1 Introduction

Organic soils are soil deposits containing organic matter. They exhibit high compressibility, low shear strength and large primary and long-term settlement. It has an adverse effect on the settlement of foundations and stability of slopes. As a result, the construction of foundations, embankments, excavations, and other ground works often becomes very difficult in the presence of soils with organic matter.

Even though there has been several studies in the past regarding the index properties and engineering properties of soils containing organic matter, there appears to be a significant void in some of the areas that needs to be addressed. There are two commonly used methods available today, put forward by standard codes of practice like American Society for Testing and Materials (ASTM), British Standards (BS) and Indian Standards (IS). A comparison of these types of methods on the same soil samples has rarely been done and consequently only a little data is available about the effectiveness of these methods. Improving estimates of soil carbon, and increasing understanding of factors that influence variability in these estimates, are important in understanding the role of soil carbon in the global carbon cycle (Bhatti and Bauer 2002). Primary aim of this paper is to analyze all the possible factors, which affect the results of organic matter determination in soils based on procedures put forward by ASTM, BS and IS codes.

2 Reported Studies on Methods for Determination of Organic Matter in Soils

The loss on ignition (LOI) approach has been used widely to estimate soil organic matter (SOM) or soil organic carbon (SOC) in agricultural and forest soils and sediments (Wang et al. 2012; Wang et al. 1996; David 1988; Vos et al. 2005; Wright et al. 2008; Konen et al. 2002; and Ball 1964). The LOI method, often referred to as ashing, is a simple and relatively inexpensive method for determining organic matter. However, reported ignition temperatures and heating times for LOI measurements vary widely, and this brings into question the accuracy of one specific combination of ignition temperature and heating time over another (Konen et al. 2002; Bhatti and Bauer 2002). Another one criticism of the LOI technique is that it may result in weight losses from structural water and carbonate during heating (Chatterjee et al. 2009 and Szava 2009). However, weight loss due to dehydroxylation of clays will be very small as suggested by Wang et al. (2012).

The dichromate oxidation method is the traditional method used in soil science for estimating organic matter. This method provides a good estimate of SOM where levels are less than 3% of SOM, and where free carbonates are present as reported by Bowman (1998). This technique is less expensive (Wang et al. 2012). In spite of its ease, this method is in disfavor because of the use of hexavalent chromium ion (Cr6+) which poses serious health hazard, due to generation of strong sulfuric acid waste (Bowman 1998).

There are numerous other methods to quantify SOM, but the methods put forward by the standard codes of practice are much preferred in routine investigations.

3 Experimental Program

3.1 Materials and Sample Preparation

Three different types of soils were taken for the present study. They are Kaolinite, Bentonite and sand (passing 2 mm sieve). All the tests were conducted on these soil specimens since, they contain no organic matter. The physical properties of soils used for the study, has been summarized in Table 1.

Table 1 Physical properties of soils used for the study
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The liquid limit, plastic limit and plasticity index were determined as per the procedure laid down by IS 2720 Part 5 (2006). The liquid limit test was conducted using Casagrande’s apparatus, starting from a water content that required around 10 blows only for the groove to close. The paste was then spread over glass plate to allow evaporation. This was then mixed thoroughly for the next test. Liquid limit test was carried out to secure a minimum of five points for plotting the flow curve. The moisture content corresponding to 25 drops as read from the curve rounded off to the nearest whole number was reported as the liquid limit of the soil. Plastic limit was determined using thread rolling method and the water content at which cracks appeared on 3 mm diameter soil threads was reported as the plastic limit. The plasticity index was calculated as the difference between liquid limit and plastic limit.The sedimentation analysis was done using hydrometer according to IS 2720 Part 4 (2006) in order to determine the grain size distribution. The grain size distribution curve was plotted and the percentage of clay, silt and sand sized particles were calculated as shown in Table 1.

Starch soluble laboratory reagent (LR) grade was used in the study as an artificial organic matter. Starch is a polysaccharide. It is a granular organic chemical produced by all green plants. When starch is mixed with soil, it induces organic content to the soil by chemical interactions with the clay mineral (Varghese et al. 2019). The properties of starch as provided by the manufacturer are presented in Table 2.

Table 2 Properties of starch used for the study
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In the preliminary study, the soil samples were tested for presence of organic matter without adding starch into them. All the samples were free from organic matter. In the second phase of the experimental program, starch was added in three different percentages viz. 10%, 20% and 30%. The amount of organic matter present in the artificially prepared organic soil samples were determined by making use of procedures put forward by ASTM, BS and IS codes.

3.2 Test Procedure

The ASTMD 2974–87 and BS 1377:3–1990 method II is based on loss on ignition method. For measurement of LOI, 5 g of soil mixed with definite quantity of starch was added to glass beakers, placed in a muffle furnace at 450 °C for 4 h, cooled to room temperature, and weighed. LOI was measured as the difference between the oven dry soil mass and the soil mass after combustion, divided by the oven dry soil mass. The IS 2720:22 (1995) and BS 1377:3 (1990) method I follows the titration procedure using potassium dichromate. In this procedure, 1 g of soil specimen was taken and organic matter present in it was oxidized using potassium dichromate and concentrated sulfuric acid. It was then titrated against ferrous sulphate solution until the color of the solution changed from blue to green.

4 Results and Discussion

4.1 Comparison of Organic Matter Content Obtained from Different Methods

In the present study, artificial organic soil samples were prepared by adding starch to sand, bentonite and kaolinite at varying proportions. Three specimens were tested for each type of soil sample and average of their results was taken. The actual organic matter was calculated based on the dry weight of soil. ASTM method, BS code method and IS code methods were used to estimate the amount of organic matter present in the soils. The percentage deviation was calculated in each case. The results are tabulated in Table 3.

Table 3 Comparison of organic matter content by various methods
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The results indicated that the ASTM and BS code method II employing loss on ignition procedure yields better results with an average percentage recovery of 98.2%.The percentage recovery was only 66.33% for BS code method I and IS code method. These results are true indications that loss on ignition method suggested by ASTM and BS codes is the most reliable method for routine estimation of organic matter.

4.2 Factors Influencing the Determination of Organic Matter

4.2.1 Ignition Temperature and Duration

The ignition temperature and duration has a significant influence on the loss on ignition test results. The temperature is the dominant factor affecting LOI, and its relationship with SOC (Abella and Zimmer 2007). Therefore, it is critical to choose a temperature that is high enough to completely remove SOM but low enough to prevent dehydroxylation of clay minerals and oxidation of carbonate. Literatures have reported ignition temperatures ranging from 360 to 800 °C and ignition duration ranging from 1 to 24 h. ASTM code and BS code clearly specifies that the ignition temperature in muffle furnace should be 440 ± 25 °C. BS code has also mentioned that the ignition time inside the furnace should not be less than 3 h. Hence, in the present study for fixing the ignition time, sand and starch samples were tested at 450 °C for 3 h and 4 h. Three specimens were tested from each type of soil samples and the average values were computed. The results are given in Table 4.

Table 4 Effect of duration of Ignition on organic matter determination
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From the present study, it was observed that an ignition temperature of 450 °C and ignition duration of 4 hours was sufficient for completely removing the organic matter from the soil since the average percentage of recovery of organic matter was 99.37%. Ignition temperature beyond 450 °C and duration beyond 4 hours did not yield much effect on results, as there was no reduction observed in the mass of soil samples. Cambardella et al. (2001) also reported an ignition temperature and time of 450 °C and 4 hours respectively.

4.2.2 Amount of Soil Sample

The most important factor potentially affecting the results of the titration procedure is the amount of soil sample. The amount of soil sample for chemical analysis will vary with the amount of organic matter present in the soil. There is a definite range of soil sample size within which efficiency of oxidation is complete (Bowman 1998). BS code and IS code specifies to take 5 g of soil sample for soils having low organic matter content and as little as 0.2 g for very peaty soil. In order to study the influence of sample size on titration test results, experiments were done using sample sizes 0.3 g, 0.5 g, 1.0 g, 1.5 g and 5.0 g by adding 10% starch to sand. The results are tabulated in Table 5.

Table 5 Effect of amount of sample on organic matter determination
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As the sample size increases, the quantity of dichromate is reduced and the indicator turns green at once, but never turns to deep blue color. Hence, sample size should be strictly restricted to 0.5 g while using titration method for determining organic matter in soil, which is in agreement with the observations of Goldin (2008) and Bowman (1998).

4.2.3 Amount of Organic Matter Present in Soil Sample

There is a definite range within which the oxidation is complete in the dichromate oxidation method (Bowman 1998). The dichromate oxidation method is restricted to the determination of organic matter in soils with an organic content up to 10%.This is because, complete oxidation will not take place in this procedure if organic content exceeds this value. Hence, the results may be under estimated. This was verified by preparing soil samples containing 10% and 20% organic matter and testing it using dichromate oxidation method. The results are presented in Table 6.

Table 6 Effect of amount of organic matter content in soil
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The results indicate that dichromate oxidation method gives reliable results when organic content is within 10%. The percentage deviation was seen to be less than 25%. However, loss on ignition method can be adopted for organic matter above 10% also, as evident from the results shown in Table 3.

4.2.4 Organic Carbon Factor

IS code and BS code specifies the use of a correction factor for converting organic carbon into percentage of organic matter present in the soil. The value of this factor is of prime concern in the determination of organic matter by titration method. In the procedure put forward by IS code and BS code, it is assumed that the SOM contains an average of 58% of carbon by weight. A conventional factor of 1.724 is used based on this assumption to convert organic carbon into percentage of organic matter present in the soil. Studies published since the end of the nineteenth century have consistently shown that the factor of 1.724 is too low for most soils. A factor of 2 based on the assumption that organic matter is 50% carbon, would be more accurate than the conventional factor of 1.724 in almost all cases as observed by Pribyl (2010).

4.2.5 Type of Indicator used for Titration

An indicator that undergoes a definite color change at a specific electrode potential is called redox indicator or oxidation–reduction indicator. IS code and BS code specifies the use of sodium diphenylamine sulphonate as the indicator for titration. Its chemical formula is C12H10NNaO3S.But the endpoint was less visible in certain soil samples.

Diphenylamine is an organic compound with the formula (C6H5)2NH. It is also used as a redox indicator. Instead of sodium diphenylamine sulphonate when diphenylamine was used as the indicator, endpoint was clearly observed. Normally an intense dark blue color is formed on addition of the indicator and as the titration proceeds, this color changes to dirty green. Towards the end of titration, the color passes through violet to a clear deep blue and at the endpoint, a bright clear green color is observed. Diphenylamine is a colorless solid and dissolves well in many common organic solvents. It can provide color change at the endpoint more rapidly than sodium diphenylamine sulphonate. Hence, diphenylamine indicator is a better option than sodium diphenylamine sulphonate as suggested by Hesse (2002).

5 Conclusions

The popular methods suggested by ASTM, BS and IS codes were studied to estimate the organic matter content in soils. Methods suggested by ASTM and BS code method II consisted of heating the soil samples at high temperatures to ignite organic matter and measuring the weight losses whereas method suggested by IS code and BS code method I used dichromate for oxidizing organic matter. The major conclusions drawn from the present study are given below:

  • Loss on ignition method can be adopted for all soils regardless of the amount of organic matter content present in it.

  • The titration method can be adopted only when organic content in soils is less than 10%, otherwise results will be underestimated.

  • While comparing the results obtained, ASTM and BS code method II based on loss on ignition gives most reliable results. Hence, this method can be used for the routine estimation of organic matter content in soils. An ignition time of 4 h and ignition temperature of 4500C was obtained as the optimum values for loss on ignition method.

  • The dichromate oxidation method can be used for determination of organic matter in soils by adopting appropriate precautions against disposal of its byproduct.

  • While preferring titration methods suggested by BS and IS codes, care should be taken to use appropriate carbon correction factor and indicator.

  • The dichromate oxidation method suggested by BS code and IS code can be adopted only if the amount of soil sample is limited to 0.5 g and diphenylamine indicator is used.