Jeju island is situated in the southern part of the Korean Peninsula and covers a lava plateau with a shield volcano at an elevation of 1,950 meters above sea level. The biosphere reserve is located at the center of the island, comprising in its core area Mt. Halla, two stream corridors and three islets.
<Figure 1．Monthly mean temperature and rainfall in Jeju>
The Jeju sheet is bounded between the latitude 33°00’ ~ 34°00’ and longitude 126°00’ ~ 127°00’. The Jeju island has an elliptical shape of 74 km in a long diameter and a short diameter of 32 km. The Mt. Halla, the highest peak located at the central part of the island, rises 1,950 m above sea level. The number of pyroclastic cones in Jeju sheet is about 360. The name of scoria cones is called Oreum in Jeju. Most of them are scoria cones and about 10 are tuff cones and rings. The former are largely concentrated in inland area, whereas the latter are distributed along the coastal regions.
Jeju island is a shield volcano, presently lying on a carbon 100m deep continental shelf off the Korean Peninsula. The island is composed mainly of basaltic lava flows and subordinate amount of pyroclastic and sedimentary rocks. The lavas of the Jeju island are known as basalt, trachybasalt, basaltic trachyandesite, trchyandesite and trachyte. The geochemical studies supported that the volcanic rocks have characteristics of oceanic island basalt and the eruption which is the hot-spot related to the mantle-plumes.
<Figure 4. Location (top) and geological map (below) of Jeju island>
There are 2,001 kinds of subtropical, temperate, and polar plants, distributed vertically with Mt. Halla which is located in the center of the island and has a rich repository of plants.
<Figure 5. Vertical distribution of vegetation in Mt. Halla>
Volcanic ash soils are generally named based on parent materials; ejecta of volcano. Major Soils in Jeju island are largely dark brown soils and black soils; dark brown soils are non-volcanic ash soil weathered by basalt, dark brown ash soil and black soils are volcanic ash soils from volcanic ash.
Jeju volcanic ash soils are more barren compared to ones of inland soils. Another innate characteristic of Jeju volcanic ash soils is greater P retention within soils. However, these problems have resolved by easy access to fertilizer. Moreover, Jeju volcanic ash soils become good soils to grow crops because of greater organic matter contents/CEC, better moisture retention and soil aeration. Dark color of the soil is drawn from organic matter in the soil. Darker soils that contain more organic matter tend to have lower allophone content and higher content of Al-organic matter complexes. The higher content of Al-organic matter complexes may cause problems to crops, but so far, research found that there is no Al toxicity to crops because of lower exchangeable Al content.
According to detailed soil map of Jeju island, there are 63 soil orders in the main island, an area of 178,343 ha. The average area of each soil order is about 2,876 ha; Gujwa series occupies the largest area of 14,165 ha and Iho series does the smallest area of 49 ha. The distribution of soil orders in Jeju are shown in the Figure 6.
<Figure 6. Soils in Jeju island classified by order(A), suborder(B), and great group(C)>
Summary of physical and chemical properties are shown in Table 3. Jeju soils tend to have higher pH as soil depth increases and lower pH as altitude increases (Table 3). Because of basic parent materials, Jeju soils have relatively higher pH (higher than 5.0).
Organic carbon tends to increase as altitude increases and subsurface soils at the higher ground (brown forest soils) have great amount of organic matter. Previous study reported that Al- or Fe- ions bond to organic matter and create stable organic matter complexes (Parfitt et al., 1977; Tate and Theng, 1980). This causes organic matter accumulation within soils.
Exchangeable base capacity decreases as altitude increases volcanic ash soils of Jeju have very high acidity, exchangeable charge and CEC, since -COOH from organic matter and OH- from allophane surface are ionized. This results in increase in soil pH.
Typically, bulk densities of Jeju soils are very low because of high organic matter content and low particle density (1.4 – 1.8 Mg/ m3). Another characteristic of Jeju volcanic ash soils is a high content of silt and great water retention (Table 4).
<Table 3. Chemical properties by different characteristics of Jeju soils.>
|Soils||Depth||pH||Org.C||NH4OAc ext. bases||Ext.Al||Acidity||CEC|
|cm||mg/kg||--------------------------- cmol(+)/kg ---------------------------|
|Dark brown soils||0 - 20||5.9||27||9.4||3.3||0.9||0.3||0||25.0||25.7|
|20 - 40||6.3||36||12.7||5.0||0.6||0.4||0||25.3||29.9|
|40 - 60||6.3||15||5.9||3.2||0.6||0.4||0||18.8||21.2|
|Very dark brown soils||0 - 20||5.2||92||2.5||1.6||0.8||0.6||0.9||62.2||33.2|
|20 - 60||5.7||46||2.0||1.6||0.8||0.6||0||49.0||30.0|
|Black soils||0- 20||5.2||154||1.3||0.6||05||0.4||2.6||96.7||47.3|
|20 - 60||4.8||173||1.6||0.8||0.7||0.7||1.7||119.1||62.8|
|Cinder cone soils||0 - 10||5.1||104||1.7||1.7||0.7||0.3||0.2||64.9||33.7|
|10 - 60||5.9||20||1.3||1.0||0.3||0.4||0||26.6||26.7|
|Brown forest soils||0- 25||5.3||140||0.6||0.3||0.3||0.4||2.8||84.9||42.6|
|25 - 50||5.5||141||0.9||0.3||0.3||0.5||1.0||97.8||47.2|
<Table 4. Physical properties by different characteristics of Jeju soils.>
|Soils||Depth||B.D.||Particle Size Dist.||Texture||Water Retention|
|Dark brown soils||0-20||1.41||20.9||67.9||11.2||SiL||27.1||20.8||8.8|
|Very dark brown soils||0-20||0.76||5.5||70.9||23.6||SiL||71.5||66.8||32.5|
|Cinder cone soils||0-20||0.62||8.4||64.2||27.4||SiL||105.0||86.9||46.6|