There is a variety of methodologies used in the aquatic sciences and soil sciences for extracting different forms of Si from sediments and soils. However, a comparison of the published extraction techniques is lacking. Here we review the methodologies used to extract different Si fractions from soils and sediments. Methods were classified in those to assess plant-available Si and those to extract Si from amorphous silica and allophane. Plant-available Si is supposed to comprise silicic acid in soil solution and adsorbed to soil particles. Extraction techniques for plant-available Si include extractions with water, CaCl 2 , acetate, acetic acid, phosphate, H 2 SO 3 , H 2 SO 4 , and citrate. The extractants show different capabilites to desorb silicic acid, with H 2 SO 3 , H 2 SO 4 and citrate having the greater extraction potential. The most common extractants to dissolve amorphous silica from soils and aquatic sediments are NaOH and Na 2 CO 3 , but both also dissolve crystalline silicates to varying degrees. In soils moreover Tiron is used to dissolve amorphous silica, while oxalate is used to dissolve allophanes and imogolite-type materials. Most techniques analyzing for biogenic silica in aquatic environments use a correction method to identify mineral derived Si. By contrast, in the soil sciences no correction methods are used although pedologists are well aware of the overestimation of amorphous silica by the NaOH extraction, which is most commonly used to extract silica from soils. It is recommended that soil scientists begin to use the techniques developed in the aquatic sciences, since it seems impossible to extract amorphous Si from soils completely without dissolving some of the crystalline silicates.

The Podzol has been elected "Soil of the year 2007" in Germany. This article reviews the present knowledge on the development, functions, and threats of Podzols. The main theories on mobilization and transport of organic matter, Fe, Al, and Si are (1) metal-organic migration, (2) metal reduction, and (3) inorganic sol migration. Immobilization theories include precipitation or polymerization due to increasing pH/ abundance of base cations with depth, mechanical filtering in soil pores, oxidation of metal-organic complexes, biodegradation of the organic part, decreasing C-to-metal ratios during translocation, adsorption to soil particles, and flocculation at the point of zero charge. Podzolization is discussed also on the catena scale, where vertical and lateral translocation processes (across pedon boundaries) need to be considered to understand Podzol patterns in landscapes. Chronosequence studies show that incipient podzolization usually becomes visible between 100 and 500 y and mature Podzols develop in 1,000-6,000 y. The occurrence of Podzols worldwide is concentrated mainly on the boreal zone and mountain regions within the humid temperate zone. Smaller Podzol areas are found in some perhumid tropical and subtropical regions. In Germany, Podzols occur in the Alps, in the glaciofluvial valleys and heathlands of N Germany, and in the mountain ranges. They fulfil several ecological functions, especially for groundwater recharge. Main threats for these mostly sandy soils are wind erosion and surface mining of sand. Two pedons which were chosen to represent the "Soil of the year 2007" are presented. Finally, some conclusions about podzolization processes are drawn, which may explain the diverse observations reported in the literature.

SummaryMany studies have highlighted the importance of the Amorphous Silica (ASi) pool to the overall mass balance in the biogeosphere. In order to advance our knowledge of measurements and quantification of this pool, it is necessary to compare the ability of different extractants to dissolve ASi in soils and to test methods developed in the aquatic sciences to soils systems. The methods used in this work included three acid extraction techniques (0.2 M NH 4 -oxalate, 0.1 M NH 4 -citrate and 0.5 M NH 4 -acetate) and two alkaline extraction techniques (0.094 M Na 2 CO 3 and 0.5 M NaOH), which are more commonly used for the measurement of ASi in aquatic sediments. Our results indicate that the amount of Si extracted from phytolith samples with the acid methods was two orders of magnitude less than the amount of extracted by alkaline extractions. When applied to natural soil samples, these extractions show that the acid techniques are only able to extract loosely-bound components such as adsorbed Si and Si bound in amorphous matrices with Al and Fe. While Na 2 CO 3 or NaOH extracted the same amount of ASi in Podzols, Na 2 CO 3 was able to extract only part of the ASi extracted with NaOH in Chernozems. Pretreatment of the samples with 0.1 M HCl before the Na 2 CO 3 extraction did not increase amounts of ASi extracted. The present work suggests that alkaline methods used commonly for ASi on aquatic sediment samples can be used on a wide variety of soils.

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