1. Lab 8 – Free Iron and OM in Forest Soils
Relationship between soils, FE and OM

2. Free Iron - AAS
Use atomic absorption spectrometry (AAS) in order to analyze free iron in soil.
- Absorption of optical radiation by free ions in gaseous state.
Beer-Lambert Law:
Linear relationship between light absorbance and concentration of the compound solution.
- if there is no absorption, all light will be transmitted.

3. Free Iron - Extractions
Extractions needed to test free iron:
1 gram soil + 50 ml sodium tartrate acetate solution
Mechanically shaken ~15 min to disperse clay
+ 2 grams sodium dithionite
Hot water 40°C for 1 hour.
Filter solution.
Dilute into a flask to be used with AAS
Page 140 in soils manual
Sodium tartrate acetate: disperse aggregate soil particles, solubilizing Fe
Sodium dithionate: extracts Fe from soil by reducing Fe3+ to Fe2+

4. Atomic Absorption Spectrometry
Selects specific wavelengths of light absorbed by sample
Air-acetyle flame atomizer.
Converts light signal into electrical signal proportional to light intensity.
Ionized gas which discharges radiation of the element through flame.
Takes the liquid solution and creates a fine aerosol spray into the flame to evaporate it.

5. Atomic Absorption Spectrometry
Hollow cathode lamp
- Sealed lamp filled with argon/neon with a metal cathode with an element and an anode.
- Ionized gas which discharges radiation of the element through flame.
Flame
Monochromator
Detector

6. Atomic Absorption Spectrometry

7. OM and Carbon % OM = % C x1.72 Methods of OM Determination:
LOI (Loss of Ignition)
Muffle furnace burns away organic matter and the difference in weight from before and after burning gives OM %.
Subject to errors: volatilization of crystalline H20, salts, inorganic carbons, etc...
Determination of a constituent percentage of OM
% OM = % C x1.72
* Known relationship between carbon and organic matter
Soil contains ~ 58% carbon (Walkley 1946) = 1.72

8. CNS 2000 LECO Induction Furnace CNS 2000
Measures Carbon, Nitrogen, and Sulphur %
Dry Combustion Method
Screen for entering data / output of results
Opening to combustion chamber

9. CNS - Samples Sample placed into a ceramic boat.
Sample is pushed into combustion chamber (1020°C). It’s combusted with oxygen.
97% to C02
3% to C0
Water is vaporized and removed from oxygen stream by magnesium perchlorate (anhydrone) to stop it from interfering with C detector.
Thermally resistant substances completely oxidized.

10. CNS - Analysis
Gases pass through an oxidation catalyst (chromium trioxide) -> C02, N2, N0x and water
Gases then pass through reduction catalyst (metallic ~650°C) which reduces the nitrogen to element
C02 in gas is detected by a C02 detector.
Light source (IR Infrared wavelength)
C02 absorption causes loss of signal which is proportional to concentration of C02
Displayed as %C

11. LOI and Carbon
LOI (loss of ignition) is an indirect measure of organic matter.
Samples are burned in a muffle furnace for 6 hours at 600°C, then weighed after burning to see the difference in weight. This shows how much OM (and other elements) have been burned off. Mineral soil remains behind.
- Depending on ignition temperature LOI can cause loss of salts, structural water, and inorganic carbons which effect results.
Close relationship between LOI and C%
(MF’s data from central and northwestern NB)

12. LOI and Carbon