Hot Water Extractable Boron
Leticia Sonon and David Kissel
Application and Principle
Boron (B) is a naturally occurring element that is found in nature in the form of
borates in the oceans, sedimentary rocks, coal, shale, and some soils. Boron is an
essential micronutrient required for the normal growth of plants. The amount of total B in
soils may range from 7 to 80 mg/kg but the total B content in soils is not necessarily
correlated with its availability to plants (Keren, 1996). In most soils, the amount of soil B
available for plant uptake is <5% of the total B in the soil (Gupta, 1967). Boron is not
recommended for application in soils when its hot water extractable B exceeds 0.5
mg/kg, and most crops are adequately supplied with an extractable B of 0.15 mg/kg.
Extraction of soil B with hot water was originally developed by Berger and Troug
(1939) which involves refluxing soil with hot water for 5 min using a soil:water ratio of
1:2. The procedure was modified by Gupta (1967) who found that increasing the
refluxing time from 5 to 10 min resulted in significant increase in the amount of B
extracted from soils. Over the years, many soil testing laboratories that routinely test soils
for B made some modifications to the Berger and Troug procedure. The University of
Georgia procedure extracts soil B using a 1:5 soil:water ratio and a suspension boiling
time of 30 min.
The extracted B can be measured colorimetrically using reagents such as carmine
(Hatcher and Wilcox, 1950), Azomethine-H (Wolf, 1971) or the most commonly used
method in recent years is by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) (Keren, 1996). The latter tool is the technique of choice among laboratories in
the Southern region of the U.S. because it has a wider dynamic concentration range, is
less sensitive to interferences especially in turbid filtrates, it is simple and rapid. Gestring
and Soltanpour (1981) found a strong correlation between B determination by ICP-AES
and the Azomethine-H for soil extracts. The method below describes the ICP-AES
technique used in measuring B extracted from soil with hot water.
Equipment and Apparatus
1. Calibrated 5-gram scoop or balance to weigh to the nearest 0.01 gm
2. 125 mL Nalgene or plastic Erlenmeyer flasks
3. 7.5 cm plastic funnels
4. Filter paper (Whatman #1, 15 cm diameter)
5. 250 mL Nalgene or plastic beaker
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6. Deionized water dispenser
7. Reciprocal hot water shaking bath
8. Instrumentation for boron analysis
a. Thermo Jarrell-Ash model 61E ICP
b. Thermo Jarrell-Ash Auto sampler 300
c. 48 position rectangular sample rack
Reagents
1. Boron Stock Solution (1000 mg/L): Use NIST traceable single element plasma
grade standard.
2. Boron Stock solution (1000 mg/L): Use NIST traceable single element plasma
grade standard.
3.
Boron Stock solution (10 mg/L): Pipette 10 mL of the 1000 mg/L B stock
solution into a 1 L volumetric flask. Make to volume with deionized water and
mix well. Store in a plastic bottle.
4. Boron Working Standards: Pipette 0, 1, and 3 mL of the 10 mg/L B stock
solution and bring to 100 mL volume with deionized water. This results to
standard B concentrations of 0, 0.10, and 0.30 mg B/L respectively.
Procedure
Extraction
1. Scoop 4 cm3 or weigh 5 g of air-dried, 2-mm sieved soil into 125 mL plastic
flasks.
2.
Add 25 mL deionized water.
3. Load the flasks on a reciprocal hot water shaking bath and shake the slurries for
30 miniutes at 80oC.
4. Filter into 125 mL plastic Erlenmeyer flasks, using Whatman #1, 150-mm
diameter filter paper. Oftentimes, the supernatant is turbid due to colloidal
materials that pass through the filter, check the filtrate for clarity and refilter if
necessary.
Analysis
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1. Calibrate the ICP with deionized water (Type 1) as blank, and the 0.30 mg/L B
standards.
2. Use the 0.10 mg/L standard as a curve verification check. Analyze this standard
immediately after calibration and after the last soil sample.
3. Analyze the filtrate on the ICP emission spectrograph.
4. Prepare one duplicate sample and one quality control sample with each set of
samples analyzed.
Calculations
1. Soil Bmg/kg = ICP extract B reading x 25/5
2. Soil Blbs/acre = soil Bmg/kg x 2
where 25 is the volume of deionized water added, 5 is the weight of soil sample,
and 2 converts mg/kg to lbs/acre (for those laboratories that give reports in
lbs/acre).
Analytical Performance
Range and Sensitivity
1. Boron was measured in ICP-AES (Thermo Jarrell-Ash Enviro 61E ICAP
Spectrometer) at a wavelength of 249.6 nm with a calculated method detection
limit of 0.01 mg B L-1. The ICP technique proved to be simple and fast, and
suitable for B determination from an aqueous extract.
Precision and Accuracy
1. Soil samples from the North American Proficiency Testing Program (NAPT) with
known B concentrations were re-analyzed following the hot water extraction and
ICP method. Extraction and B measurement were done in eight replications, and
data were compared with values reported by NAPT (table below). The data were
highly precise between replications, and the median values were comparable with
those with NAPT implying its accuracy.
UGA Lab Measurements
Soil
NAPT Median
mg kg-1
No. of
Measurements
Median
mg kg-1
Std.
deviation
NAPT 2009-101
0.500
8
0.535
0.026
NAPT 2009-104
0.200
8
0.226
0.010
3
NAPT 2009-116
UGA Lab Check Soil
0.715
8
0.685
0.046
-
8
0.093
0.025
2. For routine soil B analysis, laboratories may establish limits of acceptability such
as values of duplicate samples must agree within 20% of the average of the two
values.
3. The low level of boron generally extracted from soils poses a challenge on
accuracy of measurement as it gets closer to the instrument’s detection limit. It is,
therefore, recommended that determining boron needs by plant analysis is equally
or even more important than soil analysis.
Interferences
1. Care must be taken to filter samples properly to avoid clogging of the nebulizer
by colloids in the extract.
2. If glassware is used, it should be washed with a 1:1 mixture of boiling HCl and
deionized water before use.
Interpretation
1. The University of Georgia uses the following soil test ratings for hot water
extractable B:
Crop
All crops
Soil B Test Level, mg/kg
Low
Adequate
0.0-0.10
0.15
2. Boron is routinely recommended for alfalfa, cotton, peanuts, all commercial
vegetable crops, reseeding clover or where clover seed are to be harvested. It is
not advisable to exceed the rates recommended for the specific crops, as boron
toxicity can occur from excessive applications. When the soil test boron level
exceeds 0.5 mg/kg, boron should not be applied irrespective of the crop. Crops
differ in the amount of boron they can tolerate. Sensitive crops are soybeans,
peaches, and strawberries. Some of the tolerant crops are alfalfa, clovers, and
root crops. Corn, cotton, tobacco, tomatoes and small grains are intermediate.
Effects of Storage
1. For accurate and reproducible B analyses, it is important to use non-borosilicate
containers or low-B (aged) glasswares for B standards and extracts.
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Safety Disposal
1. The chemicals used in this procedure pose no safety risk and therefore can be
stored and disposed of according to routine laboratory procedures.
References
Berger, K.C. and Troug, E. 1939. Boron determination in soils and plants using the
quinalizarin reaction. Ind. Eng. Chem. 11:540-545.
Gestring, W.D. and P. N. Soltanpour. 1981. Boron Analysis in Soil Extracts and Plant
Tissue by Plasma Emission Spectroscopy. Comm. Soil Sci. Plant Anal. 12(8): 733-742.
Gupta, U. C. 1967. A Simplified Method for Determining Hot Water-soluble Boron in
Podzol Soils. Soil Sci. 103:424-428.
Hatcher, J.T. and Wilcox, L.V. 1950. Colorimetric determination of boron using carmine.
Anal. Chem. 22:567-569.
Keren, R. 1996. Boron. In D.L. Sparks (ed.) Methods of soil analysis, Part 3. Chemical
methods. Soil Science Society of America, Book series no. 5.
Wolf, B. 1971. The determination of boron in soil extracts, plant materials, composts,
manures, water, and nutrient solutions. Soil Sci. Plant Anal. 2:363-374.
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