Exchangeable Bases - Ammonium Chloride (15A1)

Method for measuring exchangeable bases (Ca²⁺, Mg²⁺, Na⁺, K⁺) - 1M ammonium chloride at pH 7.0

Suited for use on all soils, irrespective of whether acidic or alkaline, but preferred on acidic to weakly alkaline soils not dominated by solid-phase carbonates. Method 15A1 has no pre-treatment to remove soluble salts, with alternatives to remove them chemically (15A2) or to adjust for the presence of soluble sodium (15A3).

DTPA Trace Elements (12A1)

Soils are extracted with 0.005M DTPA, at a 1:2 soil:solution ratio, and shaken for 2 hr at 25^oC.  Analytical finish is either ICP-AES or Flame AAS.

Colwell extractable K (18A1)

The Colwell extraction was developed for measuring available P, but can also be used for determining available K. The extractant is freshly prepared 0.5M sodium bicarbonate @ pH 8.5, with a soil:extractant ratio of 1:100 and a shaking time of 16 hr.  The sodium cation is considered the primary extractor, displacing the similar potassium cation from exchange sites.

pH: 1:5 SOIL:0.01M CaCL₂ Suspension (4B1)

This pH test on milled air-dry sample is suitable for use on all soils, irrespective of whether acidic or alkaline. Values are usually unaffected by fertilisation prior to sampling, as changes to the soil’s ionic strength is masked by the calcium chloride. Code 4B1 indicates direct use of 0.01M CaCl₂, at a soil/solution ratio of 1:5, with mechanical shaking for 1 h prior to pH measurement using calibrated electrodes positioned in the unstirred supernatant after settling of the suspension. Code 4B2 provides a similar measurement outcome but relies on the addition of 0.21M CaCl₂ to a 1:5 soil/water suspension to achieve 0.01M CaCl₂ prior to measurement of pH as for 4B1. Codes 4B3 and 4B4 are identical to 4B1 and 4B2, respectively, except the soil/CaCl₂ suspensions are stirred during measurement. Method 4B5 codes for an MIR surrogate measurement. There is merit in separate use of both water and calcium chloride to measure soil pH.

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

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 (105^oC) basis.

DE-11

Microwave digestion - closed system with HF, and final medium HNO₃ and/or HCl.  AAS-Flame with deuterium background correction, using air-acetylene.

(DE-09)