Sunday, February 18, 2018

Nutrient addition as a forest restoration management strategy for Yakal yamban seedling establishment in ophiolitic soils



by Johannes R. G. Asio
Institute of Tropical Ecology and Environmental Management (ITEEM),VSU, Baybay City, Leyte, Philippines
Introduction
Dipterocarp trees (Dipterocarpaceae) have crucial ecological roles such as in the prevention of landslides, sequestration of atmospheric carbon, and biodiversity. They are also economically important in terms of timber production. These native trees are also adapted to a variety of climatic conditions and geographic locations (e.g. areas prone to heavy typhoons, marginal lands). However, the sustainable management of dipterocarp forests is still poorly understood due to the limited studies conducted on the subjet. This is particularly so in terms of the ability of these forest trees to thrive in marginal lands like those naturally contaminated with heavy metals and those soils with very low nutrient status such as ophiolitic and serpentinite areas (Corlett&Primack, 2006; DENR, 2012; Appanah, 1998; Walpole, 2010).
Ophiolite rocks are widespread in Leyte, Samar, Cebu and Palawan.These rocks generally underlain marginal lands. A typical ophiolite complex is a stratified igneous rock complex that consists of different rock layers: an upper basalt member, a middle gabbro member, and a lower peridotite member (Ishiwatari, 2016). The fertility of Ophiolite rocks in the Philippines has not yet been studied in detail, however, according to some literatures, it is generally moderately acidic to neutral, low soil organic matter, low nitrogen (N), phosphorus (P), and potassium (K), which are the major nutrients needed for plant growth, and it contains high amounts of heavy metals, such as chromium, nickel, iron, and cobalt among others (Quiñones& Asio, 2015; Ocba, 2016).
Mineral fertilizers have been used in agriculture and forestry to improve crop yield, enhance the soil fertility, and soil health. Thus, this study hypothesized that the addition of N, P, and K to an ophiolite soil could enhance the growth of Yakalyamban (Shorea falciferoides Foxw.) in problematic areas. This dipterocarp species was chosen for this research as it has been known to thrive in the ophiolitic and serpentinite areas of Samar and it is critically endangered, thus the need to preserve this dipterocarp to prevent it from becoming extinct (Fernando et al., 2009, 2008).


This study aimed to test whether the addition of nutrients enhanced the seedling growth of yakal yamban grown in an ophiolitic soils, determine the optimum nutrient combination level for yakalyamban seedling quality; and assess and evaluate whether fertilization could very well be adopted as a nutrient management practice in using yakal yamban as a rainforestation species for forest restoration in problematic soils.
Methodology
The potting medium was selected based on the soil data obtained by the VSU-OXFAM Project (2015). Detailed soil analysis done by the project showed that the soils in Barangay Padang, Hernani, Eastern Samar developed from ophiolitic rocks and have low levels of N,P,K, and Mg, but high levels of Ca. Twenty sacks of topsoil (0-30cm depth) were collected and transported to the Terrestrial Ecosystems Division of the Institute of Tropical Ecology and Environmental Management for this screenhouse experiment. The bulk soil samples were mixed, air-dried thoroughly, pulverized and sieved using a 4-mm mesh sieve. About 1.5 kg of the air-dried soil was weighed; 0.75 kg sieved soil (from the 4-mm sieve) and 0.75 kg unsieved soil to avoid soil compaction.


This one-year study was conducted using a 5 x 3 Randomized Complete Block Design (RCBD) with five treatments and three replicates, wherein each treatment per replication consisted of 10 seedlings. The treatment are as follows: T1- No fertilizer application, T2- Application of 3.65 g of Urea, 9.33 g of Solophos, & 2.8 g of Muriate of Potash, T3- Application of 3.65 g of Urea, 9.33 g of Solophos, T4- Application 9.33 g of Solophos& 2.8 g of Muriate of Potash, T5- Application of 3.65 g of Urea & 2.8 g of Muriate of Potash. Placement application was done wherein the exact amount of fertilizer for each seedling was applied a few centimeters below the soil surface. Tap water was used. About 400 mL was added as required.
Three (3) randomly selected seedlings in each replication were harvested after 3 months and 6 months from fertilizer application. The selected seedlings were photographed before and after harvest, documenting each plant part and making notable observations. Thereafter, each individual seedling was cut; each leaf was photographed in preparation for leaf area analysis. Then, each plant part (roots, stem, and leaves) was separated and placed into the corresponding paper bags ready for oven drying. The soil samples in each replication were mixed and placed into labelled plastic bags ready for air-drying and analysis.
Major Findings
Results revealed highly significant differences in leaf area, percent biomass allocation, and root-shoot ratio between treatments 6 months after sampling. In terms of leaf area, treatment 4 showed the highest leaf area value. All treatments added with phosphorus (treatments 2,3 and 4) had leaf area values that were statistically the same. This indicates that P is the most critical nutrient in the soil and that this tree species is sensitive to the P levels in the soil.

There were also significant differences in terms of the percent biomass allocation between treatments in the root, stem, and leaves, with treatment 5 showing the highest allocation in the roots; plants in P-deficient environments enhance root growth as it is their adaptive mechanism that enables them to thrive in these conditions. The result also coincides with the root-shoot ratio as study plants in treatment 5 had the highest root-shoot value.


Soil nutrient analysis was done to determine the nutrient status of each treatment. The analyses concur with the fact that ophiolitic soils are deficient with N, P, & K, thus the high values of the nutrients were due to the fertilizers added prior to destructive harvesting. It was also observed that the fertilizer treatments have not yet fully dissolved even after 6 months of application.
Plant nutrient concentration was also done to determine the nutrient content of each plant part. In terms of nitrogen (N), there were high values of N in the leaves as it is needed for photosynthetic activity. However, it was below the optimum concentration needed for plant growth (Marschner, 1995). With regards to P, there were high values of the nutrient in treatments not added with P. It may be due to the mycorrhizae present in the roots of the study plants after 6 months of application. For K, solubility played a factor since there was an inhibition of nutrients to be taken up especially between N and K.

The presence of ectomycorrhizae (EcM) was also observed in the roots of the study plants of the control (T1) and NK (T5). Various studies have proven that mycorrhiza aids in the growth of a plant as it enhances the absorption of nutrients and water (Marschner, 1995; Read, 1991). The result also coincides with the study of Turner et al., 1992 as EcM infection may serve as a purpose when dipterocarps are grown in nutrient-poor conditions.
Implications
Nutrient addition could very well be adapted as a nutrient management strategy for the seedling establishment of Yakal yamban in ophioitic soils; Treatment 5 enhanced the root-shoot ratio of the study plants, thus these seedlings are of good quality. This implies that during establishment of the seedlings in an open area, they are most likely to survive due to its adaptive mechanism (e.g. enhance root growth in p-deficient environments) and the potential fungus-root association in the soil.
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The above article is a summary of the BSEM thesis by the author which won as 2017 Phi Delta Outstanding Thesis in Applied Biological Sciences at VSU, Baybay City, Leyte. More information can be obtained from the author. Email: johannes.asio@vsu.edu.ph

Friday, September 29, 2017

Heavy metals in vegetables sold in some cities in the Visayas, Philippines


Every time we buy vegetables in the market, we do not doubt the quality of these farm products. We think they are clean, safe, nutritious and good for our health.

But the worsening environmental pollution due to the overuse and misuse of agricultural chemicals such as pesticides, the improper waste disposal, the manufacturing industry, and the transportation system may be affecting the quality of the food crops we eat everyday. Specifically, heavy metals most of which are toxic to humans at elevated concentrations, are starting to contaminate the vegetables we love to eat.

The scientific principle is simple: a contaminated soil will generally produce contaminated crops.



An interesting and very relevant student research conducted a few years ago revealed such alarming reality. Conducted to determine and compare the Pb, Cu and Zn contents of Alugbati (Basella rubra), Ampalaya (Momordica charantia), Kalabasa (Cucurbita maxima), Kangkong (Ipomoea aquatica), Pechay (Brassica rapa), and Talong (Solanum melongena) sold in markets in the cities of Baybay, Ormoc, and Tacloban (Leyte, Philippines), the study revealed that Ampalaya from Tacloban and Baybay contained excessive levels of Cu and may pose health problems to consumers. 

Likewise, Pechay from Baybay, Ormoc and Tacloban exceeded the safe level for Zn. All vegetable samples collected from the three cities were not contaminated with Pb. Cu and Zn levels varied with crop (vegetable) species and origin (production area). 

The results are very relevant in that they support and confirm the fear among consumers that some food crops sold in the local markets are not safe and may be one of the reasons for the various health problems experienced by many people.

The study was conducted in 2012 by Anna Luisa Ventulan, Christine Gay Cala, and Johannes Reiner Asio, all senior students at VSU Laboratory High School. The research adviser was Luz Geneston Asio of the Central Analytical Services Laboratory, Visayas State University, Baybay City, Leyte.

Tuesday, April 18, 2017

Ecological quality, macroinvertebrate communities and diversity in rivers in Leyte, Philippines


Researchers from the Laboratory of Environmental Toxicology and Aquatic Ecology, Ghent University, Belgium, in collaboration with researchers from the Institute of Tropical Ecology and Environmental Management of Visayas State University in Leyte have published scientific evidence of a strong link between ecological quality and macroinvertebrate communities and diversity in rivers in Leyte.

In a paper published this year (2017) in the prestigious journal Ecological Indicators, Vol. 77 and pages 228-238, Marie Anne Eurie Forio and colleagues assessed the macroinvertebrate communities, diversity, and ecological quality of 85 rivers on Leyte island. Specifically, they evaluated the biological (macroinvertebrates), chemical, physical and hydromorphological characteristics. Canonical Correspondence Analysis (CCA) and multivariable linear regression (LRM) were performed to relate the environmental variables and macroinvertebrates.

Eurie Forio and Daphne Radam during the field sampling in Cabintan, Ormoc
(at the central highlands of Leyte) in 2015
The researchers found several taxa of snails, shrimps, dragonflies, beetles, bugs and caddisflies. Although many sites had good to very good ecological quality and high diversity, about 41% had moderate to very bad ecological quality and low diversity. Based on CCA, the researchers concluded that macroinvertebrate communities were associated with velocity, sediment, conductivity and dissolved oxygen. They also observed that sensitive and tolerant taxa were encountered at high and low flow velocities, respectively. Moreover, LRM indicated that macroinvertebrate diversity and ecological quality were associated with physical (turbidity), chemical (chlorophyll), hydromorphological characteristics (bank slope & pool/riffle class), habitat degradation (gravel/sand quarrying, erosion) and the presence of logs and twigs.

Eurie Forio (lead author) and Prof. Peter Goethals (lead scientist)
This ecological study, the first of its kind (i.e. covering 85 rivers of an entire tropical island) to be conducted in the Philippines, supports the use of invertebrates as indicators of certain environmental conditions and the results of this investigation can serve as a basis to set up dedicated experiments to further prove the causality of these discovered relations. 

The study also revealed that organic pollution, as reflected by biological oxygen demand and chemical oxygen demand, was weakly related to invertebrate composition, diversity and ecological quality. This was linked to the low input in most sites and the relatively short rivers which are closely connected to the marine system. Thus, typical midstream and downstream systems were not encountered and the accumulation of these pollutants along the river is less likely. Although the island encounters intensive natural disturbances (e.g. severe typhoons), the taxa (families) were similar to those in other tropical systems and the effects of the environmental conditions were comparable.

The findings of this collaborative research are relevant and valuable in understanding the ecology of tropical islands. They also provide insights into the effects of environmental conditions on stream invertebrates, which aids in protecting and conserving tropical insular systems.

Reference:


Forio, M.A.E., K. Lock, E.D. Radam, M. Bande, V.B. Asio and P.L.M. Goethals (2017). Assessment and analysis of ecological quality, macroinvertebrate communities and diversity in rivers of a multifunctional tropical island. Ecological Indicators 77 (2017) 228–238

Tuesday, January 3, 2017

Cape Bojeador, Paoay Sand Dunes, Kapurpurawan White Rocks, Bangui Windmills, Pagudpud Resorts, and Cagayan River: Just a few of North Luzon’s wonders worth visiting

A tour around the northern tip of Luzon is really an amazing experience. For those interested in our natural environment, it is a must. This short report describes briefly some of the wonderful places that one can visit starting from Vigan City in the west (1) to Cagayan Valley in the east (10).


Vigan City
Vigan is an old city in Ilocos Sur with well-preserved Spanish colonial and Asian architecture. This UNESCO World Heritage site reminds me of some small towns in southern Spain. It is without question the most beautiful old city in the Philippines.


The Paoay Sand Dunes
The sand dunes area appears to be a raised sea bed. First evidence of this is the fact that it is directly adjacent to the sea. Second reason is the presence of shells of marine organisms in many parts of the area. Third, a road cut exposure east of the sand dunes reveals thick sandstones (formed under the sea) which could be the source, through weathering, of the sand particles. The subduction of the Sunda Plate underneath the Philippine Mobile Belt produced the Manila Trench in the South China Sea and has resulted in the uplift of northwest Luzon. Decades of soil mismanagement and the resulting soil erosion have without doubt contributed to the spread of the sand dunes.   


Cape Bojeador
This is the headland at the northwestern tip of Luzon in Burgos, Ilocos Norte. The raised marine terrace is flanked to the east by steep-sided volcanic and greywacke hills and to the west by the turbulent South China Sea. Perched on top of the hill at about 90m asl is the famous Cape Bojeador Lighthouse constructed in 1892 during the Spanish Colonial period.



Kapurpurawan White Rock Formation
The famous white rock formation in Kapurpurawan, Burgos, is actually a raised reef limestone that has been carved by the waves through time. The raised limestone is associated with the uplifting of northwest Luzon as a result of the subduction of the Sunda Plate underneath the Philippine Mobile Belt as has been mentioned above.



Bangui Windmills
Standing 70 m tall on the black sand beach of Bangui, Ilocos Norte, are 20 giant wind turbines (giant fans) facing the South China Sea in the direction of Taiwan. The area is so windy making it ideal for such a modern energy harvesting facility.


Pagudpud Resorts
A large resort and convention center called Hanna’s Beach Resort is found near the northernmost tip of Luzon and hidden from the highway by the mountain range in Balaoi, Pagudpud, Ilocos Norte.  The bay, which has fine white sand, is unique in its location and local geography. A chat with the locals would reveal controversial stories about the real owner of the resort.



Patapat Viaduct
One of the many major infrastructure projects of former President Marcos is the Patapat Viaduct in Pagudpud, Ilocos Norte. The 1.3 km bridge is elevated about 30 meters above the sea and connects the Maharlika Highway from Laoag, Ilocos Norte  to the Cagayan Valley Region. It is now part of the Asian Highway network (AH 26).


Anahaw Plantations
Anahaw or anahau (Livistona rotundifolia) is an erect palm that is widespread in northern Luzon particularly in Claveria, Cagayan. This makes the landscape in this part of Luzon different to the monotonous coconut plantations in the Visayas.


The Cagayan River
The Cagayan River (the Rio Grande de Cagayan) is considered as the second largest river in the Philippines next to the Rio Grande de Mindanao. The mighty river in the Cagayan Valley region traverses the provinces of Nueva Vizcaya, Quirino, Isabela and Cagayan and has a drainage area of about 27,300 square kilometers. The picture below shows the river from the Magapit Bridge in Magapit, Cagayan.


Soil Degradation in Cagayan Valley
Deforestation followed by decades of grazing (pasture) have caused widespread soil degradation in Cagayan Valley. This is clearly observable from Tuguegarao City down to Enrile and Sta Maria, Cagayan where the traveler sees an endless view of denuded and degraded hills and mountains.

Sunday, October 23, 2016

What is soil analysis?

President Rodrigo R. Duterte emphasized the conduct of soil analysis in the country during his first State of the Nation Address (SONA) on July 15, 2016. He said that “we shall also conduct a nationwide soil analysis to determine areas most suitable for rice farming to optimize production with the use of effective soil rehabilitation and fertilization.”

As an effect of this presidential pronouncement, many people including professionals from various academic fields have been wondering what soil analysis is. Several readers of this blog suggested that I write about this topic hence, this article.

Soil analysis refers to the measurement of soil physical, chemical, and biological properties. It is done, depending on the type of soil analysis, for the following purposes: 1) to evaluate the origin and formation of the soil; 2) to assess the level of contamination of the soil; 3) to characterize the soil as a habitat of soil organisms; 4) to assess the soil fertility status; and 5) to evaluate the soil’s suitability for certain crops. Soil analysis is generally synonymous with soil testing. The major steps of soil analysis are soil sampling (and field soil examination) and laboratory analysis.

Soil profile examination and sampling to evaluate the origin of the soil
The first type of soil analysis is the most difficult and complex type. It is carried out by soil specialists called pedologists.  It involves detailed field description of the soil using standard procedures such as the Guidelines for Soil Description (4th edition by Jahn et al., 2006) published by FAO, Rome. Soil description is done on newly dug soil pit at least 1.5m deep or fresh road cuts. Collection of soil samples for intensive laboratory analysis is done on every soil layer (soil horizon) down to the bedrock. Laboratory analyses include the physical, chemical and mineralogical properties of the soil. Geochemical analysis of rock samples is also necessary.

The second type is conducted by soil scientists interested in soil pollution or contamination. Soil samples are usually collected in areas where soil contamination is suspected. Soil sampling is done at random or at fixed interval. Only the top soil layer (0-10 or 0-20 cm) is sampled using a soil auger or similar sampling tool. Soil samples are analyzed for their contents of soil pollutants (e.g. heavy metals) and are compared with published threshold values to know if the sample is contaminated or not.
Soil sampling to assess the contamination of Taft River in E. Samar
The third type of soil analysis is conducted to know if the soil is favorable for certain soil organisms of interest (e.g. earthworms). This is popular among soil ecologists. Soil samples are collected usually from the top soil layer and then they are analysed for soil physical and chemical properties. Correlation analysis is then done between the population of the soil organism and the different soil properties to know which among the soil properties influences the population of the organisms.

The fourth type is the most well-known and commonly done type of soil analysis to support crop production. The main purpose is to know if the soil is fertile or not. Specifically, it is performed to assess, using high-tech laboratory equipment,  if the soil contains sufficient amounts of the essential nutrients required by plants (crops) to grow well and produce good yield (grain, tubers). The essential nutrients that the pant takes up from the soil include: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), chlorine (Cl) and nickel (Ni). Since N, P and K are required by the plant in largest amounts, these three are usually the nutrients that are of limited supply in the soil. So farmers need to know how much of these nutrients must be applied to the soil through fertilizers. The final result of this type of soil analysis is a fertilizer recommendation.
Soil sampling for fertility evaluation
The fifth and last type of soil analysis is carried out to assess if a certain soil is suitable for crops such as rice, corn, vegetables, fruit trees, and others. This involves field soil examination to know the soil’s texture, structure, drainage, slope and depth using the methods of the first type of soil analysis. The soil samples are analysed for soil chemical properties such as soil pH, organic matter content, nutrient holding capacity as well as the amounts of the major nutrients (similar to the third type of soil analysis). The soil properties are then matched with the ecological requirement of the crop. The final result is a suitability map or table showing the suitable crops for each soil. It also indicates the soil constraints (or problems) if a crop is grown in a soil that is not suitable for that particular crop.

From the president’s pronouncement, it looks like that he meant the fifth type of soil analysis. Due to the tremendous amount of field and laboratory works, it cannot be done by the Bureau of Soil and Water Management alone. It needs the participation of universities with strong soil science program throughout the country such as Central Luzon State University, University of the Philippines Los Banos, Visayas State University, Central Mindanao State University, and University of Southern Mindanao.
Soil science students at VSU performing laboratory analysis of soil samples
The laboratory step of soil analysis or soil testing is tedious, time-consuming, and costly because the chemicals and apparatus required are very expensive. Although there are some rapid soil test kits available, they are not reliable. Also, the laboratory analysis has to be done according to accepted procedures and by trained personnel. Examples of accepted procedures of laboratory analysis of soils are:

Carter M.R. and E.G. Gregorich (Eds.). 2008. Soil Sampling and Methods of Analysis (2nd ed). CRC Press, Boca Raton.
International Soil Reference and Information Center (ISRIC). 1995. Procedures for Soil Analysis (L.P. Van Reuwijk, Editor). Wageningen, the Netherlands.
Jones J. B. Jr. 2001. Laboratory Guide for Conducting Soil Tests and Plant Analysis. CRC Press, Boca Raton.
Margesin R. and F. Schinner(Eds.). 2005. Manual for Soil Analysis – Monitoring and Assessing Soil Bioremediation. Springer Verlag, Berlin.
Pansu M. and J. Gautheyrou. 2006. Handbook  of Soil Analysis. Mineralogical, Organic  and Inorganic Methods. Springer Verlag, Berlin.
Schlichting E., H.P. Blume and K. Stahr. 1996. Bodenkundliches Praktikum (Soil Science Practicum). Blackwell Wissenschaftsverlag, Berlin.
Sparks D.L., A.L. Page, P.A. Helmke and R.H. Loeppert (Eds.). 1996. Methods of Soil Analysis Part 3—Chemical Methods. Soil Science Society of America, Madison, Wisconsin.
Westerman R.L. (Ed.). 1990. Soil Testing and Plant Analysis (3rd ed). Soil Science Society of America, Madison, Wisconsin.

Thursday, September 1, 2016

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 about tropical soils (Jahn and Asio , 2006):

  1. The tropics,  the world’s largest ecological zone, have 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.
  1. 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.
  1. 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.
  2. 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).
  3. 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.
  4. Due to severe chemical limitations, proper management of nutrients is the main challenge for effective land use systems in the tropics.
  5. 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.
  6. 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.
  7. 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.