Exploring the Uniqueness in Taiwan's Soil

Soil is formed from “parent rock” over a long period of “time,” influenced by the “climate” and “living organisms,” and shaped by the “landscape.” Therefore, the five major soil-forming factors include climate, organism, relief, parent, and time. Taiwan is a fascinating island with deep gorges, valleys, high mountains, and a diverse range of climates, resulting in a rich variety of plants and encompassing almost every type of soil in the world except Gelisols in permafrost.

According to the following picture, various types of soil in Taiwan are displayed as monoliths in the soil museum of National Taiwan University, arranged from right to left, representing initial weathered to the highest weathered soil profiles. During the weathering process, rocks release iron oxides as they gradually disintegrate. This iron oxide dyes the soil red, making the soil increasingly red from the beginning to the end. These characteristics fully demonstrate that Taiwan is a beautiful natural museum with soil diversity. Here, we introduce their special characteristics, as well as the making procedures of soil monolith.

Soil Classification System

During the Japanese colonial period (1920s), the soils in Taiwan were frequently used for growing crops, leading to the beginning of soil research. This included studying basic soil properties, such as soil texture and pH, to facilitate soil management. In order to manage soil and agricultural practices effectively, we need to know the rules of the soil classification system and understand what it represents in soil science, which is a “common international language” and the foundation for academic research and land use.

After 1945, Taiwan adopted the soil classification system from the United States Department of Agriculture (USDA) for soil survey. The soil classification system consists of six categories, and the sequence from highest to lowest is Order, Suborder, Great group, Subgroup, Family, and Series. Based on the information and basic soil characteristics provided in these surveys, Taiwan's soils are classified into eleven soil orders in Soil Taxonomy. Over 50% of soil types in Taiwan are Inceptisols, followed by Alfisols (22%), Ultisols (10%), Entisols (6.8%), Andisols (1.2%), Mollisols (1.1%), Oxisols (0.3%), Histosols (0.1%), Spodosols (< 0.1%), Vertisols (< 0.1%), and Aridisols (< 0.1%).

Soil Monolith Making

Soil is a natural body in a 3D form called “soil pedon.” Before introducing the different soil types in Taiwan, we must first understand how a soil pedon is collected. When digging a soil pit, one of the surfaces we see is called the “soil profile.” To ensure the reality of the soil monolith, taking photos, observing and recording the profile collection site, and describing the soil color, structure, and other physical properties of the profile are important steps in sample collection.

The soil monolith-making process is shown in the following picture. First, dig a soil pit downward from the surface or use a natural section to clear a soil profile vertically toward the ground. The depth of the soil profile usually ranges from the surface to the parent rock layer, and the integrity of the soil profile must be ensured during the process. Then, spray glue on the surface of the soil profile until it is wet and stick the burlap on the soil profile. After gluing the burlap, carefully dig it out and fix it with a wooden board. Finally, the soil profile specimens were removed and transported to the exhibition hall.

Eleven Soil Orders in Taiwan

According to the various climates and topography, we can find varied soil types in Taiwan, and the only soil that does not exist in Taiwan is Gelisoils. The following figure shows the general degree of weathering in different soil orders classified in Soil Taxonomy.

Now, let’s learn about the characteristics of different soil types in Taiwan!

Histosols

In a humid environment, fallen leaves were decomposed slowly, leading to the continuous accumulation of organic matter. Histosols have an organic layer thickness greater than 40 centimeters or an organic carbon content exceeding 12%, which are commonly found in wetlands. In Taiwan, Histosols are mainly distributed near mountain lakes.

Spodosols

Spodosols are commonly found in cool and humid areas in coniferous forests in central and northern Taiwan, such as the Alishan Mountain, and the texture of the soil is usually sandy. The upper layer of Spodosols often has a “bleached horizon” that appears off-white color, which is one of the important characteristics of Spodosols. This is caused by leaching that moves iron, aluminum, and organic complex downward with the soil solution and accumulates in the “spodic horizon”, which usually appears yellow or brown in color.

Andisols

Andisols formed by volcanic ash are black in color. More than 60% of the soil profile has volcanic properties, which are strongly acidic and have a smaller density (< 0.9 Mg/m3) than other types of soil. Andisols are distributed in Yangmingshan National Park in Taiwan, and the parent rock is andesite.

Inceptisols

Inceptisols are soils that are weakly weathering from the parent rock, beginning to show significant changes in structure and color, known as the “B horizon.” Inceptisols are widely distributed and can be found in almost every county in Taiwan, except for Taoyuan, Yunlin, and Penghu. The alluvial plains and hill land soils in western Taiwan, such as the Chianan Plain, Changhua Plain, and Gaoping Plain, are classified as Inceptisols and are one of the largest agricultural production areas in Taiwan.

Vertisols

Because of the high clay content and some special clay minerals such as montmorillonite, which have swelling properties influenced by water content, we can usually find the “surface crack” in Vertisols, and this is a very unique structure in this type of soil. Vertisols can be found along the east coast of Taiwan during the dry season, such as Chang-Bin, Du-Lan, and Cheng-Gong.

Mollisols

Mollisols are usually distributed in semi-arid to humid grasslands and are often found in the same areas as Vertisols in Taiwan, but they have a “mollic epipedon,” which is an important characteristic of Mollisols. Mollic epipedon means they contain rich organic matter content, making the soil dark in color. Besides, high basic cations make Mollisols very fertile and suitable for farming.

Alfisols

Alfisols are characterized by neutral to weakly alkaline pH from the moderate weathering of mafic rocks and an “argillic horizon,” in which we can see clear “clay coatings” because of the large amount of annual rainfall in Taiwan and the accumulated silicate clay from illuviation. Alfisols are distributed in Penghu Island (basalt) and in aged alluvial plains in southwestern Taiwan, such as Chianan Plain, Liuying, and Xinying. Alfisols are also one of the largest production areas for agriculture in Taiwan.

Ultisols

Compared to Alfisols, we can also find “clay coatings” in Ultisols, but because of the tropical humid climates, there is strong leaching in the soil, and the pH and basic cations are lower than in Alfisols. In Ultisols, there are high clay content and low basic cations, and thus, the fertility is low. They are usually found in flat hills, such as Linkou, Dadu Mountain, and Luye.

Oxisols

Oxisols are the oldest soil, which means that they are the most highly weathered soils (more than 100,000 years) in the soil classification system. Similar to Ultisols, Oxisols have low pH, basic cations, and fertility. Additionally, the only substances left in Oxisols are iron and aluminum oxides, and highly weather-resistant minerals such as kaolinite and quartz. Oxisols are almost red or reddish-brown in color, and they are distributed in the highest level of the Quaternary terraces, such as the lateritic terraces in Taoyuan, Nantou, and Pingtung.

Entisols

Opposite of Oxisols, Entisols are the youngest soil in the soil classification system. There is no “B horizon,” and the soil pedon showed shallow depths and appeared dark grayish brown or dark gray. Entisols are distributed widely in rural, hill, and forest lands, and because of their high fertility, Entisols are an important agricultural production area in Taiwan.

Aridisols

In general, there is no arid climate in Taiwan, but the “saline soil” found in the southwestern coastal areas can be classified as Aridisols. Soil salinization may occur in some conditions, including seawater intrusion or evaporation exceeding rainfall. The solid structure of saline soils makes plants very hard to grow, and plowing soils is one of the improvements.

Soils play a crucial role in nature and our lives by providing habitat for creatures, supporting nutrients, and recycling water and the atmosphere. As the global population continues to increase and climate change worsens, soil issues are emerging, such as acidification, salinization, contamination, and deficiencies in trace elements and organic matter. Soil management is crucial for maintaining soil fertility and permeability to raise environmental and sustainable awareness. Besides, safe and high-quality soil is essential for protecting the environment and the health of plants and animals. Therefore, understanding soil is one of the most significant aspects of sustainable development and management worldwide. We hope that this introduction to Taiwan's soil will draw attention to soils, leading to better maintenance in the future.