Method 51 Determination of extractable boron

The predominant form of boron in soil solution is H3BO3, but above pH 9.2, H2BO3 may predominate. Hot-water extraction is the most widely accepted procedure for determining the amount of boron that is available to plants, and correlated best with the incidence of black spot in garden beets (Missouri Agricultural Experiment Station, 1998). The final determination is best performed using an ICP spectrometer, but this may not always be available, so a colori-metric method will be described. Methods using either curcumin or azome-thine-H are possible, but the latter will be suggested here. It is not only the reagent used in the MAFF/ADAS (1986, pp. 20-22) handbook, which is the method to be described (with Crown Copyright permission), but has been adopted by the Delaware Cooperative Extension (1995) as being rapid, reliable and requiring less sample preparation and handling than the curcumin method. The American methodology, however, omits the removal by ashing of any organic matter in the filtrate, which might interfere with the determination; it also adds 0.1% m/v CaCl2.2H2O to the water extractant to promote soil flocculation. Also, the filtration step is replaced by centrifugation in a plastic centrifuge tube at 2700 g for 15 min. The following method could be modified similarly if appropriate, but once adopted, should be adhered to for future comparison of results.

Boron is obviously a component of borosilicate glassware, which should therefore be avoided. Apparatus should therefore be made of PTFE, soda

© 2002 CAB International. Methods in Agricultural Chemical Analysis: a Practical Handbook (N.T. Faithfull)

glass or silica. A fibre digestion apparatus may be suitable. It may even be possible to extract the soil by boiling with the extracting water in a sealed plastic bag or pouch (Mahler etal., 1984). Having said that, however, silica (or quartz) apparatus is expensive, especially for educational purposes. The comment by Bingham (1982) should therefore be noted: 'We have not found it necessary to use special low-B glassware for the analysis of water, soil, or plant samples. Pyrex glassware or plastic ware has been entirely satisfactory.' Presumably the magnitude of the blank reading would show whether there was a contamination by extraneous boron.


• Flasks, 250 ml, conical with ground joint - silica (quartz), or soda glass.

• Condenser - either silica cold finger condensers, effective length 140 mm, or soda glass air condensers, approximately 750 mm.

• Evaporating basins - 20 ml, translucent silica, shallow form, with round bottom and spout.

• Polyethylene tubes - 20 ml with hinged cap.

Reagents. Note: all reagents must be stored in polyethylene containers.

• Azomethine-H reagent - Dissolve 0.45 g of azomethine-H in 100 ml of 1% m/v L-ascorbic acid solution. Prepare fresh weekly and store in a refrigerator.

• Boron stock standard solution, 100 pg B ml-1 - Dissolve 0.572 g of boric acid (H3BO3) in water and dilute to 1 l and mix.

• Boron intermediate standard solution, 20 pg B ml-1 - Pipette 20 ml of the boron stock standard solution into a 100 ml volumetric flask and make up to the mark with water and mix.

• Boron working standard solutions, 0-3 pg B ml-1 - Pipette 0, 1.0, 2.0, 5.0, 10.0 and 15.0 ml into 100 ml volumetric flasks and make up to the mark with water and mix. This will provide solutions containing 0, 0.2, 0.4, 1.0, 2.0 and 3.0 pg ml-1 of boron.

• Buffer masking reagent - Dissolve 250 g of ammonium acetate and 15 g of EDTA, disodium salt, in 400 ml water. Carefully add 125 ml of glacial acetic acid.

• Calcium hydroxide solution, saturated.

• Hydrochloric acid, approximately M - Dilute 85 ml of hydrochloric acid, approximately 36% m/m HCl, to 1 l with water.

Procedure. Transfer 40 ml (2 x 20 ml plastic scoopfuls, struck off level without tapping) of air-dry soil, sieved to ^2 mm, into a flask. Measure 80 ml of cold water into a boron-free container and bring to the boil. Transfer the boiling water to the flask containing the soil and attach a condenser. Reheat to boiling as quickly as possible, and continue to boil for exactly 5 min. Remove the flask from the heat source, and allow to stand for exactly 5 min. Filter under reduced pressure through a 125 mm Hartley funnel fitted with a 125 mm

Whatman No. 2 filter paper, collecting the filtrate in a boron-free tube inside the filter flask. Terminate the filtration after 5 min, and retain the filtered extract for the determination of boron. Carry out a blank determination.

Pipette 5 ml of each boron working standard solution into a silica evaporating basin, and add 0.25 g of sucrose and 2 ml of satd calcium hydroxide solution. Evaporate to dryness on a boiling water bath. Place the basin in a cold muffle furnace, slowly increase the temperature to 450°C and maintain this temperature for 2 h. Allow to cool, then add exactly 5 ml of approximately M hydrochloric acid and dissolve all soluble material. Filter through a 90-mm Whatman No. 541 filter paper. Transfer 1 ml of the filtrate to a polyethylene tube. Add 2 ml of buffer masking reagent, mix and add 2 ml of azomethine-H reagent. Mix well and allow to stand for 45 min. Measure the absorbance in a 10 mm optical cell at 420 nm. Construct a graph relating absorbance to pg of boron present. The absorbances corresponding to 0 and 3.0 pg of boron are approximately 0.2 and approximately 0.7 and should differ by approximately 0.45.

Pipette 5 ml of each soil extract into a silica evaporating basin, add 0.25 g sucrose and 2 ml of satd calcium hydroxide solution, and proceed as above as far as measuring the absorbance at 420 nm.

Calculation. Read from the standard graph the number of pg boron equivalent to the absorbance of the sample, and the absorbance of the blank. Multiply the difference by 2 to give the mg l-1 of boron in the air-dry soil sample. To express in terms of oven-dry soil, see Method 5.2, Calculation (2).

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