As this is looking like that crushing of soil aggregates in fine particles would increase the soil surface area and also give more reactive site for the chemical interaction and may lead to a higher estimation of organic carbon.
In my experience, and admittedly this was some time ago, all soil analyses began by passing the sample through a 2-mm sieve. This size was chosen because it preserved most soil aggregates while leading to a uniform particle size and removing larger organic debris and stones. I have reservations about the logic of this step in non-agricultural settings, where aggregate size may vary and organic detritus may be a significant source of soil carbon. However, if you are going to make comparisons among soils from different sites, I think the important consideration is to use a common sieve size for all. If the efficiency of the Walkley-Black extraction is affected by crushing large aggregates, at least your bias would be uniform across sites. Also, if you use 2-mm sieve size, your data should be comparable with most literature.
In Poland, at least in past, we used Tiurin method, which actually is identical with the Walkley-Black method. In that time we sieved soil by 1mm, but it is clear, that it is necessary to use 2mm sieve to assure comparison of results at international level. However, there were one additional practice - the soil, sieved by 1mm (now it should be 2mm) were milled (ground) and after it used for analysis of organic carbon.
As Barry and Michał said before it's common to use the 2mm sieve for all soil analyses. For SOC it's advisable to mill the soil to achieve a more homogeneous sample (specially when you use less than 500mg of soil)
In routine soil testing all the determinations are made on 2 mm sieved soil.But it is in practice to further grind the sample before organic carbon determination.It is recommended to grind 5 g sample of 2mm seived to 0.5 (35 mesh) or 0.2(80 mesh)mm size and take o.5 g-1 g sample for organic carbon estimation probably for better interaction of ground soil with conc. H2SO4 and K2Cr2O7 solution compared to aggregates.
Simple sieving through 2mm sieve will do as far as estimation of organic carbon by Walkley and Black method is concerned . But for soil total carbon , ground soil must be passed through 100-150 mesh sieve ( 0.100-0.150 mm openings ) .
Thanks to all of you,
This is good to take 2 mm sieve size as a standard for soil organic carbon analysis.But the question is that, what happens in the case of organic carbon analysis of a particular soil aggregate size.Because when I have analyzed soil of a particular aggregate size by crushing and same soil as an intact aggregate.There comes a variation in organic carbon percentage (in the range of 0.04-0.1). What may be the reason, i.e due to variation in sample of same soil or due to differences in estimation of organic carbon content.
Dear kumari
I agree with Rao.
It is recommended to grind 5 g sample of 2mm seived to 0.5 (35 mesh) or 0.2(80 mesh)mm size and take o.5 g-1 g sample for organic carbon estimation probably for better interaction of ground soil with conc.
Dear kumari
I agree with Rao.
It is recommended to grind 5 g sample of 2mm seived to 0.5 (35 mesh) or 0.2(80 mesh)mm size and take o.5 g-1 g sample for organic carbon estimation probably for better interaction of ground soil with conc.
In soil, micro aggregates form into macro aggregates through polysaccharides of plant or microbial origin and fungal hyphae.If one grinds soil samples up to o .2 -0.25 mm size most of the macro aggregates breakdown to micro aggregates(
There is a lot of papers about the different distribution of soil carbon in aggregates of different size because the size of the aggregates is related to their turnover and therefore to the potential they have to stabilize organic matter. So, no matter the size, you should mill or grind the aggregates to make all material pass through a 0.5 mm sieve. As the Walkley Black method is a chemical wet oxidation, its efficiency depends on the surface area; thus having the same size for the analysis, independently of the original size of the aggregates is imperative. In case a full aggregate distribution is available (sequential sieving of the whole bulk sample) and you analyse all aggregate sizes after milling to 0.5 mm, the weghted sum of C in aggregates agree fairly well with the C analysed on the 0.5 mm bulk sample. With an elementar analyser the agreement is almost perfect. With the Walkley Black method the recovery is more variable, but it's a typical problem of the wet oxidation that was already known in the '30s when the method was developed. Have a look at a good book describing soil chemical analyses. For example:
Nelson, D.W., and L.E. Sommers. 1982. Total carbon, organic carbon and organic matter, pp. 539-579, in: A.L. Page, R.H. Miller, and D.R. Keeney, eds., Methods of Soil Analysis, part 2, 2nd edition. American Society of Agronomy, Madison, Wisconsin, USA.
Please look into the paper
Effect of soil sample grinding intensity on carbon determination by high-temperature combustion. Cihacek,L and Jacobson,K.A.2007. Commun. in Soil Sci.Plant Anal. 38,1733-1739.
In high high temperature combustion method,grinding to ) 0.15 mm(100 mesh) is recommended by the authors to reduce variability in the results of carbon estimated in soils differing widely in texture.
2 mm could be appropiated. I often use this sieve size. Moreover, for example, we separate microaggregates (53-250 micres) and the particles with
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