Ammonium Fixation by High-Charge Smectite in Selected Texas Gulf Coast SoilsCy-Chain Chen, Fred T. Turner,* and Joe B. Dixon

ABSTRACTThe Beaumont (fine, montmorillonitic, thermic Entic Pelludert)

and Lake Charles (fine, montmorillonitic, thermic Typic Pelludert)soils along the Texas Gulf Coast produce only about 2 Mg rice(Oryza saliva L.) ha ' without N fertilizer, while the Nada soil (fine,montmorillonitic, thermic Typic Albaqualf) frequently produces 5Mg ha"'. In studying differences between these soils, data showedthat NaCl applied to the Beaumont soil did not reduce rice yield,but equivalent amounts of KC1 did. The KCl-induced yield reductionmay have been the result of NHJ entrapment in clay minerals causedby added K. Clay mineral characterization showed that the Beau-mont soil fixed more NHJ than the Nada soil because the Beaumontsoil was higher in soil K, high-charge smectite [i.e., 0.76 equivalentsper (Si,AI)4O10(OH)2l and charges in the tetrahedral sites. The 8-wk incubation of Beaumont soil in the rice root zone resulted inpartial release of added NhJ and no release of native NH; when theBeaumont soil had been Ca saturated. The K-saturated Beaumontsoil did not release fixed NhJ during incubation as the Ca-saturatedsoil did. The Lake Charles soil showed clay and fixation character-istics similar to the Beaumont soil, while the Nada soil did not fixbeyond its native level or release any upon incubation. The presenceof 2:1 layer silicates in Beaumont and Lake Charles soils with x-ray diffraction characteristics similar to smectite, and NHJ fixationcharacteristics similar to vermiculite, was recognized.

DIFFERENT N RESPONSE characteristics have beenobserved in rice yield in the soils of the Texas

Gulf Coast. Without fertilizer N, the clayey soils pro-duce 1 to 2 t ha~' while some of the sandier soils canproduce 5 t ha-1. Typical clayey soils are Beaumontand Lake Charles. The coarser textured soils in theregion, known to supply a significant amount of N torice crops, are typified by the Nada sandy loam. Un-derstanding native soil N and the fate of fertilizer Nin these soils is a crucial step toward increasing Nfertilizer efficiency and lowering production costs.Sowden (1976) observed that the nitrification of thefixed NH;} was slower than that of the readilyexchangeable NH} under laboratory conditions. Prac-tically all fixed NH} from fertilizer was released fromthe clay after about a year. The native fixed NH} inthe surface soil of the cropped area was found to belower than that of the uncropped area or that of thesubsoils, suggesting that cropping reduced the amountof the fixed NH} in several typical soils of EasternCanada (Sowden et al., 1978). Allison et al. (1953)reported a 7% recovery of the total fixed NH} by mil-let, Setaria italica (L.) P. Beauv., grown in a green-house in six different illitic, vermiculitic and/or mont-morillonitic soils. Keerthisinghe et al. (1984) labeledfixed NH} with I5N and placed packages of suchtreated soils containing vermiculite and montmoril-lonite in the root zone of rice. The nonexchangeableC.C. Chen, Energy and Mining Res. Organization, Industrial Tech-nology Res. Inst, Taiwan, Republic of China; F.T. Turner, TexasAgric. Exp. Stn., Rt. 7, Box 999, Beaumont, TX 77713; and J.B.Dixon, Dep. of Soil and Crop Science, Texas A&M Univ., CollegeStation, TX 77843. Received 26 Apr. 1988. Corresponding author.

Published in Soil Sci. Soc. Am. J. 53:1035-1040 (1989).

NH} released from clay upon rice growth ranged from0.7 to 3.4 mmol kg"1 in the labeled soil. It appears thatnewly fixed NH} from fertilizer is more available toplant uptake. Native fixed NH} does not have signif-icance in plant nutrition and is depleted graduallyupon successive croppings of cereals in the field ac-cording to Black and Waring (1972).

Apparently the factors influencing the availabilityof fixed NH} are many. Different crops may have dif-ferent capabilities to compete with clay minerals forthe absorption of NH}. Properties of the clay mineralsin the soil, as discussed in the previous section, maybe the factors influencing the amount of NH} that isheld tightly in the interlayer and is truly unavailableto plants. The presence of K+ and organic compoundsreleased from plant roots may be a third factor con-trolling the availability of the fixed NH}. Ammoniumfixation by clay minerals may limit N availability inthe clayey soils relative to coarse textured soils. Thisstudy examined the relationship between NH} fixationand clay mineralogy by: (i) field experiments with in-duced NH} fixation, (ii) soil clay characterizations,and (iii) the release of artificially fixed NH} under fieldconditions.

MATERIALS AND METHODSBeaumont clay, Lake Charles clay, and Nada sandy loam

were studied, with emphasis given to the contrasting char-acteristics between Beaumont and Nada. Two other soils,Edna loam (Vertic Albaqualfs, fine, montomorillonitic,thermic) and Katy fine sandy loam (Aquic Paleudalfs, fine-loamy, siliceous, thermic), were included in the originalstudy (Chen, 1985). These two soils have N-supplying char-acteristics similar to those of the Nada soil and will be dis-cussed in this paper.

Field ExperimentsOnly the data from field experiments in Beaumont and

Nada soils will be included in this paper. Field experimentswere conducted on Beaumont soil located at the Texas A&MUniversity Agricultural Research and Extension Center nearBeaumont, TX, during 1982-1983 and on the Nada soil atEagle Lake, TX, during 1983-1984, and at El Campo, TX,in 1984.

In 1982, N at 90 kg ha"1 as (NH4)SO4 was applied to allrice plots approximately 6 wk after seeding. Potassium asKC1 was applied with (NH4)2SO4 at 176 and 352 kg ha-'. Anunbalanced (variable number of replications among treat-ments) experimental design was used due to limited fieldarea availability. Yield data were obtained after harvestingand weighing of the air-dried rice grain samples.

In 1983, the common N rate for all treatments in theBeaumont soil was 135 kg ha'1 as (NH4)2SO4 and the Ktreatments included rates of 190 and 380 kg ha~'. Sodiumas NaCl was applied at rates of 112 and 224 kg ha-'. TheNada soil received the same K and Na treatment as theBeaumont soil, however, without N fertilizer. The purposeof the Na treatment was to determine whether K inducedN-uptake reduction was caused by entrapment of NH4 incollapsed layer silicates (Nommik, 1957, 1981) or by re-duced NHJ adsorption at root surfaces in the presence of K(Lucking et al., 1983). Sodium ions will not collapse clay

1035

1036 SOIL SCI. SOC. AM. J., VOL. 53, JULY-AUGUST 1989

interlayers that result in NHJ fixation, but will compete withNHj for adsorption at root surfaces. Unlike the first year(1982), all treatments were installed preplant and were in-corporated into soil with a cultivator. There were four rep-lications for the Beaumont soil and five for the Nada soilin a randomized complete block design. Identical experi-ments were conducted with the Nada soil at Eagle Lake andEl Campo in 1984 except six replications were used.

Soil and Clay AnalysesClay Mineral Separation and Identification

Fine clay (<0.2 /tm) and coarse clay (2-0.2 /urn) fractionswere individually separated from Ap horizons of the Beau-mont, Nada, Katy, Lake Charles, and Edna soils using pro-cedures described by Jackson (1974). Carbonates, organicmatter, and Fe and Mn oxides were removed using N Na-acetate solution adjusted to pH 5,30% H2O2, and dithionite-citrate-bicarbonate (DCB) treatments, respectively, usingJackson's (1974) standard procedures. Fractionation of theclay separates was achieved by repeated centrifugation. Theclay fractions were then dialyzed to remove interstitial salt,then freeze-dried. Coarse and fine clay contents were deter-mined for each soil by weighing the freeze-dried fractions.

Fifty milligrams of each clay fraction were saturated withMg or K by washing five times with Cl solutions followedby 95% ethanol washings to remove interstitial salt. The Mg-saturated samples were treated with 10% glycerol dispensedfrom a spray can before x-ray diffraction (XRD) analysis.The K-saturated samples received sequential XRD analysisafter drying at 298 K and each 2-h heating at 573 and 823K. A North American Philips XRD unit (Philips ElectronicInstruments, Mahwah, NJ) with a graphite monochrometerwith CuK a radiation was used.

Layer Charge CharacterizationThe alkylammonium method for layer charge density de-

termination (Ruehlicke and Kohler, 1981) was used to char-acterize the soil smectites. Charge location (octahedralcharge vs. tetrahedral charge) was determined by theGreene-Kelly (1955) method where Li+ is assumed to neu-tralize octahedral charge on heating at 493 K for 24 h. Thepercentage of smectite with tetrahedral charge was calcu-lated from the 1.0-nm XRD peak area reduction upon glyc-erol treatment after Li saturation and heating. The 1.0-nmpeak area was normalized against the 0.7-nm kaolinite peakarea to adjust for the overall intensity loss due to the ad-dition of glycerol (Chen, 1985).

Extractable Soil PotassiumAmmonium acetate extractable K in the Beaumont and

Nada topsoil (Ap horizon) represented the exchangeable Kand was obtained by three separate 15-min shakings of soiland 1 M NH4OAc followed by nitrations. The filtrate wasmeasured by K+ using atomic absorption analysis.

Ammonium Fixation and ReleaseAmmonium Treatments

A procedure modified from that of Keerthisinghe et al.(1984) was employed to study the release of artificially addedfixed NH$ under field conditions. Beaumont, Lake Charles,Katy, and Nada topsoils were used in this experiment. Threereplicated samples of approximately 100 g each were satu-rated with 0.5 M 15N depleted (NH4)2SO4. ExchangeableNHj and interstitial solution NHj were removed by washingthe wet samples three times with 0.5 M CaCl2. For the Beau-mont and Nada soils, an additional three replications were

prepared by the same procedures except the washing wasdone with 1 M KC1. All samples were then washed withdeionized distilled water until dispersion began. Water wasremoved from the thick suspensions by vacuum filtrationthrough Buchner funnels lined with Whatman no. 1 filterpaper. Air-dried samples were in the form of round cakesapproximately 10 cm in diam. and 1 cm in thickness. Eachcake was enclosed in Whatman no. 1 filter papers and placedin a bag made of fine nylon gauze approximately 25 meshcm"1.

Field IncubationThe wrapped soil cakes were placed in a vertical position

between two rows of rice plants in the control plots of thefour soils used in the 1984 field experiments. The top of thecakes was about 5 cm below the soil surface. Incubationbegan approximately 8 wk before harvest and after the riceplants had already developed a full root system. The soilsamples were recovered from the field at harvest and freeze-dried.

Laboratory AnalysesThe incubation samples were analyzed for total fixed

J using the procedures described by Silva and Bremner(1966). Untreated soils and NHj-treated soils were analyzedin duplicate. The titrated distillates, the end products of thequantitative NHj determination, were concentrated by air-drying and then analyzed for 15N/I4N ratio. Mass spectrumanalysis was performed by Isotope Services, Los Alamos,NM.

RESULTS AND DISCUSSIONField Experiments

In the 1982 Beaumont experiment (Fig. la), in-creases in K rates caused proportional decreases inrice yields. Although the higher K treatment yieldedapproximately 1000 kg ha"1 lower than the N onlytreatment, it could not be concluded that the yieldreduction was due to NHJ fixation by clay minerals.It is possible that the competition between K andNHJ could result in N uptake reduction as suggestedby Leuking et al. (1983).

8^x

Tffl 7

0 6

O0 5O

C 4TJ.1 3^0> 2OE 1

O

1aBeaumont, '82-..

.a

-

c

I I

aj>b"~

1bBeaumont, '83

a,b

c

~ b 0|*»

a a*

1CNada, '83 & '84

b^

a

b b b b

N 0 90 80 9OK O O 176 352Na O O O 0

0 135135135136136O 0 100380 0 00 0 0 0 112224

Treatments (kg ha"1)

0 135 O O 0 0O 0 190380 0 00 0 0 0 112214

Fig. 1. Rice yield in Beaumont and Nada soils. Data in Fig. Ic areaverage of three separate experiments at Eagle Lake, TX, in 1983and 1984, and at El Campo in 1984. Yield bars with the sameletter within a given year are not significantly different at the 5%level of probability according to Duncan's multiple range test.

CHEN ET AL.: AMMONIUM FIXATION IN TEXAS GULF COAST SOILS 1037

In the 1983 Beaumont experiment (in spite of theincreased number of replications), the K and Na treat-ments were not different from the N only treatmentat 5% a level, although the 112 kg ha~' Na treatmentyielded significantly higher than that of the 190 kg ha"1

K treatment (Fig. Ib). This observation appeared tosupport the NHJ fixation explanation for yield reduc-tion, because the large hydrated Na ion would notcollapse the clay interlayer and entrap NHJ, but wouldpossibly displace NHJ from the interlayer. The hy-pothesis that yield reduction is caused by competitionfor adsorption sites at the root surface is not supportedby these results. These results establish that K addi-tion (176 to 380 kg K ha*1) to the Beaumont soil canreduce rice yield, probably because of NHJ fixation byclay minerals as suggested by Nommik (1957, 1981)for other soils.

The same K and Na treatments described for the1983 experiments on Beaumont soil did not yield sig-nificant differences on the Nada soil (Fig. Ic). At the5% level the means of all K and Na treatments werenot different from the control, suggesting that NHJfixation by clay minerals is insignificant in the Nadasoil. The high native N supply of the Nada soil wasclearly illustrated by the high yield of the control plots(5542 kg ha"1) as compared to the Beaumont soil. Ad-ditional N fertilizer, however, did cause a significantyield increase of 1523 kg ha"1 on the Nada soil.

Clay Mineral CompositionsThe Nada soil clay contains only kaolinite and

smectite as identified by XRD (Table 1). A largeamount of quartz is present in the coarse clay. TheKaty fine clay fraction has abundant fine kaolinite andsmectite. A small amount of halloysite is present. TheKaty coarse clay fraction was dominated by quartz. Asmall amount of mica is present in both clay fractionsof the Edna soil and the coarse clay fractions of theBeaumont and Lake Charles soils. Vermiculite hasbeen reported in the Beaumont soil (Texas Agricul-tural Experiment Station, 1971) but was not identifiedin this study. It is, however, present in the LakeCharles coarse clay in a very small amount. In general,2:1 type (mainly smectite) clays are the most abundantlayer silicates in the Beaumont and Lake Charles soils,and 1:1 type clays predominated in the Nada and KatyTable 1. Weight percentage and mineralogical composition of soil

clays.

Soil

Beaumont

Edna

Katy

LakeCharlesNada

Fraction

2-0.2<0.22-0.2<0.22-0.2< 0.22-0.2< 0.22-0.2<0.2

Claycontent

13.540.43.5

13.52.13.69.2

37.33.44.2

Layer silicatesf

Kl > Mi = SmSm > Kl

Kl > Mi > SmSm > Kl > Mi

Kl> SmKl > Sm > HI

Kl = Mi > Sm > VmSm> KlKl > SmKl = Sm

Othersf

Qz, An, Ru

Qz, An, Ru, Fs

Qz, An, Ru,Qz

Qz, An, Ru, Fs

Qz, An, RuQz

soils. The Edna soil appears to have some of both the2:1 and the 1:1 types of clay.

Vermiculite and mica (Douglas, 1977; Fanning andKeramidas, 1977) are usually considered to be K andNHJ fixing minerals. Their presence or absence maybe one of the reasons why the soils differ in degree ofNHJ fixation. The native fixed NHJ level does not,however, appear to be solely a function of the presenceof mica and vermiculate. Since mica has only limitededge sites to fix NHJ, and vermiculate is present onlyin the Lake Charles soil, it is believed that smectitemay be the main NH^-fixing phase in the soils westudied. This is supported by the extensive occurrenceof native fixed NHJ in all five soils (Table 2). Regres-sion of native fixed NH$ vs. clay content yields an R2

of 0.97 among the five topsoils.

Soil Smectite Charge DensitiesOnly smectite from the Katy soil exhibited a clear

plateau of bilayer alkylammonium in the d-spacing vs.alkylammonium C number diagram, which suggests auniform charge distribution (Fig. 2a). Both Nada andBeaumont soil smectites failed to form a clear plateau,thus indicating a heterogeneous charge distribution(Fig. 2b and c, respectively). For these two soil smec-tites, bilayer alkylammonium structure is identifiedbased on the theoretical d-spacing of 1.77 nm. To en-hance the accuracy of the data, an average was takenover the charge density values calculated from the ran-domly interstratifed bi- and tri-layer structures (La-galy and Weiss, 1976). Table 3 summarizes the datapoints used and the results of such calculations.

The d-spacing vs. C number diagrams for Edna andLake Charles soil smectites are very similar to that ofthe Beaumont soil. The average values of charge den-sities for all the soil smectites vary within a narrowrange (0.69-0.84). The differences are small consid-ering the possible inaccuracy in the determinations.All measured charge densities are in the range gener-ally considered typical of vermiculite (0.6-0.9) as re-ported by the SSSA, Subcommittee S801-68-DivisionS-9 (1969). Additional evidence of high layer chargein the Nada soil smectite may be seen in the datapoints for C number 11 and higher (Fig. 2b). Thesepoints agree with the equation, d-spacing (nm) =1.1+ 0.098n of Lagaly (1982), representing a paraffintype structure of alkylammonium molecules in ver-miculite interlayers. Therefore, these samples containhigh-charge smectites based on their 1.8-nm XRDspacing upon glycerol treatment and their layer charge

Table 2. Fixed NHJ levels (mg kg"1) as influenced by chemical treat-ment and field incubation.

SoilBeaumontNadaLake CharlesKaty

Native148ft52g

148f54g

NCa251c

57g303a43g

TreatmenttNCal172e54g

210d50g

NK

267b54g

NKI263b,c

54g

t Kl = kaolinite, Mi = mica,miculite, Qz = quartz, Ru

Sm = smectite, HI = halloysite, Vm = ver-rutile, An = anatase, Fs = feldspar.

f Native = not treated, NCa = NHJ saturation followed by Ca saturation,NCal = NCa followed by 8-wk field incubation, NK = NHJ saturationfollowed by K saturation, NKI = NK followed by 8-wk field incubation.

$ Means followed by the same letter are not significantly different at the 5%level of probability according to Duncan's multiple range test.

1038 SOIL SCI. SOC. AM. J., VOL. 53, JULY-AUGUST 1989

3.0 _ 2a. Katy

2.5

O)_c'5« 2.0O.

CO•o

1.5

_ 2 b. Nada

- 2c. Beaumont

678 0 1 0 1 1 1 2 1 3 1 4 1 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 6 7 8 9101112131415161718

Alkylammonium Carbon Number (n)Fig. 2. Soil smectite d-spacing variation as a function of saturating alkylammonium carbon chain length. Solid dots = approximate integral

layer spacings. Open dots = nonintegral spacings.

density. The content of high-charge smectite in theBeaumont and Lake Charles soils is much greater thanthat in the other three soils, which accounts for thegreater capacity of the former to fix NHJ.

The percentage of smectite containing tetrahedralcharge estimated by the Greene-Kelly procedureshows a decreasing order of Lake Charles > Edna >Beaumont > Nada > Katy. Since tetrahedral (—)charge is believed to have a greater attraction for in-terlayer cations than the octahedral charge, these datamay indicate another factor that causes the clayeysoils to be NH^-fixing.

Mineralogical analyses suggest that the five soilsmay be divided into three categories with regard toNHJ fixation potentials. The Beaumont and LakeCharles soils contain large clay fractions characterizedby high-charge smectite. About 50% of each of thesesoil smectites is tetrahedrally charged. The presenceof small amounts of mica and vermiculite may con-tribute to NHJ fixation. Potassium-rich clay mineralsin these soils may release K and enhance NH$ fixation.

The Katy and Nada soils have small fractions ofclay that are mainly of the 1:1 type. Their smectitesalso are high-charged, but the major portion of charge

Table 3. Soil smectite layer charge characteristics.

Soil smectite

BeaumontEdnaKatyLake CharlesNada

Data pointsused

Cno.t9^,10,11,12

9t,10,lliot

9t,10,ll, 12,13

Charge TetrahedralCharge density distribution chargeper 0,0(OH)2

0.760.740.690.730.84

HeterogeneousHeterogeneousHomogeneousHeterogeneousHeterogeneous

4858147029

t No. of C atoms in carbon chain of reference molecules employed to deter-mine separation of interlayer charge sites (Lagaly and Weiss, 1976).

t Data points considered as the result of bilayer alkylammonium in smectiteinterlayer.

is located in the octahedral sheet. Mica and vermic-ulite are either absent or low in content. The overallevaluation of NHJ fixation potential in these two soilsis low.

The Edna soil smectite has charge characteristicssimilar to the Beaumont and Lake Charles soils.NHJ fixation in this soil, however, is expected to beless than in the more clayey Beaumont and LakeCharles soils.

Extractable Soil PotassiumAddition of K caused Beaumont soil to fix more

NHJ as indicated from the field experiments (Fig. la).Naturally occurring exchangeable K should poten-tially have the same effect. The Beaumont soil has 216mg kg"1 extractable K, which is much higher than the54 mg kg"1 of the Nada soil. Potassium treatments inthe field experiments are equivalent to 150-350 mgkg-1 in the soil. Therefore, the native K level has con-tributed significantly to the total K in those treat-ments. The extractable K level may be a function ofclay-sized mica and feldspar found in Beaumont,Edna, and Lake Charles soils (Table 1).

Ammonium Fixation and ReleaseAt the time of recovery from incubation in the field,

the soil samples still retained the original round shape.The filter paper wrappings, however, were almostcompletely decomposed. Abundant roots had pene-trated the nylon gauze, and some roots were found inthe soil sample. Only data from Beaumont and LakeCharles soils are presented in Fig. 3. Fixed NHJ-N inthe Beaumont soil increased from the 148 mg kg"1

native level to 251 mg kg-' after saturation withNHJ and the sample subsequently was washed withCaCl2 solution (Table 2). After incubation the total

CHEN ET AL.: AMMONIUM FIXATION IN TEXAS GULF COAST SOILS 1039

Fixed Ammonium (mg kg~1)Ml Pxol I IU Native KM Added I_(Released

•p 300O)^0)E,£ 200D'EOE< 100T30)XiT

0Satu. Cation

P"

-

Ca

Field Incu. | fSoil Beaumo

K

LIEnt Lake

,; •

Ca

kc.

Fig. 3. Native and added fixed NHJ as influenced by chemical treat-ment and field incubation in Beaumont and Lake Charles soils.

fixed NHJ was reduced to 172 mg kg-'. This releaseof fixed NHJ was from the chemically added fractionas shown by the 15N analysis (Fig. 3). Some addedNHJ still remained fixed after the 8-wk incubationduring active rice growth. Black and Waring (1972)concluded that such remaining fixed NHJ may still bereleased during successive croppings. Any release dur-ing off-season, however, should be considered lost tononcrop vegetation or lost by some other soil N de-pleting mechanism.

Different saturating cations have prominent influ-ences on the fixation and release of NHJ. When a KC1solution was used to wash the NHJ saturated Beau-mont soil sample, 267 mg kg-1 soil NHJ remained thatis significantly higher than the case where CaCl2 so-lution washings were employed (Table 2). The greaterfixation caused by K suggests that a rapid collapse ofclay interlayers entraps more NHJ that results in littleexchange between K and NHJ. This is reflected in thenative fraction of fixed NHJ, which is slightly higherin the nonincubated K than Ca saturated sample. TheCa cation replaced more native fixed NHJ than didK. Although the incubation condition was identical tothe Ca saturated sample, the release of fixed NHJ fromthe K saturated sample was nil. This observationagrees well with the yield-reduction effect of K foundin the field experiment in the Beaumont soil.

The fixation and release of NHJ in the Lake Charlessoil are similar to that of the Beaumont except a sig-nificantly greater magnitude of fixation has occurredperhaps because of the greater tetrahedral charge inthe Lake Charles soil smectite. The retention of fixedNHJ after incubation is also higher than the Beaumontsample. Considering these data and the clay layercharge characteristics, it appears that Lake Charles hasan NHJ fixation equal to or greater than Beaumont.

The Nada and Katy soils responded differently tothe chemical and incubation treatments when com-pared to the Beaumont and Lake Charles soils. Allentries in Table 2 for these two soils represent nativefixed NHJ based on N isotope analysis. The data in-dicate that these two soils will not fix any moreNHJ than their native level. It may be that the Nadasoil has all its specific fixation sites already occupiedby NHJ or K, whereas in the Beaumont soil, such sitesare still available for NHJ fixation.

Laboratory procedures for fixed NHJ determina-tion, such as the one by Silva and Bremner (1966), donot necessarily reflect the NHJ fraction that is trulyunavailable to rice plants. In soils such as the Beau-mont and the Lake Charles, the availability of fixedNHJ may depend on several factors: clay character-istics, K in solution, the plant's ability to extractNHJ from the soil, and the duration of the extractionprocess. This study establishes the existence of K in-duced NHJ fixation in two Vertisols and its relevanceto limited rice yields.

ACKNOWLEDGMENTSThe authors express their sincere appreciation to the Ten-

nessee Valley Authority for financial support that helpedmake this research possible.

1040 SOIL SCI. SOC. AM. J., VOL. 53, JULY-AUGUST 1989