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.

Impact of intensive vegetable production on the organic matter, nitrogen, and phosphorus levels of a volcanic soil (Andisol)

by Niezel Jane D. Estrellanes

Volcanic soils such as Andisols have many unique properties not usually found in soils derived from other parent materials (http:/www.ctahr.hawaii.edu). They are often very young and acidic depending on which type of volcanic materials they come from. They also largely consist of non-crystalline minerals such as allophane and imogolite (Nanzyo et al., 2002) which form strong bonds with organic matter thereby protecting it from decomposition. Andisols are usually light textured and are easy to till. However, they form strong complexes with phosphorus, rendering this element unavailable to plants.

One of the sampling sites in the central volcanic highlands of Ormoc, Philippines

This thesis research, a component of the Australian Center for International Agricultural Research (ACIAR) Soil Project based at the Visayas State University, evaluated the impact of intensive vegetable production on the nitrogen and phosphorus levels of volcanic soils in the central highlands of Leyte specifically in Cabintan, Ormoc City, Philippines. Twelve sampling sites (vegetable farms) including a reference site were chosen for this study. The objective was to find out if the continuous and heavy application of fertilizers for intensive vegetable production have caused the accumulation of nitrogen and phosphorous in Andisols.

Location of the sampling sites in Cabintan, Ormoc City

Soil samples were taken from the following soil depths 0-20, 20-40, 40-60, 60-80, and 80-100 cm using a soil auger. They were air-dried and passed through 2-mm sieve and analyzed for selected physical and chemical parameters such as pH (H2O, KCl, and NaF), available phosphorus, and particle size distribution. Enough 2-mm samples were also ground further and allowed to pass through 0.425-mm for organic matter content and total N analysis. 

Soil OM contents of the sampling sites. Sites 2, 4, 6 and 7 have much 
higher values than the reference site 12.

Soil N contents of the sampling sites. Most of the sites have much higher 
N levels than the reference site 12

Soil available P contents of the sampling sites. Reference site (12) has 
much lower P levels compared to all other sites.

Results revealed that the majority of the vegetable farms sampled showed higher amounts of organic matter (OM), nitrogen (N), and available phosphorus (P) than the reference site (secondary growth forest). This clearly indicates the impact of the high and continuous application of chemical and organic fertilizers (chicken manure). The very high nitrogen contents of the soils point out to nitrogen pollution which could have a serious negative impact on the groundwater quality. The high available phosphorus contents of the soils suggest improvement of the supply of this nutrient to the crops considering the fact that it is the major limiting nutrient to crop production in Andisols.

Based on the results it can be concluded that:

a. The intensive vegetable production has increased the nutrient status of the Andisol; 

b. The frequent and high rates of fertilizer application for the intensive vegetable production have greatly increased the nitrogen and phosphorus contents of the Andisol soil.

c. Nitrate and phosphate pollution of the groundwater and the nearby rivers may result from intensive vegetable production.