Jones, R.L. 1969. Determination of opal in soil by alkali dissolution analysis. Proceedings – Soil Science Society of America 33.
Problem statement. Quantitative estimates of biogenic Si (BSi) have been accomplished by microscope counts or the density separation technique (heavy liquid floatation). Microscope counts can be time consuming and inaccurate. The density separation technique is also time consuming and cannot remove BSi smaller than 5 µm. For these reasons these techniques are better suited for reconstructions in which a quantitative estimate of BSi is not required. Therefore, a new technique is required.
Goals. The authors introduce a new BSi extraction technique which involves boiling the soil sample in hot NaOH. This will actually dissolve the BSi (and some inorganic Si; see explanation below) into solution. The concentration of dissolved Si can then be measured spectroscopically. The goal of this paper is to test this new approach against microscope count and density separation estimates of the same soil samples to which is more accurate.
Study area. Some soils of Illinois (Mollisols and Alfisols).
Methods. 1.000 g of soil per sample (20-50 µm fraction). 100 mL of 0.5 N NaOH. Cook at a rolling boil for 20 minutes. Place an ice-filled beaker on top of the NaOH beaker to act as a condenser. After 20 minutes transfer NaOH beaker to ice bath to stop cooking. Transfer supernatant through #4 filter paper. Rinse sample residue and condenser beaker bottom through filter as well. This will ensure that all of the dissolved Si will be kept. Proceed with steps to prepare the sample for spectroscopic measurement (see Jones & Dreher: Silicon determination by light absorption spectrometry).
Results. The authors measured the solubility of the Si in the sample as it dissolved in ten minute increments up to one hour after immersion. The dissolution rate was linear (Fig. 1), which obeys dissolution kinetics. The authors chose a cooking time of 20 minutes for convenience sake, and presumably because the longer the sample cooked, the more inorganic Si would be acquired.
Compared to the density separation method, the dissolution technique matches up well. Fig. 2 shows r=0.97. The intercept of the scatterplot is 3.52 mg SiO2, which the authors say is close to the solubility of quartz. The regression equation is y=2.93x+3.52. For every 1% increase in particulate BSi from the density separation technique, the same sample shows an increase of 2.93 mg of SiO2 by way of the dissolution technique. Thus, a BSi sample of 0% would equal a SiO2 concentration of 3.52 mg: quartz. Indeed, when there is no BSi to be found, then the only SiO2 present must be from other silicates. In this case, the authors state that quartz is the dominant silicate.
Limitations. The authors only process the 20-50 µm fraction of the soil samples. According to Sommer et al. (2006), 18-65% of total BSi concentrations may be smaller than 5 µm. Piperno (2006) states that BSi larger than 50 µm may be significant as well. Jones et al. probably only dissolved the 20-50 µm fraction to be consistent with the other two extraction techniques. For example, density separation typically removes particles smaller than 5 µm because these small particles cannot be removed by differing density. Due to the high surface area and low volume of fine silts and small clays, these particles will typically float along with the BSi. The authors may have also decided to discard the smaller fraction because of concerns of clay dissolution. As stated before, clays have a large surface area and are almost always secondary minerals. For these reasons, clays can be highly reactive. Thus, the authors may have been trying to avoid contamination of their BSi concentration from inorganic clays. The authors also probably removed the larger sand-sized fraction because typically few BSi particles are found in this size range in mid-latitude soils. However, if a researcher is attempting to estimate total BSi, I think every attempt should be made to account for all size ranges, not just the most convenient.
Herbauts et al. (1994) make the point that this study only attempted to minimize Si dissolution from quartz, and ignored Si additions from other silicates such as feldspars and micas. I think this is a valid argument, but the solution offered by Herbauts (density separation followed by dissolution) cannot separate BSi from clays, which are often highly reactive silicates. DeMaster (1981) made the point that BSi will dissolve quickly within the first two hours. Silicates typically dissolve slower and linearly. Thus, it is possible to estimate the amount of BSi present, even if silicates are dissolving (see Fig. 1 in Koning et al. 2002 or Fig. 1 in Saccone et al. 2005). However I am concerned that this technique cannot account for all silicates. This method will only give a “best fit” or average of the silicate dissolution rate. Since there are a myriad of silicates out there, some are bound to have higher dissolution rates than BSi. How does one account for these? What happens if a large proportion of the soil sample is made of highly soluble silicates?
Conclusions. This technique in some ways overcomes some problems associated with density separation. The high solubility of acidic volcanic glass particles will hamper this technique (could be trouble for my research). The dissolution technique is much quicker than density separation and microscope counts.