By Maria Cristina A. Loreño & V.B. Asio
Figure 3. Changes in the sand, silt, and clay contents with soil depth due to landslides.
Figure 5. Changes in soil organic matter content with soil depth due to landslide.
soil and its relation to environment, agriculture, global warming, and human health
By Maria Cristina A. Loreño & V.B. Asio
Soils are formed from the weathering of rocks as influenced by climate, parent rock, topography, living organisms, and time. Among these factors, climate and topography appear to be the dominant factors that have influenced the properties and distribution of soils in Benguet, Northern Luzon.
Benguet together with Abra, Apayao, Baguio City, Ifugao, Kalinga, and Mountain Province comprise the Cordillera Administrative Region (CAR). Benguet has a mountainous topography consisting of peaks, ridges, and canyons ranging in elevation from about 900m to 2,840m above sea level.
The highest point of the Philippine highway in Cattubo, Atok, Beneguet |
The subtropical highland climate (Cwb based on Köppen climate classification) with annual average highs of 25.3 °C in April and lows of 13.3 °C in January and an average precipitation of 1,829mm (Wikipedia) promotes moderate rock weathering and soil formation rates. The steep slopes on most mountain sides enhances rapid leaching and runoff, the latter results in severe soil erosion on cultivated and bare slopes.
Outcrops of metasedimentary rocks in Atok, Benguet |
The high soil erosion rates result in poorly developed and thin soils (Inceptisols). On more stable surfaces such as on summit positions, old soils can be found which may qualify as Ultisols. Regardless of the stage of soil development, most soils are acidic with pH below 5.0 (Laurean et al., 2015. Benguet State University Research Journal 74: 10-34).
Red and old soils on summit positions in the mountains. |
Where intensive vegetable production is found, the landscape can be called Anthropocene landscapes due to the considerable soil and landscape modification resulting from human activities such as land use conversion from forest to agriculture, terracing, fertilizer and pesticide application, liming and others.
The beautiful Anthropocene vegetable landscape in Natubling, Buguias, Benguet. |
In general, the rates of fertilizer and lime application by the vegetable farmers are not based on recommended rates. This necessitates soil fertility assessment of vegetable farms to be able to determine the appropriate rates of fertilizer and lime application for improved vegetable production. This is one of the objectives of our ACIAR SlAM Project (2020117) on managing heavy metals and soil contaminants in vegetable production led by Dr. Steve Harper of the University of Queensland, Australia.
Our ACIAR Slam Project Team from the Univ Queensland, UPLB, BSU, VSU & USTP |
The Baybay landslides on April 11, 2022, brought about by the Tropical Depression Agaton, have already claimed 116 lives, and many victims are still missing. Social media is buzzing with explanations about what caused the landslides. As usual, deforestation is claimed as the top culprit. And as always, the deforestation issue is politicized. But is it the cause of the landslides? Several factors cause landslides. In the case of the Baybay landslides, the most important are geology (rock type), topography (slope), soil characteristics, land use (and vegetation cover), and rainfall. Let me explain the role of each factor.
Geology
All the landslides in Baybay are located on the western slopes of the central highlands of Leyte, also called the Leyte Cordillera. This mountain range is volcanic, and the rocks consist mainly of pyroclastic rocks, specifically basalt and andesite. Pyroclastic rocks are fragmented or unconsolidated rocks produced by volcanic eruptions. Consequently, the slopes underlain by these materials are generally weak and prone to slope failure. This situation is aggravated by the presence of the Philippine fault line along the central highlands, which has caused the shearing of the rocks.
The common type of pyroclastic rocks in the central highlands of Leyte |
Topography
The western slope of the central highlands is generally rugged and mountainous. The steepness of a mountain slope is a major determining factor in whether the slope will fail or not. The steeper the slope, the less stable it is. In the presence of a triggering factor such as a heavy rainfall event, steep slopes (>25%) may fail, thereby causing landslides. From the topographic maps available on the internet, one can easily see that the source areas of the Baybay landslides have steep slopes.
Topographic maps show the steep slopes of the source areas of Bunga & Kantagnos landslides |
Soil
Soils vary in the stage of development from young (poorly weathered) in the plains to old (highly weathered) soils in the mountains. Young, very porous, and unstable volcanic soils (65% porosity) prone to land sliding are widespread in the upper mountain slopes, generally above 300 meters elevation, such as in Mailhi. Except for the Mailhi landslide, most of the Baybay landslides occurred on the old and highly weathered soils. These soils are highly friable, clayey, and prone to shallow landslides. When saturated with water, the clay serves as a lubricant for the sliding mixture of soil and rock debris. And also, regardless of soil type, the soil can turn into a liquid state when supersaturated with water resulting in mudslides.
The source area of the Bunga landslide with its highly weathered soil, deep-seated characteristic, and mixed vegetation cover. (Photo Source: jbatravelvlog) |
The source area of the deep-seated Kantagnos landslide with its highly weathered soil and relatively thick mixed vegetation. (Photo Source: Dan Michael Castanares) |
The deep Mailhi landslide with its unstable young volcanic soil and mixed vegetation cover. (Photo Source: jbatravelvlog) |
Land use
Vegetation cover, particularly trees, can prevent soil erosion and shallow landslides, which generally occurs within the root zone. No doubt, forest trees can minimize shallow landslides better than shallow-rooted plants like coconut and grasses like cogon. Studies have revealed that roots increase water permeability and the mechanical stability of shallow pyroclastic soil cover (Alfonso-Dias, 2019). Zhang et al. (2019) found that the 23-year-old reforest in the mountain in Tacloban, Leyte, positively affected the hillslope hydrological functioning. But deep landslides that occur below the root zone are beyond the control of the roots of the vegetation cover (Zhuang et al., 2022). In such a case, the failure of the land surface is controlled more by the steep slope and by the weak geological and soil foundation. This explains why landslides can occur under any type of vegetation cover or land use. For example, on a clear day, one can see several old landslide scars in the forest on Mt. Pangasugan. In the Bunga and Kantagnos landslides, the largest Baybay landslides, the source area in the upper part of the mountain is still covered with mixed vegetation consisting of trees and coconuts. And according to Forbes and Broadhead (2013), the forest cover will not affect the occurrence of landslides during extreme events such as heavy rainfall.
Rainfall
Excessive soil water content from heavy rainfall is generally considered the primary cause of slope failure (Forbes and Broadhead, 2013). The tremendous volume of rain dumped by Agaton in three days from April 9 to 11 was 907mm, which is one-third of the annual rainfall in Baybay (Source: VSU-PAGASA Agromet Station). This is close to a meter deep water poured into the land surface in Baybay in only 72 hours. This volume of water is equivalent to 9,000 cubic meters of water per hectare or 200 gallons per square meter. No vegetation type can absorb and evaporate this tremendous amount of water in so short a time. Likewise, no soil can either hold or percolate this volume of water in just 72 hours. Even the very porous young volcanic soil in Mailhi, which has an average porosity of 65% and moisture content at a field capacity of 40%, can only potentially absorb 58 gallons of water per square meter calculated to a depth of 1 meter. The old soil under the forest can potentially hold a maximum of only 61 gallons per square meter to a depth of 1 meter, while the old soil under coconut can hold only 55 gallons per square meter. These values are based on the assumption of a flat surface and fast infiltration rates which are not the case in the landslide areas. This means that the actual amount of water that the soils on the mountain slopes can hold is much lower than the values mentioned. As soon as the soil is saturated with water, the incoming rainwater cannot be accommodated in the soil pores and flows as surface runoff, causing the landslides and the flooding, for the first time in about four decades, the city center of Baybay.
Summary
The Baybay landslides were triggered by the extremely heavy rainfall and enhanced by the unstable and highly weathered soils (or the young unstable volcanic soil in the case of Mailhi), weak geologic material composed of unconsolidated pyroclastic rocks, and the steep slopes. The role of vegetation cover is not straightforward since it can partly prevent shallow landslides but not deep-seated landslides such as those in Bunga, Kantagnos, and Mailhi. This means that the coconut's shallow root system (about a meter deep) may not have effectively prevented the landslides, but the same can be said of the trees with their 2-3 m deep rooting zone since the landslides are several meters deep in their source areas. Landslides are natural geologic processes on the land surface. Catastrophic landslides like those we have witnessed in Baybay may occur again anytime at any place with the above environmental conditions regardless of the vegetation cover. I suspect that several incipient landslides (landslides in the initial stage) were produced in various areas during Agaton but are hidden by the vegetation cover. Another typhoon may trigger these incipient landslides to become full or even catastrophic landslides. Thus, it is crucial that residents living in landslide-prone areas be given proper and timely advice.
References:
Alfonso-Dias 2019. Dissertation, University of Montpellier, France.
Forbes, K. and J. Broadhead. 2013. RAP Publication 2013/02. FAO Regional Office, Bangkok.
Zhang et al. 2019. Geoderma 333: 163-177.
Zhuang et al. 2022. Engineering Geology 298
In March 2020 last year, because of the worsening Covid-19 pandemic, the Philippine education officials suspended the classes in all schools, and students were advised to go home. Suddenly, the normally vibrant university and college campuses have turned into empty and lonely places. At the Visayas State University in Baybay City, Leyte, where more than half of the university student population reside in dormitories inside the campus, the impact of the indefinite suspension of classes has been dramatic. The campus appears desolate. The only positive effect of the pandemic is the regeneration of the natural environment. One wonders where the students have gone, and when they are coming back. Below are some photos of some parts of the 100-hectare campus that I have taken during this pandemic:
The University of the Philippines Diliman is the country's best research university (Photo source: www.goodnewspilipinas.com) |
Humboldt fellows from Indonesia, Malaysia, Philippines, Thailand and Vietnam as well as some guests from SE Asia and Germany |
The Philippine delegation (L-R): Dr. Hallare, Dr. C. Bernido, Dr. M. Bernido, Dr Sese, Dr. Vasquez, Dr. V. Asio, Dr. Navarrete, Dr. Espiritu, Dr. L. Asio and Dr. Macabeo |
Victor B. Asio is a professor of soil science & geo-ecology at Visayas State University (VSU), Philippines. A recipient of the world renowned & highly competitive Humboldt Research Fellowship, he has published papers in international journals & has co-authored the Guidelines for Soil Description (2006, 4th ed) published by FAO of the United Nations in English, Spanish, Polish, Russian & Turkish. He studied at VSU, Univ Philippines Los Banos, Justus Liebig University Giessen, & University of Hohenheim, Stuttgart. He did postdoctoral studies at the Univ Halle-Wittenberg & the National Taiwan University. He was a Research Fellow at the International Rice Research Institute (IRRI) & Vice-Chair of Soil Geography Commission, International Union of Soil Sciences (IUSS). He is an elected member of Sigma Xi The Scientific Research Honor Society.