Neutralizing Soil Acidity Under Bermudagrass Sod1

FRED ADAMS AND R. W. PEARSO.N2

ABSTRACTThe effectiveness of surface-applied lime in preventing sub-

soil acidification from residually acid N sources used on Coastalbermudagrass (Cynodon dactylon L. Pers.) was influenced by(i) soil type, (ii) lime rate, (iii) N source, and (iv) N rate.Lime applied at a rate equivalent to acidity of the N sourcemaintained both pH and exchangeable bases in a loamy sandbut not in a clay loam soil. Lime applied at three times the Nfertilizer equivalent acidity was more effective in correctingsubsoil acidity than were lower rates, and the effect was morepronounced in the coarse- than in the fine-textured soil.

Calcium gluconate was highly effective in increasing subsoilpH and exchangeable base level under Coastal bermudagrassbut caused an undesirable fungal bloom. Surface-applied'NaNO3 was effective in correcting subsoil acidity in both acoarse- and a fine-textured soil without accumulation of Na,and the effect was relatively uniform throughout the depth ofprofile sampled (45 cm). Calcium nitrate was also effectivein increasing subsoil pH and exchangeable Ca in mediumtextured soil to a depth of 45 cm, with less pronounced effectsdown to 75 cm. Residual basicity of Ca(NO3)2, as measuredby increase in exchangeable Ca in the soil profile, ranged from1.7 to 2.5 kg CaCO3 per kg N, depending upon rate ofapplication.

Additional Key Words for Indexing: lime, nitrogen, pH,subsoil.

THE USE of high rates of acid-forming fertilizers, with-out a counteracting liming program, results in rapid

development of strongly acid soil profiles, especially onhighly leached, poorly buffered soils (1, 2, 3, 6). Thisacidity can be effectively neutralized by lime on cultivatedsoils where tillage thoroughly mixes the lime with soil toa depth of several inches. Adams et al. (4) found thatdolomite used at about twice the equivalent acidity ofNH4NO3 applied to a Cecil sandy loam over a 7-yearperiod failed to maintain pH or exchangeable bases in theO- to 30-cm layer but increased pH and bases in some deeplayers. Calcitic limestone at the same rate maintained orincreased both pH and exchangeable bases throughoutthe 90-cm profile. They also found no difference betweenmixing the lime in the O- to 15-cm layer and applying itto the soil surface prior to sprigging the grass. However,just how effective surface-applied lime is in preventing orneutralizing soil acidity beneath an undisturbed sod hasnot been adequately investigated. Nor has the effectivenessof residually basic forms of N in preventing and correctingsubsoil acidity been defined.

1 Joint contribution from the Alabama Agr. Exp. Sta., Au-burn University, and the Soil and Water Conservation ResearchDivision, ARS, USDA, Auburn, Ala. Received April 10, 1969.Approved May 21, 1969.2 Professor of Soils, Auburn University, and Research SoilScientist, USDA, Auburn, Ala., respectively.

Many Coastal bermudagrass (Cynodon dactylon L.Pers.) fields in the Southern USA are fertilized with rela-tively high rates of N, especially those harvested for hay.It has been possible to continue this practice because ber-mudagrass, as shown by numerous experiments, is highlytolerant of low soil pH. The current practice for preventingor correcting soil acidity on an established grass sod is tobroadcast lime on the surface with no mechanical mixingof lime with soil. The inefficiency of lime in such a pro-gram would seem to be self-evident. Since lime movesdownward extremely slowly in soil, and N moves down-ward very rapidly, it is easy to visualize a thin lime layerat the soil's surface through which much of the NH4

+

leaches before nitrification takes place, and the subsequentdevelopment of strongly acid soil beneath the surface limelayer.

This practical problem resolves itself into two separateliming questions for the farmer: (i) What is the best lim-ing program for the prevention of excessive soil acidityunder sods receiving high N rates; or (ii) What is the bestprogram for the correction or neutralization of excess sub-soil acidity that developed because of a previously inade-quate liming program? The objectives of the field experi-ments reported here were to determine the feasibility ofsuch liming programs in the prevention and correction ofsoil acidity resulting from high rates of N on bermudagrasssods and to compare the effects of different N sources onsubsoil acidity.

Three experimental sites on established stands of high-yielding bermudagrass were selected—Suwanee bermuda-grass on Lakeland loamy sand, Coastal bermudagrass onLloyd clay loam, and Coastal bermudagrass on Greenvillesandy loam. These sites represent widely different soil typesof the area on which bermudagrass is commonly grownfor grazing and hay.

LAKELAND LOAMY SAND AND LLOYD CLAYLOAM EXPERIMENTS

Materials and MethodsThe experiments on Lakeland and Lloyd soils were con-

ducted simultaneously for 5 years. Each experiment had twomajor phases—an "acidifying" phase and a "neutralizing"phase. The acidifying phase consisted of treatments designedto develop different degrees of acidity in the soil profile. Thisphase lasted for 2 years on Lakeland loamy sand and for 3years on Lloyd clay loam. The neutralizing phase consistedof treatments intended to correct soil profile acidity and wasin effect for 3 years on Lakeland soil and for 2 years on Lloydsoil. The treatments were the same for both experiments; onlythe duration of the phases differed.

During both phases of each experiment, preventive limingtreatments were carried out concurrently with the acidifyingand neutralizing treatments. These will be described separately.

The experimental design was randomized block with splitplots. There were 80 plots, each measuring 2.44 x 2.44 m(8 x 8 ft). The data were analyzed statistically according toDuncan's new multiple range test.

737

738 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

ACIDIFYING PHASE

The treatments were designed to give four rates of acidifi-cation by applying 560 and 1,120 kg/ha (500 and 1,000Ib/acre) of N annually as NH4NO3 or as (NH4)2SO4 to dif-ferent plots. The N was applied in five equal applications eachyear, beginning in April, prior to first growth and immediatelyfollowing each clipping. Grass was clipped at 4- to 5-weekintervals and the forage removed from the area. The remainingfertilizer was broadcast uniformly to all plots annually priorto growth in early spring and consisted of 224 kg/ha of P (200Ib/acre of P2O5) from ordinary superphosphate and 181 kg/haof K (200 Ib/acre of K2O) from KC1. All unlimed plots on theLakeland soil also received annual applications of 28 kg/ha(25 Ib/acre) of Mg as MgSO4.

NEUTRALIZING PHASE

At the conclusion of the acidifying phase, soil samples weretaken to a depth of 30 cm from each plot for pH determina-tions. Various treatments of surface-applied lime were thenimposed on different plots (Table 3). The N rates and sourcesused during the acidifying phase were discontinued; the newN rate was the same for all plots, namely 448 kg/ha (400Ib/acre). The new sources consisted of one treatment with(NH4)2SO4, three treatments with NaNO3, and the remaining16 treatments were with NH4NO3. An annual application of a0-10-20 fertilizer was made at a rate of 1,120 kg/ha '(1,000Ib/acre). The liming material was a high-grade dolomitic lime-stone except for two treatments of basic slag and one treatmentof calcium gluconate. Two grades of limestone were compared:an agricultural grade (50% passed a 60-mesh screen) and anextra-fine grade (a mine dust with 100% passing a 100-meshscreen). Lime rates ranged up to 6,720 kg/ha (6,000 Ib/acre)annually. Some plots received a single application of lime atthe beginning, while others received annual applications of lime.

Fertilizer was applied in two applications: one-half of the Nand all P and K in April, the remaining N in mid-July. Clip-ping procedure of forage remained unchanged. The neutralizingphase was continued for 3 years on the Lakeland loamy sandbut for only 2 years on the Lloyd clay loam because of itsinaccessibility after that time. Although the experiment on thefine-textured soil was discontinued prematurely, the data fromit were sufficient to report and to verify trends established onthe coarse-textured soil.

PREVENTIVE LIMING TREATMENTS

To inhibit the development of soil acidity from the N fer-tilizers, certain plots were limed annually during late winter.Lime rates were based upon the amount and source of N to beadded during the subsequent growing season. Liming treatmentconsisted of two rates and two grades of dolomitic limestone.One rate was that calculated to equal the acidity produced bythe particular N treatment; the other rate was three times thisamount. Each rate of lime was, in turn, applied in two gradesof fineness; one was of agricultural grade (50% passed a 60-mesh screen), and one was extra fine (all passed a 100-meshscreen).

Soil Analysis—Soil pH was determined in a 1:1 soil-watersuspension. Exchangeable bases were extracted with neutralN NH4OAc and Ca determined by EDTA titration, Mg byatomic absorption spectrophotometry and K and Na by flamephotometry.

Results and Discussion

PREVENTIVE LIMING TREATMENTS

Specific amounts of lime required to neutralize acidityfrom various N fertilizers, first proposed by Pierre in 1928(5), served as useful guides in preventing a buildup of

Table 1—Average effects of five annual surface applications ofdolomitic limestone on soil pH and exchangeable

cations under a bermudagrass sod fertilizedat high rate of N

Lime rateOriginal

Soildepth

cm

soilSoilpH

sampleExchange-able bases

meq/100 g

EquivalentN acidity

SoilPH

Exchange-able bases

meq/100 g

Equivalent to3 X

SoilPH

N acidityExchange-able bases

meq/100 gLakeland Loamy Sand

5-1515-3030-45

5.55.3

0.730.54

5.45.14.9

0.660.450.41

6.05.55.2

1.320.630.52

Lloyd Clay Loam4-15

15-3030-45

6.36.1

3.293.13

5.45.65.8

1.361.432.07

5.85.96.0

2.332.502.54

excessive soil acidity from ammonia fertilizers duringthe three decades following recognition of the problem.Whether these values can be used as general liming guidesunder field conditions that are quite different from thoseof 1928 remains to be seen. The experiments reported hereoffer a striking contrast in soil acidity developed from therates of N used now and those used 40 years ago. Theyalso provide a measure of efficiency of surface-appliedlime in neutralizing or preventing acidity beneath an undis-turbed sod. Most "lime-N" experiments have reported onthe effectiveness of lime when mixed with soil and usuallyat much lower rates of N fertilizers.

The results of these treatments for the Lakeland andLloyd soils are summarized in Table 1. The overall effectof the low lime rate (equivalent to acidity from applied N)was to maintain both pH and exchangeable bases on theLakeland soil but to maintain neither of these on the Lloydsoil. The average loss in exchangeable basic cations at thelow lime rate was over 50% of the original basic cationsin the top 30 cm (12 inches) of Lloyd soil. Even the highlime rate (three times the equivalent acidity from appliedN) failed to prevent loss of exchangeable cations and de-velopment of acidity in the Lloyd soil profile. However,this rate did increase both exchangeable basic cations andsoil pH in the Lakeland soil profile. Thus, lime effective-ness was much greater on the coarse-textured soil.

The average values in Table 1 do not tell the wholestory, however. The rate and source of N during the firstphase also influenced the effectiveness of surface-appliedlime. For example, the average soil pH in the 15- to 30-cm(6- to 12-inch) layer of the Lloyd soil, limed at the rateequivalent to acidity from N fertilizer, was 6.2 when Nrate was 560 kg/ha (500 Ib/acre) from NH4NO3 but wasonly 4.9 when N rate was 1,120 kg/ha (1,000 Ib/acre)from (NH4)2SO4. The corresponding exchangeable basiccations were 2.56 and 1.07 meq/100 g. The other two Ntreatments were intermediate in their effects on soil acidity.Although all soil pH and exchangeable cation values werehigher under the high lime rate, these values were influ-enced by N source and rate in the same manner as theywere under the low lime treatment. Only the magnitude ofthe values was different.

In comparing coarseness of limestone, the material thatmet Alabama's minimum specifications was as effective asextra-fine mine dust on both soils.

ADAMS AND PEARSON: NEUTRALIZING SOIL ACIDITY UNDER BERMUDAGRASS 739

Table 2—Effect of N source and N rate annlied with noconcurrent liming treatment to undisturbed

bermudagrass sod on pH of two soilprofiles at conclusion of each phase

of the experiment (sampledin February)

pH of Lakeland loamy sand pH of Lloyd clay loam

Depthcm

HighN

End of Acidifying Phase*2.5-7.57.5-1515-30

5-1515-3030-45

5.9 4.75.6 4.95.3 4.8

4.94.94.9

4.54.54.4End4.64.64.6

4.54.44.3

of Neutr,4.94.74.8

4.3 6.14.3 6.14.2 6.3

lizine Phaset4.64.74. 7

5.25.66.0

5.25.86.0

4 7 4.544

444

7 4.78 5.1

8 4.77 4.98 5.6

4.54.54.6

4.74.74.7

* Both fertilizer materials were applied at annual rate of 560 and 1,120 kgAa (500 and1,000 Ib/acre) of N at low and high N, respectively, during acidifying phase, t Nadded as NH4NO3 to all plots at rate of 448 kgAa (400 Ib/acre) of N during neutraliz-ing phase.

Thus, the effectiveness of preventive lime treatments wasinfluenced by (i) soil type, (ii) lime rate, (iii) N source,and (iv) N rate. Lime was less effective on the fine-tex-tured soil than on the coarse-textured soil, possibly becausesmall lime particles moved downward more readily in thesandy soil. It is also probable that more NH4

+ was leachedfrom the profile (without nitrification) in the sandy soilthan in the more impervious clay soil. A major reason forthe ineffectiveness of lime in some treatments could havebeen that much of the nitrification occurred in the soilbelow the concentrated surface layer of lime, and hence,the resulting acidity was "beyond the reach" of lime aboveit. In general, lime was most effective at the lower N ratefrom NH4NO3 and least effective at the higher N rate from(NH4)2S04.

ACIDIFYING PHASE

Data in Table 2 for Lakeland and Lloyd soils show thatat the conclusion of the acidifying phase, all N treatmentson the sandy soil and three of the four N treatments onthe clay soil had resulted in extremely acid profiles to adepth of at least 30 cm (12 inches). The most acid pro-file in each soil was produced by the high N rate (1,120kg/ha or 1,000 Ib/acre) from (NH4)2SO4, and the leastacid profile resulted from the low N rate (560 kg/ha or500 Ib/acre) from NH4NO3.

NEUTRALIZING PHASE"No Lime" Plots—None of the unlimed soil profiles

increased in acidity during the second phase of the experi-ment, the period during which N was added as NH4NO3at the rate of 448 kg/ha (400 Ib/acre) of N. Contrary toexpectations, the pH of plots receiving (NH4)2SO4 duringthe first phase actually increased during the second phasewhen they received NH4NO3 (see Table 2); while pH ofplots receiving NH4NO3 during both phases did not changeduring the latter phase. The reason soil pH remained aboutconstant or slightly increased during the latter phase wasprobably the manifestation of two separate phenomena:(i) nitrification rate was inhibited by low pH, and (ii)residual fertilizer salt remaining in the soil at sampling timewas less during the second phase than during the first phasebecause of the lower N rate. Higher residual salt levels insoil samples result in lower measured soil pH values, eventhough total soil acidity remains unchanged.

Corrective Liming Treatments—The neutralizing phaseincluded several treatments of surface-applied lime onundisturbed sod fertilized annually with 448 kg/ha (400Ib/acre) of N from NH4NO3. The results of these treat-ments at the conclusion of the experiment are summarizedin Table 3. Results of each treatment are not reportedseparately because results from several were the same.Where there was no significant difference between resultsof treatments, pH and exchangeable base values for thesetreatments were averaged, and only the averaged valuesare listed. For example, a single application of 6,720kg/ha (3 tons/acre) of dolomite gave the same results asannual applications of 2,240 kg/ha (1 ton/acre) over a3-year period. Although the extra-fine dolomite at the twohigher rates showed a definite trend for greater effective-ness than did comparable rates of agricultural grade, resultswere combined because differences were never large andwere seldom statistically different.

Assuming the acidity from NH4NO3 to be equivalentto 1.8 kg of CaCO3 per 1 kg of N, 2419 kg/ha (2,160Ib/acre) of lime would have been required during theneutralizing phase to maintain an unchanged acidity on theLakeland soil (3 years) and 1,613 kg/ha (1,440 Ib/acre)on the Lloyd soil (2 years). This approximate amount,2,240 kg/ha (1 ton/acre), of dolomitic limestone applied

Table 3—Effect of surface-applied lime during neutralizing phase on soil pH and exchangeable bases by depth beneathbermudagrass sod fertilized at annual rate of 448 kg/ha (400 Ib/acre) of N from NH4NO3

(CaOOjequivalent)

None2,240*2,240*6,720t6.720J6,720}

20,2325

Limesource

dolomitebasic slagdolomitebasic slagcalciumgluconate

dolomite

5-15cm

4.84.95.05.35.46.1

5.6

Soil pH15-30cm

4.84.95.05.15.25.8

5.2

Lakeland k>amy sand Llovd clav loamExchangeable bases

30-45cm

4.84.84.95.05.05.4

5.1

5-15cm

0.300;390.500.620.781.51

0.88

15-30cm

0.290.310.310.380.400.91

0.43

30-45cm

0.330.290.300.330.490.95

0.42

5-15cm

4.94.95.05.15.25.5

5.2

Soil pH15-30cm

5.05.15.25.15.35.4

5.2

Exchangeable bs30-45cm

5.35.35.55.45.45.5

5.5

5-15cm

0.750.730.730.951.242.07

1.48

15-30cm

1.131.091.371.291.341.37

1.40

ises30-45

cm

1.401.491.841.881.682.33

1.89A single application of lime at beginning of neutralizing phase. Averages of eight plots (four acidification regimes and two grades of lime).

t Averages of single 6, 720-kg/ha applications at beginning of neutralizing phase and annual 2,240-kg/ha applications during same period. Averages of 16 plots (four acidifica-tion regimes and two grades of lime).

J Annual 2,240-kg/ha applications during neutralizing phase. Averages of four acidification regimes.§ Annual 6,720-kg/ha applications during neutralizing phase. Averages of eight plots (four acidification regimes and two grades of lime).

740 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

at the beginning of the neutralizing phase had no apparenteffect on the pH and exchangeable base levels below 5 cm(2 inches) because values were similar to those of unlimedsoil at the end of the experiment (Table 3).

The neutralizing effect of high rates of lime is quite obvi-ous on the Lakeland soil from the data in Table 3. The twohigher rates of dolomitic limestone and the higher rate ofbasic slag all restored subsoil pH and exchangeable basesin this soil to approximately the original levels (Table 1)prior to the experiment. Decreases in acidity were greaterin the 5- to 15-cm (2- to 6-inch) zone than at lower depths.

The effect in the subsurface of high rates of lime wasnot so striking on the Lloyd soil. Admittedly, the neutraliz-ing phase lasted only 2 years on the Lloyd soil as comparedto 3 years on the Lakeland soil, and the significance of the"time" factor cannot be assessed. However, there was atrend toward higher soil pH and exchangeable base valuesat the higher rates of lime on the fine-textured Lloyd soil,especially in the 5- to 15-cm (2- to 6-inch) layer.

Differences in soil pH and exchangeable bases resultingfrom equivalent rates of dolomitic limestone and basic slagwere generally not great enough to be statistically signifi-cant for any two comparisons. However, the overall effectshowed basic slag to be slightly more effective than dolo-mitic limestone in these experiments, probably because ofthe somewhat greater solubility of the carbonates in basicslag.

The most effective liming material by far was calciumgluconate. This material was used to provide a readilydecomposable, nontoxic organic anion with Ca so that itsresidue following microbial decomposition would be basic.In Lakeland loamy sand, 6,720 kg/ha (3 tons/acre) ofCaCOg equivalent from calcium gluconate resulted in soilpH values which were slightly more than 1.0 pH unitgreater than at the beginning of the neutralizing phase andin exchangeable base levels which were approximatelydouble those at the beginning of the experiment. The sametrend was evident on the Lloyd soil, but the experimentwas not continued on this soil long enough for the effec-tiveness of calcium gluconate to be so striking. Althoughthis material was highly effective in neutralizing soil aciditybeneath the grass sod, it was not conducive to good plantgrowth. It was applied annually at the rate of 2,240 kg/ha(1 ton/acre) of CaCO3 equivalent, and by the 3rd year,stand and yield of the bermudagrass were noticeablyreduced. Plants growing on these plots were, partially cov-ered with an unidentified fungus, or fungi, in early springand summer, and this appeared to be the reason for a lossof stand. Nevertheless, these data demonstrate clearly thatsubsoil acidity can be rapidly neutralized by a surface-applied, readily decomposable, organic Ca salt.

SODIUM NITRATE AS N SOURCEThree treatments received NaNO3 as the source of N

instead of NH4NO3 during the neutralizing phase follow-ing each of the acidification regimes. The N rate in allcases was 448 kg/ha (400 Ib/acre), which is adequate forhigh yields of bermudagrass. The reason for using NaNO3was twofold: (i) plants might absorb nitrate in excess of

Table 4—Effect of NaNOs at rate of 448 kg/ha (400 Ib/acre)of N during neutralizing phase on soil pH andexchangeable bases beneath bermudagrass sod

Lakeland loamy sand

Limerate

kg/haNone

2,240*

6, 720t

Soildepth

cm5-15

15-3030-45

5-1515-3030-45

5-1515-3030-45

SoilPH

5.55.75.55.85.95.85.95.85.5

Exchange-ablebases

meq/100 g0.540.560.360.720.580.580.800.550.50

Exchange-ableNa

meq/100 g0.080.090.080.060.160.170.040.110.12

Lloyd clav loam

SoilpH

5.45.45.65.55.55.85.55.65.8

Exchange-able

basesmeq/100 g

0.951.441.96

0.991.531.981.191.331.60

Exchange-ableNa

meq/100 g0.030.020.100.100.130.150.020.040.06

* A single application of agricultural-grade dolomitic limestone at beginning ofneutralizing phase. Average of four plots,

t Annual 2,240-kg/ha applications of agricultural-grade dolomitic limestone duringneutralizing phase. Average of four plots.

equivalent cations, and (ii) nitrate would be subjected todenitrifying action of soil microbes. Either of these biolog-ical actions leaves a basic residue in the soil, resulting inhigher soil pH values and greater retention of bases.

Assuming the basicity from NaNO3 to be 1.8 kg ofCaCOg equivalent per 1 kg of N, the basicity from NaNO3was 2,419 kg/ha (2,160 Ib/acre) of CaCO3 during neu-tralizing phase on Lakeland loamy sand and 1,613 kg/ha(1,440 Ib/acre) on Lloyd clay loam. The quantitativebasicity of NaNO3 cannot be accurately assessed in theseexperiments, but the data in Table 4 show that NaNO3 wasvery effective in neutralizing subsoil acidity. Unlike cor-rections from high rates of lime or from calcium gluconate,the neutralizing action of NaNO3 was not concentrated inthe upper soil layer. It seems to have been as effective inthe 30- to 45-cm (12- to 18-inch) layer as in the 5- to15-cm (2- to 6-inch) layer.

Neither of the two rates of dolomitic limestone usedappeared to enhance the effectiveness of NaNO3 verymuch. All treatments with NaNO3 on the coarse-texturedsoil resulted in pH values that were even greater than thoseprior to the acidifying phase. Exchangeable base valuesfor this soil had also been restored to their original levelsby the conclusion of the neutralizing phase. Although thisphase lasted only 2 years on Lloyd clay loam, and therewas considerably more acidity to be neutralized in this soilthan in Lakeland loamy sand, a significant increase in bothsoil pH and exchangeable bases had occurred in the clayloam soil throughout the sampled profile at its conclusion.

To determine whether a significant amount of exchange-able Na remained at the conclusion of the neutralizingphase, samples from all NaNO3 plots and comparableNH4NO3 plots were analyzed for exchangeable Na. Thedifference in Na contents between comparable Na andNH4 plots was assumed to be the Na remaining from theNaNO3 fertilizer. The laboratory procedure itself is fraughtwith low levels of Na contamination from glassware sothat only wide differences in exchangeable Na have mean-ing. Data in Table 4 show that Na was fairly .evenly dis-tributed throughout the sampled profile and was about0.1 meq/100 g in all plots fertilized with NaNO3. Thisdoes not suggest an appreciable accumulation of exchange-able Na, but rather that Na had been leached from thesesoils about as fast as it had been added.

ADAMS AND PEARSON: NEUTRALIZING SOIL ACIDITY UNDER BERMUDAGRASS 741

GREENVILLE SANDY LOAM EXPERIMENT

Materials and MethodsIn this experiment surface applications of Ca(NO3)2 and

NH4NO3 were made over a 4-year period to an establishedCoastal bermudagrass sod. Main plots were irrigation level,and subplots were N treatments.

Two irrigation levels after each N addition were used (threeper year):

1) irrigated to displace water to a depth of 38 cm (15inches),

2) irrigated to displace water to a depth of 114 cm (45inches).

The N treatments were:1) NH4NO3 at 896 kg/ha (800 Ib/acre) of N,2) NH4NO3 at 896 kg/ha (800 Ib/acre) of N + 3,920

kg/ha (3,500 Ib/acre) of CaCO3,3) Ca(NO3)2 at 448 kg/ha (400 Ib/acre) of N,4) Ca(NO3)2 at 896 kg/ha (800 Ib/acre) of N,5) Ca(NO3)2 at 1,344 kg/ha (1,200 Ib/acre) of N,6) No N; 3,920 kg/ha (3,500 Ib/acre) of CaCO3.

Plots were surrounded by galvanized iron borders sunk 15cm (6 inches) into the soil and extending 5 cm (2 inches)above soil level to retain water and amendments within plotboundaries. The CaCO3 was applied before sprigging Coastalbermudagrass and was disked into the soil. An annual appli-cation of 1,120 kg/ha (1,000 Ib/acre) of a 0-10-20 fertilizerwas made in early spring. Nitrogen applications were made inthree equal additions—the first just before growth started inthe spring, and the second and third after the first and secondclippings of grass. All clippings were removed from the plots.

Soil samples for pH and exchangeable Ca determination weretaken by 15-cm (6-inch) layers to a depth of 76 cm (30inches) at the beginning of the experiment in 1963 and at itstermination in 1968.

Results and DiscussionIRRIGATION LEVEL AND SUBSOIL ACIDIFICATION

Since the experiments on Lakeland loamy sand andLloyd clay loam showed that the magnitude of N effecton profile acidity was different for the two soils, treatmentswere included in the experiment on Greenville sandy loamto determine whether this was due, at least in part, to dif-ferences in leaching. After each application of N, either asNH4NO3 or Ca(NO3)2, water was applied in amounts towet the profile to either 15 or 45 cm. The premise was thatin the deeper wetted treatments maximum expression ofresidual acidity from the NH4NO3 would occur in the sub-surface zones. Similarly, by moving the applied Ca(NO3)2into the subsoil before an appreciable amount of NO3 couldbe taken up by the grass roots, maximum effect towardcorrection of subsoil acidity should be realized.

Final levels of subsoil (15 to 75 cm) exchangeable Cain the NH4NO3-treated plots were less, to a highly signifi-cant degree, at the higher rate of water application thanat the lower rate (Table 5). The difference in Ca contentwas reflected in consistently lower final subsoil pH valuesin the wetter plots. Thus, even the relatively small differ-ence in water applied to these plots resulted in a substan-tial difference in the rate at which Ca moved from theprofile following use of residually acid fertilizer. At thelow rate of irrigation, Ca was displaced from the O- to15- and the 15- to 30-cm zones, but tended to accumulatein the lower layers. Increasing the water application in-

Table 5—Changes in exchangeable Ca and pH in a Greenvillesandy loam profile under two levels of irrigation

following annual applications of 896 kg/ha(800 Ib/acre) of N as NttjNOs

over a 4-year period

(cm)

0-1515-3030-4545-6060-75

InitiW,*

5.85.45.25.35.2

Soilisl

W,

5.75.45.35.3

.5. 1

PHTT

W,

4.45. 15.55.65.4

Exchangeable Ca (meq/100 g)'inal

W,

4.44.95.45.35.2

In

W,

1.861.642.002 242.21

itialW,

1.721.661.772.272.03

Fi

W,

0.491.392.312.392.48

inalW2

0.621.002.042. 152.15

* 27. 7 and 67. 3 cm of water added to W, and W3. respectively, over the 4-year period.

creased the depth of Ca displacement. Based on initial andfinal exchangeable Ca contents, there was a net loss of2,492 kg/ha (2,225 Ib/acre) of CaCO3 equivalent fromthe low irrigation treatment as compared with a loss of4,169 kg/ha (3,722 Ib/acre) from the high treatment.Thus, the application of only 40-cm additional water overa 4-year period nearly doubled the loss of Ca from the75-cm profile. Using the above figures, 1 kg of N fromNH4NO3 resulted in the loss of Ca equivalent to about 0.7and 1.1 kg of CaCO3 at low and high levels of irrigation,respectively.

These results confirm the premise that the differenceobserved in subsoil acidification of Lakeland sandy loamand Lloyd clay loam was caused by the greater movementof water through the coarser textured Lakeland soil. Simi-larly, soil characteristics that restrict water movementthrough the profile should tend to limit expression of resid-ual acidity to the surface soil, and there are results whichsupport this suggestion (2).

Irrigation level did not influence downward movementof Ca from CaCO3 mixed in the O- to 15-cm layer withoutapplied N. Therefore, results from the two water treatmentswere combined and are presented in Table 6. Both Ca andpH were increased in the zone of incorporation, and asmaller, but similar, effect was obvious in the underlying15-cm zone. However, there was no indication of an effectmore than 15 cm below the mixed zone interface. Usingexchangeable Ca to calculate recovery of added lime, theCa gained was equivalent to 3,806 kg/ha (3,398 Ib/acre)of CaCO3. When this is compared to the amount initiallyapplied, 3,898 kg/ha (3,480 Ib/acre), it is obvious thatthere was very little loss of Ca from the profile in no-Ntreatment. This is in striking contrast to the 3,517 kg/ha(3,140 Ib/acre) of CaCO3 equivalent lost during the sameperiod when NH4NO3 was applied at 896 kg/ha (800Ib/acre) of N.

Table 6—Profile pH and exchangeable Ca in plots of Greenvillesandy loam receiving lime* but no N

Depth(cm)0-15

15-3030-4545-6060-75

Soil pHInitial

5.95.55.45.35.2

Flnalt6.15.85.45.45.3

Exchangeable Ca (meq/100 g)Initial1.851.731.982.452.27

Final2.572.252.102.412.23

* 3, 903 kg/ha (3, 485 Ib/acre) of lime mixed in the O- to 15-cm layer.t Initial sampling May 1964; final sampling April 1968.

742 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

Table 7—Effect of rate of application of Ca(NOs)2 on soilprofile pH and exchangeable Ca in a Greenville

sandy loam at the end of 4 yearsof fertilization

Annual rate of N

Depthcm0-15

15-3030-4545-6060-75

InitialsoilpH

5.75.45.25.25.2

Initialexchange-

ableCa

meq/100 g1.791.602.022.362.27

448 kg/haSoilPH

6.16.36.05.75.5

Exchange-able Ca

meq/100 g3.373.082. 73»2.552.43*

896 kg/haSoilpH

6.16.16.15.85.5

Exchange-able Ca

meq/100 g3.163.463.232.822.64

1, 344 kgAaSoilPH

5.9*6.15.95.75.5

Exchange-able Ca

meq/100 g2.81*3.373.273.303.00

* Significantly different (. 05) than the other treatments for the same depth.

CALCIUM NITRATE AS N SOURCE

Contrary to expectations, water regime did not influencethe effect of Ca(NO3)2 on subsoil exchangeable Ca or pH.Therefore, results from the two irrigation treatments werecombined. Final pH and exchangeable Ca, listed in Table7, show relatively little difference among the three ratesin their effect on these soil properties although the accumu-lation of exchangeable Ca was somewhat greater below 30cm where higher rates were applied. In general, the 896-kg/ha (800-lb/acre) rate was as effective at increasingexchangeable Ca as the highest rate, which indicates thatthe intermediate rate supplied as much N as the sod coulduse.

There was no difference in final soil pH among the ratesof Ca(NO3)2 for any subsoil layer, although exchangeableCa was affected significantly. This is believed to be a resultof lack of precision in soil pH measurements. It seemsunlikely that free salt could account for differences in Caaccumulation since samples were taken 6 months after thelast fertilizer addition during which time there was over66 cm of rainfall.

The changes in soil pH and exchangeable Ca over the4-year period can be seen by comparison of initial and finalvalues of these properties (Table 7). For example, plotsthat had received an annual application of 896 kg/ha(800 Ib/acre) of N showed an increase of 1.86 and 1.21meq/100 g of exchangeable Ca in the 15- to 30- and the30- to 45-cm zones, respectively. The effect persisted,though less pronounced, to the 75-cm depth, and the accu-mulation of Ca was reflected in a higher final pH through-out the profile.

It is apparent from these results that surface-appliedCa(NO3)2 can effectively counteract soil acidity at con-siderable depth in a permeable soil providing there areactively absorbing roots present in these acid zones. Thesame effect should follow use of NH4NO3 together withadequate rates of incorporated lime. Coastal bermudagrass,because of its tolerance of low pH and its capacity forutilizing large amounts of NO3, would favor maximumexpression of this effect.

It also appears that at the higher rates of applicationconsiderable amounts of the Ca(NO3)2 may have passedthrough the 75-cm profile without effect on acidity. At the

intermediate rate of application, for example, Ca equiva-lent to 12,902 kg/ha (11,520 Ib/acre) of CaCO3 wasapplied over the 4-year period, yet only 5,902 kg/ha(5,270 Ib/acre) of this can be accounted for as gain inexchangeable Ca. Assuming a near maximum irrigatedgrass yield at this location of 20,160 kg/ha (18,000Ib/acre) and a Ca content of 0.8%, Ca equivalent to only1,680 kg/ha (1,500 Ib/acre) of CaCO3 would have beenremoved in the hay. Thus, nearly 50% of the Ca appliedas Ca(NO3)2 is unaccounted for, much of which wasprobably leached out of the 75-cm profile. At the lowerrate of application Ca(NO3)2 was far more efficient inincreasing exchangeable Ca. Using the same approach fol-lowed above it can be shown that only about 2% of theapplied Ca cannot be accounted for and could, thus, beassumed lost through leaching, along with the associatedNO3~. At the highest rate of application, on the other hand,nearly 60% of the Ca cannot be accounted for.

While the results of this study do not permit an accurateevaluation of the equivalent basicity of Ca(NO3)2, it isobviously significant. If equivalent basicity is calculatedfrom increased exchangeable Ca, 1 kg N would be equiva-lent to 2.5, 2.0 and 1.7 kg CaCO3 at the 448-kg/ha(400-lb/acre), 896-kg/ha (800-lb/acre), and 1,344-kg/ha (1,200-lb/acre) rates, respectively. This is con-siderably higher than the commonly accepted figure of 1.3.The residual basicity of the Ca(NO3)2 is clearly reflectedin soil pH increases throughout the profile. These rangedfrom 0.3 unit in the 60- to 75-cm zone to 0.5, 0.8, and 0.9in the 45- to 60-, 30- to 45-, and 15- to 30-cm layers,respectively. The trend toward somewhat higher exchange-able Ca values at higher rates of Ca(NO3)2 applicationwithout concomitant increases in pH are possibly a resultof displacement of some Ca salt or dissolution of smallamounts of a difficultly soluble compound during extrac-tion.