1^

METHODS OF

SOIL AND PLANT ANALYSIS

With Special Reference

to Strontium 90 Contamination

'^''"'' '• - iSo5

CUfiHEiïï SEÜML RECÎMIÛl

Agriculture Handbook No. 288

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE
CONTENTS

Page

Preparation of samples 1

Ammoninin acetate extraction of soil 2

Ashing of plant material 3

Determinations on ammonium acetate extracts of soil and solutions of plant ash 4

Calcium and magnesium 4

Strontimn by flame photometry 8

Strontium by X-ray fluorescence emission spectrometry__ 12

Sodium and potassium 13

Determinations on separate samples 14

pHof sou 14

Organic matter content of soil 15

Exchangeable hydrogen lö

Available and fixed strontium 90 in soils 17

Strontium 90 in plants 22

Literature cited 23

Washington, D.C. Issued August 1965
METHODS OF SOIL AND PLANT

ANALYSIS

With Special Reference to Strontium 90

Contamination

By W. R. HEAU), R. G. MENZEL, H. ROBERTS, JE., and M. H. FREEE, Soil and Water

Conservation Research Division, Agricultural Research Service'^

This handbook contains procedures for determining soil properties

necessary to the uptake of strontium 90 from soil by plants. By these

methods can be determined the amount and kind of exchangeable

cation, pH, and amount of organic matter in soil, as well as the amount

of major cation and strontium content of plants. The application of

the analytical results obtained is not discussed, because it depends

primarily on the purpose for which the samples were taken.

The procedures were taken directly from the literature or were

developed in part or entirely by the U.S. Soils Laboratory. Caution

should be used in universally applying these procedures, since some of

them were developed for specific materials. This is especially true

for strontium determination, both natural and radioactive, in that

interference from other elements might have different effects on materials other than plants and soils.

PREPARATION OF SAMPLES

Soil samples are prepared for determination of exchangeable alkali

and alkaline earth cations by extraction with ammonium acetate.

Separate soil samples are used for determination of pH, organic

matter content, and exchangeable hydrogen. A much larger sample

is used for determination of available and fixed strontium 90.

Spread i\\Q field sample (at least 10 pounds) of soil on heavy paper

and allow to dry imtil it can be crushed and sieved easily. Sieve the

soil through a quarter-inch mesh screen. Mix before subsampling by

rolling on a large sheet of paper. Discard large roots, rocks, and

other extraneous material after weighing.

Samples of plant materials should be dried for preservation and

better subsampling. Air-drying may be satisfactory, but drying in

a forced-draft low-temperature oven (below 70° C.) is preferred.

Air-dried samples require a moisture determination and, in some

cases, certain parts of the plant will not dry in a reasonable time.

Cutting thick plant parts such as petioles, stems, or roots into small

segments or slices will greatly increase their drying rate. Unless the

^ The preparation of this handbook was supported in part by the U.S. Atomic

Energy Commission.

1
2 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

entire sample is to be analyzed, the plant material must be finely

ground to facilitate proper subsampling. Also, the organic fraction

of the plant material is much easier to destroy if the sample is finely

divided. The grinding is usually done in a Wiley mill ^ or similar

apparatus, using a 30- or 40-mesh screen.

Plant materials are prepared for analysis by dry combustion. This

is recommended because large samples are usually required for determination of strontium 90 and they are much easier and safer to

handle by dry combustion. The determination of alkali and alkaline earth cations may be made on aliquot parts of the sample taken

after the ash has been dissolved.

Ammonium Acetate Extraction of Soil

Equipment

(1) Carboy—18 liters.

(2) Erlenmeyer flask—250 ml.

( 3 ) Mechanical shaker.

(4) Suction flask—1 liter.

(5) Büchner funnel—No. 1 (5.6-cm. inside diameter).

(6) Filter paper—Whatman No. 42 or equivalent (5.5-cm.

diameter).

(7) Spatula.

(8) Beaker—400ml.

(9) Flutedwatchglass.

(10) Hotplate.

(11) Volumetric flask—100 ml.

Reagents

(1) Neutral ammonium acetate (NH4OAC), 1 N: Mark the

carboy at the 18-liter level. Fill it three-fourths full with distilled

water. Add 1,037 ml. of concentrated acetic acid (HOAc) and

1,215 ml. of concentrated ammonium hydroxide (NH4OH). After

mixing and cooling this solution, measure the pH of a small aliquot.

If acid, return the aliquot to the carboy and add 5 to 10 ml. of concentrated ammonium hydroxide, mix thoroughly, and take the pH

reading again. Decrease the amount of ammonium hydroxide used

as the pH approaches 7. If the solution is basic, add small increments of concentrated acetic acid in place of ammonium hydroxide.

If the solution is only slightly basic, bubbling air through the solution for several hours usually lowers the pH to 7. Dilute to a volume

of 18 liters,

(2) Nitric acid (HNO3) —concentrated, reagent grade.

(3) Perchloric acid (HCIO4)—concentrated, reagent grade.

(4) Nitric acid, 0.1 N: Dilute 114.5 ml. of concentrated nitric

acid and make up to 18 liters in a carboy with distilled water.

' Trade names are used in this publication solely for the purpose of providing

specific information. Mention of a trade name does not constitute a guarantee

or warranty of the product by the U.S. Department of Agriculture or an endorsement by the Department over other products not mentioned.
METHODS OF SOIL AND PLANT ANALYSIS ó

Procedure

Put 25 g. of the sieved sample into a 250-mL Erlenmeyer flask ; add

100 ml. of 1 A^ neutral ammonium acetate, stopper, and let stand overnight. Larger amounts of soil should be extracted if the texture is

very sandy or if the soil is very acid. Increase the amount of soil

proportionately as the amount oí extractmg solution is increased. For

best results, mix the sample by frequent shaking, either by hand or in

a mechanical shaker.

Set in place a 1-liter suction flask and a Büchner fmmel containing a

circle of Whatman No. 42 filter paper, 5.5 cm. in diameter. Add a

small amount of water to the funnel and apply suction until the paper

is set. Do not allow the paper to become dry. Rapidly swirl the

soil and ammonium acetate mixture. Pour enough of this mixture

on the set filter paper to just cover the paper, allow to stand for a

few seconds, and then turn on the suction. When this solution has

been pulled through, add the rest of the suspension rapidly. If this

procedure is done carefully, the filtrate will be clear. If the filtrate

is cloudy, turn off the suction and pour the filtrate back in the funnel.

This may have to be done several times with some soils. Mix the

remaining suspension in the flask and add to the Büchner funnel when

possible. Soil allowed to become dry may develop cracks and consequently will not leach properly. If cracks develop, use a spatula

to close them before adding more liquid.

Rinse the Erlenmeyer flask with small amounts of ammonium acetate and add the rinse to the Büchner funnel when no more liquid is

standing on the soil, but before it becomes dry. Continue rinsing until 250 ml. of leachate is collected in the suction flask. Allow the soil

to drain well before stopping suction.

Pour the leachate into a 400-ml. beaker and rinse the suction flask

several times with distiUed water, adding the rinses to the beaker.

Cover the beaker with a fluted watchglass. Evaporate the solution to

dryness on a hotplate. When the material is dry, add 20 ml. of concentrated nitric acid and 5 ml. of concentrated perchloric acid and

evaporate to dryness again. One acid treatment should destroy the

organic matter. If not, again add 10 ml. of concentrated nitric acid

and 2 ml. of concentrated perchloric acid and evaporate to dryness.

When the beaker is cool, add about 50 ml. of 0.1 N nitric acid, which

should dissolve most of the solids. Filter through Whatman No. 42

filter paper into a 100-ml. volumetric flask. Wash the beaker and

residue with small amounts of 0.1 N nitric acid until almost 100 ml.

of solution is collected in the volumetric flask. Make to volume with

distilled water and mix thoroughly.

Ashing of Plant Material

Equipment

(1) Porcelain evaporating dish—No. 9 (265-mm. diameter).

(2) Electric muffle furnace, with automatic temperature control.

(3) Watchglass (12-inch diameter).

(4) Hotplate.

(5) Funnel—long stem (100-mm. outside diameter).
4 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

(6) Filter paper—Whatman Xo. 42 or equivalent (18.5-cm.

diameter).

(7) A^olimietric flask—500 ml.

(8) Erlenmeyer flask—250 ml.

Reagents

(1) Hydrochloric acid (HCl)—concentrated, reagent grade.

(2) Hydrochloric acid, 1 Ä': Dilute 85 ml. of concentrated

hydrochloric acid to a volume of 1 liter with distilled water.

Procedure

Ash the plant material in an electric muffle furnace at 550° C. for 16

hours. Two hundred grams of plant material can conveniently be

ashed in a No. 9 porcelain evaporating dish. Tlie ash should be white

if oxidation of the organic material is complete. Gray or dark-colored

ash indicates incomplete oxidation. Eemuffling after cooling and

aeration will oxidize the sample more completely.

Place a 12-inch watchglass on the porcelain dish, leaving a small

opening tlirough which enough distilled water is added to moisten

the ash. Then add concentrated hydrochloric acid to cover the ash.

This procedure minimizes loss of ash caused by splattering. After the

reaction has subsided, rinse the material on the watchglass into the

porcelain dish. Place the uncovered dish on the hotplate and evaporate carefully to dryness. Cool and dissolve the sample with several

lumdred milliliters of 1 Ä^ hydrochloric acid. Filter through Wlmiman No. 42 filter paper, using a long-stem funnel. Wash the dish and

undissolved material collected on the filter paper with additional 1 N

hydrochloric acid until a volume of 500 ml. is collected in the volmnetric

flask. Discard the undissolved residue.

Put a 100-ml. sample from 500 ml. of the ash extract in a 250-ml.

Erlenmeyer flask. Keep for determination of alkali and alkaline

earth cations. The remaining 400 ml. is used for the determination of

strontium 90.

DETERMINATIONS ON AMMONIUM ACETATE EXTRACTS OF SOIL AND SOLUTIONS OF PLANT ASH

Calcium and Magnesium

Equipment

(1) Pipets_l,5,15,and20ml.

(2) Beakers—250 niL

(3) Microburet.

(4) Fluorescent lamp.

(5 ) Magnetic stirrer and stirring bar (optional).

(6) pH meter.

(7) Hydrion paper.

(8) Funnel—long stem (7.0-cm. outside diameter).

(9) Hotplate. ^

(10) Filter paper—Whatman No. 42 or equivalent (5.5-cm.

diameter). ^
METHODS OF SOIL AND PLANT ANALYSIS 5

Reagents

(1) Standard calcium solution : Dissolve 0.5004 g. of special ''low

magnesium" reagent grade calcium carbonate (CaCOs), dried at

150° C, in 5 ml. of approximately 6 ^V hydrochloric acid (HCl) and

dilute to a volume of 1 liter. This solution is 0.005 molar.

(2) Buffer solution: Dissolve 67.5 g. of ammonium chloride

(NH4CI) in 200 ml. of distilled water, add 570 ml. of concentrated

ammonium hydroxide (NH4OH), and dilute the solution to a volume

of 1 liter with distilled water.

(3) Masking reagents (each of the following solutions must be

stored in separate containers) :

(a) Cyanide solution: Dissolve 1 g. of ]3otassium cyanide

(KCN) in 100 ml. of distilled water.

(b) Hydroxylamine-hydrochloride solution : Dissolve 5 g. of

hydroxylamine-hydrochloride (NHsOH-HCl) in 100 ml. of distilled

water.

(c) Potassium ferrocyanide solution : Dissolve 4 g. of reagent

grade potassium ferrocyanide (K4Fe(CN)6-3H20) in 100 ml. of

distilled water.

(d) Triethanolamine—reagent grade.

(4) Eriochrome Black T. indicator : Dissolve 0.2 g. of Eriochrome

Black T. (EBT) in 50 ml. of methanol. Prepare a fresh solution

every 3 weeks.

(5) Standard EDTA solution: Dissolve 1.8613 g. of disodium

ethylenediaminetetraacetate (EDTA) (molecular weight^372.254) in

distilled water and dilute the solution to a volume of 1 liter. This

solution is 0.005 molar. Its titer will change if stored in glass containers but not in polyethylene.

(6) Standard magnesium solution: Dissolve 0.1216 g. of reagent

grade magnesium turnings in dilute hydrochloric acid and dilute to

a volume of 1 liter. This solution is 0.005 molar.

(7) Calcon indicator: Dissolve 20 mg. of Calcon in 50 ml. of

methanol. Prepare a fresh solution weekly.

(8) Sodium hydroxide, 10-percent solution: Dissolve 10 g. of

reagent grade sodium hydroxide (NaOH) in 100 ml. of distilled water.

(9) Sodium tungstate solution: Dissolve 20 g. of sodium tungstate (Na2W04-2H20) in 100 ml. of distilled water.

Procedure(ï,2, 3, 4, 9)'

Standardization of EDTA.—Pipet 5-ml. aliquots of the standard

calcium solution into each of three 250-ml. beakers and bring the

volume to approximately 150 ml. with distilled water. Pipet 15 ml. of

buffer solution, 10 drops of each masking reagent, and 10 drops of EBT

indicator. Titrate the solution with EDTA, with stirring, to a permanent blue, using a microburet under a fluorescent lamp. Although

stirring by hand is satisfactory, the use of a Teflon-coated magnetic

stirring bar activated by a magnetic stirring apparatus is much more

convenient. Eepeat titration with three 5-ml. aliquots of standard

magnesium solution. This standard serves to check the validity of the

calcium titration.

^ ItaUc numbers in parentheses refer to Literature Cited, p. 23.

Gr7l-799—^5 2
6 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

The EDTA should also be standardized, using Galeón as the indicator. Pipet three 5-inl. aliquots of standard calcium solution into

three 250-mL beakers and bring the volume to approximately 150 ml.

with distilled water. Add 10 drops of each masking reagent, then

pipet 1 ml. of 10-percent sodimn hydroxide or an amount sufficient to

raise the pH to 12 or slightly higher. Check with a pH meter or

Hydrion paper. Add 5 drops of Calcon indicator and titrate from a

red to a permanent blue with EDTA under the same conditions as

above. All microburet readmgs should agree within 0.05 ml.

Titrate three blanks with each standardization. The EDTA

should be standardized every tune a new set of samples is to be

titrated. This titration not only rechecks the standardization of the

EDTA but also indicates the presence of contamination in the microburet, pipets, and beakers.

Calchim Plus Magnesium.—Place an aliquot contaming 1 to 2 mg.

of calcium plus magnesium in a 250-ml. beaker and dilute to about

150 ml. with distilled water. Pipet 15 ml. of buffer solution and 10

drops of each masking reagent. Allow a few minutes for the reactions

to take place, especially for the formation of manganese ferrocyanide.

Warming will speed this reaction. Add 10 drops of the EBT indicator and titrate in ih.ç> same manner and to the same color as when

the EDTA was standardized.

If the end point is difficult to obtain, probably too much phosphate

is present. (See comments.) To overcome this difficulty, use the

following procedure : To a new aliquot, add 10 drops of each masking

reagent as before. Add a known excess amount of the standardized

EDTxV. Experience will indicate the amount of EDTA necessary.

Bring the solution to the necessary pH value by adding 15 ml. of the

buffer solution. The pH of this solution should be 10. If there is

any doubt because of high salt or acid content in the original aliquot,

check with the pH meter and add more buffer to bring to pH 10. Heat

the solution on a hotplate to near boiling for severalminutes to speed

up t\\Q reaction, cool, add 10 drops of EBT indicator, and titrate from

a blue to a permanent red with the standard calcium solution.

Ccdckim.—Place an aliquot containing 1 to 2 mg. of calcium in a

250-ml. beaker and dilute to a volume of about 150 ml. with distilled

water. Add 10 drops of each masking reagent and enough 10-percent

sodium hydroxide (usually about 1 ml.) to raise the pH to 12 or

slightly higher. Check with a pH meter or Hydrion paper. Add 5

drops of Calcan indicator and titrate the solution from red to blue

with standardized EDTA.

If the end point is difficult to obtain because of the presence of

phosphate, to a new sample add the masking reagents and a known

but an excess amount of EDTA. Bring the pH up slowly to 12 with

10-percent sodium hydroxide. Then heat the solution to near boiling

tor several minutes, cool it, add 5 drops of Calcon indicator, and

titrate the solution from blue to red with a standard calcium solution.

Magnesmrïi.—P\d.(^^ an aliquot containing 2 to 4 mg. of magnesium

ma 250-ml. beaker and add enough distilled water to make a total

volume of about 100 ml. Pipet 20 ml. each of the buffer solution

and sodium tungstate solution. Check with a pH meter and add more

buffer solution to brmg to pH 10. Heat the beaker and contents for

i- ^^o^i^ ^^^ boiling, cool, and filter the contents through Wliatman

^o. 42 hlter paper. Wash the paper and precipitate with a solution
METHODS OF SOIL AND PLANT ANALYSIS 7

containing 50 ml. of buffer solution per liter. Add 10 drops of each

masking reagent. Allow a few minutes for the reactions to take place.

Add 10 drops of the EBT indicator and titrate from a red to a permanent blue. Heating slightly will speed the magiiesimu end point.

Calculation of Results,—If the EDTA solution has been carefully

prepared from a pure material, the following relationships are true :

Each milliliter of EDTA is equivalent to 0.005 mM of cation whether

the cation is calcium, magnesium, or the sum of the two. Also, each

milliliter of EDTA is equivalent to 0.2004 mg. of calcium or calcium

plus magnesium expressed as calcium, or 0.1216 mg. of magnesium or

calcium plus magnesium expressed as magnesium.

Since the commercial form of EDTA may contain varying amoimts

of adsorbed water and other impurities, the following equations are

used to calculate the millimoles of calcium plus magnesium, calcium,

or magnesium in the bulk sample, depending on the conditions under

which the titration was made.

Standardization of EDTA :

(Millimoles of standard cation used in titration)

(Milliliters of EDTAstandard—milliliters of EDTAhiank)

=concentration of EDTA in terms of millimoles per milliliters

Evaluation of sample :

Concentration of standardized EDTAx (milliliters of EDTAsampie—

milliliters of EDTAbiank) Xaliquoting factor=millimoles of cation

in total sample

If an excess of EDTA was added to overcome phosphate interferences, these equations are used to obtain the total millimoles of

cation in the sample. From this value subtract the number of millimoles of standard calcium used in the back titration, giving the millimoles of cation originally in the aliquot taken.

The usual procedure is to obtain the millimoles of calcium plus

magnesium and then determine the millimoles of either calcium or

magnesium. The millimoles of the other cation are obtained by

subtraction.

Comments

The nature of the material may make some modifications necessary.

The amounts of reagents used are sufficient for most conditions that

may be encountered. However, in the presence of large amounts of

iron or aluminum, more masking reagents may be required. It may

be even necessary to remove a large excess of iron or aluminum by the

classical hydroxide precipitation. When abnormally large arnounts

of phosphate are present, a separation or removal of phosphate is recommended before the titration. Increasing or decreasing the amount

of indicator may help some workers to see the end point better.

As little as 10 p.p.m. of phosphate will interfere with the calcium

titration. This concentration is rarely found in soil extracts, but it

will be commonly found in plant tissue digests, particularly from seeds.
8 AGRICULTURE HANDBOOK 2 88, U.S. DEPT. OF AGRICULTURE

The stability constant of calcium phosphate is less than that of calciiimversenate ; therefore, EDTA will dissolve calcium phosphate, but the

process is slow. In the procedure generally used, calcium phosphate

is formed before sufficient EDTA has been added. In the titration

then a pennanent blue, which indicates the end point, cannot be mamtained while calcium phosphate is dissolving. Tliis makes the titration impractical.

In some rare instances the magnesium concentration may be as large

or larger than the calcium concentration, causing a poorly defined

color change at the end point when titrating calcimn alone. In this

case, the calcium indicator is adsorbed on the magnesium hydroxide

particles, making the color change many shaded and difficult to see.

A prote<:tive colloid such as Carbocel may be added to prevent the adsorption of the dye {10). It has also been foimd that if most of the

EDTA is added before the pH is raised, the color change at the end

point is sharp and easily seen (7).

The amounts of extractable or total barium or strontium in some

soils may be large enough to interfere. Although calcium may be separated from barium and strontium, the procedures are difficult. Both

barium and strontium form EDTA complexes of low stability ; therefore, unless their concentrations approach that of calcium, the degree

of interference will be small.

Strontium by Flame Photometry

Equipment

(1) Equipment necessary for determining calcium.

(2) Pipets—5 and 10 ml., graduated, blowout type.

(3) Centrifuge tubes—40 ml., heavy duty.

(4) Water bath—2-liter beaker, with met^l cover punched out

to support centrifuge tubes.

(5) Centrifuge.

(r>) Electric muffle furnace, with automatic temperature control.

(7) Fritted glass filter assembly: From a 15-ml., 20-mm. diameter, coarse porosity filter funnel (Corning, type 36060), remove

the part above the glass frit with a dass saAv and ß:rind level the remammg frit. Cut a 5-cm. chimney from a length of glass tubing (18-

mm. mner diameter) and gTind one end level. IMount the filter funnel

m a suction apparatus, place a filter pad on ih^. fritted 2:lass, and center

the ground end of the chimney carefullv over tlie fritted glass. During

nitration the assembly is held tightlf together by means of rubberbands and wire hooks.

(8) Suction apparatus—Nylab filter bell.

(9) Filter paper—glass-fiber or conventional paper (2.1-cm.

diameter). ^ ^ ^

(10) Flame photometer—D.U. Beckman model, with photomultiplier nttnchment and hydrogen-oxygen burner assembly.

(11) Beakers—5 ml.

Reagents

9 0 iy Strontium standard solutions: A series containing 0, 0.5,1.0,

Í mnio ' 'f^^^}^ PvP-'^i- ""^ stTontium and 500 p.p.m. of calcium

IS made up m 0. 1 N hydrochloric acid. A stock solution of strontium
METHODS OF SOIL AND PLANT ANALYSIS 9

chloride should be standardized by gravimetric determination of

strontium sulfate. Standard calcium solution, as prepared for the

determination of calcium (p. 4), may be used to make this series of

strontium standards.

(2) Keagents necessary for determining calcium (p. 4).

(3) Hydrocliloric acid, approximately 6 N: Dilute 500 ml. of

concentrated hydrochloric acid (HCl) to a volume of 1 liter with distilled water.

(4) Oxalic acid, 5 percent: Dissolve 100 g. of oxalic acid

(H2C2O4) in 2 liters of distilled water.

(5) Indicator solution: Phenol red, 0.1 percent in 1:1 ethyl alcohol: Dissolve 0.1 g. of phenol red in 100 ml. of a solution containing

52 ml. of ethyl alcohol and 48 ml. of water. Méthylène blue, 0.2 percent : Dissolve 20 mg. of méthylène blue in 100 ml. of water. Mix these

two solutions at a rate of one part of phenol red and two parts of

méthylène blue.

(6) Ammonium hydroxide: Dilute 250 ml. of concentrated ammonium hydroxide (NH4OH) to a volume of 1 liter with distilled

water.

(7) Washing solution: To make 1 liter, mix the following ingredients in the proportions gÍA^en : 20 ml. of concentrated ammonium

hydroxide, 326 ml. of distilled water, 326 ml. of ether, and 326 ml. of

ethyl alcohol.

(8) Hydrochloric acid, 0.1 N: Dilute 17.2 ml. of concentrated

hydrochloric acid to a volume of 2 liters with distilled water.

Procedure

The strontium determination is made on a solution containing 500

p.p.m. of calcium, which for convenience should have a volume of

about 15 ml. From the following calculationsthe number of milliliters

of soil or plant extract (see Preparation of Samples) needed is

obtained.

(1) If milliequivalents of calcium per 100 g. of soil is known,

(milliequivalents of calcium per 100 g. of soil) ^^

4

=milligrams per 100 ml.

(milligrams per 100 ml.) X10=parts per million

-7 '-7j^.—-=number of milliliters needed. (parts per million)

Therefore,

150 V . 7——-; -.— —-—; — z;—TTY=aliquot. (milliequivalents 01 calcium per 100 g. 01 soil)

(2) If parts per million of calcium in the extract is known,

7,500 ^^j. ^^^

(parts per million of calcium)

For convenience, take the aliquot to the nearest 0.5 ml. and adjust the

final volmne to contain 500 p.p.m.
10 AGRICULTURE HANDBOOK 2 88, U.S. DEPT. OF AGRICULTURE

Pipet the calculated aliquot into a 40-nil. centrifuge tube and

dilute to about 20 ml. with distilled water. Add 1 ml. oí 6 N hydrochloric acid and 2 ml. of 5-percent oxalic acid and place in a gently

boiling water bath. Remove the tube from the bath after its contents

have reached about 90° C. (equilibrium with boiling water bath) and

add 5 drops of the mixed indicator solution. Add ammonium hydroxide dropwise, mixing continually with a swirling motion until the

solution just changes from a green to a blue. Calcium oxalate should

start precipitating shortly after the hydroxide is added. Replace the

tube in the boiling water bath for 30 minutes. The addition of small

amounts of ammonium hydroxide may be necessary from time to time

to keep the suspension slightly basic (just blue). Cool the tube and

suspension and centrifuge for 20 minutes at about 1,500 r.p.m.

Decant the supernate carefully and lay the tube on a slant for

10 minutes to facilitate drainage. Wipe the lip of the tube with a

tissue to remove the last drops and suspend the oxalate precipitate by

stirring with a jet of 5 ml. of washing solution. Centnfuge, decant,

and drain once again.

Allow the precipitate to air-dry overnight. Heat the samples at

least 1 hour at 525° C. in a muffle furnace. At this temperature calcium and strontium oxalate are converted to carbonates, also any iron

or aluminum present is converted to the dehydrated oxides. When the

samples are cool, dissolve the precipitate in 0.1 N hydrochloric acid.

The number of milliliters of hydrochloric acid necessary is calculated

to the nearest 0.01 ml. as follows :

(Parts per million of calcium in original sample),, ,. , .„.,.,

TQö —^^ aliquot =milliliters

of 0.1 N hydrochloric acid required

If iron, manganese, and aluminum were present in the original sample m any appreciable amoimts, they will appear as precipitates in the

sample and must be removed. The precipitates are easily removed by

filtermg. The removal is essentially quantitative if done shortly after

the carbonate sample is dissolved. A fritted glass filter assembly with

suction apparatus is convenient^ in that the assembly may be rapidly

washed with acid and dried with ether or alcohol between samples.

ihe actual filtermg surface may be either glass-fiber or conventional

mter paper Glass-fiber paper has the advantage of retaining little

of the liquid fraction. ^ ^

If the filtered sample is cloudy or colored, the sample should be

rehltered or dried and muffled. ^Vlien a clear solution is obtained,

üetermme the calcium as previously described. If the initial titration

or estimation of calcium, pipeting, and precipitation procedure were

Clone properly, the calcium concentration should be 500 p.p.m. If the

present titration is not within 20 p.p.m. of 500 p.p.m., the sample must

be concentrated or diluted. i i' ? f

A D.U. Beckman photometer is used to estimate strontium in a hy-

^^^^'""""W^ ^^''''^* .^"^'^ ""^ l^^st a 30-minute warming-up period

nn 1 \î 1 ' r^""'' '''^'^'^^' ^^ ^^^^^^- Set the oxygen pressure at 10 p.s.i.

fZ\Zt ^'y^^^^f^^ pressure at 3 p.s.i., with the control associated with

the mstrument, and light the flame. For stability of the pressures, it
METHODS OF SOIL AND PLANT ANALYSIS 11

is best to have at least 20-p.s.i. pressure showing on the low-pressure

gages of the oxygen and hydrogen tanks. Use full photomultiplication and set the smtch accordingly. The blue tube is set into position

according to the instructions furnished by the manufacturer. Always

ke^ the galvanometer at zero with the dark current control.

The following settings are made initiaUy: Wavelength dial to

460.7 m/x, slit width at 0.03 mm., transmittance reading at 100, and

sensitivity control at midpoint. Aspirate several 5-ml. beakers full

of water. Place a 5-ml. beaker of the most concentrated standard

solution in position to be aspirated. Open the shutter and move the

wavelength dial slightly. A deflection of the galvanometer needle

to the left indicates that the original wavelength setting was wrong.

Adjust until a very slight turn of the dial in either direction results

in the galvanometer needle deflecting to the right. Close the shutter

after each reading and check the dark current setting.

Produce a standard curve as follows : Set the transmittance dial at

100 and aspirate the most concentrated standard. Zero the galvanometer needle with the dark current control, open the shutter, and

zero the needle with the sensitivity control. Close the shutter, remove

the sample, and rinse the burner with distilled water for a few seconds. Put the next most concentrated standard solution in position.

Make the reading by moving the transmittance dial until the galvanometer needle is at zero. Reset the instrument at 100-percent transmission with the most concentrated standard solution before each

new reading is made. Obtain transmittance readings for standards

by this process. Plot the transmittance reading against the concentration of the standard and calculate the equation of the line.

Pour the unknown into a 5-mL beaker and aspirate through the

burner assembly for a few seconds. Discard the remainder, refill

the beaker, and replace under the flame. Obtain the reading by

turning the transmittance dial until the galvanometer needle is at

zero. After every six to eight determinations of unknown samples,

adjust the instrument with a standard sample in the range of the

unknowns.

Comments

Adjustments to be made and precautions to be taken before and

during every determination :

( 1 ) Oxygen pressure, 10 p.s.i.

(2) Hydrogen pressure, 3 p.s.i. A change in pressure, as indicated by the width of the hydrogen pressure gage needle, is sufficient

to give incorrect measurements.

(3) Dark current must be zero before and after each measurement.

If not, measure again.

(4) Any yellowish tint to the flame usually indicates the presence

of sodium.

Possible factors contributing to incorrect results are as follows :

( 1 ) Phototube positioning switch not completely in or out.

(2) Plugged burner is usually indicated by aspiration continuing

a short time after the solution is removed from the siphon tube. Also,

the measurements are approximately the same value. Unplug the

burner as follows: Turn off the hydrogen, leaving the oxygen on.

Carefully push a wire (found with instrument) up the siphon tube;
12 AGRICULTURE HANDBOOK 288, U.S. DEPT. OF AGRICULTURE

never start from the top. If this does not solve the problem, the

burner assembly may need repair and should be returned to the

manufacturer.

(3) Slit width may have been accidentally changed.

Strontium by X-ray Fluorescence Emission Spectrometry

Equipment

(1) Mortar (aójate) and pestle.

(2) Pipet—ImL

( 3 ) Glass stirring rod.

(4) Spatula.

(5) Sample holder: Modified so that an aluminum alloy frame,

0.05 inch thick, can be slid into the holder and supported along the

edges only. The center of the frame is cut out to match the opening in the holder. The bottom side of the frame is covered with

0.00025-inch Mylar film stretched tight and cemented to the frame

with rubber cement. The irradiation beam is carefully centered so

that no part of the holder or frame is irradiated.

(6) Microscope slide.

(7) Philips Norelco X-ray spectrograph, with FA-60 tungsten

X-ray tube. The X-ray beam excites secondary X-rays from the

sample. These are collimated to an analyzing crystal, then to a scintiUation detector with linear amplifier and fast sealer. The X-ray

tube is operated at 50 kv. and 40 ma.

Reagents

(1) Strontium standard solutions: From a stock solution (see

reagents for Strontium by Flame Photometry) prepare dilutions

containing 0, 20, 40, 60, 80, 100, 150, and 200 [xg, of strontium per

milliliter of solution.

(2) Ethyl alcohol (C2H5OH)—95 percent.

Procedure

Ash 100 g. of j)lant material, as described under Preparation of

Samples. Kecord the> weight of the ash and transfer the ash to an

a^ate mortar, grind with a pestle, and mix to obtain a homogeneous

mixture.

Prepare standard samples in an identical manner, except pipet 1 ml.

of standard strontium solution (e.g., 0, 20, 40, 60, 80, 100, 150, and

200 Jig, of strontium) into a mortar containing a slurry of 1 g. of ash

and 20 ml. of 95-percent ethyl alcohol. Mix the slurry thoroughly

with a glass stirring rod, dry overnight at 70° C, and mix thoroughly

with mortar and pestle to obtain a homogeneous fine powder.

Transfer the powdered ash with a spatula to the aluminum alloy

frame, backed with Mylar film, of the sample holder, level off with

the edge of a glass microscope slide, and smooth with the flat side of

the microscope slide to give a uniform surface for reflectance. About

one-half gram of ash is required to fill the frame. It is not necessary

to weigh definite amounts of the sample, nor is it desirable to pack the

sample into the frame.
METHODS OF SOIL AND PLANT ANALYSIS 13

Place the filled frame in tlie sample irradiation chamber of the

spectrograph. Scan the first sample of a series by settmg the goniomet-er to rmi automatically over the desired range (about 20° or 30°

2Ö) in order to obtain a picture of the area for a background and thus

avoid choosing an angle for background with interference from other

lines, up to the fifth order. The strontium Ka line appears at 25,15°

2Ö, whereas the position of 25.90° is usually used for a background

reading.

For quantitative analysis, use the peak-to-background ratio (P/B)

of intensities as a measure of the strontium concentration. Obtain this

ratio by setting the goniometer at 25.15° and recording the time necessary to measure 51,200 counts, then setting the goniometer at the

background position (25.90°) and recording the time required for

51,200 counts. Convert the recorded times to counts per second, and

calculate the ratio of the counts per second at the strontium peak to

the counts per second at the background position.

Prepare a standard curve from measurements of the P/B in samples

with added strontium. On the scan that was made, draw a straight

line connecting the background on one side of the strontium peak to

the background on the other side. Usually the background directly

under the peak (25.15°) is higher than the one measured at 25.90°.

This difference must be accounted for in determining the unknown

amount of strontium originally present in the standard samples. The

ratio of the count rate ofthe background at 25.15° to the count rate at

25.90° is called B/B. Subtract it from the P/B for the samples with

added strontium. Then plot the net P/B versus the added strontium.

Draw a straight line through the points and extrapolate to the negative X axis, which has the same scale as the positive X axis. This

intercept gives the micrograms of strontium originally present.

Make a standard curve of the observed P/B to the total strontium

(original plus added). Determine the strontium content of the unknown samples from their measured P/B in terms of micrograms of

strontium per gram of ash. Multiply this by the weight of the ash

and divide by the weight of the plant material to obtam the micrograms of strontium per gram of plant material.

The sensitivity for strontium is estimated to be between 2 and 3

p.p.m., and the error is estimated to be about plus or minus 8 percent.

Sodium and Pofassium

Equipment

Flame photometer—D.U. Beckman model, with photomultiplier

attachment and hydrogen-oxygen burner assembly.

Reagents

(1) Sodiiun standard solutions: Dissolve 2.542 g. of oven-dry

reagent grade sodium cliloride (NaCl) in distilled water and dilute

to a volume of 1 liter. This stock solution contains 1,000 p.p.m. of

sodium. Dilute appropriate volumes of stock solution with distilled

water to prepare a series of solutions containing 0, 1, 2.5, 5, 10, 15, 20,

30,40, and 50 p.p.m. of sodium.
14 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

(2) Potassium standard solutions : Dissolve 1.907 g. of oven-dry

reagent grade potassium chloride (KCl) in distilled water and dilute

to a volume of 1 liter. This stock solution contains 1,000 p.p.m. of

potassium. Dilute appropriate volumes of stock solution with distilled

water to prepare a series of solutions containing 0, 1, 2.5, 5,10,15, 20,

30,40, and 50 p.p.m. of potassium.

Procedure

Sodium and potassium may be estimated by using the red or blue

phototube. The settings for each tube are given in table 1.

TABLE 1.—Settings for red and Hue phototubes for estimating sodium

and potassiwn

Item

Eed phototube Blue phototube

Sodium Potassium Sodium Potassium

Wavelength

Slit width

Photomultiplier setting

RfîSiRtor Rwitp.h -noRition

m/i-_

mm__

587

15

Off

3

767

0.04

Off

3

587

0.02

2

2

767

0.05

Full

2

^

In all cases oxygen pressure is 10 p.s.i. and hydrogen pressure is

Sp.s.i.

In the U.S. Soils Laboratory the blue phototube is used and all

procedures are the same as for strontium, except the settings given

here.

DETERMINATIONS ON SEPARATE SAMPLES

pH of Soil

Equipment

(1) Beaker—50 ml.

(2) pH meter,

(3 Glass '- stirring rod.

Procedure (8)

To a 20-g. sample of soil in a 50-ml. beaker, add 20 ml. of distilled

water and stir the suspension several times at regular intervals for

about an hour.^ Measure the pH of the soil suspension with the glass

electrode, stirring well just prior to immersing the electrode deep into

the suspension.

Comments

Soil-water ratios ranging from 1: 0.5 to 1:10 have been proposed for

the determination of the pH of soils. The DH of most soils increases

with dilution, usually attaining approximately a constant value at
METHODS OF SOIL AND PLANT ANALYSIS 15

1:5. There is little justification for using soil-water ratios wider

than 1:1. The pH measurements of soils at a moisture content below

that of the moisture equivalent are subject to several inherent errors

and also make mandatory the use of a special glass electrode and pPI

meter.

Organic Matter Content of Soil

Equipment

( 1 ) Erlenmeyer flask—500 ml.

(2) Burets—50- and 100-ml. dispensing burets for acid.

( 3 ) Pipet—automatic 10 ml.

(4) Asbestos.

(5) Dropping bottle.

Reagents

(1) Potassium dichromate, 1N: Dissolve 49.04 g. of reagent grade

potassium dichromate (K.CroOr) in water and dilute to a volume of

1 liter.

(2) Sulfuric acid (H2SO4)—not less than 96 percent.

(3) Phosphoric acid (H3PO4)—85 percent, U.S.P. grade.

(4) Barium diphenylaminesulfonate indicator: Prepare a 0.16-

percent aqueous solution.

(5) Ferrous sulfate, 0.5 N \ Dissolve 140 g. of reagent gTade ferrous sulfate (FeSOi-THsO) in water, add 40 ml. of concentrated sulfuric acid (H2SO4), cool, and dilute to a volume of 1 liter. Standardize this reagent each day by titratmg against 10 ml. of 1 A^ potassiimi

dichromate, as given mider Procedure.

Procedure (8)

^ Transfer a weighed quantity of soil (ground to pass a 0.5-mm.

sieve), containing 10 to 25 mg. of organic carbon into a 500-ml. Erlenmeyer flask, and add 10 ml. of 1 A" potassium dichromate. Then add

rapidly 20 ml. of concentrated sulfuric acid from the 100-ml. dispensing buret, directing the stream into the solution. Innnediately swirl

vigorously by hand for 1 minute and let the flask stand on a sheet of

asbestos for about 30 minutes. Then add 200 ml. of water, 10 ml.

of phosphoric acid from the 50-ml. dispensmg buret, and 10 drops of

barimn diphenylaminesulfonate indicator.

Proceed with the titration as follows : Add the ferrous sulfate until

the solution is purple or blue, then add the ferrous sulfate in small

amounts of about 0.5 ml. until the color flashes to green with little or

no warning. Add 0.5 ml. of 1 A^ potassium dichromate to restore an

excess of dichromate, and complete the titration by adding ferrous

sulfate drop by drop to a light-green end point. If more than 8 ml,

of the available 10 ml. of potassium dichromate is reduced, the determination should be repeated with less soil.

Percentage of organic matter in soil sample=

(milliliters of 1 A^ potassium dichromate (reduced) ) s^ A /^g

(grams of sample)
16 AGRICULTURE HANDBOOK 288, U.S. DEPT. OF AGRICULTURE

Comments

The estimation of soil organic matter based on the use of the conventional empirical factor for converting organic carbon to organic

matter may be subject to considerable error, even when the organic

carbon is determined accurately by the more tedious dry-combustion

method. For this reason, the more rapid chromic acid-titration

method is now widely used. Despite its simplicity, it has been shown

to give resuhs that agree closely with those obtained by the drycombustion method when applied to a wide variety of soils.

In this method the soil is digested with a mixture of chromic and

sulfuric acids making use of the heat of dilution of sulfuric acid.

Consequently, it is necessary to add the sulfuric acid rapidly and to

cool the flasks uniformly on a sheet of asbestos. The unpublished

results of M. Peech indicate that the volume of the potassium dichromate solution and the concentrated sulfuric acid may be reduced to

half that specified in the procedure without significantly affecting the

results, provided, of course, that the size of the soil sample is proportionately reduced.

The color change of the optional ö-phenanthroline indicator occurs

at a much higher oxidation-reduction potential, and tliis indicator is

theoretically superior to diphenylamine. With certain soils, however,

the color change of diphenylamine is more easily observed than that

of (9-phenanthroline; hence, both indictors are recommended. The

aqueous solution of barium diphenylaminesulfonate indicator is

much more stable than that prepared by dissolving diphenylamine in

concentrated sulfuric acid.

Nitrates mterfere only if present in quantities in excess of 5 percent

of the carbon content. Carbonates, even when they constitute 50

percent of the soil, do not affect the results. Since elemental carbon,

as charcoal or coal, is practically unattacked in this method, this

source of error is eliminated. Interference due to significant quantities of chlorides can be overcome by adding silver sulfate to the

sulfuric acid, or a correction can be made if the quantity of chlorides

is known. When clilorides are present in quantities less than the

molecular equivalent of carbon, 1.25 g. of silver sulfate should be

dissolved in each 100 ml. of concentrated sulfuric acid.

Exchangeable Hydrogen

Equipment

( 1 ) Erlenmeyer flasks—125 and 250 ml.

(2) Gooch-type filter funnel—15 ml., 2-cm. diameter, coarse

porosity (Coming, type 36060).

(3) Filter paper—glass-fiber or conventional paper (2.1-cm.

diameter).

Reagents

(1) Buffer solution: Barium cliloride (BaCl2-2H20), 0.5 N,

and tnethanolamme, 0.2 N: Dilute 100 ml. of commercial triethanolamme (specific gravity 1.126, about 8 N) with 1,000 ml. of distilled

water and partially neutralize with hydrochloric acid (HCl) to
METHODS OF SOIL AND PLANT ANALYSIS 17

adjust to pH 8.1-8.2 (this requires approximately 360 ml. of 1 iV

hydrocliloric acid). Make up to a volume of 2 liters with water and

mix with 2 liters of a solution containing 250 g. of barium chloride.

Protect from the carbon dioxide of the air.

(2) Replacement solution: Barium chloride: Dissolve 250 g. of

barium chloride in 4 liters of distilled water, add 10 ml. of buffer

solution, and mix.

(3) Sodium hydroxide, 0.01 N: This solution need only be approximate. Weigh 0.4 g. of sodium hydroxide (NaOH) and dissolve

in 1 liter of distilled water.

(4) Bromcresol green, 0.04 percent : Triturate 0.1 g. of the mdicator with 14.3 ml. of 0.01 N sodium hydroxide and dilute the mixture

to a volume of 250 ml. Avith distilled water.

(5) Methyl red, 0.02 percent: Triturate 0.04 g. of the indicator

with 1.5 ml. of 0.01 N sodium hydroxide and dilute to a volume of

200 ml. with distilled water.

(6) Standard hydrochloric acid, 0.10 N: Dilute 8.35 ml. of concentrated hydrochloric acid to a volume of 1 liter with distilled water.

Standardize this solution agamst a standard base by accepted

procedures.

Procedure (8)

Place 10 g. of soil in a 125-ml. Erlenmeyer flask, add 25 ml. of buffer

solution, and allow the flask to stand for one-half hour, mixmg the

contents occasionally by swirlmg. Transfer to a Gooch-type filter

funnel containing a moist filter paper disk and filter into a 250-ml.

Erlenmeyer flask. Use an additional 25 ml. of buffer solution to

aid in the transfer of all the soil to the funnel. Leach the soil with

100 ml. of the replacement solution by adding small increments. This

filtration and leacliing should not be completed in less than 30

minutes.

To the leachate, add 10 drops of bromcresol green and 2 drops of

methyl red. Titrate with 0.10 N hydrochloric acid. The end point

can be chosen as any i)oint during the progressive color change from

a bluish green through violet to pink. This end point should be

checked against a blank containing 50 ml. of buffer solution and 100

ml. of repracement solution and titrated to the same end point with the

0.10 A^ hydrochloric acid. This end point should be reached when

titrating the soil extracts. All calculations are made with this blank

determination as a reference.

Available and Fixed Strontium 90 in Soils

Equipment

(1) Erlenmeyer flask—2 liters.

(2) Biicliner funnels (6- and 8-inch diameter).

(3) Filter paper—^Wliatman No. 42 or equivalent.

(4) Suction flasks—1 and 2 liters,

(5) Beakers—150 and 800 ml., 2 and 3 liters.

(6) Plotplate.

(7) Funnel—long stem (9-cm. diameter).
18 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

FIGURE 1.—Assembly of sample holder.

(8) Platinum evaporating dish—50 ml.

(9) Electric muffle furnace, with automatic temperature control.

(10) Mortar and pestle.

(11) Glass stirring rod.

(12) Indicator paper.

(13) pH meter.

( 14 ) Ice bath—crushed ice in water.

(15) Mechanical stirrer.

(16) Glass wash bottle.

(17) Fritted glass filter assembly (see equipment for Strontium

by Flame Photometry).

(18) Glass-fiber filter pad—2.1 cm.

(19) Sample holder (see fig. 1) : Machine an aluminum disk

38 mm. m diameter and 4 mm. thick so that it has a 3-mm. well (wall

thickness 3 mm.) and a hole in the center of the well 23 mm. in diameter (F). Make an aluminum ring (B) approximately 32-mm. outer

diameter, 29-mni. inner diameter, and 3 mm. high so that it fits loosely

m the well. Use a steel disk (D) 28 mm. in diameter and 3 mm. thick

as backmg for the sample. Assemble the parts as follows : Glue the

Saran film (A) to the aluminum ring (B). Center the 2.1-cm. glasshber niter pad (C), on which the sample has been collected, on the

steel disk (D). Place the aluminum ring (B) over the steel disk (D)

so that the Saran film holds and protects the sample. Invert the ring

{B) and the disk (D) and cover with the Saran fihn (E). Place the

well ot the machined aluminum disk (F) over the Saran film (E)

and press down so that all other parts fit into the well. Trim any

excess Saran fihn (E) with a sharp knife. The exact size of the

sample holder may be varied to fit the automatic sample-changing

equipment The sample is then ready to count.

(20) Low-background G.M. counting equipment.
METHODS OF SOIL AND PLANT ANALYSIS 19

Reagents

(1) Strontium nitrate, 2 N: Dissolve 3,810 g. of strontium nitrate (Sr(N03)2) in distilled water and dilute to a volume of 18

liters with distilled water.

(2) Hydrochloric acid: For 6, 2, 1, and 0.2 TV, dilute 9 and 3

liters and 1,500 and 42 ml., respectively, of concentrated hydrochloric

acid (HCl), each one to a volume of Í8 liters with distilled water.

(3) Nitric acid (HNO3)—concentrated, reagent grade.

(4) Yttrium carrier: Dissolve 4 g. of yttrium oxide (Y0O3) in

5 ml. of concentrated nitric acid. After the reaction is complete, dilute to a volume of 200 ml. Check the radioactive purity of this

reagent. Add 1 ml. of reagent solution to 200 ml. of distilled water,

add 5 ml. of concentrated nitric acid, allow to stand for 14 days, and

milk as given under Procedure. If contaminated, the yttrilmi nitrate solution can be purified by following the procedures used in

the Health and Safety Laboratory (6).

(5) Sulfuric acid (H2SO4)—concentrated, reagent gTade.

(6) Sodium carbonate (]Sra2C03)—reagent grade.

(7) Ammonium hydroxide, 4 N : Dilute 670 ml. of concentrated

ammonium hydroxide (NH4OH) to a volume of 2,500 ml. with distilled water. Ammonium hydroxide, 1 percent: Dilute 180 ml. of

concentrated ammonium hydroxide to a volume of 18 liters with

distilled water.

(8) Acetic acid, glacial (HOAc).

(9) Sodium dichromate, 1 M: Dissolve 298 g. of sodium dichromate (jN'a2Cr207-2H20) in water and dilute to a volume of 1 liter.

(10) Barium chloride, 0.5 M: Dissolve 61 g. of barium chloride

(BaCl2'21120) in water and dilute to a volume of 500 ml.

(11) Potassium cyanate (KCNO)—reagent grade.

(12) Ammonium oxalate: Add approximately 50 g. of ammonium oxalate ((]Nn4)2C204*H20) to 1 liter of water. An excess of

salt should remain on the bottom after equilibrium is obtained by

frequent shaking.

(13) Ethyl alcohol (C2H5OH)—95 percent.

(14) Standard strontium 90 : Kadioactive solution standard type

ES 90 obtained from Nuclear Chicago Corporation, Standards Section.

(15) Potassium chloride (KCl)—reagent grade.

Procedure

Extraction of Soil.—Place 1 kg. of soil in a 2-liter Erlenmeyer flask

with 1 liter of strontium nitrate solution. The concentration of strontium nitrate is adjusted so that 3 liters of it contains strontium equivalent to 10 times the total exchangeable cations of the soil, which were

previously determined by the ammonium acetate method for bases

and the barium chloride-triethanolamine method for hydrogen (p.

16). The strontium nitrate solution is easily made up for each sample

from a stock solution of 2 N strontium nitrate. Stir the soil and

solution well and allow to stand overnight. Filter through an 8-inch

Büchner funnel, using Whatman No. 42 paper, into a 2-liter suction

flask. After the soil has drained, resuspend it in 1 liter of the strontium nitrate solution for that soil, and filter again after 2 to 6 hours.

This time the filtration will be much slower, and the sample may be

divided about equally between two Büchner funnels. Wash the soil
20 AGRICULTURE HANDBOOK 2 8 8, U.S. DEPT. OF AGRICULTURE

with the third 1-liter part of the strontium nitrate solution. Combine

the extracts in a o-liter beaker and evaporate to dryness on a hotplate.

These extracts contain the available fraction of the strontium 90.

Place the extracted soil in a 3-liter beaker and add 1.5 liters of 6 iV^

hydrochloric acid. Stir well and allow to stand for 5 days on a hotplate at 70° to 80° C. Filter through an 8-inch Büchner funnel, washino- with 1 liter of 1 A^ hydrochloric acid. Combine the extracts in a

3-Rter beaker and evaporate to dryness on a hotplate. This material

contains the fixed fraction of strontium 90. Discard the extracted

soil.

Treatment of Extracts,—The small amount of organic matter in

the strontium nitrate extract can be destroyed easily by adding 100

ml. of concentrated hydrochloric acid and evaporating to dryness.

Dissolve the sample in 500 ml. of water containing 5 ml. of concentrated

nitric acid and filter through AVliatman No. 42 paper into an 800-ml.

beaker. Add 1 ml. of yttrium nitrate solution containing 20 mg. of

yttrium. The sample is now ready for removal of natural radioactive

contaminants.

If the soil sample contained approximately 200 meq. or greater of

calcium and magnesium, the strontium nitrate extract will need to be

carried through the strontium sulfate precipitation procedure as described below for the hydrochloric acid extract. The large amount

of strontium present will require splitting the sample to attain complete strontium precipitation. To check this, more sulfuric acid may

be added to the filtrate to see if additional strontium sulfate forms.

The strontium sulfate from all subsamples is combined for the fusing

process.

The hydrochloric acid extract contains large quantities of aluminum, iron, silicon, and organic matter, as well as the desired strontium,

and requires the following treatment: Add 100 ml. of concentrated

nitric acid. After the reaction has subsided, begin heating gently to

avoid loss of the foaming sample. Evaporate the solution to dryness.

Dissolve the precipitate in 200 ml. of 6 N hydrochloric acid, heating

as necessary. Dilute with 400 ml. of water and filter through a 6-inch

Biiclmer funnel into a 1-liter suction flask. Transfer the filtrate into

a 2-liter beaker and add 50 ml. of concentrated sulfuric acid to precipitate the strontium sulfate. Digest the precipitate on the hotplate for

several hours, and filter through a long-stem funnel with Whatman

No. 42 filter paper.

Transfer the paper and precipitate to a platinum evaporating dish

and burn off the filter ]:)aper in a muffle furnace with the door open

and temperature about 200° C. After the filter paper is charred, close

ih^ door and increase ih^ temperature to 500° for 1 hour. Weigh the

strontium sulfate and mix thoroughly with an equal weight of sodium

carbonate, using a mortar and pestle.* Place the mixture in a platinum

evaiwratino; dish and fuse it in a muffle furnace at 900*=^. After 1

hour at this temperature, turn off the furnace and allow the dish to

cool mside the furnace. The cake then comes easily out of the dish.

Place It ni a 800-ml. beaker with 400 ml. of water, heat gentlv, and

break up the cake with a glass stirring rod. When the cake is completely broken up, an undissolved residue of strontium carbonate

remains. Filter through Whatman No. 42 filter paper in a 9-cm.

funnel. Dissolve the strontium carbonate in 200 ml. of 2 N hydro-
METHODS OF SOIL AND PLANT ANALYSIS 21

cliloric acid, and remove the small undissolyed residue by filtration

through Whatman No. 42 paper in a 9-cm. funnel. Add 1 ml. of

yttrium nitrate solution containing 20 mg. of yttrium.

Removal of Natural Radioactwe Contaminants.—Some soils contain

radioactive contaminants that interfere with the determination of

strontium 90, particularly in the hydrochloric acid extract. These are

prmcipally daughters of radium 226 and thorium 232. Scavenges with

yttrium hydroxide and barium chromate remove most of the contaminants. The barium chromate scavenge is done by the method of

homogeneous precipitation {S).

Heat the extracts and precipitate yttrium hydroxide by adding 4 N

ammonium hydroxide to pH 8, using either indicator paper or a pH

meter to check the pH. Digest the samples for 1 hour after the precipitate is formed. ^ The precipitate often contains noticeable quantities

of manganese oxide, ferric hydroxide, and aluminum hydroxide. Filter through Wliatman No. 42 paper in a 9-cm. long-stem funnel, maintaining a column of liquid in the funnel stem to speed the filtration.

For the barium chromate scavenge, acidify the filtrate with 5 ml.

of concentrated nitric acid and add 1 ml. of acetic acid, 1 ml. of 1 BI

sodium dichromate, and 2 ml. of 0.5 M barium chloride. Adjust the

pH of the extracts to 2.0-2.5 by adding ammonium hydroxide. Cool

the extracts in an ice bath while using a mechanical stirrer. Stir 3 g.

of potassium cyanate, dissolved in a little water, into the extract.

After 3 to 6 hours of continuous stirring and when the pH of the extract has risen to 5.0-5.2, filter through "\Vhatman No. 42 paper and

discard the precipitate. It is necessary to repeat the barium chromate

scavenge at least once. Add 5 ml. of concentrated nitric acid, 1 ml. of 1

M sodium dichromate, 2 ml. of 0.5 M barium chloride, adjust the pH to

2.0-2.5, and proceed as for the first barium chromate scavenge. Add

5 ml. of concentrated nitric acid and 1 ml. of yttrium nitrate solution

to the extracts and heat to remove the dissolved carbon dioxide. Proceed with the final yttrium hydroxide scavenge, as indicated in the

preceding paragraph. Put aside the extracts for buildup and milking

of yttrium 90.

Milking of Yttrium 90,—The yttrium 90 radioactive daughter of

strontium 90 is easily separated from the large amount of strontium

in the two extracts. Since yttrium 90 has a half-life of 64.6 hours,

it reaches 97 percent of its equilibrium value with strontium 90 in 2

weeks after any separation. Thus, milkings can be made repeatedly.

Each time the extracts are put aside for buildup of yttrium 90, they

are acidified with concentrated nitric acid and 20 mg. of yttrium is

added.

Heat the sample and precipitate yttrium hydroxide as for the

yttrium hydroxide scavenge. After digestion and filtration, dissolve

the yttrium hydroxide in a minimum quantity of hot 0.2 N hydrochloric acid, which is directed on the precipitate as a stream from a

wash bottle. Collect the yttrium chloride solution in a 150-ml. beaker.

Heat, add 5 ml. of saturated ammonium oxalate solution, and adjust

the pH to 6.0. Allow the precipitated yttrium oxalate to digest about

1 hour. Then filter through a small fritted glass funnel, catching the

precipitate on a 2.1-cm. glass-fiber filter pad. Dry the precipitate by

washing with 95-percent ethyl alcohol. Transfer it to a sample holder

(fig. 1) for the radioactivity determination.
22 AGRICULTURE HANDBOOK 288, U.S. DEPT. OF AGRICULTURE

The radioactivity determination is made with an end window G.M.

tube. Use anticoincidence circuit and shielding to obtam background

counts of 1.5 to 2.0 c.p.m. . i ^ n .i

Count the sample four times over a 2-week period. Iwo o± these

coimts should be within the first 3 days after the yttrium hydroxide

precipitation. Make the third count approximately 5 days after the

precipitation and the final count after 10 days.

For each count a standard deviation is calculated according to the

following equation :

o. 1 11 ' ±' V^bkg.+^sample Standard deviation-=

Where i\^bkg.=total count of background in time (t)

A^8ampie=total couut of S ample in time (t)

The count per minute plus or minus its standard deviation is plotted

against the theoretical fraction of yttrium 90 left. This fraction is

detemiined from its radioactive decay constant. For example, onehalf is left 65 hours after precipitation of yttrium hydroxide and onefourth is left 130 hours after precipitation. The time interval for

radioactive decay is taken as the midpoint of each of the four comiting

periods. If the sample is uncontaminated, a straight line will result,

which intercepts the abscissa at zero, showing no yttrium 90 left. The

extrapolated Ime to the ordinate becomes the disintegrations per minute of strontium 90 per original soil sample when the appropriate

efficiency factor is used.

The efficiency factor is obtained by taking a solution containing a

known amount of strontium 90 through the entire procedure. ^ This

factor should be rechecked every 2 months. Potassium chloride is used

as a working standard. The weight of potassium chloride used is

estimated from the count rate of the samples being analyzed. ^ One

gram of potassium is equivalent to 1,776 disintegrations per mmute.

The potassium chloride standard and background counts should be

made every 2 days.

Strontium 90 in Plants

The strontium 90 determination for plant material is basically the

same as described in the preceding section for soils. Strontium 90 is

brought into solution, interfering ions are eliminated, and yttrium 90

is separated.^ The amount of plant material used will vary according

to its strontium 90 content. With low-background counting equipment, 1 picocurie is about the minimum that can be determined accurately. Ten or more picocuries are desirable for a quicker and more

accurate determination.

^ Several hundred grams of plant material are generally used to obtain

high enough count rates for a convenient radioactivity determination.

Thus, other ions may be sufficiently concentrated so as to cause interference with the chemical procedure. Phosphate and sulfate may interfere, but they can easily be removed by following the strontium
METHODS OF SOIL AND PLANT ANALYSIS 23

sulfate precipitation procedures given in the preceding section pertaining to the treatment of the hydrocliloric acid (HCl) extract of soil.

Calcium may interfere in the filtration of the strontium sulfate. In

this case the supemate is decanted, while still hot, from the strontium

sulfate precipitate. ^ The supernate is filtered quickly and the material

on the filter paper is recombined with the strontium sulfate.

The plant material may be contaminated with fresh fission products,

which could interfere in the ji^trium 90 radioactivity measurement.

The^ interfering fission products can be eliminated by following the

yttrium hydroxide and barium oxalate scavenge procedures described

in the preceding section. The yttrium 90 milking procedure and radioactivity measurement are performed as given in the preceding

section.

LITERATURE CITED

(1) CHENG, H. H., and KUETZ, L. T.

1960. SOLL MAGNESIUM ANALYSIS, ELIMINATION OF INTERFERENCE IN EDTA

TITRATION OF EXCHANGEABLE SOIL MAGNESIUM. Jour. Agr. and

Food Chem. 8: 24-26.

(2) CHENG, K. L., and BRAY, R. H.

1951. DETERMINATION OF CALCIUM AND MAGNESIUM IN SOIL AND PLANT

MATERIAL. Soil Sci. 721 449-458.

(3) CONNERS, J. J.

1950. ADVANCES IN CHEMicAL-coLORiMETEic METHODS. Amer. Water

Works Assoc. Jour. 42: 33-39.

(4) DiEHL, H., GoETz, C. A., and HACK, C. C.

1950. THE VERSENATE TITRATION FOR TOTAL HARDNESS. Amer. Water

Works Assoc. Jour. 42: 40-48.

(5) GORDON, L.

1952. PRECIPITATION FROM HOMOGENEOUS SOLUTION. Aualyt. Chem.

24: 459-462.

(6) HEALTH AND SAFETY LABORATORY, U.S. ATOMIC ENERGY COMMISSION.

1957. MANUAL OF STANDARD PROCEDURES. N.Y. Oper. Off. G-03-02-01,

[n.p.].

(7) LEWIS, L. L., and MELNICK, L. M.

1960. DETERMINATION OP CALCIUM AND MAGNESIUM WITH (ETHYLENEDINiTRiLO) TETRAACETATE. Analyt. Chem. 32:1123-1127.

(8) PEECH, M., ALEXANDER, L. T., DEAN, L. A, and REED, J. F.

1947. METHODS OF SOIL ANALYSIS FOR SOIL-FERTILITY INVESTIGATIONS. U.S.

Dept. Agr. Cir. 757, 25 pp.

(9) PRIBIL, R.

1954. KOMPLEXOMETRISCHE TITRATIONEN (CHELATOMETRIE) DIE MASKIERING

VON ALUMINUM UND EISEN BEI TITRATIONEN GEGAN ERICOCHROMSCHWARZ T ALS INDICATOR. CoHect. Czechoslovak Chem. Commun.

19: 405-^69.

(10) SCHOUWENBURG, J. C. VON.

1960. MICRO-EDTA TITRATION OF CALCIUM, MAGNESIUM INTERFERENCE.

Analyt. Chem. 32: 709-710.

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