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Soil Science Society of America Journal 63:1258-1273 (1999)
© 1999 Soil Science Society of America

DIVISION S-5-PEDOLOGY

Soil Development and Fertility Characteristics of a Volcanic Slope in Mindanao, the Philippines

D.D. Poudela and L.T. Westb

a Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616 USA
b Dep. of Crop and Soil Sciences, Miller Plant Sciences Building Room 3111, Univ. of Georgia, Athens, GA 30602 USA

ddpoudel{at}ucdavis.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
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 µg P g-1, 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.

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 • -{Delta}K°, measures of labile K


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
VOLCANIC 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, 1984; Parfitt and Saigusa, 1985; Shoji et al., 1988; van Wambeke, 1992; West et al., 1997). Nieuwenhuyse and van 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 ({approx}0.9 g cm-3), 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 {approx}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; Sanyal 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, 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 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
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Description of the Study Area
The Manupali watershed lies between 124° 47' to 125° 08' E and 7° 57' to 8° 08' N on the island of Mindanao in the Philippines. This watershed has an area of {approx}50496 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 34°C in areas below 500 m, and 18 to 28°C in areas above 500 m. Major crops grown are corn (Zea mays L.), sugarcane (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.


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  		Table 1 Site characteristics for the pedon locations

 
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 NH+4 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 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 550°C. These slides were scanned from 3 to 32° 2 {theta} 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 L-1 P. After 10 d of equilibration, samples were centrifuged at 14500 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 García 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 Ca2+, Mg2+, K+, and Na+ 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
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
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).


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  		Table 2 Selected physical properties and NaF pH of all pedons

 
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 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 (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.


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  		Table 3 Selected chemical properties of the pedons studied

 
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 cm-3, while bulk density values for the surface layers of the remaining pedons ranged from 0.9 to 1.11 g cm-3, 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 g-1 with generally lower values for the mountain pedons. However, surface area measurements by N2 gas adsorption in volcanic ash–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 allophane-like materials. Allophane contributes to enhanced water retention due to its fine particle size and hollow spherical 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 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 L-1 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 García 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 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.


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  		Table 4 Regression coefficients, observed level of significance (P value), and number of points included in the equation C/Pa = a + m C, where, C is equilibrium P concentration and Pa is sorbed P per gram of soil, to compute P adsorption maxima (PaM = 1/m) in the Langmuir equation Pa = K(PaM)C/(1 + KC)

 


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  		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

 
Phosphate sorption maxima were positively correlated 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, imogolite and Al–humus complexes in these soils. However, the relatively higher correlation coefficient between PaM and Alo (r = 0.86) than PaM and Ald (r = 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).


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  		Table 5 Simple correlation coefficients for PO4 sorption maxima (PaM) and selected soil properties. (n = 20)

 
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 attributed 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 (ARKe), the potential buffering capacity for K (PBCK), and the measures of labile K (-{Delta}K°) were determined and are presented in Table 6 .



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  		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; {Delta}K is a measure of labile K, and ARK is the activity ratios (aK/(a Ca+Mg)1/2)

 

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  		Table 6 Degree of K availability (ARKe), buffering capacity for K (PBCK), and measures of labile K (-{Delta}K°) for the topsoil of pedons under study.{dagger}

 
The -{Delta}K° 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, -{Delta}K° values were similar to corresponding values from NH4OAc and double acid extraction (Table 7) . The relatively lower -{Delta}K° 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 correlation was found between the -{Delta}K° 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.


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  		Table 7 Extractable and solution K.{dagger}

 

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  		Table 8 Simple correlation coefficients for the Q/I parameters and selected soil properties (n = 12).{dagger}

 
Samples representing the mountains also showed the smallest degree of K availability (ARKe) (Table 6). As with -{Delta}K°, 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 ARKe values than the rest of the geomorphic units (Table 6) ranging between 0.1032 and 0.4472 (mol L-1)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 (ARKe) (Table 6). A significant negative correlation between water retention at 1500 KPa and ARKe (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 ARKe 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 -{Delta}K° values, the ARKe values were negatively correlated with Alo/Ald, suggesting that the degree of K availability increases with the degree of ash weathering and soil development.

Although the equilibrium K (ARKe) 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 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.



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  		Fig. 3 Relationships between Potential Buffering Capacity (PBCK) for K and soil pH

 
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.


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  		Table 9 Exchange capacities and selected nutrient status of the pedons studied

 
Cation-Exchange Capacity and Base Saturation
The CEC values ranged between 7.5 and 50.4 cmolc kg-1 for the mountains, 5 to 21.7 cmolc kg-1 for the upper footslopes, 7.5 to 20.4 cmolc kg-1 for the lower footslopes, and 9 to 24.2 cmolc kg-1 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 kg-1, respectively, suggesting a large amount of pH-dependent charge. The CEC in these soils was found to be most correlated with total C (r = 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 = 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 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.

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 + 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 + 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 foot-slopes (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
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
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 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.Sen 1961


   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.

Received for publication July 24, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





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