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 Fraction | Particle Size (mm) | Turnover Time (years) | Description |
|---|---|---|---|
| plant residues | ≥ 2.0 | < 5 | recognizable plant shoots and roots |
| particulate organic matter | 0.06 – 2.0 | < 100 | partially decomposed plant material, hyphae, seeds, etc |
| soil microbial biomass | variable | < 3 | living pool of soil organic matter, particularly bacteria and fungi |
| humus | ≤ 0.0053 | < 100 – 5000 | ultimate stage of decomposition, dominated by stable compounds |
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.
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
M is the product of (% Silt + % Very Fine
Sand)( % Fine Sand + % Medium Sand +
% Coarse Sand)
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)
|
Depth
|
Clay
|
Silt
|
Coarse
Silt
|
VF
Sand
|
Fine
Sand
|
Med
Sand
|
Co
Sand
|
|
0 – 6
|
8.8
|
5.0
|
6.3
|
4.8
|
4.0
|
21.1
|
50.0
|
|
6 - 30
|
28.5
|
2.4
|
5.0
|
5.0
|
4.0
|
25.3
|
30.0
|
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 :
|
Depth
|
Clay
|
Silt
|
VF
Sand
|
Sand
|
|
0 – 6
|
8.8
|
5.0
+ 6.3 = 11.3
|
4.8
|
4.0
+ 21.1 + 50.0 = 75.1
|
|
6 - 30
|
28.5
|
2.4
+ 5.0 = 7.4
|
5.0
|
4.0
+ 25.3 + 30.0 = 59.3
|
|
Weighted
Average
|
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
Code
|
Definition
of Soil Structure
(Nomograph)
|
Soil Structure
Classes
(Soil
Report - Depart. Of Agriculture)
|
|
1
|
Very Fine
Granular
|
Very Fine
Granular, Crumb
|
|
2
|
Fine
Granular
|
Fine
Granular / Crumb
|
|
3
|
Medium or
Coarse Granular
|
Medium
Granular/Crumb or Very Fine to Fine Subangular Blocky or Very Fine to Fine
Angular Blocky
|
|
4
|
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)
|
|
1
|
Rapid
|
Class 8 – 9 : Excessive to Very Excessive Drained
|
|
2
|
Moderate
To Rapid
|
Class 5 –
7 : Somewhat Imperfect to Well Drained
|
|
3
|
Moderate
|
Class
4 :
Imperfectly Drained
|
|
4
|
Slow To
Moderate
|
Class
3 :
Somewhat Poorly Drained
|
|
5
|
Slow
|
Class 2 :
Poorly Drained
|
|
6
|
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 ]
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
TEXTURE CIERO ,HYDEC,USLE
USLE
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
HYDEC
Clay : < 0.002 mm
Calculation of M (% Silt + % Very Fine Sand)(% Sand)
CIERO
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
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