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 when the 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
1
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 (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.
3. 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. Make
up/store in plastic container.
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.
Note: Deionized water may be substituted with 10 mM CaCl2 to reduce turbidity
in the filtrate, with no significant change in extractable B (Jeffery and McCallum,
1988 and Watson, 1988).
3. Load the flasks on a reciprocal hot water shaking bath and shake the slurries for
30 minutes at 80oC.
4. Filter into 125 mL plastic Erlenmeyer flasks, using Whatman #1, 150-mm
diameter filter paper. The supernatant may be turbid due to colloidal materials
that pass through the filter, check the filtrate for clarity and refilter if necessary.
Analysis
2
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
The dilution factor of 25/5 is the volume of deionized water added divided by the
weight of soil used. The factor of 2 converts mg/kg to lbs/acre (for those
laboratories that reports values in lbs/acre).
Analytical Performance
Range and Sensitivity
1. Boron was measured using a 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. Three soil samples from the North American Proficiency Testing Program
(NAPT) with known B concentrations were 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 indicated relative precision between replications, and the
median values were comparable to the NAPT values.
UGA Lab Measurements
Soil
NAPT 2009-101
NAPT Median
mg kg-1
0.500
No. of
Measurements
Median
mg kg-1
Std.
deviation
8
0.535
0.026
3
NAPT 2009-104
0.200
8
0.226
0.010
NAPT 2009-116
0.715
8
0.685
0.046
-
8
0.093
0.025
UGA Lab Check Soil
2. For routine soil B analysis, laboratories may establish limits of acceptability. For
example, duplicate values reading within 20% of the average of the two values
may be acceptable.
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 by plant tissue analysis is equally
or even more important than the soil analysis of boron.
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. It is also advisable to run replicate analysis of all
samples when using glassware, to isolate random contamination errors due to
insufficient leaching.
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 is 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, cole
crops, and root crops. Corn, cotton, tobacco, tomatoes and small grains are
intermediate.
4
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.
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.
Jeffrey, A.J. and L.E. McCallum. 1988. Investigation of a hot 0.01 M CaCl2 soil boron
extraction procedure followed by ICP-AES analysis. Commun. Soil Sci. Plant Analysis.
19: 663 – 673.
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.
Watson, M. E. 1988. Recommended soil boron test. p. 23-25. In W. C. Dahnke (ed.)
Recommended Chemical Soil Test Procedures for the North Central Region. North
Dakota Agric. Expt. Stn. Bull. No. 499.
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.
5