Development of a DTPA soil test for zinc, iron, manganese, and cropper

Development of a DTPA soil test for zinc, iron, manganese, and cropper

LINDSAY & NORVELL; DTPA SOIL TEST FOR ZN, FE, MN, AND CU 21 Aux into the atmosphere from a grazed pasture, Seienee 185609

610

L B., and D E, Kissel 1973 Ammonia volatilization from

surface applications of ammonium compounds on calcareous soils: 1

General Theory Soil Sets Soc, Am Proc 37:835.859

6 IAEA 1970, Rise ferlization Technical Report Series 108 IAEA, Vienna

1 Khind, Có S and N, P Data, 1975 Effect 'leogen application on yield’ and ferilizer nitrogen utilization by of method and timing of

favsand rice J Indian Soe, Soil Sci, 23-442-486,

8 Koyama, T., and N Niamsiichland 1973, Sailplant auction Studies on ‘ropical tice ‘VI The effect of afferent levels of

Pitrogenous @ uluzation by te plants, Soil Sei Plant Nutr 192265 fetlizer application on plant growth, g

214

9, Lemon, E, 1878 Ninous ovife (N;O) exchange sĩ the land surface:

dn Niteogen inthe catioament D R Nielson and J G MacDonald

(ed.) Kearney Foundation Workshop, 31 Jan ~ 3 Feb 1977, Lake

‘Arrowhead, Calif Academic Press, New York

10 Liss, § 1975, Chemisy of the sea surface miceolayer In (cd) 3

P Riley and G'S raphy” Vol IL, 2a ed,

Development of a DTPA $

‘Test for

1, MacRae, 1 C., anđ R, Ancajas, 1970 Volatlization of ammonia rm submerged tropical sale: Pant Soil 3397-108,

12 Mikkelsen, D.S.,.S, K Be Dana, and W.N Osemea 1977, Factors sifecting ammonis volatilization from flooded environment of rice Inn, Rive Res, Conf Apnl 1977, IRRI, Marla, Philippines

13, Sillen, L.G., and A E, Martel, 1968, Stability constams of metal jon complexes Special Publication no, 17 3rd ed The Chemical Sức, London,

14, van Dijk, J W 1943, Some experiments on the use of nitrogen as a fertilizer with ammonia dssolved im the tigation water applied (0 fice Coniribation na 12 Chuo Naoi Sikenzyoe, Bogor, In

15, Ventura, W.B., andT Yoshida 1977 Amn

1 Mooded topical soil Plant Soil 46:521-831

16, Wetselaar, R T Shaw P Firth, 1 Oupathum, and H Thitipoca

1977 Armonia volatilization from variously placed ammonia Sulphate under lowland ‘ice Feld conditions in central Thailand Proc Int Seminar SEFMIA 10-14 Oct 1977, Tokyo Japan Soc of Sci, of Soil and Manure, Tokyo, Japan

17 Willis, W HL, and M.B Sturgis 1944 Loss of nitrogen feom Foose soit as affected by changes i temperature and reaction Sil Sel Sos, Am, Proc 9:105-113,

olailizaion from

‘inc, Iron, Manganese, and Copper!

W L Linpsay anp W A NorveLt?

ABSTRACT

A DIPA soil test was developed to identify near-neutral and

calcareous soils with insufficient available Za, Fe, Mn, or Cu for

inaximum yields of erops, The extractant consists of 0.0053 DTPA

‘diethylenctriaminepentaacetic acid), 01M triethanolamine, and

‘4 plZ of 7.3 The sol est consists of shaking 10 g of

air-dry soil with 20 ml of extractant for 2 hours, The leachate is

filtered, and Zn, Fe, Mn, and Cu are measured in the filtrate by

stomie absorption spectrophotometry

“The soil test successfully separated 77 Colorado soils on the basis of

crop response to Zn, Fe, and Ma fertilizers Critical mutrient levels

must be determined separately for each crop using standardized

procedures for oil preparation, grinding, and extraction The critical

levels for com using the procedures reported herein were: 0.8 ppm for

42a, 4.5 ppm for Fe, and tentatively 1.0 ppm for Mn, and 0.2 ppm for

cu

Development of the soll test was based, in part, om theoretical

considerations The extractant is buffered at pÏf 7.30 and contains

CaCl s0 that equilibrium with CaCOs is established at a CO, level

about 10 times that of the atmosphere Thus, the extractant precludes

dissolution of CaCOs and the release of occluded nutrients which are

‘ormally not available to plants DTPA was selected as the chelating

agent hecause it can effectively extract all four micronutrient metals

Factors such as pH, concentration of chelating agent, time of shaking,

and temperature of extraction affect the amount of micronutrients

extracted and were adjusted for maximum overall effectiveness

Additonal Index Words: chelates, nutrient deficiency, mieronu-

tren,

Lindsay, W L., and W A Norvell 1978, Development of a DTPA test,

for zine, iron ‘manganese, and copper Soil Sci, Soc Am J

451-428

"Ganhihulon rơm the Deparment of Agronomy, Colorado State

Published with he approval ofthe Director ofthe Colorado State Uni

‘Agri Exp Stn a Scenic Sens Paper no 2527 Spported in pty

and Ingtis, Ine Kansas Cay, Mo Received 9 Oct 197 and

30523, and Atscste Seat, Connecticut Agric, Exp Sim, New

Haven, Conn spectively

in gieo le conducted numerous studies on the reactions of metal chelates in soils (Lindsay et al., 1967; Norvell and Lindsay, 1969, 1972;

Lindsay and Norvell, 1969; Halvorson and Lindsay, 1972, 1977; Norvell, 1972; Lindsay, 1974, 1979) As these

‘theoretical and experimental studies with metal chelates advanced, we realized that diethylenetriaminepentaacetic acid (DTPA) should be useful as an extractant for si- multancous measurement of available Zn, Fe, Mn, and Cu

in soils, A procedure was developed and tested on 77 Colorado soils and was found to be successful for separat- ing soils according to their response to Zn and Fe fertilizers The method shows promise as a soil test for Mn and Cu, but because very few Mn and Cu deficiencies are found in Colorado, the procedure could not be adequately tested for these elements

The objectives of the present paper are: (i) to present the theoretical basis for the DTPA micronutrient soil test, (ii) to report the effectiveness of the test for delineating Zn and Fe responsive soils, and (iil) to discuss the DTPA soil test following its use for 10 years

THEORETIC

BASIS FOR THE DTPA SOIL

‘TEST Chelating agents offer great promise for assessing readily available micronutrient cations in soils These agents combine With Tree metal ions in solution forming soluble complexes and thereby reduce the activities of the free metal ions in solution In response, metal ions desorb from soil surfaces or dissolve from labile solid phases to replenish the free metal ions in solution The amoupt of chelated metals that accumulates in solution during the extraction is function of both the activity of metal ions in the soil

‘Gntensity factor) and the ability of the soil to replenish those ions (capacity factor), Both factors are important in determining the availability of elements to plants

In searching for an appropriate chelating agent to use in a micronutrient soil test, we chose DTPA because it offered the most favorable combination of stability constants for the si-

42 SOIL SCI SOC AM J., VOL 42, 1978

‘Table 1—Fquilibrium conditions in soils used for the theoretical

development of the DTPA soil test

‘Equilibrium resction|

‘SoikCa = Soil + Ca™ 200 Exchange oquileia of C

from soll phases plus Ca added in extracting solution indy, 1919)

‘SoibMg = Soil + Mg" =2:70 Approximated as 1/5 (Ca)

Sfter measurements ix

‘Several sil extracta Lindsay, 1978) [Norvell & Lindsay (1978) [Norvell & Lindsey (1969, 1972)

Lindsay & Norvell (1989) Norvell & Lindsay (1962, 1972)

Lindsay & Norvell (1963)

FAOH)(soll) = Fe" + SON —38.58

SoilZn'+ 2H = Zn + Soll +62

SoitCu + 2H" = Cu" + Sol +83

MnO, (pyrolusite) + 4H" +26

{The AT; corresponds to mixed equilibrium constants where all terms are ‘expressed in concentrations at 0.1 ionic strength except H’, OH, and e

Swhichare expressed as activilies,

Lindsay (1979)

multaneous complexing of Fe, Zn, Cu, and Mn Although

number of chelating agents can effectively complex Cu and Zn in

soils, DTPA was exceptional in that it was also among the best

chelating agents for Fe and Mn (Norvell, 1972) Since Fe and Za

deficiencies are most prevalent on calcareous soils, the extractant

‘was designed specifically to avoid excessive dissolution of CaCOs

with the release of occloded micronutrients, which are normally

not available for absorption by roots This objective was achieved

in part by buffering the extractant in a slightly alkaline pH range,

‘and in part by including soluble Ca’ Trethanolamine (TEA)

‘was selected as 4 buffer because of its pKa ~ 7.8 and because it

burns cleanly during flare atomization in atomic absorption

spectrophotometry

"The extracting solution that was developed consists of 0.005

DTPA, 0.01M CaCly, and 0.1M TEA buttered at pH 7.30,

Jn the extracting solution, DTPA is fully associated with Ca as

JL? and CanL~, where L'~ represents the free DTPA ligand

Approximately two thirds of the added Ca is associated with

ITPA in these chelates while the balance remains in solution as

froe Ca'* At the selected pH of 7.3, approximately thee fourths

Of the TEA is protonated and is present as HTE

‘When the extractant is added to soils, additional Ca? and some

Mg? enter solution, largely because the protonated TEA ex-

changes with Ca?" and Mg"* from the soil exchange sites This

exchange generally raises the concentration of ionic Ca’ by two:

to threcfold and aids in suppressing the dissolution of CaCO, in

cealeareous soils, For the 77 soils of this study, the concentration

of free Ca’~ averaged close to 0.01M with more than 80% of the

extracts falling between 0.007 and 0.014M At pH 7.3, CaCO,

tends to dissolve only slightly in these solutions and would reach

equilibrium, theoretically, at a COs partial pressure of ap-

proximately 10 times that in the atmosphere Typically soils

Contain slightly higher CO» levels than found in the atmosphere

Extraction of micronuirient cations from soils depends upon the

binding strength of DTPA for the various metal ions as well as the

binding strength of the soil for these ions Previously developed

procedures (Lindsay and Norvell, 1969; Norvell, 1972) were used

for computer assisted predictions of the potential complexing of

Fe, Zn, Cu, and Mn during extractions by DTPA For these

predictions, the equilibrium relationships given in Table | were

used to estimate the solubility of micronutrient metal fons and

competing alkaline carth cations, These relationships were de-

Veloped at an ionic strength of 0.1 which approximates closely

that of the DTPA extracting solution in contact with soils

The suitability of DTPA as an extractant for micronutrient

metals is illustrated by Fig 1 This plot summarizes the ability of

to chelate Fe, Zn, Cu, or Mn in competition wih Cá?" and

‘Mg in the extracting solution Each curve represents equilib-

8

= Fig 1—The equilibrium levels of Mn, Fe, Zn, or Cụ extracted by DTPA in competition with Ca®* and Mg! (See Table 1) The numbers refer to redox expressed as pe + pH

rium between a single micronutrient solid phase and the extracting solution containing DTPA and the two alkaline earth cations The discontinuities in slope of these curves between pH 7.3 and 7.4

‘occur at the point above which Ca** solubility is depressed by CaCO

“The predicted chelation of Fe** is excellent below pH 7 and is still substantial at pH 7.3 Unlike most other chelating agents, DIPA is able to apply a moderate stress to solubilize soil Fe at a

pH where CaCOs Is not dissolved Chelation of soil Zn and Cu is Expected to he excellent over a wide pH range and should place considerable stress on labile forms of these micronutrients, Chelation of Mn by DTPA in the extracting solution is more difficult to predict because itis redox dependent (Lindsay 1978: Norvell and Lindsay, 1972), The curves in Fig 1 suggest that Mn‘? in equilibrium with MnO, (pyrolusite) would be chelated best at low pH and under reducing conditions such as represented

by pe + pH values of <17, Much less but still some potential for complexing of Mn exists in the slightly alkaline pH range under

‘oxidizing conditions as represented by pe + pH = 18

‘During actual extractions, the predicted levels of chelation shown in Fig 1 are never achieved because many of the reactions require weeks or months for equilibrium to be reached (Norvell

‘and Lindsay, 1972), and because the supplies of most extractable micronutrient metals in soils are very small relative to the DTPA used, With a 2:1 solution-to-soil-ratio, the capacity of DTPA to complex each of the micronutrient cations (expressed in terms of parts per million of metal on a dry weight soil basis) is 10 times its Atomic weight and ranges from 550 to 650 ppm depending on the

‘micronutrient cation (Table 2) Only Fe was present in sufficient

‘quantity to saturate the DTPA For the 77 Colorado soils in our Study there was enough total Fe on the average to saturate the chelate 50 times, In contrast, there was enough total Zn and Cu in these soils to occupy only 9% and 2.5%, respectively, of the total DTPA present On the average, there was sufficient total Mn (0

‘occupy 61% of the DTPA During actual extractions of the 77 Colorado soils, an average of only 3.5% of the chelating agent

‘was occupied hy the four micronutrient cations (Table 2) Thus DTPA is present in excess of the micronutrient metal cations thet are normally solubilized during an extraction This excess reduces the possibility that the extraction of one micronutrient might significantly affect the amounts of other metals extracted

MATERIALS AND METHODS

Soils Seventy-seven surface soils were collected in Colorado 10 represent the major agricultural arcas of the state Both Zn and Fe responsive and nonresponsive soils were included The soils were

LINDSAY & NORVELL: DIPA SOIL, TEST FOR ZN, FE, MN, AND CU 423

‘Table 2—The capacity of the DTPA soil test to extract micronutrient cations compared to the average total eontent in 77 Colorado sơils,

‘the average levels actually extracted from these soils, and the critical nutrient levels reported herein

mon soil dry weight ‘Asa fraction of total DTPA present

‘Average DTPA-extractable content in 77 Colorado soils 116 84 109 08T C0008 0011s 00188 00014 (Critical nutrient level for cnen reported herein os 45 1Ú 080 00012 00080 00018 00003

air-dried, crushed with a wood mallet, and sieved thưough 4 6-mm

nylon screen, Precautions were taken to avoid contamination

during sampling, drying, crushing, and storage For the laboratory

analyses a Fepresentative 2-kg subsample of each soil was further

pulverized with a wooden rolling pin and screened through &

-mm stainless steel sieve

Forty-two of the soils were collected in 1964 and are the same

soils used to develop the EDTA-ammonium carbonate soil test for

‘Za (Trierweiler and Lindsay, 1969) These soils were sclected 10

represent areas where both Za-deficient and nondeficient crops

were observed and are referred 10 as the 64-series The remaining

35 soils were collected in 1965 and were selected to include areas

where crops showed Fe, Zn, and possibly Mn deficiencies They

are referred to as the 65-seris

Characteristics of the 77 soils used to develop the DIPA

‘micronutrient soil test are summarized in Table 3 These soils are

typical of arid regions, being low in organic matter with a mean of

1,6% Most of the soils are alkaline with only 9 of the 77 soils

having pH values below 7.0 Sixty-two of the soils were

calcareous with pH values above 7.3 and with CaCOs contents

ranging from a trace to 25% having a mean of 5.0% The mean

and median values for various soil parameters are typical of many

soils in Colorado (Follett and Lindsay, 1970)

Greenhouse Study

‘The 64-series soils received the following fertilizer treatments:

@ low P, no Zn; (i) low P, 5 ppm of Zn; (ii) high P, no Zn; and

Gy) high P 5 ppm Zn Soils with < 22 ppm of NaHCO:

extractable P (Olsen ct al., 1954) were considered deficient in P;

therefore suflicient P was added to give this minimum, based

upon soil textere and previous experience with Colorado soils

This level of P was designated the low P treatment The high P

‘teatment consisted of an additional 75 ppm of P Details of the Za

and P treaiments, supplemental fertilizers, greenhouse manage-

ment, harvesting, and evaluation of Zn response on the 64-series

soil fave been reported (Trierweiler and Lindsay, 1969),

The 35 soils belonging to the 65-series were cropped in the

greenhouse to determine the need for Zn, Fe, and Ma fertilizers

The fertilizer treatments consisted of: (i) Check, Gi) Fe, (i) P,

iy) PFe, (¥) PZn, (vi) PZnEe, (vii) PZnMn, and (viii) PZnMaFe

“The rates and sources of fertilizer additions were: $ ppm of Fe as

FeEDDHA (ferric ethylenediamine di(o-hydroxyphenylacetate))

5 ppm of Zn as ZnSQ.- 7H:0, 75 ppm of P as Ca(H;PO,);-H0,

and 1S ppm of Ma as MnSO, -H-O using reagent grade chemi:

cals Airdry soil, equal to 2 ke of oven-dry soil, and the

fertilizers were mixed in a twin-shell blender and placed in

polyethylene-lined cardboard cartons, The treatments were repli

ated vice Sorghum [Sorghum bicolor (L.) Moenche) variety

RS 610 was used as the test crop Adequate N and $ were applied

{o prevent their being deficient The plants were harvested 7

‘weeks after planting

DTPA Extracting Solution

‘The DTPA extracting solution was prepared to contain 0.005M

DIPA, 0.01M CaCl, 0.1M TEA and was adjusted to pH 7.30

‘To prepare 10 liters of this solution dissolve 149.2 g of reagent

grade (HOCH.CH3)sN (TEA), 19.67 g of diethylenetriaminepen-

faacetic acid (DTPA), and’ 14.7 g of CaCl: 2H,0 in ap-

proximately 200 mi of distilled water Allow sufficient time for

‘Table 3—Characteristics of the 77 Colorado soils used to develop

the DTPA micronutrient soil test

Range

+ Bxcludes soils 65-42 and 65-48 which were old orchard soils containing 21.8 and? ppm of DTPA-extractable Cu, respectively the DTPA to dissolve, and dilute to approximately 9 liters Adjust the pH to 7.30 * 0.05 with 1:1N HCI wale stirring and dilute 10

10 liters This solution is stable for several months (Another commonly designated name and formula for DTPA is [(carboxy

‘methy))imino}bis (ethylenenitrlo)tetraacetic acid with the formula [(IOCOCH,:NCH,]:NCH:COOH with a formula weight of 393.35)

Extracting Procedure Ten grams of airsiried soil was placed in a 125ml conical flask, and 20 ml of the DTPA extracting solution was added Each flask was covered with stretchable Parafilm and secured upright

‘on a horizontal shaker with a stroke of 8.0 em with a speed of 120 coyeles/min After 2 hours shaking, the suspensions were filtered

by gravity through Whatman no 42 filter paper The filtrates were analyzed for Zn, Fe, Mn, and Cu using stomic absorption spectrophotometry and appropriate standards

RESULTS AND DISCUSSION Soil Test for Zn

‘The effectiveness of the DIPA soil test for separating soils based on crop response to Zn fertilization is sum marized in Fig 2 and 3 In Fig 2 are represented the 42 soils belonging to the 64-series that were cropped in the greenhouse with corm Zea mays L.) by Trierweiler and Lindsay (1969) The open bars represent soils that gave no growth response to Zn fertilization in the greenhouse and were considered to supply adequate Zn The black bars represent soils that gave a significant yield response (>95% probability) at the low P level while the crosshatch bars represent soils that showed @ significant response to Za when 75 ppm of P was added Soils were classified as Zn- deficient if com responded to Zn at either level of added P

A DTPA-extractable Zn level of 0.8 ppm separated the 10 soils with adequate available Zn from the 32 soils where a response to Zn fertilization was obtained

424 SOIL SCI SOC AM J., VOL 42, 1978,

ORN RESPONSE TO ZN

4 16

=

gi

Š Gg| _._ _._ S8MIGA, ĐH 2N LEYEL _

#

Oã ân San ve Sy a7 9 8 ae

SOIL NO

Fig, 2—Separation of Zn deficient and nondefcient soils with DTPA soll test from greenhouse responses on 42

‘of Za and P

| SORGHUM RESPONSE TƠ 2N

BOARDERLINE

61 72 6 a8

ặ

SOIL NO

freenhiouse with sorghum,

In Fig 3 are represented the 35 soils of the 65-series in

which sorghum was the test crop The critical level of

DTPA-extractable Zn, in this case, was estimated as 0.6

ppm reflecting a slightly lower sensitivity of sorghum to

‘Zn-deficiency compared to com The crosshatched bars

identify soils in which yields of sorghum were not in-

creased significantly by Zn, but mild visual symptoms of

Zn deficiency were noted and the Zn content of plant tops

was <10 ppm The black bars and the open bars represent

clearly identified Zn-deficient and nondeficient soils, 1e-

spectively Figures 2 and 3 show that the DTPA soil test

‘was effective in separating soils on the basis of plant re-

sponse to Zn,

3Effectiveness of the DTPA micronutrient soll test for separating 35 Zn deficient and nondeficient sols of series 65 as determined in th

Soil Test for Fe For the 35 soils of the 65-series, a critical level of 4 ppm DTPA-exiractable Fe separated the responsive fror the nonresponsive soils in the greenhouse (Fig, 4) Some ¢ these soils showed borderline response in that dry matte yields were not increased significantly at the 54 level wit two replications, but the plants showed mild visual de ficiency symptoms of Fe Soils with <2.5 ppm of DTPA extractable Fe can be expected to cause Fe deficiency i sorghum Soils between 2.4-4.5 ppm appear borderline Sorghum will likely show visual deficiency symptoms b may or may not show significant yield increases from F fertilizer Soils with >4.5 ppm are not expected to shor

LINDSAY & NORVELL: DTPA SOIL TEST FOR ZN, FE, MN,

JORGHUM RESPONSE

14 @ OEFICENT

SOIL

Fig 4—Effectiveness of the DTPA micronutrient s

‘Cropping in the greenhouse with sorghum

Fe-deficiency Comparison of visual symptoms of Fe

deficiency observed in the field with soil test results

confirms the applicability of these soil test levels for Fe

(W L Lindsay, unpublished data)

Soil Test for Ma

Manganese fertilizer treatments were also included in the

greenhouse study of the 35 soils of the 65-series None of

these soils showed a response to Mn confirming previous

experience that Colorado soils contain adequate Mn for

crops The DTPA-extractable Mn in these soils ranged

from 1.2 to 32 ppm suggesting that the critical level was

<1.2 ppm

In order to characterize further the possible usefulness of

the DTPA soil test for Mn, a correlation was made between

DTPA-extractable Mn and hydroquinone-ammonium ace-

tate-extractable Mn (Adams, 1965) The correlation cocfii-

cient was 0.80 with a regression equation of y = 10x Thus

the DTPA soil test gives similar information to the

hydroquinone-ammionium acetate extraction except that

only about 0.1 as much Mn is extracted by DTPA, which

may be advantageous Tentatively the critical level of

DTPA-extractable Mn is set at 1.0 ppm until further

information is available

Soil Test for Cu Copper deficiencies in crops in Colorado ate not known,

Recently Proskovec? found that 8% of 443 soils from the

Golden Plains area of easter Colorado had <0.2 ppm of

DTPA-extractable Cu, Selected soils were cropped in the

greenhouse, and although no responses were obtained, soils

with low extractable Cu produced crops that were very low

in Cu Earlier, Follett (1969) obtained two Florida soils that

"B,J Proskovee 1976, Potent

soils: MS Thesis, Dep of Agronomy, Colorado

Catlins, Cola

copper deficiency in easter Colorado

State Univesity, Fr

GIANG

73561 Ôn” GÀ gi

AND cu

Pid ed eee test for separating 35 Fe deficient and nondeficient soils of series 65 as determined by

were known to be Cu deficient Both soils contained 0.18 ppm of DTPA-extractable Cu On the basis of these results

a critical level of 0.2 ppm for Cu is tentatively proposed, None of the 77 soils used in this study had <0.2 ppm DIPA-extractable Cu

Experimental Variables Experimental conditions influence the amount of Zn, Fe,

Mn, and Cu extracted by DTPA The effects of shaking time, DTPA concentration, pH, and temperature were studied with five soils selected to provide a representative range of extractable metals

Effect of Shaking Time—The effect of shaking time was examined at 1, 3, 8, and 16 hours, The results reported as averages of duplicate analyses are given in Table 4

‘Table 4—The effect of shaking time on the DTPA-extractable

‘Zn, Fe, Ma, and Cu and from’ test

426 SOIL SCI SOC AM J., VOL 42, 1978

‘Table 5—Effect of different concentrations of DTPA on the

extractable Zn, Fe, Mn, and Cu in5 test soils, ‘Table 6—Effect of pH on the DTPA-extractable Za, Fe, Mn, and ‘Cuin5 test soils and final pH after extraction

A 20 08 0280 U34 28 88 92 030 0ẠI

B os 20 om 088 os 09 41 49 048 060

¢ os: 20 ĐT 035 20 lộ 103 96 09% 085

D 05 20 o4g oat lê 33 93082 94 O82

Increasing the time of extraction increased the quantity of

cach micronutrient extracted in the order Mn>Fe>Cu>Zn,

but the rate of release decreased sharply after the first hour,

especially for Zn, Cu, and Fe, Causes for the continuing

dissolution of Mn with time are not known but may reffect

some chemical reduction accompanying the longer shaking

periods Examination of duplicates indicated that the

extractions and analyses were satisfactory

‘An extraction time of 2 hours was chosen This insured

that the initial rapid dissolution of the micronutrient would

be complete and slight variations in time of preparation

and filtration would not be critical

Concentration of DTPA—The concentration of DTPA

was varied from 0.5 to 20 x 10-3M Higher chelate

concentrations obviously put greater stress on supplies of

labile metals Increasing the concentration of DTPA in-

creased the extraction of all four micronutrient cations in

the order of Mn>Fe>Cu>Zn (Table 5)

A DTPA concentration of 5 x 10°7M was selected This

concentration provides ample chelating capacity to remove

measureable amounts of all four micronutrient metals and

provided sufficient excess to prevent competitive secondary

interactions among the metals extracted, Results in Table 5

also confirm the sensitivity of Mn chelation to Ca’!

competition shown earlier (Norvell and Lindsay, 1972) At

20 x 103M DTPA, the Ca** was not adequate to control

the activity of DTPA at a low and relatively constant level

‘AS a result, the chelation of Mn increased sharply and

erratically

Effect of pH —The effect of pH of the DTPA extracting

solution was studied on the five test soils The initial pH of

the extractant was set at 7.0, 7.3, 7.6, or 7.9 Changes in

pH during a 2-hour extraction and the effect of pH on the

extractable Zn, Fe, Mn, and Cu are summarized in Table 6

‘There was a tendency for the pH to shift upward by 0.1 to

0.3 units during extraction This undoubtedly resulted from

some exchange of HTEA™ (the conjugate acid of the TEA

buffer) for soil cations and also from slight dissolution of

CaCO

pHof — pHafter

‘Testsoil extractant extraction 7n Fe Mm Cu

a

13 TẾ 155 186 048 051 37 17 103 81 100 100

Increasing pH from 7.0 to 7.9 had little effect on the amount of Zn and Cu extracted from the test soils, The amounts of Mn (and especially of Fe) extracted decreased, dramatically with increase in pH

‘A pH level of 7.30 was selected for the DIPA soil test from both a theoretical and practical basis This pH is within the natural range of near-neutral and alkaline soils, and affords rapid equilibrium with CaCOs at a CO, partial pressure approximately 10 times that of the atmosphere At higher pH values 100 little Fe is dissolved to measure conveniently while at lower pH values, too much Fe and

Mn are potentially extractable, and CaCO, is unstable Effect of Temperature—The effect of temperature during the 2-hour extraction period was studied at 15°, 25°, and 35°C Increasing temperature clearly increased extractable micronutrients (Table 7) A 10°C increase in temperature fon an average increased extractable Zn by 15%, Cu by 24%, Fe by 30%, and Mn by 54% The importance of

‘temperature control in the soil testing laboratory is clearly evident A temperature near 25°C was adopted for these studies

RESULTS WITH THE DTPA SOIL TEST DURING

THE PAST 10 YEARS Ten years have elapsed since the DTPA soil test was first proposed (W L Lindsay and W A Norvell 1967 A new soil test for the simultancous determination of available zine and iron Meeting of the Wester Soil Science Society Los Angeles, Calif., June 1967) During this period numerous laboratories have used the method to estimate the availability of micronutrient and heavy metals in soils Let

us consider some of the findings

Brown et al (1971) examined 92 California soils for extractable Zn and tested each soil in the greenhouse for plant response to Zn fertilization Using a critical level of 0.5 ppm of extractable Zn, the DTPA soil test was 83% effective in identifying soils on whi

428

using other standardized procedures, This finding affords

some degree of flexibility for conditions so long as their

procedures are consistent, and critical levels are adjusted

accordingly

CONCLUSIONS,

‘The DTPA soil test is based on sound principles of soil

and chelation chemistry The test is designed to extract

simultaneously plant available Zn, Fe, Mn, and Cu in near-

reutral and calcareous soils, Our results and those of

numerous other laboratories indicate that the soil test has

been successful The soil test has been used most exten-

sively and successfully for identifying soils low in available

zinc, Fewer laboratories have reported use of the soil test

for estimation of available iron, but these results too are

highly encouraging Further, by including soil pH in

interpretating the results, the soil test may also prove useful

for estimating metal availability in acid soils, well below

the pH range for which the test was originally designed

Standardization of sample preparation and extraction

procedures are absolutely essential Our results and others

demonstrate clearly that chemical and procedural variables

have a marked influence on the amounts of metals ex-

tracted Laboratories should be consistent in their method-

ology and if possible establish critical levels of extractable

micronutrients by calibration with soils of known m

cronutrient availability

LITERATURE CITED

Macizon, Wit

Soi, Sos Am Br 361521625,

Sine eadmumennched sewage slags: 1 Eavaon Qui

Xöân_ II

4 Brown, A L., James Quick, and J L Edings 1971 A comparison

Of anaiytical "methods for” soil zing Soil Sci Soc Am Proc

3%:105- 107

5, de Boer, G.J., and H M, Reisenauer 1973 DTPA as an extractant

Of available soil iron, Commun, Soil Sei Plant” Anal

40) 121-128,

6, Follet, R H 1969, Za, Fe, Mn, and Cu in Colorado sols Ph.D

Thesis, Colorado State University, Fort Collins Univ Microfims,

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