Tuesday, March 17, 2015

Properties of Soils from Ultramafic Rocks in the Degraded Grassland Watershed in Basper, Leyte

by
Cecille Marie O. Quiñones
Instructor, Department of Soil Science
Visayas State University

Ultramafic rocks are those containing less than 45 percent silica with color indices more than 70 (Huang, 1962). These rocks, being igneous in nature, could have been derived directly from the earth’s mantle or indirectly from other mantle-derived material such as basaltic magma (Wyllie, 1969). Most of them are outcrops of oceanic lithosphere as ophiolite belts and typically found in areas within the Circum-Pacific margin and Mediterranean Sea (Oze et al., 2004). In the tropical Far East, the Philippines has considerable number of ultramafic areas that are within Ophiolite complexes. However, extensive studies pertaining to the plant and soil chemistry of ultramafic soils have been mostly focused in the Palawan region and Sibuyan Islands (Proctor, 2003). Modest attention is given to other ophiolitic areas such as those enclosed within the Tacloban ophiolite/ophiolitic complex in Leyte. 
The Ophiolite complex in northeastern Leyte (Dimalanta et al., 2006)
Dimalanta et al. (2006) reported that Leyte is within the Philippine arc island system wherein the island itself is composed of Cretaceous arc rocks and ophiolitic rocks. According to them, these rocks are oceanic lithosphere fragments that are covered by sedimentary carapace which imply deep marine environment and associated metamorphic rocks such as metamorphic soles of amphibolite, serpentinite, regionally metamorphosed rocks, and ocean floor metamorphosed rocks. Two ophiolitic areas found in Leyte are the Malitbog and Tacloban Ophiolite complexes. The Tacloban Ophiolite complex is composed of residual peridotite particularly harzburgites, layered to massive gabbros, sheeted diabase, and basalt dike complex. The identity unit of the Tacloban Ophiolite complex is the isotropic gabbro section. 

The soil that developed from an ultramafic rock, especially after it has been metamorphosed, is generally branded with low concentrations of plant nutrients and high content of potentially toxic elements such as Nickel (Ni), Chromium (Cr), and Cadmium (Cd) (Oze et al. 2004; Cheng et al., 2011). Moreover, Chardot et al. (2007) reiterated three specific pedogenic processes that occur in ultramafic rocks. First is the relatively fast Ca leaching, second is the formation of black humus mainly mull type on the topsoil, and third is the subsoil formation of clay texture horizon structure that is generally weakly structured. 

The properties of soils in degraded watersheds underlain by ultramafic rocks are relatively unknown. Thus, this study was conducted in the project site of the ACIAR Hydrology Project of Professor L. Bruijnzeel (Free University of Amsterdam) in Basper, Tacloban, Northeastern Leyte. Seven soil profiles were studied in various slope positions in the grassland watershed to characterize the area in terms of their soil morphological, physico-hydrological, and chemical properties as influenced by its parent material. Stream water samples were also collected for selected property determination including the contribution of sources to the cation concentration.  

The Basper watershed in Tacloban, Leyte 
Results showed that soils in the study site ranged from poorly to moderately developed and from shallow to deep profile. The soil profile development appears to be related to the slope positions. Those on the footslope and summit positions had thinner solum while those in the lower backslope developed into deep soils. In addition, the soils are generally clay loam to sandy clay loam in the upper horizon. They tend to be sandy in the lower section of the profile due to the saprolite coming from the ultramafic parent rock. Measurements of physico-hydrological properties indicate that bulk density values increased with depth accompanied by a decrease in total and air-filled porosity. Furthermore, saturated hydraulic conductivity of surface soils are moderately low to moderately high than the subsurface soils.


The soil in the summit position of the watershed showing the greenish ultramafic rock in the lower soil profile.
In terms of chemical properties, results showed soil pH values varying from 5.9 to 7.10 indicating near neutral soil reaction due to high Ca and Mg of the ultramafic parent material. Soil organic carbon is generally low as well as the total N content. Available P varies from very low to high. Exchangeable Ca and Mg are high while the exchangeable K and Na are low. Because of the high exchangeable Ca and Mg, the base saturation values of the soils are generally high which also resulted in the low values of exchangeable H and Al in the soils. Effective CEC measures of the soils range from 10.61 to 25.81 cmolc kg-1 indicating low values.

The study also revealed a baseflow electrical conductivity ranging between 111.30 to 317.00 µS cm-1. Calculated weathering index using Na/(Ca+Na) showed that weathering of the ultramafic rock is the major source of cations in the stream water in the watershed. This study overall displayed the dominant role of parent material and slope position in the development and properties of the soils from ultramafic rocks in the area. 

Acknowledgment

The article is based on my MSc thesis. I am grateful to the Department of Science and Technology (DOST) for the ASTHRDP-NSC graduate scholarship, my Graduate Advisory Committee, and Prof. Bruijnzeel.

References

CHARDOT, V., G. ECHEVARRIA, M. GURY, S. MASSOURA AND J. L. 2007. Nickel bioavailability in an ultramafic toposequence in the Vosges Mountains (France). Plant Soil 293: 7-21.
CHENG, C., S. JIEN, Y. IIZUKA, H. TSAI, Y. CHANG AND Z. HSEU. 2011. Pedogenic chromium and nickel partitioning in serpentine soils along a toposequence. Soil Sci. Soc. Am. J. 75: 659–668.
DIMALANTA, C. B., L. O. SUERTE, G. P. YUMUL, R. A. TAMAYO AND E. G. L. RAMOS. 2006. A cretaceous supra-subduction oceanic basin source for Central Philippine ophiolitic basement complex: Geological and geophysical constraints. Geosciences 10 (3): 305-320.
HUANG, W. T. H. 1962. Petrology. McGraw-Hill Book Company. USA. 480 pp.
OZE, C., S. FENDORF, D.K. BIRD AND R.G. COLEMAN. 2004. Chromium geochemistry in serpentinized ultramafic c rocks and serpentine soils from the Franciscan complex of California. Am. J. Sci. 304:67–101.
PROCTOR, J. 2003. Vegetation and soil and plant chemistry on ultramafic rocks in the tropical Far East. Perpectives in Plant Ecology, Evolution and Systematics. Urban & Fischer Verlag. 6 (1-2): 105-124.
WYLLIE, P. J. 1969. The origin of ultramafic and ultrabasic rocks. Tectonophysics 7(5-6): 437-455.

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