Review of Herbauts et al. (1994)

Herbauts, J.; Dehalu, F.-A.; Gruber, W. 1994. Quantitative determination of plant opal content in soils, using a combined method of heavy liquid separation and alkali dissolution. European Journal of Soil Science 45.

Problem statement. A previous study (Herbauts et al. 1990) had found that the alkali dissolution technique of Jones (1969) did not account for all forms of silicate dissolution. While Jones (1969) did measure silicate dissolution from quartz, the study failed to recognize Si contributions from other forms, namely feldspars and mica among others.

Goals. To introduce and test a new alkali dissolution technique, which accounts for other forms of silicate dissolution.

Study area. Soil samples from the Belgian Ardennes (low phytolith content) and savannah soils from east, central, and west tropical Africa (high phytolith content).

Methods. Four steps: isolate the 20-50 µm fraction from the samples; density separation of the phytolith content (which presumably includes at least some silicates); dissolution of phytolith extracts in hot alkali solution; and determination of Si content via atomic absorption spectrometry (AAS). The 20-50 µm was isolated by sieving and gravity sedimentation. The authors justify this decision by stating that soil fractions <20 µm pose certain problems when attempting to isolate the phytoliths by density separation. Namely, these small particles tend to “clump” together. Also, fine silt and clay particles tend to readily float in density separation due to their high surface area relative to their volume. Phytoliths were floated in ZnBr2. 0.5-2.0 g of digested, sieved and gravity sedimented sample was used. The standard specific gravity is set at 2.3 g cm-3, but the authors set their heavy liquid at 1.92 g cm-3. The authors claim that, for their samples at least, that the phytoliths still float at this density. They claim that this further reducing the contamination hazard from other particles. Floating phytoliths were removed using a peristaltic pump. Herbauts et al. (1990) found that five floating rounds were required to remove 95% of phytoliths. The authors then filtered the phytolith extract through 5.0 µm pore size filters. The phytolith extract and the filter were then placed in a PTFE-lined pressure vessel with 15 cm3 0.5 M NaOH and placed overnight in an oven at 150˚C. Si concentrations achieved by this method was then compared to the density separation technique via weighing and microscope counting.

Results. In the Jones (1969) paper, that author found that 3.26 mg SiO2 must be taken out of the final SiO2 concentration due to partial quartz dissolution. In the Herbauts et al. (1994) study, those authors point out that this correction factor is quite large when compared to the actual phytolith Si amount (40-400%). Thus, a standard correction factor may not be suitable for all soils. Further, the Jones et al. (1969) paper did not take into account other silicates such as feldspars and micas; both of which can be highly weatherable and occur in large quantities.

To determine whether these factors are significant, the authors tested the dissolution technique on quartz separates. According to table 3, quartz does not dissolve at 105˚C. It dissolves slightly at 120˚C, and more at higher temperatures. Thus, if an NaOH dissolution technique were carried out at temperatures below 105˚C, no quartz should dissolve.

The authors next tested the mineral separate (i.e. the residue left over after the phytoliths had been extracted) to determine if other silicates were dissolving. According to Fig. 1, both Si and Al concentrations increased with time. Dissolution was undertaken at both 60˚C and 105˚C. This makes clear that mineral dissolution of aluminosilicates does occur, even when steps are taken to avoid it. The authors then conclude that the only way around this is to perform a density separation technique to extract the phytoliths prior to NaOH dissolution.

Dissolution of the phytolith extracts showed that almost all of the phytoliths were dissolved overnight at 150˚C. Fig. 2 shows the dissolution rate curve. Note that the majority of the phytoliths dissolved within one hour: perhaps the other stuff is inorganic??

Following this, the authors reported a highly significant correlation between the first density extraction and the total amount of phytoliths in the sample (r2=0.987). From this, the authors state that successive extraction steps are not necessary; the total phytolith concentration can simply be computed from the first extract (Figs. 2, 4).

The extraction+dissolution technique was found to have a high correlation with extraction+weighing (r2=0.908) and a moderately high correlation with extraction+counting (r2=0.731).

Limitations. As with Jones (1969), only the 20-50 µm soil fraction was used. This effectively makes this technique unacceptable for precise total biogenic Si (BSi) measurements. While the <20 µm fraction is problematic for density separation techniques, I believe the authors should have pointed out that a new technique must be developed to account for this.

The use of a specific gravity of 1.92 g cm-3 is suspect. While it may have worked for their samples, I know from my own experience that it will not work on many soils.

The authors state that there is no way to account for aluminosilicate dissolution in the samples. I would disagree: if the dissolution kinetics were known for each of the aluminosilicates in the sample, correction factors could be introduced on a sample-by-sample basis. The authors do point out that the across-the-board 3.26 mg SiO2 correction factor used by Jones (1969) is too broad, and seem to be suggesting that each sample must be measured for aluminosilicates. I would agree that the extraction+dissolution technique is a novel way around this, but it in truth impractical. A researcher cannot simply ignore BSi found in “inconvenient” soil fractions.

I don’t think I would use only the first phytolith extraction to estimate total phytolith concentration. The authors did use a peristaltic pump, which will aid in precision, but there is simply too much potential error in only using one extraction.

Conclusions. This paper succeeded in highlighting the shortcomings of the Jones (1969) paper. However, instead of producing a new technique which is practical, the authors created a “quick and dirty” method which skirts the true issue: that it is very difficult to completely separate BSi from other silicates (whether by density separation, dissolution, or both). This method could be used in studies where the total phytolith concentration is not the primary focus, such as in reconstructions.