Inorganic Constituents in Soil

Soil plays a major role in ecosystem services (an ecological term referring to the benefits granted to humans by ecosystems). Among the various ecosystem services, provisioning services are important providers of foods, fibers, wood, and other naturally sourced materials. To increase biological production while maintaining sustainable soil and ecosystems, we must understand element cycling in soils and ecosystems and its high dependence on inorganic constituents. This monograph describes the fundamentals, along with visual aids, of inorganic soil constituents for beginning students of soil science, environmental science, biogeochemistry, and interested readers in other disciplines. The visual aids include optical photographs, electron microscope images, and element maps acquired by energy dispersive X-ray analyses.

This chapter introduces the primary minerals that are relatively common in soils. It first presents the accepted views on the elemental compositions of the Earth’s crust, rocks, and minerals. Soils at the top of the Earth’s crust are also within the rock cycle. Silicate and silica minerals, which constitute more than 90% of the minerals in the Earth’ crust, are outlined. Samples of relatively un-weathered and weathered primary minerals were obtained from new volcanic ash and soils derived from granitic rocks, respectively. Quartz is highly resistant to weathering, whereas biotite in soil is altered in moist climates. The composition of primary minerals in soils is affected by the types of parent rocks, weathering, sorting, and other soil-forming factors, resulting in mineral compositions that deviate from the average mineral composition of the Earth’s crust.

This chapter introduces secondary minerals such as clay minerals, hydroxides and oxides in soils. Clay minerals, which are major secondary minerals in soils, are phyllosilicates that have 1:1 or 2:1 type layers. The 1:1 type minerals are kaolinite and halloysite. The 2:1 type minerals are smectite, vermiculite, micaceous minerals, and chlorite. Gibbsite and manganese oxides are introduced as examples of hydroxides and oxides, respectively. This chapter also constitutes the basic part of treatment of the inorganic constituents in soils. Secondary minerals have an effect on the chemical, physical, and biological functions of soils. As the size of secondary minerals is approximately 2 μm or less, electron micrographs and X-ray diffraction (XRD) are effective for characterizing these minerals. Schematic diagrams are used to interpret the chemical structure of phyllosilicate clay minerals.

Non-crystalline inorganic constituents of soil, such as volcanic glasses, phytoliths, laminar opaline silica, allophane, and imogolite are introduced using optical and electron microscope images and energy dispersive X-ray (EDX) analysis. The Al-humus complex and Al-rich Sclerotia grains are also introduced. The volcanic glasses are formed from magma and can be categorized as primary. All of these non-crystalline inorganic constituents are found in volcanic ash soils. Among these, phytoliths can be found under vegetation in many other soils than volcanic ash soils. Formation of allophanic materials from fresh pumice is shown stepwise using polished sections to demonstrate microscopic distribution of elements and inorganic constituents. Allophane and imogolite are rich in Al whereas their parent material, volcanic ash, is silica-rich. Changes in morphological property and element concentration of volcanic ash or volcanic glass during the formation of these secondary non-crystalline constituents are discussed.

Inorganic soil constituents sensitive to varying redox conditions, such as hydrated iron oxide, vivianite, siderite, iron (II) sulfides, and jarosite, are analyzed using optical and electron microscopes, energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Many of these minerals are sourced from paddy field soils, which undergo reducing and oxidizing conditions in the plow layer every year. Iron mottles formed at the soil redox interface in the presence of reducing and oxidizing conditions provide significant visual evidence of varying redox conditions in soil. Polished sections were used to examine the elemental distributions and morphological properties of the mottles. One type of iron mottles is formed around rice roots by oxygen diffusion from the roots. They are cylindrical in form and include soil matrix minerals. Other type of iron mottles is formed on the surfaces of irregular or vesicular pores by oxygen diffusion through soil pores after drainage. These mottles contain few soil matrix minerals. In association with iron, the distribution of phosphate is strongly affected by changes in redox conditions in paddy field soils with low active Al content.

Three topics are introduced to exemplify the important roles of inorganic soil constituents—the effects of tsunami on soil in Japan in 2011, the dynamics of radiocesium in the soil environment, and phosphates related to a soil–plant system. With respect to tsunami inundation into paddy field soils, soil erosion by seawater flow, sedimentation of soil transported by the seawater flow, precipitation of evaporites, and sodification are discussed. Removal of the deposited sediments and soil washing by rain and irrigation water were effective for restoration of the salt-affected farmlands. Radiocesium was effectively trapped by soil, which regulated its transfer to agricultural products. Among inorganic soil constituents, weathered biotite has a high fixation capacity for radiocesium. The biotite might have been released from granitic rock and volcanic ash. Apatite is the key phosphate in both natural and farmland soils, although it is converted to more soluble forms in the fertilizer industry. Fixation of phosphate by active Al materials is so high in Andisols that the recovery of phosphate by agricultural crops is low, and phosphate accumulation in plow layer soil is continuing. Struvite plays a role in cycling phosphate in the soil–plant system of farmlands.