Page 503 - Biosystems Engineering

Agricultur e Management 475

Bray–Kurtz P1 Method The dilute acid–fluoride extractant easily removes
acid-soluble potassium from phosphates bound to Al, Fe, and Ca.
Phosphates in the extract are determined calorimetrically as phos-
phomolybdenum blue with ascorbic acid as a reducing agent and
antimony added to the solution to give a stable Mo-P-Sb compound.
Reagents of this method are prepared as follows:
1. Bray–Kurtz P1 extracting solution: Dissolve 22.2 g of NH F and
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41.6 mL of concentrated HCl making 20 L. This gives us a
solution of 0.03-M NH F in 0.025-M HCl. This should be
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stored in a polyethylene bottle.
2. Reagent A: As described in Olsen’s method.
3. Reagent B: As mentioned earlier.
4. Sulphuric acid 2.5 M.
5. Standard stock potassium solution: As mentioned in Olsen’s
method. Prepare a standard curve as described under Olsen’s
method. Weigh 2.5 g of 2-mm air-dried soil into a 150-mL
Erlenmeyer flask. Add 25 mL of extracting solution. Place a
stopper and shake the suspension for 5 min on a reciprocat-
ing shaker. Immediately filter through a Whatman No. 42 fil-
ter paper. If the filtrate is turbid, quickly pour the filtrate back
through the same filter. To avoid the interference of fluo-
ride, add 7.5 mL of 0.8-M boric acid (50 g of H BO /L) to 5 mL
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of extract, if necessary. Place a 5-mL aliquot of the extract in
a 25 mL volumetric flask. Add distilled water to 20 mL and
add 4 mL reagent B to the volume. After 10 min, read the
intensity of blue color as described under Olsen’s method:

R × 25× 2. 24
Bray s P (kg/ha) = ×μ gP × 448
’
.
× .
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where R = μg of P in the aliquot.
Determining Exchangeable Potassium The term “available potassium”
(K) conventionally refers to exchangeable and water-soluble K.
Exchangeable K constitutes the major portion of available K except in
saline or saline sodium soil. Available K or exchangeable K, along with
Ca and Mg, are usually determined in neutral normal ammonium ace-
tate (in NH OAC) extract. K is estimated using a flame photometer and
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Ca and Mg either by EDTA titration or by using an atomic absorption
spectrophotometer.
To determine potassium, reagents are prepared as follows:
1. Ammonium acetate (1 N, pH 7.0): Mix 700 mL of distilled water
and 57 mL of 99.5 percent glacial acetic acid and 69 mL of con-
centrate ammonium hydroxide. Dilute to a volume of 900 mL