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European Journal of Applied Sciences – Vol. 10, No. 2
Publication Date: April 25, 2022
DOI:10.14738/aivp.102.12049. Anand, S., Diarra, S., & Sharma, A. C. (2022). Soil Organic Carbon: Associating the Walkley-Black Wet Oxidation Method to Loss on
Ignition (Gravimetric Method) and Clay Content for Selected Samoan Inceptisols. European Journal of Applied Sciences, 10(2). 219-
227.
Services for Science and Education – United Kingdom
Soil Organic Carbon: Associating the Walkley-Black Wet
Oxidation Method to Loss on Ignition (Gravimetric Method) and
Clay Content for Selected Samoan Inceptisols
Sanjay Anand
University of the South Pacific
School of Agriculture, Geography, Environment
Ocean and Natural Sciences, Samoa Campus
Siaka Diarra
University of the South Pacific
School of Agriculture, Geography, Environment
Ocean and Natural Sciences, Samoa Campus
Ami Chand Sharma
Ministry of Agriculture, Fisheries and Forestry, Fiji Islands
ABSTRACT
Precise estimation of soil organic carbon (SOC) is vital in many scientific fields
including natural and applied sciences, as well as climate science researches.
Ordinary methods of measuring soil organic carbon (SOC) aren’t sufficient for
accurate estimation of C sequestration nor to meet environmental safety standards.
Methods that allow for accurate assessment of SOC, which are eco-friendly, and yet
practicable are more desirable for use in developing countries. Loss on ignition
method (LOI) of appraising SOC offers an alternative method to the traditional
Walkley-Black (W-B) method of organic carbon determination. A quantitative
relation between LOI and Walkley-Black methods was studied for selected Samoan
inceptisols. For this study, 36 soil samples from 0-15 cm depth were collected from
12 sites to determine the levels of SOC (%) by both the methods. Clay contents were
also determined and used to develop simple and multiple linear regression
equations. Highly significant positive associations (P<0.001) were observed
between %OM-WB and %OM-LOI (r = 0.92); %OM-WB and %Clay content (r = 0.91);
and, %OM-LOI and %Clay content (r = 0.76). Multiple rectilinear regression
equation using clay content of soil and LOI as independent factors and W-B as
dependent factor provided reliable estimates of SOC since significant multiple
relationship was established (R2 = 0.69, p < 0.001).
Keywords: Loss on ignition, Walkley and Black, Soil organic carbon, Soil organic matter.
INTRODUCTION
Soil organic carbon occurs as plant and animal matter at various stages of decomposition and
the final product, the humus. It is the chief element of soil organic matter and plays an important
role within the global carbon cycle and climate change [1]. Naturally, SOC forms are derived
from the decomposition of living organisms and contamination through anthropogenic
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European Journal of Applied Sciences (EJAS) Vol. 10, Issue 2, April-2022
Services for Science and Education – United Kingdom
activities [2]. Most of the terrestrial carbon is held in soils, more than twice as much as in
vegetation or the atmosphere, and changes in SOC content can have a significant effect on the
global carbon budget [3].
The determination of total SOC is an essential part of site characterization because it can
distinctly influence the chemical and biological reactions in a soil [4]. Soil organic carbon can
be used for assessing soil quality; it indicates the level of productivity of a given soil. It has
tremendous impact on the chemical, physical, and biological properties of soil. Three plant
macronutrients, nitrogen, phosphorus, and sulphur, are constituents of soil organic matter [5].
Soil organic matter plays a critical role in the conservation of fertility, especially in the
extremely coarse textured soils [6]. In these soils, SOC is both is a source of nutrients and
provides a mechanism for nutrient retention through increased cation exchange capacity. It
affects water infiltration as well as increases retention of plant available water, and it provides
favorable conditions for soil biota. Because of these properties, organic matter has been termed
as the ‘lifeblood’ of soil.
Thus, large amounts of SOC will function as a source of sink for nutrients, maintain soil fertility,
and reduce leaching of soluble nutrients [7, 8]. Estimates of SOC are often needed for
applications in ecology, soil science, and global temperature change.
Agricultural sustainability improves with increases in organic matter in soils. Soil organic
carbon, derived from the decay of plants and animals in various forms, is a key component of
organic matter playing a great role in increasing soil fertility. Thus, an improved management
of soil organic carbon is needed to enhance soil fertility. As has been emphasized elsewhere,
farmers and producers of tomorrow may need to not only farm soil judiciously but to also “farm
carbon” [9].
Accurate, rapid and cost-effective soil carbon determination is also important to the
development of a soil carbon accounting system [10]. Many methods are available for
measuring SOC, each with its own advantages and disadvantages in terms of accuracy, expense,
and convenience [11].
In developing countries, unfortunately, accurate SOC determinations are challenging because
approaches are either not ecologically acceptable or are too expensive, requiring costly
instruments and highly trained personnel.
Loss on ignition or gravimetric method offers a fast and simple method of estimating SOC [12-
15]. The method assumes that the decline in sample mass upon ignition is merely due to loss of
organic matter. However, it is vulnerable to errors from ignition period and temperature and
the mass of soil to be ignited. An underestimation results from low ignition temperatures while
increased ignition duration overestimates organic carbon levels [16].
Conversely, W-B has significant uncertainties regarding the proportion of total soil organic
carbon that is oxidized as well as oxidation of other substrate constituents in the soil. Soil
organic carbon recovery is highly variable and strong acid and chromium are generated as
environmental wastes [17, 18]. In the W-B procedure the active or decomposable soil organic
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Anand, S., Diarra, S., & Sharma, A. C. (2022). Soil Organic Carbon: Associating the Walkley-Black Wet Oxidation Method to Loss on Ignition
(Gravimetric Method) and Clay Content for Selected Samoan Inceptisols. European Journal of Applied Sciences, 10(2). 219-227.
URL: http://dx.doi.org/10.14738/aivp.102.12049
matter are oxidized, while the greater recalcitrant carbon present as graphite or charcoal is not.
However, W-B method for SOC determination has been proposed as the most suitable for
volcanic soils [19].
A very little variance was reported between the LOI and W-B methods of SOC estimation [20].
The LOI method gave good estimates of SOC and SOM, however, is generally considered
unreliable for soils with low organic carbon contents. When LOI and W-B are compared with
the reference method (i.e. dry combustion), an excellent relation (with a mean conversion
factor of 1.25) existed between them, which compared well with the recommended value of
1.32 [21, 22]. Many different regression equations and models have been developed for the
estimation of SOC by various methods. Regression equations between LOI and SOC determined
by elemental carbon analyzer showed very strong relationships (R2 = 0.94 to 0.98) [15]. Clay
content and clay mineralogy significantly influences these equations. In the multiple regression
equations, clay content explained 78% (0-15 cm) of the variation in SOC [18]. Clay content of
the soil samples improved the predictions in the multiple regression equations. A conversion
factor of 14.9 based on clay content was obtained [23]. This high mean conversion factor
reflected significant water loss by dehydration of Fe, Al, and Mn oxides clay minerals. The LOI
method has been widely used by many authors for estimating SOM and SOC using different
temperatures for the muffle furnace: at 360oC for 2 h [15, 24-27] and at 300, 450, and 600oC for
2 h [18].
Precise fertilizer and pesticide management practices which are definitive for precision
agriculture require rapid and accurate estimation of SOC to optimize field productivity and
minimize groundwater contamination risks [28, 29]. With the interest in studying the carbon
cycle on continuously cropped land in the pacific island countries, there is a need to quantify
SOC of volcanic soils in order to assess the most efficient method. Hence, a rapid and easy
method for SOC estimation is beneficial. The main objective of the study was to propose a
simple, easy and reliable method for estimating SOC in the laboratory.
MATERIALS AND METHODS
Study Area
Soil samples for the study were collected from 12 sites including; cocoa plantations, taro fields,
pasture fields, banana, sparse natural growth and arable land under cultivation (mixed
cropping). The soils belong to the Moamoa series and are classified as Inceptisols. The site
location experiences a humid tropical climate with an annual rainfall varying from 2,500 to
3,500 mm with a strong seasonality of distribution (Maatuis and van Meer, 2003) [30].
Soil sample Collection and Preparation
Thirty-six soil samples each of soil depth, 0-15 cm were collected from the 12 different sites of
the crop plantation. The soil samples were air-dried for 10 consecutive days in the soil
chemistry laboratory and later sieved through a 2 mm sieve to remove debris. Part of the air- dried soil samples was oven-dried to a constant mass at a temperature of 105oC for 24 hours.
Laboratory Analysis
Particle size analysis was determined following the hydrometer method using the correction
for temperature fluctuations. Organic carbon was estimated using W-B by wet oxidation