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.

The origin of the catena concept

The catena concept in soil science comes from the Latin word “catena” which means chain. So it is a chain of soils linked by topography. It also refers to a sequence of soils in different positions in the landscape. It was introduced to the scientific literature by Geoffrey Milne (1898-1942) in a paper entitled “Some suggested units of classification and mapping particularly for East African soils” published in Soil Research-Bodenkundliche Forschung, Supplement to the Proceedings of the International Union of Soil Science Vol. IV No. 3 (1935), pp: 183-198. He noted “the regular repetition of a certain sequence of soil profiles in association with topography” in East Africa which was also observed earlier (in 1911 and 1912) by the German Peter Vageler. Milne wrote that "a distinctive word is needed in referring to this phenomenon" hence, he proposed the word catena.

Ernst Schlichting (1923-1988) who was a professor at the University of Hohenheim in Stuttgart pioneered the approach of considering the soil always as part of the landscape. He proposed that soils in different positions in the catena exchange materials through transport processes and thus could be compared to the transfer processes between horizons in a soil profile. He and his students have shown that the downward transport of solids or solutions may lead to a direct or indirect linkage between catena elements (Sommer and Schlichting, 1997; see above figure). In Schlichting’s view, the genesis of soil can only be understood if its relation to the other soils in the catena is taken into consideration.

Typical catena in the volcanic areas of Leyte, Philippines

Catena is now also widely used in other sciences particularly ecology albeit with a slightly different meaning (e.g. a catena of terrestrial ecosystems).

Soil as component of landscapes and ecosystems

Landscape is a three dimensional section of the Earth’s surface with specific pattern of topography, rocks, soil, water and flora and fauna. E. Schlichting (1923-1988) proposed that soils in different positions in the landscape (or catena) exchange materials through transport processes which could be compared to the transfer processes between horizons in a soil profile (Schlichting, 1964). Landscape pedology is an emerging science focusing on soil as part of the landscape particularly on the variability of soil properties at the landscape scale (1-10km) (Sommer, 2006).

Ecosystem is a natural system consisting of a biosystem (community of organisms) interacting with the geosystem (its physical environment). The geosystem includes soil, water, relief, and climate. Soil is a major component of geosystem in that it provides nutrients, water and living space to the organisms in the ecosystem. Two emerging fields of science are ecopedology and geoecology. The former focuses on the ecological role of soil while the latter on the geosystem (soil, rock, water) component of ecosystems.

The two major types of ecosystems are the terrestrial ecosystem (or ecosystem on land) and aquatic ecosystems. In the humid tropics, a common landscape consists of the following terrestrial ecosystem types: forest, agricultural (agro-ecosystem), wetland, urban and mangrove. All the terrestrial ecosystem types are linked by the soil. The transfer of water, nutrients and soil material occurs largely in the soil. The soil also determines to a great extent the biological system that develops in each terrestrial ecosystem type. Further, degradation of the soil in the terrestrial ecosystem also affects the health of the aquatic ecosystem nearby. For instance, in many places in the Philippines, severe soil erosion in the uplands causes heavy siltation of the nearby marine area and thus affecting the survival of marine organisms.

References

Schlichting, E. 1964. Einführung in die Bodenkunde. Verlag Paul Parey, Hamburg, 93pp.

Sommer M. 2006. Influence of soil pattern on matter transport in and from terrestrial biogeosystems- a new concept for landscape pedology. Geoderma 133: 107-123.

What is soil?

Soil or pedosphere is the thin and fragile skin of the Earth. It is a dynamic natural body that results from the transformation of rocks and sediments by various physical, chemical and biological processes under the influence of climate, topography, organisms and time. Human activities have altered the environment and soil processes in many parts of the Earth that is why man is considered the sixth factor of soil formation.

Soil is the foundation of human civilization. In ancient times, productive soils (i.e. good quality soils) have supported thriving civilizations but soil degradation due to misuse caused the downfall of several of them such as the Mesopotamian and Lydian kingdoms in the Mediterranean region and the Mayan civilization in Central America (Lal, 2006). In recent decades, hunger in certain parts of the world like Africa has been largely caused by unproductive soils. In the Philippines, most of the poorest rural communities are located in degraded lands.

Reference

Lal, R. 2006. Soil science in the era of hydrogen economy and 10 billion people. The Future of Soil Science. IUSS. pp: 76-78.