Forms Of Iron Oxides And Magnetic Susceptibility Of Representative Soils In Fujian Province And Theirs Genetic Significance

Fujian province is located between the middle subtropical and subtropical zone. Influenced by the subtropical maritime monsoon climate, the climate is different regionally. Changing climate and diversity topography and rich geological resources formed the rich types of soil. So it provide a good condition to study the classification of soils. Iron oxides are widespread in various types of soils within Fujian province. Its activity is very strong and affected by environmental conditions easily. It is easy to observed the activities of iron oxides through the change of level and color in profile. At the same time, the magnetic susceptibility of soils change with the different characteristics of iron oxides. In this paper, we make typical soil profiles as the research object. After a section of observation and record of the field, collection of soil sample and analysis of the physical and chemical properties, we make sure that the influence factors in the process of soil formation. Combining the feature of profile distribution and morphology and quantity of iron oxide and the distribution of magnetic susceptibility, it is studied the important significance of pedogenesis in the iron oxide and magnetic susceptibility within this paper. The results show that:(1) Parent materials and environment of soils and the way to used and development time are important to the morphology and quantity and distribution characteristics of iron oxides in Fujian. The total content of iron is between12.9～178.2g·kg-1. Due to the difference of soil types, the content of free iron and active iron and its freeness and activation degree also have big difference. Relationship between content of free iron and its freeness roughly is Ferrisol soil> Entisols soil and Cambisols soil> Paddy soil. Distribution of the activation degree are to the contrary roughly. Due to the influence of long-term planting rice, free iron have no obvious regular change with altitude in paddy soil. But the freeness of iron in upland and forest land increase with the decrease of the latitude.(2) The types of soil is abundant within fujian province. And the type and quantity of iron-bearing mineral is different also. Color of soil in moist state showed red brown and yellow brown mainly, and they have a distribution between2.5YR～10YR. Part of the soil present more than10R red or more2.5Y yellow color. Due to seasonal drowned for a long time, iron oxide have a reaction that reduction leaching and oxidez deposition under the influence of organic matter and water in paddy soil. So it make the color give priority to black, grey and brown between7.5YR～5Y, and the chromaticity and color values are smaller relative to the dry land soil.(3) The variation of magnetic susceptibility is large also. Magnetic susceptibility is higer in soil which is rich in iron. Magnetic susceptibility range in500～900×10-8cm3kg-1. Because of the forming time is short, the susceptibility of entisols is about100×10-8cm3kg-1. Magnetic susceptibility of cambisols is between20-40×10-8cm3kg-1magnitude. Paddy soil is generally low, but there are differences between the different types of paddy soil magnetism. Usually the magnetic susceptibility in gley type of paddy soil (e.g., FJ-69, FJ-78, FJ-93) are below8×10-8cm3kg-1. It is lower than waterloggogenic paddy soil (e.g., FJ-129, FJ-129) significantly, which the average of magnetic susceptibility is35.1×10-8cm3kg-1(4) The parent material is the most important factors which affectted magnetic susceptibility. In addition, moisture and organic matter are also the important factors that affect soil magnetism. Influence of the kinds of ferromagnetic minerals in the soil parent material and the quantity are large for the soil magnetic susceptibility in the study area. The general regularities of the variation is basite(peridotite, basalt, etc.)>acidic magmatic rocks (granite, diorite, etc.)> metamorphic rock and sedimentary rock(marble, sandstone, limestone, etc.)> water sediment. The influence of moisture conditions is also very obvious. The magnetic of paddy soil is lower than upland and forest land in fujian province. When the upland and forest land developped from strong magnetic materials used to paddy field, magnetic susceptibility decreased obviously. The magnetic is the type of idiomorphic soil>semi-hydromorphic soil> hydromorphic soil generally. The influence of organic matter is also very important, especially it occurred in the surface of soil riching in organic matter. The decomposition of organic matter make iron oxide restored, then it will oxidized to maghemite, etc., resulting the enhancement phenomenon of magnetic susceptibility in surface soil.