Air dry moisture content (2A1)

This method is required to adjust soil chemical results based on air-dry samples to an oven-dry (105oC)
basis. When the air-dry moisture content (M%) is known, the correction from air-dry
to oven-dry is as follows:


            Oven-dry result  =   [Air-dry result  x  (100+ M%)]/100


Nitrate Nitrogen (7C2)

Initially, milled air-dry soil is extracted for 1 h with 2M KCl at a 1:10 soil/solution ratio. For 7C1a to 7C1h methods, mineral-N components are quantified by steam distillations and subsequent titrations. For 7C2 methods, mineral-N fractions in the clarified soil extract are determined by automated colorimetric procedures. Ammonium ions (NH4+) are measured by a modified Berthelot indophenol reaction, while the Griess-Ilosvay reaction is used for NO3-N (and NO2-N). The methods specify reporting results for NH4-N and NO3-N [plus NO2-N if present], respectively, as mg N/kg on an oven-dry (105oC) basis. Specifically, method 7C2a relates to an automated colour, continuous segmented flow analytical finish, while Method 7C2b codes an automated colour finish by flow injection analysis. Little difference is expected in results due to choice of colorimetric finish.


Extractable Phosphorus - Olsen (9C2)

This extractable P test on milled air-dry sample uses freshly prepared 0.5M sodium bicarbonate @ pH 8.5 as the extractant. The soil/extractant ratio of 1:20 and a relatively short extraction time of 30 min favours readily available forms of soil P, while suppressing the solubility of basic calcium phosphates often found in neutral and alkaline soils. Method 9C1 involves a manual, molybdenum-blue colorimetric finish with a preferred absorbance at 882 nm. Method 9C2a defines an equivalent automated molybdenum-blue continuous segmented flow analytical finish, while Method 9C2b codes an automated colour finish by flow injection analysis. No significant difference is expected in results due to the analytical finishes described. The methods specify reporting results as mg P/kg on an air-dry basis.

pH of 1:5 soil/water suspension (4A1)

This test on milled air-dry sample involves mechanical shaking with deionised water in a closed system for 1 h at a soil/water ratio of 1:5 prior to pH measurement using calibrated electrodes, while stirring the soil/water suspension. The method is suitable for use on all soils, irrespective of whether acidic or alkaline. Values may be lower than expected on recently fertilised soils due to a temporary increase in soil solution ionic strength.

Total organic carbon - high frequency induction furnace, volumetric (6B2)

This method for total soil OC involves production, purification and measurement of CO2 evolved when soil carbon is ignited in a stream of O2. Because all C compounds are converted to CO2, the C from carbonates, charcoal, undecomposed wood, etc, will be included, as no soil pre-treatment is specified. In the volumetric sub-method 6B2a, concentrated KOH solution is used to absorb the CO2 released. The difference between the original volume of gas in the burette and the volume produced after ignition equals the volume of CO2 evolved from the sample, after correction for gas temperature and pressure. Sub-method 6B2b is similar to Method 6B2a, except the CO2 produced by ignition is measured via infrared / thermal conductivity detection. Both 6B2a and 6B2b use finely-milled air-dry sample, with weights varying with expected C concentrations. Surrogate estimates can be obtained by NIR (method 6B4a) or MIR (method 6B4b) reflectance spectroscopy. The methods specify reporting as %C on an oven-dry (105oC) basis.


Total soil N – Dumas high-temperature combustion (7A5)

This method utilises automated and/or microprocessor controlled instrumentation, which mostly is able to measure at least total C, N and S in the same sample. Dumas-N dry oxidation includes all forms of soil N, without the need for lengthy pre-treatments, although results can occasionally differ from those expected in soils with high levels of fixed ammonium-N (lower results) and when soils are organically rich (high results, due to incomplete combustion resulting in the formation of methane rather than CO2). Typically, dry, finely-ground sample is subjected to high-temperature combustion (e.g. 950–1,250oC) in a stream of purified O2. An aliquot of the gases produced by combustion is carried by helium gas to a thermal conductivity cell for measurement of any N2 generated, a process typically taking 3–5 min. A heated copper catalyst reduces NOx to N2. The method specifies reporting results as %N on an oven-dry (105oC) basis.