Analytes Certified for in 2022
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.
Exchangeable bases - 1M ammonium acetate at pH 7.0.
This rapid method for exchangeable cations in non-saline acidic through to slightly alkaline soils has no pre-treatment for soluble salts. It should yield similar data to those of method 15A1, except it can overestimate exchangeable Ca in soils containing calcium carbonate.
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.
Sulfate sulphur is extracted in the absence of activated charcoal from air-dry soil <2 mm particle size, by 0.01M Ca (H2PO4)2 at pH 4.0 using a soil/solution ratio of 1:5 and an extraction time of 17 h at 25 degrees C. This extracted sulphur is then determined in an aliquot of particle-free soil extract by ICP-AES.
This measure of soil organic carbon (OCW&B; expressed as %C) usually yields a lower figure than the true total organic carbon value. The method uses finely-milled air-dry sample. It involves wet oxidation by a dichromate-sulphuric acid mixture and relies only on heat of reaction. Soil weight should take account of the expected concentration of OC, and it is expected that allowance will be made for positive soluble Clâ€‘ interference in soils containing >0.5% Cl. The method specifies reporting on an oven-dry (105oC) basis. Nowadays this method is less preferred than 6B methods.
This index of soil P sorption embraces adsorption as well as precipitation reactions. Equilibrated soil extracts are obtained by shaking milled air-dry soil continuously for 17 h at a ratio of 1:10 (w/v) with a P equilibrating solution initially containing the equivalent of 1000 mg P/kg in 0.01M CaCl2. The residual P in the extract is determined using the Murphy and Riley colorimetric finish. In other related methods (9I2a, 9I3a) the PBI value is modified to allow for the soil fertility level. In this method, there is nosuch adjustment. The methods specify reporting results on an air-dry basis.
Tests for water-soluble chloride (Cl) on milled air-dry sample are suitable for use on all soils. For method 5A1, Clâ€‘ in clarified 1:5 soil/water extracts is determined by potentiometric titration with AgNO3 in conjunction with an Ag/AgNO3 electrode array. For method 5A2a, Clâ€‘ in clarified 1:5 soil/water extracts is determined by an automated, continuous flow colorimetric procedure based on the formation — in the presence of ferric ions and free thiocyanate ions — of highly coloured ferric thiocyanate in proportion to the Clâ€‘ concentration. Method 5A2b is similar, except it pertains to the use of flow injection analysis (FIA). For 5A1 and 5A2 methods, it is assumed there are no chemical interferences of significance. Moreover, Method 5A2a has proven more precise than method 5A1, particularly at soil concentrations <50 mg Cl/kg. Other analytical finish options involve chemically-suppressed ion chromatography (5A3a), single-column electronically suppressed ion chromatography (5A3b), and direct measurement by ICPAES (Method 5A4). The methodology specifies reporting results on an air-dry basis.
Wet digestion - open system without HF - diacid (HNO3/HClO4). ICP-AES finish.
Wet digestion - open system without HF - diacid (HNO3/HClO4). AAS-Flame with pulsed hollow cathode lamp background correction, using air-acetylene.