Soil Survey Staff, 1996).
The significantly positive correlation between P sorption maxima and both citrate-dithionite extractable Al
(Ald) and oxalate extractable Al (Alo) (Table 5) suggest
that the P sorption is largely due to oxides and oxyhydroxides of Al, noncrystalline Al oxides, allophane, imo-
Fig. 1. Langmuir adsorption isotherms for P sorption of the surface layers of selected pedons (a) Pedon 3 (b) Pedon 4 (c) Pedon 10 and (d)
Pedon 13 representing the mountains, the upper footslopes, the lower footslopes and the alluvial terraces, respectively; Pa is the amount of
P sorbed by unit weight of soil and C is the concentration of P remaining in solution.
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POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
Table 5. Simple correlation coefficients for PO4 sorption maxima (PaM) and selected soil properties. (n 5 20)
PaM
Clay estimate
Total C
CEC
Ald
Feo
Sio
Alo
1500 KPa water
PaM
Clay estimate
Total C
CEC†
Ald‡
Feo§
Sio
Alo
1500 KPa water
1
0.57**
0.62**
0.57**
0.72***
0.52*
0.54*
0.86***
0.62**
1
0.06
0.09
0.03
0.09
20.07
0.09
0.97***
1
0.97***
0.75***
0.88***
0.59**
0.86***
0.14
1
0.66***
0.93***
0.64***
0.85***
0.19
1
0.62**
0.67***
0.88***
0.08
1
0.73***
0.86***
0.18
1
0.83***
20.02
1
0.17
1
*,**, and *** Significant at the 0.05, 0.01, and 0.001 levels of probability, respectively.
† CEC is cation-exchange capacity.
‡ Subscripted d is citrate-dithionite extractable.
§ Subscripted o is oxalate extractable.
golite and Al–humus complexes in these soils. However,
the relatively higher correlation coefficient between
PaM and Alo (r 5 0.86) than PaM and Ald (r 5 0.72)
(Table 5) may suggest that P sorption is more closely
associated with noncrystalline Al oxides, allophane, imogolite and Al–humus complexes than oxides and oxyhydroxides of Al. Both the Ald and Alo content of the
mountain soils were higher than those of the other geomorphic units (Table 3).
Although PaM between the upper two layers from
each pedon were not significantly different, PaM for
the second horizon was consistently higher than the
surface horizon in all the pedons from the upper and
lower footslopes (Table 4). This increase in PaM is at-
Fig. 2. Immediate quantity/intensity (Q/I) relations for K of the surface layers of selected pedons (a) Pedon 3 (b) Pedon 4 (c) Pedon 10 and
(d) Pedon 13 representing the mountains, the upper footslopes, the lower footslopes and the alluvial terraces, respectively; DK is a measure
of labile K, and ARK is the activity ratios (aK/(a Ca1Mg)1/2).
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SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
tributed to higher Alo or clay or both in the subsurface
layers (Tables 2 and 3). In contrast, PaM was lower in the
subsurface layers than in the surface layers for pedons
representing the alluvial terraces.
This high P-fixing capacity of these soils requires special consideration in the development of a P fertilization
plan. Martini and Luzuriaga (1989) suggested band application of P fertilizers and using sources of P with low
solubility. Another important consideration would be
to minimize soil erosion, which removes the surface
layers that generally had P buildup due to P fertilization
(Gachene et al., 1997).
Immediate Quantity/Intensity Measures
for Potassium
All surface layer soil samples studied showed a characteristic Q/I curve, that is, linear upper portion and
asymptotic lower portion. The immediate Q/I relationships for the same pedons used to develop Langmuir
adsorption isotherms are presented in Fig. 2a, 2b, 2c,
and 2d. Based on the Q/I relationships for K from all
surface samples studied, the values for Q/I parameters:
the degree of K availability (AReK), the potential buffering capacity for K (PBCK), and the measures of labile
K (2DK8) were determined and are presented in Table 6.
The 2DK8 values, which are a rough measure of the
size of the pool of labile K in the soil (Beckett, 1994b),
were smaller for the mountains compared with those of
other geomorphic units (Table 6). Except for pedons
from the mountains, 2DK8 values were similar to corresponding values from NH4OAc and double acid extraction (Table 7). The relatively lower 2DK8 compared
with NH4OAc and double acid extraction for the mountains is believed to be due to K fixation by amorphous
constituents such as allophane and imogolite (van Reeuwijk and Devilliers, 1968). In this study, a negative correTable 6. Degree of K availability (AReK), buffering capacity for
K (PBCK), and measures of labile K (2DK8) for the topsoil of
pedons under study.†
Pedon
AReK
(mol
L21)1/2
PBCK
kg21/(mol/L21)1/2)
(cmol
Mountains
DK8
(2)
cmol
kg21
1
3
0.0195
0.0128
1.8
1.4
Upper footslopes
0.0143
0.0031
4
5
6
7
0.1032
0.4472
0.3439
0.2463
4.4
2.7
2.5
3.2
Lower footslopes
0.4496
1.2102
0.8449
0.7783
8
9
10
11
0.1684
0.0376
0.1110
0.0768
5.6
3.6
3.6
2.5
Alluvial terraces
0.9392
0.1330
0.3931
0.1889
12
13
0.0898
0.0805
4.6
7.0
0.4049
0.5576
† Q/I measures for K in Pedon 2 were not determined due to lack of
sample.
lation was found between the 2DK8 values and the Alo/
Ald ratio (Table 8), suggesting that the size of the pool of
labile K increases with the degree of mineral weathering
and soil development.
Samples representing the mountains also showed the
smallest degree of K availability (AReK) (Table 6). As
with 2DK8, this lower degree of K availability is attributed to the stage of soil development, parent materials,
and K fixation by alluminosilicates. Since this geomorphic unit has the least-developed soils, there has not
been enough time for ash weathering and K release.
Also, these soils are derived mainly from siliceous parent materials having a very low level of K2O (0.82%)
(Heiken and Wohletz, 1985). Third, since these soils
still have andic soil properties (West et al., 1997), a
higher amount of K fixation by alluminosilicates is expected (van Reeuwijk and Devilliers, 1968).
Soil samples representing the upper footslopes
showed higher AReK values than the rest of the geomorphic units (Table 6) ranging between 0.1032 and 0.4472
(mol L21)1/2. The higher level of K availability in this
geomorphic unit is believed to be due to enough time
for ash weathering and a lower degree of K leaching
down the profile. Since soils are Oxisols, it can be safely
stated that the ash parent materials have had enough
time for weathering and K release.
The lower footslopes and the alluvial terraces showed
very similar levels of equilibrium K values (AReK) (Table
6). A significant negative correlation between water retention at 1500 KPa and AReK (Table 8) also indicated
that soils with andic properties, which are characterized
by a high water retention (Soil Survey Staff, 1996), have
a lower degree of K availability, especially if they were
developed from K-poor ash parent materials. As expected, the AReK values for these soil samples were found
to be strongly positively correlated with the size of the
pool of labile K and extractable K (Table 8), suggesting
that soils having higher extractable K values also have
higher degree of K availability. As with 2DK8 values,
the AReK values were negatively correlated with Alo/Ald,
Table 7. Extractable and solution K.†
Pedon
1 M NH4OAc (pH 7)
0.05 M HCl 1
0.01 M H2SO4
cmolc kg21
Mountains
Distilled water
(solution K)
cmol kg21
1
3
0.1721
0.1328
0.3295
0.3197
Upper footslopes
0.1205
0.1869
4
5
6
7
0.1771
1.2764
1.0567
0.5902
0.3738
2.5071
1.8097
1.2285
Lower footslopes
0.1180
1.2768
0.7421
0.2191
8
9
10
11
0.4181
0.1131
0.3738
0.3098
1.1665
0.2951
0.8571
0.6246
Alluvial terrances
0.2484
0.0639
0.2287
0.1475
12
13
0.3639
0.2361
0.9269
0.4377
0.1697
0.0369
† Analysis for Pedon 2 was not done due to lack of soil sample.
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POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
Table 8. Simple correlation coefficients for the Q/I parameters and selected soil properties (n 5 12).†
K
e
AR
K
e
AR
PBCK
2DK8
NH4OAc
extracted K
HCl 1 H2SO4
extracted K
H2O
extracted K
pH-H2O
ECEC
BS
1500 KPa
water
Ald
Alo/Ald
Extractable Al
PBC
K
1
20.06
1
0.89***
0.35
0.97*** 20.18
2DK8
NH4OAc
H 2O
extracted HCl 1 H2SO4 extracted
K
extracted K
K
pH-H2O
1
0.79**
1
0.97*** 20.09
0.87***
0.97***
1
0.89*** 20.31
0.67*
0.94***
0.91***
BS
Ald
Alo/Ald
Extractable
Al
1
0.28
0.66*
1
0.15
1
1
20.17
0.14
0.24
20.63*
0.77**
0.1
0.49
0.18
0.60*
0.35
20.06
20.55
20.18
0.1
0.21
20.63*
20.14
0.05
0.2
20.56
0.32
0.05
0.05
20.57
20.32
20.63*
20.25
20.66*
0.05
20.59*
20.25
20.52
20.22
20.26
20.57
20.26
20.09
20.41
0.03
20.47
20.65*
20.43
ECEC
1500
KPa
water
1
0.65*
0.80**
0.002
1
0.89***
1
20.28
20.22
20.59*
20.72**
0.37
0.25
20.80*** 20.52
20.84***
0.02
20.84***
1
0.29
0.05
0.09
*, **, and *** Significant at the 0.05, 0.01, and 0.001 levels of probability, respectively.
† AReK is degree of K availability; PBCK is buffering capacity for K; 2DK8 is measures of labile K; ECEC is effective cation-exchange capacity; BS is base
saturation; subscripted d is citrate-dithionite extractable; subscripted o is oxalate extractable.
suggesting that the degree of K availability increases
with the degree of ash weathering and soil development.
Although the equilibrium K (AReK) is indicative of
the degree of K availability in soils, it is the potential
buffering capacity for K (PBCK) which measures the
ability of soils to maintain the labile K against depletion
(Beckett, 1964b). The potential buffering capacity for
K is found to increase in the following order: mountains , upper footslopes , lower footslopes , alluvial
terraces (Table 6). Soil samples representing the alluvial
terraces showed almost four times higher PBCK values
compared with those of the mountains, suggesting that
soils in the mountains have very limited capacity to
maintain their labile K against depletion compared with
their counterparts on the alluvial terraces. The potential
buffering capacity of these soils was found to be positively correlated with soil pH and BS (Table 8), suggesting that acidification and basic cation depletion results in lower potential buffering capacity for K on this
Fig. 3. Relationships between Potential Buffering Capacity (PBCK)
for K and soil pH.
volcanic slope. Soil pH was the property most highly
correlated with PBCK (Fig. 3) in these soils. Beckett
(1964b) also reported a good linear correlation between
PBCK and soil pH. Because the ECEC and BS were
found to be significantly positively correlated with soil
pH (Table 8), acidification will not only lower the PBCK
values but it will also lower the ECEC and BS percentage of these soils.
The poor buffering capacity for K in the higher elevations suggests a relatively larger crop response to applied fertilizers than in lower elevations, and intensified
crop production without sufficient K fertilizer application will result in poor crop yields. The exchangeable
K for all pedons largely decreased by depth (Table 9),
indicating the need for practicing erosion control measures to protect K-rich surface layers, especially on sloping lands.
Cation-Exchange Capacity and Base Saturation
The CEC values ranged between 7.5 and 50.4 cmolc
kg21 for the mountains, 5 to 21.7 cmolc kg21 for the
upper footslopes, 7.5 to 20.4 cmolc kg21 for the lower
footslopes, and 9 to 24.2 cmolc kg21 for the alluvial
terraces (Table 9). High values in mountains reflect
CEC from “amorphous materials” in the recent ash cap.
The average CEC of these soil samples at pH 8.2, pH
7 and field pH (ECEC) were 19.5, 12.7, and 2.5 cmolc
kg21, respectively, suggesting a large amount of pHdependent charge. The CEC in these soils was found
to be most correlated with total C (r 5 0.94, P # 0.001).
The surface layer CEC that reflects CEC contributions
from organic matter were higher than those for other
horizons, and the CEC values decreased by depth (Table
9). Martini and Luzuriaga (1989) also reported a high
correlation between the CEC and organic matter (r 5
0.88) for volcanic ash–derived soils in Costa Rica.
The CEC of volcanic ash–derived soils decreases with
the degree of soil development, as Inceptisols were
found to have higher CEC values than Ultisols and
Oxisols. Because the CEC was highly associated with
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SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
Table 9. Exchange capacities and selected nutrient status of the pedons studied.
Exchangeable
Horizon
Ca
Mg
Na
Sum of
bases
K
CEC
pH 8.2
pH 7.0
ECEC
Exch.
Al
cmolc kg21
Mountains
Pedon 1
Ap
AB
Bw1
Bw2
Bw3
BC1
BC2
Pedon 2
A
BA
Bt1
Bt2
Bt3
BC1
BC2
BC3
BC4
C
Pedon 3
A1
A2
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon 4
Ap
B/A
Bw1
Bw2
Bw3
C1
C2
C3
ROCK
Pedon 5
Ap
BA
Bt1
Bt2
Bt3
BC
Pedon 6
Ap1
Ap2
Bt1
Bt2
Bt3
BC
Pedon 7
Ap
BA
Bt1
Bt2
Bt3
Bt4
Bt5
1.6
0.7
0.7
0.8
0.6
0.5
0.4
0.5
0.3
0.2
0.0
0.0
0.4
0.5
0.0
0.1
0.1
0.1
0.1
0.3
0.2
0.2
0.2
0.1
0.0
0.1
0.0
0.0
2.3
1.3
1.1
0.9
0.8
1.2
1.1
42.6
36.3
27.7
17.8
19.6
12
11.6
33.7
32.2
18
11.8
12.2
11.4
10.9
4.3
2.5
1.4
0.9
0.8
1.2
1.9
2.0
1.2
0.3
0.0
0.0
0.0
0.8
4.3
1.3
0.4
0.1
0.1
0.1
0.1
0.0
0.1
0.1
1.1
0.2
0.2
0.1
0.2
0.2
0.3
0.2
0.2
0.2
0.1
0.2
0.3
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.2
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5.7
1.8
0.9
0.4
0.5
0.4
0.5
0.3
0.4
0.4
55.5
29.4
16.7
15
13.5
12.1
12.8
12.7
13.2
11.7
50.4
22.6
12.7
10.3
9.2
8.8
9.1
8.4
9.7
9
8.6
3.9
2.6
2.2
1.6
1.4
1.4
1.1
1.7
1.1
2.9
2.1
1.7
1.8
1.1
1.0
0.9
0.8
1.3
0.7
1.1
0.4
0.6
0.1
0.1
0.1
0.2
0.4
0.2
0.1
0.0
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.1
0.1
0.1
0.2
0.1
0.1
0.1
0.0
0.1
0.0
45.7
36.8
17
14.7
15.2
14.5
14.2
37.3
25.2
7.5
8.5
8.1
7.6
8
4.1
1.8
0.6
0.3
0.3
0.4
0.4
2.3
1.0
0.0
0.0
0.0
0.0
0.0
5.0
5.1
1.4
0.5
0.3
0.1
0.2
0.3
1.1
1.0
0.9
0.4
0.1
0.2
0.1
0.1
0.1
0.2
0.1
0.2
0.1
0.1
0.2
0.1
0.1
0.2
0.1
0.2
0.2
0.1
0.0
0.0
0.0
0.0
0.0
0.0
6.3
6.4
2
0.7
0.7
0.3
0.4
0.6
1.4
33.4
32.3
15.9
11.2
10.3
9.8
15.1
14.2
1.4
21.7
21.2
8.9
7
6.9
5.8
12
9.9
33.6
7.5
7.3
2.1
0.9
0.9
0.4
0.5
0.6
1.4
1.2
0.9
0.1
0.2
0.2
0.1
0.1
0.0
0.0
2.8
1.6
0.8
0.2
0.1
0.0
1.4
0.5
0.3
0.4
0.3
0.1
0.0
0.3
0.1
0.1
0.2
0.1
1.3
0.2
0.1
0.1
0.1
0.0
5.5
2.6
0.11
0.7
0.7
0.2
31.4
13.3
14
11.7
11.4
14
18.8
10
5
7.2
7.1
9.6
6.8
2.6
0.11
0.8
0.7
0.2
1.3
0.0
0.0
0.1
0.0
0.0
3.2
1.6
1.1
2.9
2.2
1.0
0.7
0.3
0.3
0.8
1.0
0.7
0.0
0.0
0.1
0.1
0.1
0.0
1.0
0.5
0.5
0.1
0.1
0.1
4.9
2.4
2
3.9
3.4
1.8
29.4
27.2
19.4
15.2
13.3
11.9
17.5
16.9
12.4
10
7.6
7.3
5.9
4.6
2.3
3.9
3.4
1.8
1.0
2.2
0.3
0.0
0.0
0.0
3.4
2.3
1.7
0.3
0.2
0.3
0.3
1.3
0.7
1.0
0.8
0.5
0.3
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.7
0.3
0.1
0.2
0.2
0.2
0.3
5.5
3.4
2.9
1.4
1
0.9
0.9
30.1
18.9
16.3
15.3
15
15
14.7
19.2
11.8
9.2
8.1
8.1
8.2
8.7
5.9
3.4
2.9
1.4
1
0.9
0.9
0.4
0.0
0.0
0.0
0.0
0.0
0.0
1.8
0.8
0.6
0.3
0.3
0.4
0.4
Upper footslopes
Continued next page.
total carbon, the removal of organic matter–rich surface
layers by soil erosion in sloping lands exposes subsurface
layers with a lower organic matter content that results
in decreased CEC values.
Soil samples representing the mountains showed
lower base saturation calculated from CEC at pH 8.2
and higher exchangeable Al saturation (calculated from
ECEC) than soil samples representing the upper
footslopes, the lower footslopes, and the alluvial terraces (Table 9), suggesting that the soils in the mountains are base poor compared with soils at lower elevations. This lower level of base content in the mountain
soils is attributed to their being developed in young ash
parent materials.
POUDEL & WEST: SOIL DEVELOPMENT AND FERTILITY CHARACTERISTICS OF A VOLCANIC SLOPE
1271
Table 9. Continued.
Exchangeable
Horizon
Ca
Mg
Na
Sum of
bases
K
CEC
pH 8.2
pH 7.0
ECEC
Exch.
Al
cmolc kg21
Lower footslopes
Pedon
Ap
AB
Bt1
Bt2
Bt3
Bt4
Bt5
Pedon
Ap
BA
Bw1
Bw2
BC
CB1
CB2
Pedon
Ap
BA
Bt1
Bt2
Bt3
BC1
BC2
Pedon
Ap
BA
Bt1
Bt2
Bt3
Bt4
BC
8
1.1
0.7
1.1
0.1
0.4
0.2
0.2
0.5
0.3
0.4
0.3
0.2
0.2
0.1
0.0
0.0
0.0
0.1
0.1
0.1
0.0
0.6
0.8
0.2
0.0
0.3
0.5
0.3
2.2
1.8
1.7
0.5
1
1
0.6
25.9
20.7
18
16.7
16.6
15.4
14.5
14.6
11.8
9.8
8.3
7.8
7.5
8
3.2
1.9
1.7
0.6
1
1
0.6
1.0
0.1
0.0
0.0
0.0
0.0
0.0
3.2
3.9
1.3
0.2
0.2
0.1
0.3
0.3
1.1
0.6
0.3
0.1
0.1
0.1
0.0
0.0
0.1
0.1
0.0
0.1
0.1
0.2
0.1
0.1
0.2
0.2
0.3
0.4
3.7
5.1
2.1
0.8
0.5
0.6
0.9
32.6
23.6
21.3
19.8
19
17.9
17.6
18.8
13.9
11.5
10.5
10.3
9.8
9.4
6.1
5.1
2.3
1.7
1.5
1.2
1.4
2.4
0.0
0.2
0.9
1.0
0.6
0.5
4.4
2.4
1.2
0.5
0.3
0.3
0.3
0.7
0.5
0.9
0.5
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.1
0.2
0.4
0.2
0.1
0.1
0.1
0.1
0.1
5.6
3.2
2.3
1.2
0.8
0.6
0.7
27.6
18.9
15.8
15.7
15.7
14.9
15.1
16
11.3
9.9
9.3
10.1
9.9
10.2
6
3.2
2.3
1.5
1.5
1
1.2
0.4
0.0
0.0
0.3
0.7
0.4
0.5
2.5
0.5
1.1
0.8
0.3
0.2
0.2
1.4
0.1
0.2
0.3
0.3
0.2
0.1
0.1
0.0
0.0
0.0
0.1
0.0
0.0
0.3
0.0
0.0
0.0
0.1
0.1
0.1
28.3
18.1
15.2
15.5
14.7
14
13.2
20.4
12.5
9.3
8.7
9.3
8.9
9.1
5.3
1.2
1.3
1
1.2
1.1
0.7
1.0
0.6
0.0
0.0
0.4
0.6
0.3
4.3
1.5
1.2
0.5
0.2
2.6
1.9
1.5
0.6
0.1
0.0
0.1
0.2
0.1
0.1
0.5
0.1
0.1
0.1
0.1
7.4
3.6
3
1.3
0.5
29.2
18.2
15.4
15.7
15
19.4
10.6
9.3
9
9.8
7.4
3.6
3
1.3
0.7
0.0
0.0
0.0
0.0
0.2
5.2
4.8
2.0
2.1
1.2
1.3
3.1
3.5
2.0
2.6
1.7
1.7
0.1
0.1
0.2
0.3
0.3
0.4
0.3
0.3
0.5
0.3
0.2
0.2
8.7
8.7
4.7
5.3
3.4
3.6
31.8
31.3
20.8
18.3
16.6
17.3
24.2
21.8
14.2
12.1
12.5
13
8.8
8.7
4.7
5.3
3.4
3.8
0.1
0.0
0.0
0.0
0.0
0.2
9
10
11
Pedon 12
Ap
BA
Bt1
Bt2
Bt3
Pedon 13
Ap1
Ap2
Bt1
Bt2
Bt3
Bt4
4.3
0.6
1.3
0.9
0.8
0.5
0.4
Alluvial terraces
Calcium and Mg values were generally higher for the
lower elevations than mountains, and they decreased
by depth (Table 9). Although Ca represented 50 to 86%
of the sum of basic cations, the low (Ca 1 Mg)/K ratios
(2.7–30.0) for the surface layers of all pedons indicated
possible higher response of crops to Ca application than
K. Martini and Luzuriaga (1989) reported higher yield
response to Ca application than K in Andepts with low
(Ca 1 Mg)/K ratios in Costa Rica. According to Tisdale
et al. (1993), Mg saturation should generally be .10%
for optimum plant growth. In this study, none of the
pedons except those representing alluvial terraces have
Mg saturation .10% in their surface horizons (Table
9), suggesting that soils in the mountains, the upper
footslopes, and the lower footslopes are deficient in Mg.
Since a high Ca/Mg or K/Mg ratio can also cause Mg
deficiency (Tisdale et al., 1993), liming or K fertilization
plans should take into account the Mg status of the soils.
Fertility characteristics varied with the degree of soil
development across the four geomorphic units: the
mountains (Inceptisols and Ultisols), the upper footslopes (Oxisols), the lower footslopes (Oxisols), and
the alluvial terraces (Oxisols). However, soils in all the
geomorphic units were surprisingly uniform with respect
to their morphological and physical characteristics.
CONCLUSIONS
Volcanic ash–derived soils on a slope of Mt. Kitanglad
in the Manupali watershed in Mindanao, the Philippines
showed a gradient in soil development which increased
from the mountains to the lower footslopes. Differences
in chemical and fertility characteristics varied with the
degree of soil development. Soils from mountains had
higher CEC values and lower ECEC values than soils
on other parts of the landscape, suggesting that the CEC
1272
SOIL SCI. SOC. AM. J., VOL. 63, SEPTEMBER–OCTOBER 1999
in young volcanic ash–derived soils is largely due to
“amorphous materials” and soil organic matter. Base
saturation increases with the degree of soil development
due to ash weathering. Although all soil samples studied
indicated high P-fixation capacity, mountain soils had
higher P fixation, suggesting that the higher the degree
of soil development the lower the P fixation by volcanic
ash–derived soils. Similarly, the Q/I parameters for K
were also closely related to the degree of soil development. Of all the geomorphic units, the mountains had
the smallest pool size of labile K. They also had the
lowest degree of K availability and lowest potential buffering capacity. These differences are interpreted to be
due to the presence of a thin recent capping of ash on
the soils in the mountain geomorphic unit.
The morphological and physical characteristics of
these soils suggest that soils in this landscape provide
an excellent environment for plant growth through their
good internal drainage, infiltration, rooting depth, aeration, water-holding capacity, and soil tilth. However,
high P-fixation capacities and lower K availabilities in
these soils require special consideration from a fertilizer
management point of view. Similarly, these soils have
a small pool of basic cations which may be depleted
rapidly under intensified agricultural production if appropriate measures are not taken. Except for the alluvial
terraces, Mg deficiency is likely to occur in most crops,
as Mg saturation of the exchange complex of samples
representing these geomorphic units was found to be
very low. Soils in all the geomorphic units were acidic,
which may necessitate amendments with chemical fertilizers for continuous agricultural production.
ACKNOWLEDGMENTS
We acknowledge the SANREM-CRSP funded by the
USAID under Grant No. LAG-4198-A-00-2017-00 for supporting this research study. We are grateful to two anonymous
referees for their invaluable comments.
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