Silicon fractions in Histosols and Gleysols of a temperate grassland site

The importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects on many crops. Therefore, it is important to determine the soil Si status and to examine different extractants for testing plant-available Si. Little information is available about the Si status of Histosols and C-org-rich Gleysols in temperate climate. This study was undertaken (1) to characterize different Si pools in C-org-rich groundwater soils of an experimental site and (2) to study the influences of small-scale variability on element distribution. At the experimental site, the thickness of the C-org-rich layer ranges between 4 and 5 dm overlying fine-sandy fluvial sediments. Four extractants were evaluated: 0.01 M CaCl2, 0.5 M acetic acid, 0.1 M sodium pyrophosphate, and 0.1 M Tiron (C6H4Na2O8S2 center dot H2O). Further, total element content was determined following HNO3/HF digestion. Calcium chloride-soluble Si shows no significant relations to other parameters analyzed. On the basis of published data, the soils investigated could be classified as Si-deficient. The Si fraction extracted with acetic acid displays relations to C-org content of the soil and a weak correlation to CaCl2-soluble Si, indicating that both solutions extract overlapping but not the same fractions. Sodium pyrophosphate extracts mainly organo-mineral Fe and Al complexes in the soils studied, which is reflected in a highly positive correlation to C-org. Pyrophosphate-soluble Si showed a negative relationship to Corg, which means a closer relation of this Si fraction to mineral matter than to C-org. The Tiron solution extracted most Si of all extractants, but this amounts only 1% of the total Si content. Taking into account the element-specific relationship between pyrophosphate and Tiron-extractable Fe, Al, and Si, it can be concluded that Tiron dissolves mainly the opaline silica present in Histosols and C-org-rich Gleysols. The distribution of C-org and ash content shows clear spatial trend at the experimental site, which is correlated to pyrophosphate-extractable as well as total Si. This small-scale variability of soil parameters itself is related to a distinct microrelief.