|Main authors:||Abdallah Alaoui and Gudrun Schwilch
|Source document:||Alaoui, A. and Schwilch, G. (2016) Soil quality and agricultural management practices inventory at case study sites. iSQAPER Report 28 pp|
The labile organic carbon can be measured in the field by a prior solution preparation (CaCl2 and KMnO4).
The labile fraction of soil carbon is the component of organic matter that feeds the soil food web and is closely associated with nutrient cycling and other important biological functions in the soil. Weil et al. (2003) have developed a field kit method for the determination of KMnO4 oxidisable Carbon. In this test a dilute solution of KMnO4 is used to oxidize OC. Generally, in the course of the experimental procedure the greater the loss in colour of the KMnO4, the lower the absorbance reading will be, hence the greater the amount of oxidisable Carbon in the soil.
- Prepare stock solution of CaCl2 (0.1 M)
- Prepare KMnO4 (0.02 M) in 0.1 M CaCl2, adjust solution to pH 7.2 with 0.1M NaOH.
- Transfer 2 ml of above solution into graduated polypropylene tube
- Add distilled water to till 20 ml mark swirl to mix
- Add 4.9±0.3 g (or 5 ml scoop of soil volume) of soil to the above solution [soils should be air dried in the sun for 15 mins and crumbled]
- Wrist-Shake mixture for 2 mins and allow to stand/settle for 5 mins
- Pipette 0.5 ml from upper 1 cm depth into another polypropylene tube and add 45 ml of distilled water
- Make solution up to 50 ml mark with more distilled water
- Transfer 15 ml of the above solution into a glass cuvette
- Measure absorption (at 550 nm) with colorimeter(1)
- Calibration: measure the absorbance of the following
a. Distilled water filled in glass cuvette (blank) and set to zero.
b. 0.50 ml 0.005 M KMnO4 to a graduated tube+ 45 ml distilled water and made to 50 ml mark and shake. Transfer 15 ml to a cuvette and measure absorbance
c. Repeat the above procedure but with 0.01 M and 0.02 M KMnO4.
d. Make a calibration curve with absorbance (x-axis) and conc. (y-axis)
- Determine the active labile carbon in soil using the equation of Weil et al., 2003:
Labile carbon (mg/g) = [0.02 mol/l – (a+b x absorbance)] x (9000 mg C/mol) x (0.02l solution / 5 g soil).
where a is the intercept and b is the slope of the calibration curve you have determined.
Table: Permanganate oxidisable carbon contents (mg/g) considered to be low, moderate and high for soils of various textures.
|Soil organic carbon status||Sand||Sandy loam||Loam||Clay loam/Clay|
|good||> 1||> 1.4||> 1.8||> 2.0|
|moderate||0.5 – 1.0||0.7 – 1.4||0.9 – 1.8||1.2 – 2.0|
|poor||< 0.5||< 0.7||< 0.9||< 1.2|
Values (mg/g) of labile carbon considered to be “good”, “moderate” and “poor” for soils of different textures. The table is taken from Moody and the values are based on several hundred laboratory-based organic matter determinations.
Ray R. Weil, Kandikar R. Islam, Melissa A. Stine, Joel B. Gruver and Susan E. Samson-Liebig. 2003. Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use: (https://www.enst.umd.edu/sites/default/files/_docs/Weil_et_al_2003_corrected.pdf)
(1) http://www.hach.com/pocket-colorimeter-ii-wavelength-specific-model-550- nm
(2) A Methodology of a Visual Soil - Field Assessment Tool - FAO.org ftp://ftp.fao.org/agl/agll/lada/vsfast_methodology.pdf