Friday, February 1, 2013


Total Organic Carbon

What it is: Total organic carbon (TOC) is the carbon (C) stored in soil organic matter (SOM). Organic carbon (OC) enters the soil through the decomposition of plant and animal residues, root exudates, living and dead microorganisms, and soil biota. SOM is the organic fraction of soil exclusive of nondecomposed plant and animal residues. Nevertheless, most analytical methods do not distinguish between decomposed and non-decomposed residues. SOM is a heterogeneous, dynamic substance that varies in particle size, C content, decomposition rate, and turnover time.
Soil Organic Carbon (SOC) is the main source of energy for soil microorganisms. The ease and speed with which SOC becomes available is related to the SOM fraction in which it resides. In this respect, SOC can be partitioned into fractions based on the size and breakdown rates of the SOM in which it is contained (see table 1). The first three fractions listed are part of the active pool of SOM. Carbon sources in the active pool are relatively easy to break down.
SOM contains approximately 58% C; therefore, a factor of 1.72 can be used to convert OC to SOM. There is more inorganic C than TOC in calcareous soils. TOC is expressed as percent C per 100 g of soil.
Table 1. Size and breakdown rates of various soil organic matter fractions.
Soil Organic Matter
Particle Size
Time (years)
plant residues≥ 2.0< 5recognizable plant shoots and roots
particulate organic matter0.06 – 2.0< 100partially decomposed plant material, hyphae, seeds, etc
soil microbial biomassvariable< 3living pool of soil organic matter, particularly bacteria and fungi
humus≤ 0.0053< 100 – 5000ultimate stage of decomposition, dominated by stable compounds
Why it is important: SOC is one of the most important constituents of the soil due to its capacity to affect plant growth as both a source of energy and a trigger for nutrient availability through mineralization. SOC fractions in the active pool, previously described, are the main source of energy and nutrients for soil microorganisms. Humus participates in aggregate stability, and nutrient and water holding capacity.
OC compounds, such as polysaccharides (sugars) bind mineral particles together into microaggregates. Glomalin, a SOM substance that may account for 20% of soil carbon, glues aggregates together and stabilizes soil structure making soil resistant to erosion, but porous enough to allow air, water and plant roots to move through the soil. Organic acids (e.g., oxalic acid), commonly released from decomposing organic residues and manures, prevents phosphorus fixation by clay minerals and improve its plant availability, especially in subtropical and tropical soils. An increase in SOM, and therefore total C, leads to greater biological diversity in the soil, thus increasing biological control of plant diseases and pests. Data also reveals that interaction between dissolved OC released from manure with pesticides may increase or decrease pesticide movement through soil into groundwater.
Specific problems that might be caused by poor function: A direct effect of poor SOC is reduced microbial biomass, activity, and nutrient mineralization due to a shortage of energy sources. In non-calcareous soils, aggregate stability, infiltration, drainage, and airflow are reduced. Scarce SOC results in less diversity in soil biota with a risk of the food chain equilibrium being disrupted, which can cause disturbance in the soil environment (e.g., plant pest and disease increase, accumulation of toxic substances).
What you can do: Compiled data shows that farming practices have resulted in the loss of an estimated 4,400,000,000 tons of C from soils of the United States, most of which is OC. To compensate for these losses, practices such as no-till may increase SOC (figure 1). Other practices that increase SOC include continuous application of manure and compost, and use of summer and/or winter cover crops. Burning, harvesting, or otherwise removing residues decreases SOC.
For more information go to Soil Management Practices.
total organic carbon
Figure 1. Effect of 10 years of conventional till and no-till on OC (calculated from SOM data in Edwards et al., 1999).
Measuring total organic carbon:
Presently, no methods exist to measure TOC in the field. Attempts have been made to develop color charts that match color to TOC content, but the correlation is better within soil landscapes and only for limited soils. Near infrared spectroscopy has been attempted to measure C directly in the field, but it is expensive. Numerous laboratory methods are available.

Organic Matter and Erosion 

 K          =          [ 2.1M1.14(10-4)(12-OM) + 3.25(S – 2) + 2.5(P – 3)]/100 x 0. 317

K         is  the Soil Erodibility Index
       is the product of (% Silt + % Very Fine Sand)( % Fine Sand  + % Medium Sand  +
            %  Coarse Sand)
OM     is the percent of Organic Matter
S        is the Soil Structure Code
P        is the Soil Hydraulic Permeability Class  

Step 2.1 :  Calculation of  M  (% Silt + % Very Fine Sand)(% Sand)

Ø  Definition of Soil Particle Size Classes.  The particle size classes definition used in the USLE nomograph are as below :

                        Clay                 :           <   0.002 mm
                        Silt                   :           0.002 – 0.05 mm
                        Very Fine Sand  :          0.05 – 0.1 mm
                        Sand                 :           0.1 – 2.0 mm

Ø  The soil type in Block 3B is Rengam/4 Soil Series. The Soil Particle Analyses (7 Fractions) for this soil is given below (see appendix)

Coarse Silt
VF Sand
Fine Sand
Med Sand
Co Sand
0 –  6
6 - 30

Ø  Usually the weighted average soil particle analyses to a depth of 25 centimeter are used in the computation of the K Index.

Ø  Based on the above data, the weighted average content for the “Silt” fraction, “Very Fine Sand” fraction and “Sand” fraction as required in the computation of the USLE are calculated as below :

VF Sand
0 –  6
5.0 + 6.3 = 11.3
4.0 + 21.1 + 50.0 =  75.1
6 - 30
2.4 + 5.0 =  7.4
4.0 + 25.3 + 30.0 =  59.3
8.8 x 6/25 +
28.5 x 19/25
= 23.77
11.3  x  6/25 +
7.4  x 19/25
=  8.34
4.8 x 6/25 +
5.0 x 19/25
=  4.95
75.1 x 6/25 +
59.3  x 19/25
= 63.07

Ø  The value of  M              =         (% silt  +   % Very Fine Sand)( % Sand)
                               =         ( 8.34 + 4.95)(63.07)
                              =          13.29 x 63.07
                              =          838.2

Step 2.2 :  Calculation of Average Percent Organic Matter

From the soil analytical report, the organic matter content of the Rengam/4 Series of the estate  within 25 cm depth are 0.49 % Organic Carbon (0 – 6 cm) and 1.63  % Organic Carbon (6 -  25 cm).  The percentage of organic matter content is derived by multiplying % organic carbon content with a factor of 1.72.

Ø  Calculation of the weighted average Organic Matter (OM) content of 25 cm depth  :

Organic Carbon (weighted average)         =          0.49 %  x  6/25  +  1.53 %  x  19/25
                                                            =          0.118 %            +  1.163 %
                                                            =          1.28  %
Organic Matter (Weighted Average)         =          1.28 x 1.72       
=          2.20 %

Step 2.3 :  Determination of Soil Structure Parameter

The equivalent soil structure classes used in the USLE and in Malaysian soil reporting as suggested by the Department of Agriculture, Malaysia is given as  :

Ø  From the soil report of Rengam/4, this soil series have Fine to Medium Subangular Blocky Structure.

Ø  With reference to the above below, the Soil Structure (S) Code for the soil is  4

Soil Structure


Definition of Soil Structure

Soil Structure Classes

(Soil Report - Depart. Of Agriculture)
Very Fine Granular
Very Fine Granular, Crumb
Fine Granular
Fine Granular / Crumb
Medium or Coarse Granular
Medium Granular/Crumb or Very Fine to Fine Subangular Blocky or Very Fine to Fine Angular Blocky
Blocky, Platy or Massive
Medium to Coarse Subangular Blocky,Angular Blocky, Prismatic, Columnar or Massive

Step 2.4 :  Determination of Soil Hydraulic Permeability Parameter

The drainage classes used in mapping Malaysian soils is used to as equivalent to the soil hydraulic permeability that are used in the USLE Nomograph as suggested by the Department of Agriculture, Malaysia below  :

Soil Permeability Value
Soil Profile Hydraulic Permeability
(USLE Nomograph)
Soil Profile Drainage Classes
(Soil Report – Depart. Of Agriculture M’sia)
Class 8 – 9 : Excessive to Very Excessive Drained
Moderate To Rapid
Class 5 – 7 : Somewhat Imperfect to Well Drained
Class 4  :  Imperfectly Drained
Slow To Moderate
Class 3  :  Somewhat Poorly Drained
Class 2  :  Poorly Drained
Very Slow
Class 0 – 1 :  Very Poorly to Somewhat Poorly

Ø  From the soil report of the farm, the Rengam/4 Series has a Drainage Class 7 which is   Well Drained

Ø  With reference to the above Table, the Soil Hydraulic Permeability (P) Value of the soil is  2

Step 2.5 : Computation of the Soil Erodibility (K) Index

Ø  Using the values that have been derived (as shown above) viz.

M         =          838.2                [ Refer Step 2.1 ]
OM       =          2.2                    [ Refer Step 2.2 ]
S          =          4                      [ Refer Step 2.3 ]
P          =          2                      [ Refer Step 2.4 ]

Ø  The computation of K index  for the Rengam Soil Series is represented by the following algebraic equation :

K          =          [ 2.1M1.14(10-4)(12-OM) + 3.25(S – 2) + 2.5(P – 3)]/100 x 0.317

Ø  The above algebraic equation is written in the Excel software in the diskette provided for a rapid computation by the computer. Load the file “Computation of K Nomograph Value“ from the A : directory into the computer.

Ø  Filled in the values (%) for the weighted average values for the “Clay”, “Silt”, “V.Fine Sand”, “Sand” and “Organic Matter” columns.

Ø   Similarly, filled in the values for the “Soil Structure Parameter” and the “Soil Hydraulic Permeability Parameter.” Columns.

Ø  The Excel program first computed the M value. Compared the computed M value obtained by computer with the value of 838.2  that you have obtained in Step 2.1.

Ø  The next column in the Excel program indicated the Soil Erodibility (K)  Value as computed by the computer  :-

K   =     [ 2.1 x 838.21.14(10-4)(12 - 2.2) + 3.25(4 – 2) + 2.5(2 – 3)]/100 x 0.317
                  =    0.0267    



USLE nomograph are as below :

                        Clay                 :           <   0.002 mm
                        Silt                   :           0.002 – 0.05 mm
                        Very Fine Sand  :          0.05 – 0.1 mm
                        Sand                 :           0.1 – 2.0 mm


                       Clay                 :           <   0.002 mm
                        Silt                   :           0.002 – 0.06 mm
                        Very Fine Sand  :          0.06 – 0.2 mm
                        Sand                 :           0.2 – 2.0 mm

Calculation of  M  (% Silt + % Very Fine Sand)(% Sand)


 Clay <2μmn  =   <   0.002 mm
Fine Silt 2-20μm =0.002-0.02mm  
Coarse Silt  20-50μm =0.02-0.05mm
Fine Sand  50-200μm =0.05-0.2mm
Coarse Sand 200-2000μm =0.2-2mm

Calculation of  M  (% Silt + % Very Fine Sand)(% Sand)



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