Climate, Geology and Soils of the Tropics with Special Reference to Southeast Asia and Leyte (Philippines)

Transcription

1 Climate, Geology and Soils of the Tropics with Special Reference to Southeast Asia and Leyte (Philippines) Reinhold Jahn 1 and Victor B. Asio 2 1 Soil Science & Soil Protection, University of Halle-Wittenberg, Germany 2 Soil Science Division, Visayas State Unviersity, Baybay, Leyte, Philippines vbasio@vsu.edu.ph In: Proceedings of the 11th International Seminar-Workshop on Tropical Ecology, August 2006, Visayas State University, Baybay City, Leyte 6521-A, pp

2 23 CLIMATE, GEOLOGY AND SOILS OF THE TROPICS WITH SPECIAL REFERENCE TO SOUTHEAST ASIA AND LEYTE (PHILIPPINES) Reinhold Jahn 1 & Victor B. Asio 2 1 University of Halle-Wittenberg, Germany 2 Visayas State University, Baybay, Leyte, Philippines Abstract Based on the FAO Agro-Ecological Zones Project, the tropics are described by their different growing period zones and the soil resources differ between the different continents. Tropical ecosystems have a distinct role in the global biogeochemical cycles of elements as exemplified by the global carbon budget. The major soil constraints for plant growth, derived per region with special attention to SE- Asia, include low cation exchange capacity, low base saturation, and high P-retention capacity. This leads to a high demand for nutrient cycling in land use systems. While the availability of N is coupled to the interaction of atmosphere, biosphere and the upper part of the pedosphere, the availability of P, K, Ca and Mg is dependent on pedogenic processes and soil mineralogical properties. For these nutrients (i.e. P, K, Ca & Mg), the external fluxes which differ from landscape to landscape, have to be considered. There is a need to include organic matter in nutrient cycles because of its crucial role in tropical soils. 1. Occurrence of tropical ecosystems and climatic variation Tropical regions occur, by common geographic definition, in the region lying between the Tropic of Cancer and the Tropic of Capricorn. Thus, the tropics include approximately 40% of the land surface and is the largest ecozone of the earth (Fig. 1). According to Köppen (1931), the tropics are characterised by an annual mean air temperature above 18 C throughout the whole year. The largest climatic variation is introduced by the variability of precipitation, reaching from nearly 0 mm in the Saharan and Atacama Desert to 11,700 mm on Mt. Waialeala in Hawaii (Eswaran et al., 1992). Eswaran et al. (1992) define tropical soils as such with a difference between summer and winter soil temperature of less than 5 C, providing the mean annual soil temperature is >5 C (according to the iso-temperature regime of the Soil Taxonomy, Soil Survey Staff 1975). They estimate that more than one third of the soils of the world are tropical soils. In the Agro-Ecological Zones Project of the FAO (1978, 1980, 1981) the definition of Köppen has been used to map the tropical regions with their corresponding growing period zones (Table 2). In situations where temperature does not constrain plant growth, the growing period is only determined by the availability of water. Different terms and definitions, mostly related to vegetation type, precipitation and length of dry/moist season, are used to divide the tropics into different tropical ecosystems (Table 1). Since decades ago, the most widely used system of classification uses the length of the growing period to divide major types of climatic regions with an ecological implication for plant growth (FAO 1978, 1980, 1981, 1993). In an environment like the tropics, where temperature is not limiting to plant growth, the growing period is only determined by the availability of water and nutrients. Table 2 and Figure 2 to 4 show on a world wide scale the extent and areas of the tropical regions with different growing periods. For SE-Asia (including the subtropical part) a more detailed list of the distribution of growing period zones is given in Table 3. Area of zones in percent of land surface Tropical regions cover the humid tropics, the Arid Zone and the seasonally Dry Tropics (Table 1). More than 90% of these are tropical lowlands. About 40 % of the warm tropics are moist or seasonally dry for a maximum of 3 months, about 40% have a dry season of 3 to 9 months and about 20 % have a growing period of less than 3 months including the semiarid and arid parts of the Tropics (Table 2, Figure 2).

3 24 tropics subtropics subtropics San Francisco 160 New York Tropic of Cancer Dakar Bombay Lima Santiago de Chile Tropic of Capricorn Kapstadt polar / subpolar zone ice deserts tundra and areas of frost debris boreal zone moist mid-latitudes dry mid-latitudes desert and sub-desert grass steppe tropic / subtropic dry regions desert and sub-desert thorn savanna (transition area) (to summer moist tropics) thorn steppe (to winter moist sub-tropics) after J. Schultz Tokio Rangun Manila 20 0 Jakarta 20 Melbourne winter moist subtropics summer moist tropics dry savanna moist savanna permanent moist sub-tropics permanent moist tropics polar/ sub-polat zone 15 tropic / sub-tropic dry regions 11 winter moist sub-tropics 2 boreal zone 13 deserts and sub-deserts 21 summer moist sub-tropics 16 moist mid-latitudes 10 permanent moist subtropics 4 dry mid-latitudes 11 permanent moist tropics 8 sum sub-tropics ~20 Figure 1: The Ecozones of the World (Schultz, 1995) sum-tropics ~30

4 25 Table 1: Widely used terms, characteristics and definitions to divide the warm tropics (see Schultz, 1995; FAO, 1993) Desert Semidesert Thornbush Dry Moist Moist Tropics Savanna (Inner Tropics) Rain period (month) Rain (mm) ~125 ~250 ~500 ~1000 ~2000 > high variability > Runoff/Precipitation After FAO (1993) Arid Zone Seasonally Dry Tropics Humid Tropics Length of growing period 1) (days) ) Time where P > 0.5 PET plus time where storage water (soil) is available Table 2: Extent of tropical climates with different growing period zones (according to FAO, 1978, 1980, 1981) Major climate S and C America Africa SE Asia Growing period 1) M ha % M ha % M ha % M ha % Warm Tropics 2) 3) , , , ,9 >330 days , , , , days , , , , days , , , , days , , , ,4 <90 days 53 3, ,0 30 4, ,5 Cool Tropics 4) (tropical highlands) ,7 97 4,5 13 1, ,2 Cold Tropics 5) (tropical mountains) 32 2,2 3 0,1 2 0,3 37 0,9 Tropics (34 %) (50 %) 1) Time where P > 0.5 PET plus time where storage water (soil) is available 2) All months with monthly mean temperatures, corrected to sea level, above 18 C 3) 24-hr Mean (daily) temperature regime during the growing period >20 C 4) 24-hr Mean (daily) temperature regime during the growing period 5/10-20 C 5) 24-hr Mean (daily) temp. regime during the growing period <5 C, <6.5/10 for Africa 682 (16 %) (100 %) The length of the growing period as based on the average of long-term climatic observations, may be of little importance in certain years. In the semiarid tropics, years with extended drought periods are well known. Large variations are also known for the perhumid tropics. Figure 5 shows an example from Leyte (Philippines), where a dry period of four months occurred during the El Niño year of The climate of Leyte The present climate of Leyte (Figure 6) is characterised as a humid tropical monsoon climate (tropical rainforest climate, monsoon type after Köppen 1931). Because of the presence of the high central mountain range, the climate in the eastern part of the island is a little bit different to that in the western part. The former has pronounced maximum rain period with no dry season (Type II) in the Coronas system (Coronas, 1920) while the latter has no pronounced maximum rain period with no dry season (Type IV). The following discussion is based on climatic data taken principally by different stations of the Philippine meteorological stations (PAG-ASA) located in Baybay, Maasin, Ormoc and Tacloban. Additional information were taken from published reports like Barrera et al. (1954) and Coronas (1920). Rainfall: The rainfall distribution generally shows a more pronounced maximum rain period of up to about 500 mm per month in the eastern side of Leyte than in its western side with only about 300 mm per month. Although both areas have no pronounced dry season, they experience the lowest rainfall during the months of March, April and May. In terms of total amount, Ormoc Highland (>700 m asl) has higher rainfall (about 3300 mm/year) compared to Baybay (about 2600 mm/year) near sea level. The higher amount of rainfall in the Ormoc Highland can be attributed to its higher elevation and its proximity to the central mountain range which tends to enhance cloud formation. The lower amount of rainfall recorded in Baybay is due to its lower elevation (7 m asl) and greater distance to the mountain range. It has always been observed by the second author (who has been living in the plains for the last more than 20 years), that oftentimes rain falls in the mountain but not in the plains. It is also important to note

5 26 that the Ormoc Highland data are the average of only four years (there has been more rainfall in recent years) while the Baybay data are the average of 19 years. Thus, the actual difference in rainfall between the two sites is considered much less than that shown by the available rainfall data. Temperature: Average temperature at sea level in Leyte is around 27 C and seems to be higher in the eastern side than in the western side. Day and night temperatures differ by about 5 C around the mean whereas the coldest and warmest month differ only in the range of 2 C. Analysis of the temperature data given in Müller (1979) for different sites representing different elevations in the Philippines like Aparri, Baguio, Manila Legaspi, Iloilo, Davao and Zamboanga showed an average decrease of 0.6 C per 100 m rise in elevation. Therefore for the highest mountains (1,250 m) in Leyte an average temperature of about 20 C can be assumed being just at the border between the warm tropics and the moderately cool tropics (FAO 1980). Wind pattern, monsoon and typhoons: Leyte, in particular, and the Philippine archipelago, in general, experience two types of monsoonal (seasonal) winds which have tremendous effect on the over-all climate. From June to October, a southwest monsoon (locally called Habagat ) occurs. This seasonal wind blows from the southwest direction causing extreme cloud development and rainfall at the western section of the island or of the archipelago. From November to February, the northwest monsoon (locally called Amihan ) occurs which blows from the northwest direction hence causing cloud development and rainfall at the eastern sections of the island or of the archipelago. Aside from their effect on rainfall distribution, these monsoonal events could have considerable influence on the soil development through salt spray. Yaalon (1983) discussed the importance of salt spray in soil development in areas near the coast. A typhoon is another climatic event which is very important in Leyte as well as in most parts of the archipelago. It is generally characterised by strong winds and heavy rainfall such that it does not only damage the vegetation but it also enhances erosion, landslides and floods. During a strong typhoon, many trees are uprooted or their branches are broken enhancing light penetration in forested or thickly vegetated areas. In Mt. Makiling, Philippines, Cuevas and Sajise (1978) concluded that wind velocity influences very significantly the amounts of litter produced in the forest. Soil moisture and temperature regimes: Figure 6 suggests that there is available moisture in the soil in most months of the year in Leyte. This easily qualifies for the udic moisture regime. According to Soil Taxonomy (Soil Survey Staff, 1975), udic moisture regime implies that in most years the soil moisture control section is not dry in any part for as long as 90 cumulative days. In some years, for example the El Niño year of 1983 (Figure 5) a dry period of some months can occur. Soil temperature measured at a depth of 10 cm at the meteorological station located at 7 m asl elevation showed generally the same pattern in the morning (measured at 8:00 AM) and in the afternoon (measured at 2:00 PM). It is, however, lowest (about 26 C) in December and January and highest in May (30 C). This measurement was done in an open area with pure grass stand of about 20 to 30 cm high. Soil temperature in the mountains is believed to be slightly lower due to elevation and vegetation cover (except in shifting cultivation). These data as well as the data in Figure 6 reveal that the average temperature is higher than 22 C and the difference between mean summer (June-July-August) and mean winter (December-January-February) does not exceed 5 C. Thus, the soil temperature regime is classified as isohyperthermic according to Soil Taxonomy (Soil Survey Staff, 1975).

6 Tr. of Cancer Equator Tr. of Capricorn Tropics Subtropics Growing Period >330 days days days days <90 days km after FAO (1978, 1980, 1981) Cool and Cold Tropics (Tropical Highlands and Mountains) Figure 2: The Tropics and their different growing period zones S- & C-America Mio. ha Africa Mio. ha Figure 3: Growing period zones of the Philippines ( SE-Asia 682 Mio. ha Length of Growing Period (tropical lowlands, world-wide) 0 50 % 100 Cold (Mountains) Cool (Highlands) after FAO, 1978, 1980, day 2 1 / desert semidersert savanna thornb. dry moist Natural Vegetation Type 11 rain forest 12 months Figure 4: Distribution (%) of Growing Period Zones in the Tropics

7 28 Table 3: Distribution of tropical and subtropical climates and zones of different growing periods in SE Asia (according to FAO, 1980; total area is 898 million ha) Days of growing period cold dry Total Major Climates Warm tropics Cool and moderate cool tropics Cold tropics Warm & mod cool subtr., sum. rain Cool subtropics, summer rain Cool subtropics, winter rain Cold subtropics All major climates forests are among ecosystems with the highest NPP per square meter. Figure 5: Comparison of the monthly rain in the El Niño year 1983 with the long-term average in ViSCA, Leyte, Philippines (Data from ViSCA) C 50 max. ø min. 25 Ormoc Highland (700 m a.s.l.) 24.2 C, 3391 mm ( ) Ormoc (-s.l.) 26.3 C ( ) 2265 mm ( ) VISCA (7 m a.s.l.) 27.4 C, 2621 mm 500 ( ) mm 0 JFMAMJJASOND 100 pot. Evapotr Biliran Island Baybay Maasin (-s.l.) 27.1 C ( ) 2536 mm ( ) Limasawa Island Manila Tacloban (-s.l.) 27.8 C ( ) 2545 mm ( ) Panaon Island Pronounced maximum rain period, no dry season No pronounced maximum rain period, no dry season km Figure 6: Climatic pattern in Leyte, Philippines (Asio, 1996) 3. The role of the tropics in biogeochemical cycles of elements: the example of carbon Rough estimates of global net primary productivity (NPP) show the distinct role of the tropics for the global carbon cycle. Tropical and subtropical rain Regarding global change, it is believed that increasing the carbon in the atmosphere is one of the most important reasons for further changes of ecosystems. Aside from the excessive burning of fossil carbon (coal, oil, gas) in the industrialised countries, the decrease of plant biomass, soil carbon and burning of biomass are also reasons for the increase of CO 2 in the atmosphere. The active sources of C are, on a world wide scale, in the order of g soil carbon, g atmospheric carbon and g carbon in plant biomass. Carbon in the biomass of aquatic ecosystems ( g) is of significantly lower amount (Schlesinger, 1997). Estimates of global net primary productivity (NPP) show the distinct role of the tropics for the global carbon cycle. Tropical ecosystems cover less than 30% of the land surface (8% of the globe) but carry more than 50% of the vegetation mass (Table 3). Tropical forests are, aside from wetlands, the ecosystems with the highest NPP per square meter. Forests of the humid tropics carry more than one quarter of the earth s plant biomass despite covering only 7% of the land or 2% of the total surface of the earth. Carbon in tropical soils is stored within the range of the world soils average. However, distinct differences are found between different soil groups. Results of a worldwide inventory showed that due to their mineralogical characteristics, Andosols store in the 0-100cm depth more than twice (25 kg/m 2 ) the organic carbon of Ferralsols (11 kg/m 2 ) or Acrisols (9 kg/m 2 ) (Batjes, 1996). The same tendency was observed in Leyte for Andisols (near Ormoc) and Alisols (near Baybay) by Asio (1996) and Jahn & Asio (1998).

8 29 Table 3: Area, plant biomass and net primary productivity of terrestrial ecosystems (Schlesinger, 1997) Area Mean plant biomass C in vegetation Mean net primary production Net primary productivity Ecosystem Mio. km 2 % kg C/m 2 Gt % kg C/m 2 /yr Gt C/yr % Tropical wet and moist forest Tropical dry forest Tropical woodland and savannah Tropical ecosystems total Desert Temperate steppe Temperate forest Boreal forest Tundra Rock and ice Wetland Cultivated land Terrestrial ecosystems total Gt = g 4. Geology of SE Asia and Leyte As early as 1910, it was already believed that the Philippine archipelago together with Formosa (Taiwan), Japan Celebes, Borneo, New Guinea and most islands of the Malaya group were the rough shattered loose ends of the Asiatic continent and were believed to mark the border of the continental plateau (Smith, 1910). This early hypothesis seemed to agree with paleogeographic models of Asia for the Miocene (Masarowitsch, 1958) which showed that the archipelago was above water and appeared to be part of the Eurasian continent. But Wernstedt and Spencer (1967) advocated the theory of some geologists that the islands of the Philippines are merely the higher portions of submerged mountain masses arranged along lines of structural weakness (e.g. submarine volcanoes, horst and anticlinoria). Other models of plate tectonics (e.g. Nunn, 1994) show the Philippine archipelago and other SE Asian island at the horn of the Eurasian plate which converges or collides with the northwestward-moving Pacific plate. Presently, this Eurasian continental plate protrusion (Hutchison, 2005) is called the Sunda Plate (Fig. 7) which has been confirmed to be moving independently at 10 mm/yr eastward relative to Eurasia (Wikipedia). The Philippine Sea Plate, pushed westwards by the Pacific Plate, converges on the Eurasian Plate at Taiwan in a 307 direction at 86 mm/yr. The present-day physical geography of Southeast Asia is governed by these basic characteristics (see Hutchison, 2005). Figure 7. The Sunda Plate (Wikipedia) The Sunda Plate includes the South China Sea, the Andaman Sea, southern parts of Vietnam and Thailand along with Malaysia and the islands of Borneo, Sumatra, Java, and part of Celebes in Indonesia, plus the south-western Philippines islands of Palawan and the Sulu Archipelago. The eastern, southern, and western boundaries of the Sunda Plate are tectonically complex and seismically active. Relative to the African Plate, the Sunda Plate moves mm/yr (Wikipedia). Rammlmair (1993) published the most recent and comprehensive analysis of the evolution of the Philippine archipelago. Among the points in his report which are of relevance to this lecture are:

9 30 1) the archipelago lies in the field of interaction of three major plates- Eurasia, Pacific and Indo- Australia (compare Figure 8); 2) it is composed of subparallel ridges which alternate with basins more or less parallel to the bordering trenches. The ridges consist mainly of upthrust or uplifted belts of ophiolites and volcanic/plutonic complexes; 3) several collisional events locally led to strong uplift and intense folding; 4) the Philippine Fault is a young feature which postdates the collision of the North Palawan continental fragments with Panay and Mindoro (in the Miocene); 5) the evolution of the archipelago is complex and has to be seen as dynamic interaction between platelets and major plates. The fluctuations of the sea level during the Pleistocene could have helped shape the present nature of the Philippine islands. Scott (1984) presented an estimate that a rise of five to ten meters above the present levels occurred in the Philippines during the interglacial periods. For Southeast Asia, Kadomura (1995) cited some studies which put the lowering of the sea level at about 120 m during the Last Glacial Maximum or about 20,000 to 15,000/14,000 years before present. Figure 8. Relationship of the Sunda Plate to the Philippine Sea Plate. It shows the subduction of the Philippine Sea Plate below the Sunda Plate as well as the resulting Philippine Fault and the Philippine Trench. The tectonic movement and the subsequent plate convergence could have resulted in the uplift of Leyte and the other islands of the archipelago which were believed to be still under water until the middle of the Miocene or about 16 million years ago (G. F. Becker as cited by Smith, 1910) and which probably existed (as archipelago) at least since the beginning of the Tertiary (Wernstedt and Spencer, 1967). This could have also resulted in the extensive folding, faulting and volcanism throughout the archipelago. The Philippine Fault Line which runs from Northwest Luzon down to Eastern Mindanao is a major effect and evidence of this geodynamism. This left-lateral Philippine Fault, along which the slip-rate has been measured at around 26 mm/yr (McCaffrey 1996 as cited by Hutchison, 2005), must have also enhanced extensive volcanic activity during the Tertiary and the Quaternary which explains the widespread occurrence of eruptive rocks from different phases of volcanic activity. The collision and associated geologic uplift (some geologists believe that there have been several periods of widespread subsidence and uplift during and subsequent to the Tertiary) continues until the present time (Rammlmair, 1993) and explains the presence of exposed sedimentary

10 31 rocks primarily coralline limestones, conglomerate, breccia, sandstone, siltstone and mudstone which range in age from Miocene to Holocene. In many areas these are overlain by recent volcanic material particularly of acidic and basic geochemistry. Figure 9 shows the occurrence of major rock formations in the island. As can be observed, the oldest rock (Cretaceous) represented by ophiolite complex with pelagic sediments, occur in the northeastern and southern part of the island. Tertiary rock from different epochs are also widespread. These include Eocene and Paleocene altered volcanic flows and breccia, Miocene sedimentary (limestone, sandstone, stilstone and claystone) and volcanic (andesitic, basaltic and dacitic) rocks and Pliocene-Miocene volcanic sediments and pyroclastics. The youngest deposits, the Quaternary rocks, are the most widespread. This includes Pleistocene volcanic and limestone rocks which cover a wide portion of the central highlands. Also, Holocene alluvial sediments are found in the plains. Biliran Island Ormoc Samar Sea Strait of Juanico Samar Quaternary holocene alluvial sediments corraline limestone intermediate volcanics volcanic sediments Baybay Pliocene - Miocene volcanic sediments intermed. conglomerate & pyroclastics Miocene corraline limestone conglomerate, tuff breccia, sand-, silt- & mudstone calcareous conglomerate, shale & sandstone serpentinized rocks andesitic, basaltic & dacitic flows & breccia Oligocene & Eocene diorite Eocene & Paleocene Maasin altered volcanic flows & breccia Cretaceous km Ophiolite complex with pelagic sediments redrawn from DENR ' Manila Tacloban Leyte Gulf Limasawa Island Figure 9: Geological map showing the major rock formations in Leyte (Asio, 1996) Philippine Fault Line Panaon Island The very limited published information about the geomorphology of Leyte include a brief description of its physiography in the Soil Survey Report of Leyte (Barrera et al., 1954) and some broad landform and geomorphological maps prepared by the Bureau of Soils and the DENR. The information in these materials are so general hence, a new interpretation particularly of the study sites is necessary. Leyte Island is characteristically rugged and mountainous. A central mountain range (also called Leyte Cordillera) with an average elevation of more than 1000 m rises from the north and extends down south where it grows into a complex mountain range in the whole of southern Leyte. Two parallel minor mountain ranges exist in the island s northwest composed primarily of sedimentary rocks, and the other in the islands northeast composed of pre-tertiary igneous and metamorphic rocks (among the oldest rock formations in the island). These two minor mountain ranges are separated from the central mountain range by the two most important valleys of Leyte. The central mountain range is characterised by steep slopes and abundant fault scarps and is composed basically of volcanic materials of various ages except in some parts especially in the northeast (e.g. Palompon) and in the south (e.g. Maasin, Matalom, etc. ) where exposed limestones occur resulting in the formation of Karst topography (low Kegel-Karst) common in Southern Leyte. The geomorphology of the island owes its present form to the combined effects of or interactions of endogenic processes such as plate tectonics (and the resulting uplift and faulting) and volcanism, and exogenic processes such as weathering, erosion, transport and deposition. Volcanism, faulting and uplift are probably the major processes which have created the mountain ranges (compare Figure 10 and 11). Volcanism deposited the volcanic materials and the existence of the major fault and its accompanying secondary faults, produced the steep and rugged nature of the mountains. Weathering, erosion, transport and deposition could have played major role in creating the rivers and streams as well as the alluvial plains. It is hypothesised here that the central mountain range was originally much higher than its present height and that weathering and erosion have reduced it with most of the undissolved materials deposited

11 32 into the lower areas resulting in the present alluvial plains. According to the Davisian cycle of erosion (Small, 1972), landforms could either be in stages of youth, maturity and old age. Leyte, in particular the central highlands, could easily belong to the youth stage. Small (1972) said that at this stage, a system of streams develop and cut rapidly downwards and in due course form deep valleys bounded by slopes of 30 or more. On these slopes, weathering and slumping operate but at quite slow rate compared with the speed of river incision. For a long period, the valley cross-profiles approximate V-shaped, except in areas of complex geological structures where stepped profiles develop. This description fits well to the landform characteristics of most parts of Leyte. This is consistent with the geological age of the island which is relatively young. The streams which are mostly of second order morphometry, generally follow the subparallel drainage pattern although in some areas some streams tend to have angular bend pattern indicating that they follow secondary fault lines. In contrast to the unstable and periodically eroded sides of the valleys, the surfaces in between the streams seemed to have persisted for a long time, meaning they are relatively preserved which explains the occurrence of deep weathering profiles in such preserved surfaces as exemplified by the research sites particularly in Baybay. Such preservation of original surface is possible in the youth stage. Small (1972) wrote that throughout this stage, parts of the initial land surface could be preserved on the watershed between consequent streams. Jahn et al. (1995) observed that the association of the strongly eroded sites in the valleys and the deep weathered saprolites in the high plains at the Baybay research site is one argument for the existence of old erosional surface. Tacloban Ormoc Baybay Figure 11: Topography of Leyte Figure 10: Topography of the Philippines 6. Distribution of Soils in the Tropics Soil development is a function of bedrock, climatic conditions, relief, action of organism (including man) and time of soil formation (Jenny, 1961). Consequently, soils exhibit a high diversity even within a single eco-zone. In modern classification

12 33 systems, taxonomic classes and map units are determined by observable and measurable properties (e. g. specific horizons, clay cutans, content of organic matter, CEC, ph, etc.) which fall in defined classes. The globally accepted FAO- Unesco-legend (1974) distinguishes 26 major soil groups and 106 soil units. The soil map of the world (1 : 5,000,000) proves, that 24 of the major soil groups and 90 of the soil units are located within tropical regions reflecting a very high soil diversity even on a very large mapping scale. Since some soil forming conditions are more specific for the tropics (e. g. high temperature, high precipitation, old land surfaces) than for other regions, some soils occur almost exclusively within the tropics. About 90% of the Ferralsols, 80% of the Nitosols, and 60% of the Acrisols are situated in tropical regions. As considerable data have shown, Ferralsols and Acrisols are the most weathered soils having low cation exchange capacity, low content of weatherable minerals and in most cases low base saturation. In decreasing order, Vertisols, Fluvisols, Arenosols, Gleysols, Planosols as well as Luvisols (including Lixisols) are found in parts from 42 to 35% in the tropics (compare data of Table 4 with FAO, 1993) and are therefore also typical soils in other eco-zones. According to FAO (1978, 1980, 1981), more than 50% of the tropics are covered by only four major soil groups (Ferralsols, Acrisols, Luvisols and Arenosols), giving an impression that the tropics have generally larger landscapes than other zones as a result of the higher age of the land surfaces. As Table 4 and Fig 12 would show, 74% of the area are covered by seven soil groups and about 90% of the area from eleven soil groups. Aside from such distribution, distinct differences among individual tropical regions can also be recognised. The American tropics are dominated by Ferralsols, Acrisols and Gleysols (62% of land area). Ferralsols, Arenosols, Luvisols and Lithosols (52% of land area) are important in Africa, while Acrisols, Cambisols, Luvisols and Lithosols (53% of land area) are more specific for SE Asia (Table 4). The large proportion of Cambisols and Luvisols in SE Asia reflects clearly the younger age of land surfaces and therefore reduced duration of weathering processes. In 1988, an Anthrosol major soil group was introduced into the FAO legend (FAO-Unesco, 1988) to include those soils in which human activities have resulted in a profound modification or burial of the original soil horizons, cuts and fills, secular additions of organic material, longcontinued irrigation, etc. The main occurrences of Anthrosols are known from SE Asia, subtropical China, Korea and Japan and are largely related to paddy fields. Table 4: Distribution of major soil groups in the warm tropics (after data in FAO, 1978, 1980, 1981; % based on area of the warm tropics) Region: S. and C. America Africa SE Asia Soil group: M ha % M ha % M ha % M ha % Ferralsols Acrisols Luvisols Arenosols Lithosols Gleysols Cambisols Regosols Nitosols Fluvisols Vertisols

13 S- & C-America Mio. ha Figure 12: Distribution (%) of major soil groups in the tropics Nitosols Regosols Gleysols Africa Mio. ha 0thers Arenosols Leptosols Order according to world-wide (Tropics) distribution based on World Soil map (FAO, 1978, 1980, 1981; changed to FAO-Legend 1988) Plinthosols Cambisols Ferralsols Acrisols Luvisols SE-Asia 682 Mio. ha A proper judgement of soil diversity is also a question of observation scale. The FAO soil map of the world, the best information we still have today on a global scale, dates back to the 1970s. For large regions, its compilation was based on reconnaissance work and rather general information with only few references to local soil data. Therefore, evaluations based on the generalised world soil map can result in misconception and errors (Eswaran et al., 1992). The minimum decision area (smallest area on a map from which reliable information can be derived for interpretations) in a map of a scale of 1 : is about ha. This does neither conform to the scale of the landscape nor to the farmers needs. Recognising their variability within a certain landscape, maps in scales between 1 : to 1 : should be used for the proper management of the soil resources. Such maps have a minimum decision area between 6 and 0.4 ha (FAO, 1986; Eswaran et al., 1992). 7. Soils of Leyte The occurrence and distribution of soils in Leyte is greatly affected by geology and geomorphology. Well-developed soils are found on stable old surfaces or on areas underlain by old rock formations. But poorly developed soils also occur over old rock formations when the surface is unstable and subject to erosion, landslide and other disturbances as exemplified by steep slopes. The effect of the rock type is shown by the occurrence of calcareous soils from limestones which clearly separates them from those soils developed from volcanic materials. Very young or very poorly developed soils are also found in the alluvial plains. The poor development is caused either by the youthfulness of the sediments or by waterlogging which inhibits soil development. Barrera et al. (1954) classified the soil types of Leyte into poorly drained flat lowland, moderately drained flat lowlands, well-drained flat lowland and well-drained rolling uplands. We attempted to approximate the soil orders (Soil Taxonomy) of the different soil series/types mentioned above and observed that Ultisol, Alfisol, Inceptisol (and Andisol) and Entisol appear to be the major soils in the island. Moreover, results of a characterisation study of the acid soils on upland areas of Leyte revealed (unpublished data of the second author) that many of the soils can be classified as Inceptisols (include Andisols) and Ultisols. From the 1976 FAO-UNESCO Soil Map of the World (scale 1 : 5,000,000), the major soils in Leyte include Orthic Acrisols (which are now split on the basis of their clay activity into Acrisols [low CEC] and Alisols [high CEC]) with inclusions of Humic Acrisols and Dystric Nitosols in the central mountain range, Gleyic Cambisols with inclusions of Gleysols and Pellic Vertisols in the central western lowland (Ormoc-Baybay-Bato area), Eutric Gleysols with inclusions of Gleyic Cambisols and Eutric Cambisols in the central eastern lowland (Abayog-Burauen-Carigara area), Dystric Nitosols

14 35 with inclusions of Eutric Cambisols and Orthic Luvisols in south-and northwesertn parts (Matalom-Maasin area, San Isidro-Tabango area) including a small portion in the northeastern part of the island, and Orthic Luvisols with inclusions of Orthic Acrisols and Ferrasols in the southern tip and northwestern part of Leyte (Isabel area). In Mt. Pangasugan, Alisols (Ultisols) dominate the lower slopes. These soils are acidic, red, deep with generally low nutrient status and kaolinitic and halloysitic mineralogy. In the upper slopes (from about 300 m asl), Andosols (Andisols) which are young volcanic soils are widespread. These soils have high organic matter content, contain shortrange-order clay minerals (allophane and imogolite), and have relatively high nutrient status (except P which is very low due to extremely high P retention capacity of soils) (Asio, 1996; Zikeli, 1998). Juo and Franzluebbers (2003) classified tropical soils into kaolinitic, oxidic, allophanic and smectitic soils. 1) Kaolinitic soils are highly weathered soils having a clayey texture B horizon and kaolinite (> 90%) as the dominant mineral in the clay fraction. They have an effective CEC of less than 12 cmol/kg of clay in the lower B horizon. These soils have a relatively high bulk density, especially in the clayey subsoil horizons (> 1.40 Mg/m 3 ). 2) Oxidic soils are strongly weathered red and yellowish, fine-textured soils that typically have low bulk density ( Mg/m 3 ) and friable consistency due to the large amounts of stable microaggregates throughout the profile. In addition to the dominant clay mineral which is kaolinite, these soils have moderate amounts of clay-size Fe oxides and hydrous oxides (> 5% Fe 2 O 3. The red oxidic soils contain predominantly high surface area hematite, while the yellow oxidic soils contain predominantly high specific surface area goethite. They have low water-holding capacity, low nutrient reserve, and high phosphate "fixation" capacity. 3) Allophanic soils are dark-colored young soils derived mainly from volcanic ash and they have low bulk density (< 0.9 Mg/m 3 ), high waterretention capacity (150% by weight), and contain predominantly allophanes, imogolite, halloysite, and amorphous Al silicates in the clay fraction. Theys are very productive and are restricted to areas with active volcanoes throughout the world. In tropical regions, allophanic soils are found in Central America, the Caribbean, the Pacific Islands, Indonesia, and the Philippines. 4) Smectitic soils are alluvial soils containing moderate to large amounts (30% or more) of smectite, a shrinking and swelling clay mineral, and small amounts of other 2:1 silicate minerals such as illite, vermiculite and chlorite in the clay fraction. They have high base saturation and water retention capacity and are among the most productive soils for rice cultivation. 8. Site qualities and main constraints of tropical soils Generally, site properties are very complex qualities which have to be estimated from observable and measurable site characteristics and consequently need to be evaluated for specific land uses (FAO, 1976). For the purpose of ecological land evaluation, the large number of characteristics should be summarized in statements about possible rooting depth, rootability and water-, air-, energy-, and nutrient budgets (Schlichting et al., 1995). Additionally, statements about elasticity and stability of a site under a specific use are needed to establish sustainable land use. Since many site characteristics are the result of pedogenesis, the development of site properties is also closely related to the time-dependent development of soils. Usually, land evaluation concepts qualify site characteristics as time dependent for the period of survey only, but this period is relatively short compared to the time required for soil formation. Common soil classification systems (Soil Survey Staff, 1975; legend of FAO-Unesco, 1974, 1988; ISSS, 1998) are more or less natural classification systems and based on records of soil properties. Since soil properties are derived from observed and measured soil characteristics, it is possible to develop technical soil classifications which reflect information about land use capability or needs for soil management for a certain land use. In a generalised way, it is possible to derive some main constraints for land use from soil maps. For South America this was demonstrated with the Fertility Capability Soil Classification System (FCC) of Sanchez et al. (1982). Following the example of Sanchez et al. (1982) and Smith et al. (1990), the information about soil types in the tropics from FAO (1978, 1980, 1981) are grouped in Table 7 to recognise the main constraints for land use in the tropics. For SE Asia, the same constraints are

15 36 shown in Table 8 depending on the different growing period zones. Based on the distribution of soils in the tropics, most chemical soil properties such as low cation exchange capacity and low base saturation are limiting constraints which lead to a low natural soil fertility and which makes the soils susceptible to the loss of nutrients by leaching. These properties are related to the highly weathered soils (Ferralsols, Acrisols), to leached soils (Planosols, Podzols, Podzolluvisols), very sandy or quartzitic soils (Arenosols) and in the case of low CEC also to very young soils (Regosols). Table 5 and Figure 13 show some important properties of soils mostly occurring in the tropics. Kauffmann et al. (1998) investigated the data of 149 soil profiles of the humid tropics and found that soil chemical limitations dominate over physical limitations (Table 6, Figure 14). Table5: Physical and chemical characteristics of important soils in the humid tropics (selected data from Kauffmann et al., 1998) Ferralsols Acrisols Luvisols Cambisols Arenosols Soil characteristic topsoil subsoil topsoil subsoil topsoil subsoil topsoil subsoil topsoil subsoil Sand (%) Clay (%) Bulk density (g cm -3 ) ph KCl Organic Carbon (%) C/N ratio CECpH7(cmolc kg -1 ) Base saturation (%) Exchangeable Al (cmolc kg -1 ) Figure 13: Physical and chemical characteristics of important soils in the humid tropics

16 37 Table6: Frequency (%) of constraints on land quality of soils in the humid tropics (selected data from Kauffmann et al., 1998) Degree of constraints: no weak moderate serious very serious Land quality: Rootable volume Soil moisture availability Oxygen availability Nutrient availability Nutrient retention capacity Al toxicity Land quality Degree of Constraints No Weak Moderate Serious Very serious Rootable volume Oxygen availability Soil moisture availability Nutrient availability Nutrient retention capacity Al-Toxicity after Kauffmann et al., % Figure 14: Frequency (%) of constraints on land quality of soils in the humid tropics Following the example of Sanchez et al. (1982) and Smith et al. (1990), the information about soil types in the tropics from FAO (1978, 1980, 1981) are grouped in Table 7 to recognise the main constraints for land use in the tropics. For SE Asia, the same constraints are shown in Table 8 depending on the different growing period zones. About 64% of soils in the tropics have low cation exchange capacity and/or low base saturation. In SE Asia, due to the occurrence of more fertile soils such as Cambisols and Fluvisols, the area affected by these limitations is significantly smaller (48%). High P-retention is characteristic of soils which have high contents of Low Activity Clays (LAC) and of sesquioxides and leads to the high P-fixation capacity of these soils. This property is strongly associated with Andosols, which contain high amounts of short-range-order clay minerals and Fe oxides. It is also related to most Ferralsols as well as to the plinthic and ferric soil units with very high enrichment of Fe- and Al-oxides and to many intensively red coloured soils which in many cases have also high amounts of Fe-oxides. Therefore nearly three quarters of the soils in the tropics (in SE Asia 62%) have either low CEC or low base saturation or high P-retention or a combination of these. Lowlands are a major landform in the tropics. They include vast flood plains or broad river valleys where Gleysols with a high groundwater table occur. In SE Asia these soils are widely used for lowland rice production. Soils which are limited by soil depth occur almost only in the tropical highlands and mountains. Other constraints like vertic properties, high lime content and enrichment of sodium and salts are related to the drier parts of the tropics. These constraints are generally of minor significance within the warm tropics but are severe land use limitations in landscapes with a short growing period due to low precipitation. The proportion of areas with none of the constraints listed above is generally very low in the tropics, reaching a maximum of 11 % in SE Asia. The constraints derived from the soil map of the world (Table 7 and 8) conforms very well with the findings of studies based on real soil data.

17 38 Table 7: Main constraints of soils in tropical regions (% based on area of the warm tropics) Region: America Africa SE Asia Constraints: % % % % CEC; low cation exchange capacity 1) BS; low base saturation 2) P-ret; high P-retention 3) Ground- and stagnant water 4) Low soil depth 5) Clayey (vertic) 6) High content of lime and gypsum 7) High Na-saturation 8) High salt content 9) The most important combinations CEC/BS CEC/BS/P-ret BS/P-ret None of the above constraints ) Ferralsols, Acrisols, Planosols, Regosols, Arenosols, Podzols, Podzoluvisols, ferrallic- and ferric units 5) Lithosols, Rendzinas, Rankers 2) Ferralsols, Acrisols, Histosols, Podzols, Podzoluvisols, ferralic-, ferric-, 6) Vertisols, vertic units dystric-, humic- and albic units 7) Calcic, calcaric and gypsic units 3) Ferralsols, Nitosols, Andosols, plinthic, ferric, chromic- and rhodic units 8) Solonetz, solodic units 4) Gleysols, Planosols, Histosols, gleyic units 9) Solonchaks Table 8: Distribution of tropical and subtropical climates with different growing period zones and main constrains of soils in SE Asia (after FAO, 1980; total area is 898 million ha) Major climate / growing period cold dry warm Tropics cool and moderate cool Tropics cold Tropics warm and moderate cool subtr., sum. rain cool subtropics, summer rain cool subtropics, winter rain cold subtropics All major climates Main constraints of soils CEC; low cation exchange capacity BS; low base saturation P-ret; high P-retention Ground- and stagnant water Low soil depth Clayey (vertic) High content of lime a. gypsum High Na-saturation High salt content Sulphuric (thionic) The most important combinations CEC/BS CEC/BS-P BS/P BS/gleyic None of the above constraints Selection see Table 7

18 39 Soil cold Length of Growing length ogperiod growing period constraints days < low CEC low BS high P-ret. gleyic soil depth vertic high Ca-saturation high Na-saturation salts sulphur none % 898 mio. ha Figure 15: Constraints of soils in the different growing period zones of SE Asia (Tropical and subtropical climates) 9. Ecological land use systems require intense nutrient cycling As demonstrated above, soils of the tropics need special attention regarding their nutrient status and they must be managed specifically according to the type of soil. Nitrogen The availability of N is coupled to the interaction of the atmosphere, biosphere and the upper part of the pedosphere. Under natural conditions, tropical soils have around 100 t/ha organic carbon (estimated from the data of Kauffmann et al., 1998) giving a stock of about 8 t N/ha (C/N~15) which is moderately high. Soils like Arenosols (see Table 5 and Figure 13) have distinctly lower carbon and nitrogen contents. The C/N ratio in Cambisols is more favourable than in Acrisols, Ferralsols and Luvisols (Table 5). As a rule, the turnover of carbon and nitrogen in tropical soils is enhanced by high temperature and moisture and therefore the availability of N in many soils is not the limiting factor. In addition, in the humid tropics with a high frequency of rainstorms, the natural supply of N from the atmosphere is rather high. Through soil tillage carbon and nitrogen stocks are being reduced to less than 50%. As biomass is extracted from the system during harvest in high yielding agro-ecosystems, agro-forestry, alley cropping, mulching, green manuring, minimum tillage and multiple cropping are proven techniques to stabilise the stock of carbon and nitrogen and minimise also soil erosion (FAO, 1989; Ragland & Lal, 1993; Kang et al., 1998). In general, the aim of any management should be to stabilise the level of biomass production and of soil carbon near that of natural systems. Cation exchange capacity and available K, Mg, Ca CEC is an important soil characteristic to describe the ability of soils to retain nutrients especially for K, Mg and Ca. The availability of these nutrients can be described by the base saturation. Highly weathered soils such as Ferralsols and Acrisols and the highly quartzitic soils such as Arenosols have low to very low CEC, while the less weathered Cambisols and Luvisols with considerable amounts of clay have moderate to high CEC which is associated with moderate to high base saturation (Table 5 and Figure 13). In strongly weathered soils, the dominance of two-lattice clay (kaolinite) and high leaching rates are the main reasons that these tropical soils have serious problems with the nutrient retention capacity (Table 5 and Figure 13). In agro-ecosystems, this constraint leads to significant yield reductions or even no yield at all. Traditional land use systems with periodic burning of the vegetation and long fallow periods (shifting cultivation) up to more than 20 years have been an effective method to avoid this problem under lower population pressure. Organic substances have a much higher CEC (~250 cmol c kg -1 ) than clay minerals (kaolinite <10 cmol c kg -1 ). This is another reason to stabilise the stock of organic carbon in soils. Since biomass production and harvest are always connected with the extraction of nutrients, the cycling of the unused biomass within farming systems is the best way to minimise nutrient