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.

Some notes on the soils and use of fertilizers and pesticides by vegetable farmers in Claveria, Misamis Oriental, Philippines

The gently rolling topography which typifies a large portion of the volcanic landscape in Claveria makes it ideal for intensive large-scale vegetable production. The widely grown vegetables include cabbage, beans, tomato, sweet pepper and eggplant.

The breathtaking volcanic landscape of Claveria, Misamis Oriental

But the strongly weathered soils which range from Oxisols in the lower slopes (about 400 to 600 m above sea level or asl) to Ultisols in the upper slopes (about 600 to 900m asl) are a major constraint to vegetable production in the area. Oxisols (also called Ferralsols) and Ultisols (also called Alisols and Acrisols) are clayey, reddish, acidic and nutrient-poor soils although they generally have good physical properties like good structure and moderate to high porosity. As in other volcanic landscapes, the oldest and most infertile soils (Oxisols) are formed on the older and stable lower slopes.

Dr. Apol & Nelds Gonzaga, Ruby Gabaca, Dr. Steve Harper & myself in front of an Ultisol soil at 920m asl. 

Farmers are apparently aware of the chemical and nutrient limitations inherent in these soils. That is why they apply lime and a variety of chemical and organic fertilizers. Rates of application are, however, not based on soil/plant tissue analysis but on what the farmers perceive as necessary. Thus, the rates appear to be insufficient in the case of lime, but excessive for the chemical fertilizers. This undoubtedly increases the production cost and can lead to more soil and environmental problems like acidification and groundwater pollution, respectively.

Heavy fertilizer application is done starting at planting of vegetables

Pest and diseases are also greatly affecting vegetable production in the Claveria landscape. As a result, farmers practice excessive application of pesticides which poses a serious threat to the health of the farming families, the consumers in urban centers, and the environment in general. The lack of awareness among farmers about the proper application of pesticides can be seen from their improper handling of these hazardous chemicals and from the fact that they just leave the pesticide containers at the farm borders.

It is common for farmers to mix two pesticides with water and spray the cocktail to the vegetables twice a week

The above observations strongly justify the urgent need for research on soil and nutrient management as well as integrated pest management in Claveria.

Available K in highly weathered soils in Leyte and Samar, Philippines

by Ivy L. Capilitan
Department of Agronomy and Soil Science
Visayas State University, Leyte

Soil potassium (K) may be divided into four pools: water-soluble, exchangeable, non-exchangeable, and matrix K (Darunsontaya et al. 2012). With K depletion by plants, water-soluble K and exchangeable K are lowered to minimum levels characteristic of soils. Brady (1974) reported that approximately 90 to 98% of the total soil K is in the non-exchangeable form (although some become available very slowly due to weathering) and is part of the internal structure of clay mineral particles and parent rock material. This form is not available for plant uptake. Feldspar is a general term for a group of aluminosilicate minerals containing Na, Ca or K. Along with mica, feldspar contains most of the K. The plants cannot use the K in this crystalline insoluble form but over long periods of time, these minerals weather (breakdown) and K is released. This process however is too slow to supply the full K needs of field crops yet as soon as these minerals weather, some K moves to the slowly available pool.  

Approximately 1 to 2% K is in the exchangeable form and is lightly bound or held (adsorbed) on the surface of clay particles and organic matter. This form becomes available rapidly and easily to plants when it exchanges with other cations and moves back into the soil solution. K availability also appears to be diminished by allophane through K fixation (van Reeuwijk and Devilliers, 1968 as cited by Poudel and West, 1999). Hopkins and Huner (2009) noted that positively charged K is naturally attracted to a region in the soil with the dominance of negative charges. Both the soil solution and exchangeable K are measured in a soil test as available K. But according to IPI (1977), the concentration of K in the soil solution is a better indicator of K availability than the amount of exchangeable K.

Relationship of exchangeable K and soil solution K with plant uptake of K (IPI, 1977)

Little information is available about K most specifically on its content and availability in highly weathered soils in the Philippines. Thus, we conducted a study that compared the K concentrations and availability in selected highly weathered soils in Leyte and Samar Islands and evaluated the factors affecting them. The soils we used have developed from basalt (Baybay and Biliran site), shale (Matalom site), and ultrabasic rocks (Salcedo site). In terms of the relative degree of weathering, the following sequence was observed: Salcedo soil > Matalom soil > Baybay soil > Biliran soil. The soils are deep, clayey with color ranging from brown to yellowish-brown and red to yellowish-red. They have angular to sub-angular blocky structure with friable and very friable to very firm consistency when moist but generally sticky and plastic when wet.  In terms of chemical characteristics, the soils have acidic pH values ranging from 5.0 to 5.4 (very strong acid). They have a low to medium amounts of organic matter of 2 and 6 %. The total N contents are high in the upper horizon which decreases with depth. All the soils are dominated by 1:1 type of clay minerals particularly kaolinite and halloysite as well as high amounts of iron and aluminum oxides which reflect the highly weathered nature of the soils. 

All the four soils have low exchangeable K and low soil solution K. In general, soil solution K was significantly correlated with soil pH. On the other hand, exchangeable K was generally not correlated with soil pH. The low levels of "available" soil K appear to be related to the highly leached and highly weathered nature of the soils. The study found no clear evidence of the connection between K availability and the type of parent rock of the soils studied. 

References
Darunsontaya T., Suddhaprikarn A., Kheoruenromne I, Prakongkep and Gilkes R. J.. 2012. Geoderma 170: 11-24.
Brady N.C.. 1974. The Nature and Properties of Soils. Macmillan Publishing Co. New York.
Hopkins W.G. and Huner N.P.A.. 2009. Introduction to Plant Physiology. John Wiley and Sons, Inc., New York.
International Potash Institute (IPI). 1977. Potassium dynamics in the soil. CH-3048 Worblaufen-Bern/Switzerland.
Poudel D.D. and West L.T.. 1999.  Soil Sci. Soc. Am. Journal 63:1258–1273.

Highly weathered soils from Visayas, Philippines

Weathering is the alteration by chemical, mechanical, and biological processes of rocks and minerals at or near the Earth’s surface, in response to environmental conditions.

Highly weathered soils (or strongly weathered soils) are soils that have undergone prolonged and intense weathering under the net leaching environment of the humid tropics. They are commonly found on stable and old geomorphic surfaces underlain by easily weatherable rocks such as ultrabasic and basic rocks as well as by pre-weathered sediments (Beinroth, 1982). These soils are clayey, deep, reddish, acidic, and have low nutrient status. According to Jackson et al. (1948), highly weathered soils are characterized by weathering stages of 10 to 12 wherein the clay fraction is dominated by 1:1 phyllosilicates (kaolinite & halloysite), aluminum oxide (gibbsite), and iron oxides (goethite and hematite). This mineralogical characteristic is also predicted by the “residua hypothesis” of Chesworth (1973) which states that soil composition will with time move towards the residua system composed of SiO2, Al2O3, Fe2O3, and H2O. In the USDA Soil Taxonomy, the highly weathered soils belong to the Ultisols and Oxisols orders. In the World Reference Base, these soils belong to the reference soil groups Alisols, Acrisols, and Ferralsols. These soils possess nutritional problems for crop growth and thus are a problem for agriculture.

(Beinroth, F.H. 1982.Geoderma 27(1982)-1-73; Chesworth, W. 1973. J. Soil Science 24: 69-81; Jackson, M.L. et al. 1948. J. Physical and Colloidal Chemistry 52: 1237-1260).  

Below are photos of the important highly weathered soils from Leyte, Negros and Samar islands in the Visayas. 

This is an Oxisol that formed from ultrabasic rock in Salcedo, Eastern Samar

The widespread red soil (Ultisol) in the volcanic area of Central Negros

An Ultisol on pre-weathered sediments from basalt in Silago, Southern Leyte

An Ultisol formed on pre-weathered sediments from basalt in Biliran, Leyte

The widespread soil from basalt on the lower slopes of Mt. Pangasugan, Baybay, Leyte