Landslides changed the soil characteristics in Leyte, Philippines

By Maria Cristina A. Loreño & V.B. Asio 

Landslide is defined as the downslope movement of soil mass, rocks, and debris. It is one of the most serious environmental hazards in the Philippines. On April 11, 2022, four catastrophic landslides occurred in Leyte due to tropical storm Agaton, which caused the loss of hundreds of human lives (for a detailed explanation of the causes, please see the Soil and Environment blog). Two of the landslides happened in Bunga and Mailhi in Baybay City. Until now, little research has been done on the effects of landslides on soil properties and soil development. Such information is crucial for the rehabilitation of landslide-affected areas. The objective of the study was to evaluate the changes in the morphological, physical, and chemical properties of volcanic soils due to landslides. 

The study was conducted in the Bunga landslide with old soil (Ultisol) and in the Mailhi landslide with young volcanic soil (Andisol). The sites are found on steep volcanic mountain slopes underlain by andesitic pyroclastic rocks. Vegetation in both sites is a mixture of trees, coconuts, and shrubs. Soil profiles were examined and sampled on the upper, middle, and lower portions of the landslides. The soil profiles on the upper slopes were not affected by the landslides and were used as reference (unaffected soil). Soil samples were collected from every soil horizon or layer and analyzed in the laboratory for physical and chemical properties.

 

Results revealed that the landslides changed many soil characteristics crucial to soil use and productivity. In particular, the kind and depth of soil horizons, soil color, abundance of plant roots, and presence of rock fragments were modified by the landslides. The trend was the same for both the old and young soils (Figs. 1&2). In Bunga with old soil, the landslide resulted in more clayey soil but with very irregular distribution with soil depth. In Mailhi, with young soil, the landslide led to the increased sand content in the soil profile (Fig. 3). 

Figure 1. Changes in soil morphology due to landslide in Mailhi, Baybay 

Figure 2. Changes in soil morphology due to landslide in Bunga, Baybay

Figure 3. Changes in the sand, silt, and clay contents with soil depth due to landslides.

As expected, landslides increased the soil's porosity due to the mixing and deposition of soil material. In terms of soil pH, the landslides increased the pH of both the old and young soils due to the mixing of the soil and the deposition of fertile topsoil from the upper slopes (Fig. 4). Landslides tended to decrease the soil organic matter (SOM) in the topsoil but increased it in the subsoils (Fig. 5).

Figure 4. Changes in soil porosity and pH due to landslide.

Figure 5. Changes in soil organic matter content with soil depth due to landslide.

Landslides changed the characteristics of the soils and the degree of soil development. The mixing of the soil made the soil unstable and prone to soil erosion and further slope failure. The landslides also lowered the fertility and potential productivity of the soils. Because of the instability of the soils, a few years should be allowed to pass before the landslide sites are utilized for agriculture, forestry, or other land uses.

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Note: This article is based on the poster presented by the authors at the 12th ASTHRDP Graduate Scholars Conference organized by the DOST-SEI and the National Science Consortium on 12-13 September 2024 at the Dusit Thani Resort Mactan, Lapu-Lapu City, Cebu. We thank the DOST-SEI for the ASTHRDP scholarship to MCAL and Dr. Luz Geneston Asio, and Mr. Kenneth Oraiz, GAC Members, for their valuable comments.