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
Electrical conductivity (EC) of 1:5 soil/water extract (3A1)
This test on milled air-dry sample at a soil/water ratio of 1:5 for 1 h is suitable for use on all soils, irrespective of whether acidic or alkaline. It usually underestimates the soluble salt status of soils containing natural or added gypsum, particularly if ³ 1% of gypsum is present. Such soils would have an EC of about 2 dS/m. Soil EC x 0.336 (Method 3B1) approximates percent total soluble salts, while approximate soil ionic strength (Method 3C1) at 0.1 bar (I0.1) can be calculated as follows: I0.1 = [0.0446*EC1:5 – 0.000173], where I0.1 has units of mM, and EC1:5 has units of dS/m @ 25oC.
DTPA Trace Elements (12A1)
Soils are extracted with 0.005M DTPA, at a 1:2 soil:solution ratio, and shaken for 2 hr at 25oC. Analytical finish is either ICP-AES or Flame AAS.
Mehlich3 ICP (18F1)
Analysis of Mehlich 3 extract using inductively-coupled atomic emission spectroscopy (ICP-AES)
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 nitrogen semi micro Kjeldahl, steam distillation. (7A1)
Method 7A1 provides a good estimate of total soil N in soils with little NO3: otherwise slightly lower apparent total soil N concentrations are likely. The method initially involves semi-micro Kjeldahl digestion. Subsequently, NaOH solution is added to release NH3 by steam distillation. The NH3 released is quantitatively absorbed in dilute boric acid, with total soil N determined by titration with standard acid. The method uses finely-milled air-dry sample and specifies reporting results as %N on an oven-dry (105oC) basis.
DE-23
Microwave Digestion - closed system without HF, and final medium HNO3 and/or HCl. ICP-AES finish.