Tropical soils: some important aspects of these less understood soils

Tropical regions occur between the Tropic of Cancer and the Tropic of Capricorn. The tropics include approximately 40% of the land surface and is the largest ecozone of the earth. According to Köppen (1931), the tropics are characterized 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).

An Afisol (Luvisol) soil derived from mudstone in Eastern Samar, Philippines

According to Uehara and Gillman  (1981), "tropical soils" is a common name used to identify any soil that occurs in the tropics. They noted that like most common names, the term lacks precision, but it is more readily understood by a larger audience than are the scientific names. In contrast, Sanchez (1976) argued against the use of the term "tropical soils" since it does not accurately reflect the soils in the tropics. 

Selected properties of the major tropical soils (Jahn and Asio, 2006)

The name tropical soils is now globally accepted but these soils have remained poorly understood until now. The following are some important aspects of tropical soils (Jahn and Asio , 2006):

1. The tropics,  the world’s largest ecological zone, have a very high potential for plant growth but with soil limitations in vast areas.
2. About one-third of the soils of the world are tropical soils. The most widespread are Ferralsols, Acrisols, Luvisols, Cambisols, and Arenosols.

3. The large proportion of Cambisols (Inceptisols) and Luvisols (Alfisols) in Southeast Asia re­flects clearly the younger age of land surfaces and therefore the short duration of weathering processes.

4. Some soils occur almost exclu­sively within the tropics. About 90% of the Ferralsols (Oxisols), 80% of the Nitisols (Oxisols/Ultisols), and 60% of the Acrisols (Ultisols) are situated in tropical regions.
5. The major soil limitations or soil constraints  are  low cation exchange capacity, low base saturation (low pH, high Al-saturation) and high P retention. They are most widespread in South America, Africa and Southeast Asia (in decreasing order based on area).
6. Physical constraints like high groundwater table, air deficiency, and low soil depth are of lesser significance but govern special requirements for soil management in specific landscapes.
7. Due to severe chemical limitations, proper management of nutrients is the main challenge for effective land-use systems in the tropics.
8. Internal and external fluxes of nutrients are different among soil types and different among tropical landscapes. These have to be considered in ecological land-use systems.
9. To conserve the stock of organic matter in tropical soils (and to increase it in degraded soils), biomass productivity will be a key point for ecological land-use systems.
10. To enable policy-makers as well as land users to establish sustainable and ecological land use systems in the tropics, more precise soil maps and soil information are needed.

References

Eswaran H., J. Kimble, T. Cook & F.H. Beinroth. 1992. Soil diversity in the tropics: Implications for agricultural development. In: Myths and Science of Soils in the Tropics. SSSA Special Publ. No. 29.

Jahn R. and V.B. Asio. 2006. Climate, geology and soils of the tropics with special reference to Southeast Asia and Leyte (Philippines). In: Proc. 11th International Seminar-Workshop on Tropical Ecology, 21-25 Aug 2006, VSU, Baybay City, Leyte, pp: 23-42.

Köppen W. 1931. Grundriss der Klimakunde. W. de Gruyter & Co., Berlin

Sanchez, P.A. 1976. Properties and Management of Soils in the Tropics. Wiley, New York

Uehara G. and G. Gillman. 1981. The Mineralogy, Chemistry, and Physics of Tropical Soils with Variable Charge Clays. Westview  Press, Boulder Colorado.

Melanterite Soil: A green soil in the highlands of Samar

A soil at the heart of Samar, the third largest island of the Philippine archipelago, and along the Paranas-Taft road at about 300 m above sea level (within the Samar Island Natural Park) easily catches the attention of travellers. This is because it is unique: it is green in color. As far as I know, no soil with such color has yet been reported in the scientific literature.

The melanterite soil near the Bagacay mining area in Samar island

The dominant green color is probably due to the abundance of the secondary mineral called melanterite, a hydrated iron sulphate mineral (FeSO4.7H2O) formed from the decomposition of pyrite or other iron minerals due to the action of surface waters. Melanterite is known to be stable only under highly acidic condition. It is commonly found in mines as a post-mining formation on mine walls, in sulfidic sedimentary and metamorphic rocks as well as in coal and lignite deposits. It indicates the possible presence of sulfuric acid and should not be handled with bare hands or inhaled (www.mindat.org).

Photo of the site along the highway in Central Samar where the melanterite soil occurs

The green soil we have examined in Samar developed from mudstone interlayered with coal deposit. The site is not far from an area which was mined for coal and pyrite and thus it appears to satisfy the environmental conditions favorable for melanterite occurrence.

We had the chance to examine the soil during our fieldwork in Samar on 2-3 Feb 2012 as part of my graduate course in pedology (Soil Science 212). We plan to conduct a detailed pedological and geochemical study on this soil in the near future. For easy reference, I suggest to call it “Samar melanterite soil”.

Recent updates: In the book "Assessment, Restoration and Reclamation of Mining Influenced Soils" edited by Prof. Jaume Bech (University of Barcelona) and his colleagues and published by Academic Press, London, in 2017,  the occurrence of melanterite mineral in some mining-affected soils from Spain and Portugal has been mentioned. This seems to confirm our observation about the green soil in Samar in 2012.

(Members of the team: Ariel Bolledo, Mark Moreno, Pearl Carnice, Richel Lupos, Dr. Ian Navarrete (Humboldt Research Fellow), Forester Elpidio Cabahit Jr. from the Samar Island Natural Park, and myself (VBA)).

Relation between properties and age of soils in the Amazon forest

The Amazon Basin is that part of South America drained by the Amazon River and its tributaries. It has a tropical climate with an annual rainfall of 1500-2500mm, and a day temperature of 30-35 degrees Celsius (Wikipedia).

Much of what we now know about tropical soils was derived from many years of research in the Amazon rainforest. It is now widely known that this very important rainforest is growing on largely infertile and highly wethered soils called Ferralsols in the IUSS World Reference Base classification or Oxisols in the USDA Soil Taxonomy (see photo of typical soil profile).

It has been suggested by some ecologists that the efficient nutrient cycling and the periodic dust deposition from Africa explain why the infertile soils are able to support the lush rainforest vegetation.

In the recent issue of the international journal Biogeosciences Discussions, Quesada and colleagues reported the results of their interesting study on the soils in the Amazon Basin. Highlights of their findings are as follows:

1. There were large variations of soil chemical and physical properties across the Amazon Basin. The properties varied, as predicted, along a gradient of pedogenic development or in other words with soil development. Contrary to the popular notion especially among ecologists and foresters, the study showed that the Amazon soils varied from young to old soils (e.g. Gleysols and Cambisols to Alisols, Acrisols and Ferralsols).

2. Nutrient pools increased slightly in concentration from the youngest to the intermediate aged soils after which it declined gradually in the older soils. The lowest values of nutrients were found in the most weathered (or oldest) soils.

3. Soil physical properties were strongly correlated with soil fertility, with favorable physical properties occurring in highly weathered and nutrient depleted soils. The least weathered and more fertile soils had higher incidence of limiting physical properties.

4. Soil phosphorus concentrations varied with the degree of weathering. Higher P concentrations were observed in younger than in older soils which agreed with results of earlier chronosequence studies like that of Walker and Syers (1976).

5. Phosphorus availability in the younger soils was governed by the weathering of the primary and secondary minerals (particularly apatite) which in turn was controlled by soil pH.

Reference

Quesada CA, Lloyd J, Schwarz M and co-workers. 2009. Chemical and physical properties of Amazon forest soil in relation to their genesis. Biogeosciences Discussions 6: 3923-3992.

Characteristics and formation of rain forest soils from Quaternary basalt in Leyte, Philippines

The classical view about soils of tropical rain forest ecosystems is that these soils are old, acidic, and infertile. It is now widely acknowledged that this view which has greatly influenced research and management of the fragile rain forest ecosystem during the last several decades is largely a misconception. Although highly weathered soils (Oxisols or Ferralsols) are the most dominant soils in the humid tropics, tropical soils range from relatively young fertile soils (e.g. Inceptisols) to the highly weathered infertile soils (e..g. Oxisols). The extent of highly weathered soils is less in geologically young areas like in much of SE Asia.

More detailed investigations of rain forest soils are vital for the sustainable management of this threatened ecosystems. These could also lead to a better understanding of the response of rain forests to climate change.

Navarrete et al. (2009) recently conducted a study to evaluate the physical, chemical and mineralogical characterisitics of rain forest soils in Leyte, Philippines. Some of the important findings of that study include:

1) Soils along the catena studied showed minimal variations in their morphological, physical and chemical properties. This has important ecological implications as it tends to not support the idea that high soil spatial variability at short distances in rain forest ecosystems is a major factor for its high biodiversity.

2) The dominant soil-forming processes that produced the soils in the study area are weathering, loss of bases and acidification, desilification, ferrugination, clay formation and translocation, and structure formation. The loss of bases and acidification due to rapid leaching are shown by the low base saturation, very low exchangeable bases, acidic pH, and the low contents of total Ca, Na, Mg, and K. The degree of desilification is almost unifrom in all soils and may have reached 12-19% of that found in the parent material. Ferrugination is shown by the increased loss of bases, halloysitic and kaolinitic mineralogy, high contents of iron oxides and low base saturation. Clay formation and translocation are reflected by the high clay contents particularly in the middle part of the soil profile. Soil structure formation is exhibited by the good soil physical condition.

3) The nature of the basalt parent rock and the climatic condition prevailing in the area as well as its relief appear to be the dominant factors affecting the development of the soils.

Reference

Navarrete IA, K Tsutsuki, VB Asio, R Kondo. 2009. Characteristics and formation of rain forest soils derived from late Quaternary basaltic rocks in Leyte, Philippines. Environmental Geology 58: 1257-1268.

Weathering of basalt and clay mineral formation in Leyte, Philippines

Weathering is the physical, chemical, and biological alteration of minerals in rocks, sediments, and soils at or near the Earth’s surface. It is an important link in the global rock cycle and is also an essential process for the formation of soils and landforms. Chemical weathering of silicate minerals which comprise over 90% of the Earth’s crust, removes CO₂ from the atmosphere so it helps regulate the Earth’s climate over long time scales. Basalts are among the more easily weathered crystalline rocks thus, weathering of these rocks acts as a major CO₂ sink. Chemical weathering of rocks likewise releases nutrient elements for use by the biota in the ecosystem and also produces clay minerals which are the central components of soils.

We studied the weathering of basalt by evaluating the gain and loss of elements, stream water composition, weathering indices, and clay mineral formation in the soil derived from basalt under the humid tropical conditions (average annual rainfall of 2700 mm and an average temperature of 28^oC) in Leyte, Philippines. The study site is located in the rain forest on the lower western slope of Mt. Pangasugan having an elevation of 100 m asl. The weathering profile studied is about 4 meters deep, heavy clay, acidic and yellowish red soil classified as Alisol (or Ultisol).

Results revealed that much of the basic cations Ca, Mg, K, Na, and part of Si have already been lost from the weathering product (saprolite and soil). This was however accompanied by the accumulation of Al, Fe, C, and H2O. The extent of weathering as indicated by the loss of elements based on the total elemental composition of fresh rock and of saprolite and soil was closely related to the cation composition of the stream water in the study site. Relative rates of loss of bases and silica revealed the sequence: Ca>K>Na>Mg>>Si for the soil, and Ca>Na ≥ Mg>K for the stream water. The ratio Na : (Na+Ca) of the stream water indicated that its major source of cations was rock weathering.

Results also showed that the intensive basalt weathering has resulted in the formation and abundance of kaolinite and halloysite clay minerals (see above TEM micrograph) as well as goethite in the highly weathered soil. The idea that weathering moves to a system composed of SiO₂, Al₂O₃, Fe₂O₃, and H₂O (residua hypothesis of Chesworth) appears to be supported by the results of this study.

Reference
Asio VB and R Jahn. 2007. Weathering of basalt and clay mineral formation in Leyte, Philippines. Philippine Agricultural Scientist 90 (3): 204-212.