Crop Response to Soil Acidity Factors in Ultisols and Oxisols: I. Tobacco1

FERNANDO ABRUNA-RODRIGUEZ, JOSE VICENTE-CHANDLER, ROBERT W. PEARSON, AND SERVANDO SiLVA2

ABSTRACTYields of tobacco (Nicotiana tabacum L.) on three Ultisols in

Puerto Rico increased with liming up to about pH 5.0. Therewas a highly significant inverse relationship between yield andexchangeable Al. Maximum yield was generally reached asexchangeable Al approached 0 and at base saturation valuesof around 60%. Yields on an Oxisol increased with liming toabout the same pH and base saturation level, although onlytraces of exchangeable Al were found. Leaf analysis indicatedthat Mn toxicity was a probable factor in the Oxisol. Tobaccoroot growth, studied in one of the Ultisols, was directly affectedby low soil pH. Al toxicity appeared to be the chief cause ofrestricted growth but Ca deficiency was a possible contributingfactor.

Additional Key Words for Indexing: base saturation, ex-changeable Al, root growth, calcium deficiency, aluminumtoxicity.

Ew crop yields obtained on Ultisols and Oxisols seem tobe closely related to soil characteristics such as high

aluminum and manganese concentrations, and low ex-

changeable base status accompanying low pH. The useof fertilizers that leave acid residues aggravates the prob-lem if a well-planned liming program is not implemented.These problems tend to slow down the development ofagriculture in the humid tropics where the world's greatestpotential for increased food production lies.

Most of the soils of Puerto Rico are moderately tostrongly acid (Samuels, 1962) but little information isavailable on the specific factors influencing yield and com-position of typical crops grown under intensive manage-ment on acid soils. However, sugarcane (Lugo-Lopez etal., 1959), pineapple (Pennock, 1950), and velvet beans(Bonnet et al., 1945) have been found to respond to limeon various soils of the Island. Tropical kudzu growing inassociation with molasses grass responded strongly in yieldand protein content to lime (Caro-Costas and Vicente-Chandler, 1963).

Heavy fertilization with residually acid materials as isnow used with most crops in Puerto Rico can rapidly in-crease soil acidity. Abrufia and co-workers (1958) foundthat the application of 896 kg N/ha (800 Ib N/acre) asammonium sulfate yearly over a 2-year period sharply in-creased acidity and loss of bases in typical soils of thehumid region of Puerto Rico. Similarly, Samuels and Gon-zalez (1962) report that applications of ammonium sulfateto sugarcane over long periods increased acidity of avariety of soils.

630 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

With heavy fertilization, the response of crops to limingcan be very strong. Abruna and co-workers (1964) foundthat liming doubled the yields of heavily-fertilized Napiergrass growing on an acid Ultisol. Abruna and Vicente-Chandler (1967) found that yields of heavily-fertilizedsugarcane growing on a typical Ultisol ranged from 2 to105 metric tons/ha (1 to 47 tons/acre) yearly, depend-ing on exchangeable base and Al content of the soil. Foliarcomposition of the cane was not affected by soil aciditylevel. Abruna et al., (1965) reported that intensively man-aged coffee responded strongly to liming on acid soilshigh in manganese, but did not respond to liming of acidsoils low in manganese even though exchangeable alu-minum was high.

The present series of studies investigated the effects ofvarious factors associated with soil acidity on yields andfoliar composition of major tropical crops on four typicalsoils of the humid tropics.

MATERIALS AND METHODS

Field Experiments

SOIL CHARACTERISTICSThe characteristics of the four soils are presented in Table 1.

Cation-exchange capacity was determined by the NH4OACextraction procedure. Calcium and Mg were determined byVersenate titration using Calcein as Ca indicator and Calmagiteas Ca-Mg indicator. Potassium was determined by flame pho-tometry and Mn colorimetrically after oxidation with periodate.Exchangeable Al was extracted with IN KCL and determinedby the aluminon method and soil reaction was measured witha glass-electrode pH meter using a 1:1.5 soil/water suspension.

SAMPLING PROCEDURESExperiment sites were divided into from 30 to 60 plots of

136 m2, each arranged in a complete randomized-block designwith treatments replicated five times. The soil in all plots ofall four experiments was sampled 4 and 8 months after limingto determine its acidity status. At each sampling twelve coreswere taken in each plot at 0- to 20-cm depth, composited,air-dried, and passed through a 20-mesh sieve before deter-mining exchange capacity, and exchangeable Ca, Mg, K, Al,and Mn. Leaf samples were taken 40 days after planting. Thethird leaf from the growing tip of 10 plants in the centralrows of each plot was sampled for analysis. The samples werewashed with distilled water, dried, and analyzed for N, P, K,Ca, Mg, and Mn.

CROP MANAGEMENTIncrements of Ca(OH)2 were added as required to provide

a wide range in acidity and percent base saturation. TheCa(OH)2 was thoroughly worked into and mixed with theupper 15 cm of soil.

At planting time, all plots were fertilized at a rate of 202kg of nitrogen as urea, 146 kg of P from triple superphosphate,224 kg of K from potassium sulfate, and 34 kg of Mg as mag-nesium sulfate per ha. The fertilizer was broadcast evenly overeach plot.

Tobacco seedlings. (Nicotiana tabacum L.) of the PR 1-60variety were planted 46 cm between plants in rows 91 cmapart. At the Coto site, PR 264 variety was used. The experi-ments on Corozal clay and Corozal subsoil and on Coto claywere irrigated as required.

Buggetta was broadcast around plants at planting time to

Table 1—Soil characteristicsSoiltype

Humatasclay

Corozalclay

Corozalclay subsoil

Cotoclay

Location

Orocovls

Corozal

Corozal

Isabela

Classification

Typlctropohumults

Aquictropudult

Aquictropudult

Tropeptichaplorthox

Origin

Volcanictuffs

Tuffaceousmaterial

Tuffaceousmaterial

Limestonequartzitlcsand deposits

OM%

3

4

2.5

2.0

InitialCEC pH

meq/lOOg

15 4.0

19 4.9

16 4.3

5 4.4

BDg/cn?

1.1

1.2

1.3

1.1

control slugs. Periodical spraying with 15% Parathion main-tained excellent control of leaf insects. (Mention of a trade-mark or proprietary product does not constitute a guaranteeor warranty of the product by the U.S. Department of Agri-culture, and does not imply its approval to the exclusion ofother products that may also be suitable.)

Green weights of leaves harvested periodically were recordedat each stripping. Subsamples from each plot were taken andcured at an experimental curing barn at the AgriculturalExperiment Station of the University of Puerto Rico.

ROOT GROWTH STUDIESIn order to define the specific effects of soil acidity on

tobacco root development as differentiated from overall plantgrowth, several experiments were carried out with the Humatasclay.

Experiment 1—In the first experiment, Humatas surfacesoil at pH 3.7 was limed with MgCO3, or CaCO3, or a 1:1combination of the two materials to give a pH in each caseof 5.6, at which point no Al could be detected in the displacedsoil solution. The treated soil was fertilized with (NH4)2HPO4and K2SO4, potted, and tobacco grown for 5 weeks in thethe greenhouse. Both top and root yields were measured.

Experiment 2—In the second experiment a split-root potexperiment was run in which the acid Humatas soil formedthe subsoil, overlaid by a 10-cm layer of the same soil typefrom a field plot that had been limed to over 60% basesaturation and fertilized at the rate of 1,344 kg of 14-4-10per ha. Three rates of lime were used in the subsoil materialto give pH values of 5.7, 4.8, and 4.2, corresponding toexchangeable Al levels of 0, 3.5, and 7.0 meq/100 g. Tobaccoplants were grown for 4 weeks after transplanting in thegreenhouse, after which the roots that had developed in thesubsoil were washed out on sieves, dried, and weighed.

Experiment 3—In a third experiment the Humatas soil atthree pH levels similar to those used in the preceding test(4.2, 4.8, 6.6) was placed in coarsely-woven saran mesh bags15 cm long, fitted into 13-cm diameter holes drilled betweenyoung tobacco plants in the field. The bags were cylindricaland were placed in intimate contact with the sides of theholes so that the mass of soil contained lay at a depth of5 to 20 cm. After 4 weeks the bags were recovered and theroots growing within them washed out, dried, and weighed.

Experiment 4—In the final experiment the glass-front boxtechnique was used with Humatas Ap soil material forming thesubsoil, overlaid by a 10-cm layer of well-fertilized and limedsurface soil similar to the one used in Experiment 2. There werethree treatments: (i) subsoil untreated (pH 5.1, 3% calciumsaturated), (ii) subsoil limed to pH 5.7 with MgCO3, and(iii) limed to pH 5.7 with CaCO3. Tobacco seedlings weretransplanted to the boxes and grown until radicle length inthe best treatment was about 26 cm. Radicles were measuredat regular intervals.

Soil solution was displaced by the method described byKhasawneh and Adams (1966) and analyzed for Ca, Mg, Na,K, and Al for exchangeable cations, as described above. Molaractivity of Al was calculated using the Debye-Huckel equationfor single ion activity (Kielland, 1937).

ABRUNA-RODRfGUEZ ET AL.: CROP RESPONSE TO SOIL ACIDITY: TOBACCO 631

RESULTS AND DISCUSSION

Interrelationships Between Soil Acidity Factors

Since the actual pH, aluminum, and base saturationvalues did not correspond exactly with the theoreticallyestimated levels, regression analysis was used to interpretthe results.

Data for the four soils combined show that pH valuesincreased from about 3.8 with a base saturation of around20% to about pH 5.0 with a base saturation of around80% (Fig. 1). The low pH values in relation to exchange-able base contents of these soils are explained by the proba-ble presence of free salts resulting from heavy fertilization.These results also show a strong tendency toward a lowerpH at a given percent base saturation in the Humatas claythan in the other soils.

Figure 2 shows that the level of Al saturation decreasedfrom about 40% at pH values of approximately 4.0, toless than 5% at pH values of around 5.0. Essentially noexchangeable Al was found above pH 5.0. Thus, by limingto about pH 5.0, equivalent to around 70% base satura-tion, exchangeable aluminum content of these soils wasreduced to what should be non-toxic levels for most crops.A very close relationship between percent Al saturationand pH is indicated by the r value of 0.87. Again, theHumatas soil shows definite indications of being somewhatdifferent from the others in that Al saturation in this soiltended to be lower at a given pH level. Cation exchangecapacities used in these calculations were determined withammonium acetate at pH 7.0. Thus, some pH-dependentcharge is included and base saturation levels appear some-what higher than if some of cations were used.

CROP RESPONSE

Experiment on Humatas Clay

The Humatas clay at this location is high in exchange-able Al at low pH values, but is low in exchangeable and

Table 2—Tobacco yield and foliar composition at differentlevels of base saturation of a Humatas clay

Basesaturation

121824303642485373

%- 17-23- 29- 35-41-47-53- 73-93

Yieldsof curedtobacco

236380630828

1,0431,3251,6171,6881,686

Dry weight composition ofN

3.963.863.833.623.963.763.633.553.60

P

0.430.390.400.450.470.480.450.440.46

K

4.574.724.524.764.254.264.863.903.92

Ca

1.521.711.812.001.851.771.841.851.87

leavesMg

0.150.210.250.200.240.230.260.240.25

Mn

220251274298200160220142140

easily reducible Mn. Table 2 shows that yields of tobaccoincreased with percent base saturation up to about 60%.Yields were increased over sevenfold by liming to increasebase saturation from the 12 to 17% range to the 53 to73% range. Degree of base saturation of the soil had nooverall effect on N, P, K, Mg, or Mn content of the tobaccoleaves, but Ca content showed a definite tendency toincrease as base saturation increased up to about 30%.

There was a highly significant inverse relationship be-tween tobacco yield and percent saturation of the exchangewith Al (r = —.82**). Yields of cured tobacco decreasedfrom around 1,700 kg/ha when degree of Al saturationwas less than 10%, to less than 400 kg/ha when aluminumsaturation exceeded 40%. This relationship is reflected inthe yield-soil pH curve with an r value of —.80. Yields ofcured tobacco increased from almost 0 at pH levels in theneighborhood of 3.5 to nearly 2,000 kg/ha at pH levelsof about 5.0.

There was no significant relationship between eitherMn or Ca content of the tobacco leaves and yields, orbetween soil pH or base saturation and Ca, P, or Mn con-tent of the tobacco leaves.

Experiment on Goto Clay

This soil was very high in Mn (200 ppm exchangeableand 2,000 ppm easily reducible) but had practically no

pH

— Humatos ClayA- Goto Cloy0-CorozalCldyX-CorozdCkiy Subsoil

•r-76 ,Y-3-I76+OO39X-0-OO02X2

2O 4O 6O 8OPERCENT BASE SATURATION

Fig. 1—Relationship between pH and percent base saturationof the exchange complex of four typical soils of the humidtropics.

50

40

i 30

! 20

3-5

•-Humatas Clay0-CorozalClayX-Corozal Clay Subsoil

r=-87**Y=5-I99-0-058X+0-0006X2

4-5 5-0 6-0

Fig. 2—Relationship between pH and percent Al saturationof the exchange complex of three typical soils of the humidtropics.

632 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

Table 3—Tobacco yields and foliar composition at differentlevels of base saturation of a Goto clay

Basesaturation

32 - 4041 -4950 - 5859 -6768 - 7677 - 859 6 - 9 495 +

Yieldsof curedtobacco

262463503637571539540682

Dry weight composition of leavesN

3.723.723.783.663.693.723.713.73

P

0.300.360.370.410.340.360.370.35

K

4.154.004.283.753.774.033.653.88

Ca

2.042.021.711.641.712.402.572.23

Mg

0.510.440.620.610.340.330.350.31

Mn

2,3801,4201,1701,0001,060

660760580

exchangeable Al (less than 10 ppm exchangeable Al) evenat the lowest pH level tested. It was, therefore, well-suitedto studying the effects of Mn, as affected by soil acidity inthe absence of toxic levels of Al, on crop yields.

Yields of tobacco increased from about 260 kg/hawhen base saturation of the soil was at 32 to 40% to over600 kg/ha when it was at 59 to 67% (Table 3). Base satu-ration had no consistent effect on N, P, K, Ca, or Mg con-tent of the tobacco leaves. However, Mn content of theleaves dropped from an average of 2,380 ppm at 32 to40% soil base saturation to 580 ppm at over 95% basesaturation. This compares with values of 100 to 200 ppmof Mn in tobacco leaves on the other three soils studied.

The fact that yields of tobacco tended to increase withdecreasing Mn content of leaves, and composition withrespect to other ions was not affected significantly, sug-gests that the response of tobacco to liming on this soilwas due to a reduction in Mn toxicity, although Mn contentof the tissue did not quite reach the 3,000 ppm level thatHiatt and Ragland (1963) found caused typical Mntoxicity in Burley tobacco.

Experiment on Corozal ClayYields of tobacco decreased with increasing percent Al

saturation (r = —.55**) and increased with pH (r = .56**)of the Corozal clay. Yields increased from an average of1,415 kg/ha at 25 to 35% base saturation to 1,900 kg/haat 47 to 57% saturation (Table 4). The data also showthat base saturation of this soil had no effect on N, P, K,Ca, or Mg content of the tobacco leaves. There was no

lOOr—m>——————————i———————————————————

so

60

20

_ o

x •

— HUMATAS CLAY0-COROZAL CLAYX-COROZAL CLAY SUBSOIL

I0 10 20 30 40 SO

PERCENT ALUMINUM SATURATIONFig. 3—Relationship between percent Al saturation of three

typical soils of the humid tropics (Ultisols) and percent ofmaximum yields produced by tobacco.

Table 4—Tobacco yield and foliar composition at differentlevels of base saturation of a Corozal clay

Soil basesaturation

%25 - 3536 -4647 - 5758 - 72

Table

Soil basesaturation

%22 -2829 - 3536 -4243 -4950 -60

Yieldsof curedtobaccokg/ha1,4151,4811,9001,829

Dry weight composition of leaN

3.233.303.403.34

P

0.310.320.410.34

5 — Tobacco yields and foliarlevels of base saturation of a

Yieldsof curedtobaccokg/ha

488815

1,0301,1441,384

K

-% ———3.613.003.963.48

Ca

1.912.012.241,87

vesMg

0.190.170.230.22

Mnppm151135147

83

composition at differentCorozal subsoil

Dry weight composition of leaN

3.553.453.563.473.41

P

0.440.390.430.310.36

K

3.033.333.183.683.46

Ca

2.082.152.222.122.02

ivesMg

0.290.310.230.250.22

Mnppm154116120

9393

significant relationship between pH or exchangeable basecontent of the soil and the Ca and P content of the tobaccoleaves.

Experiment on Corozal Clay Subsoil

Yields of cured tobacco on Corozal subsoil decreasedfrom over 1,500 kg/ha at less than 10% Al saturation toless than 500 kg when Al saturation approached 40%(r = —.78**). Similarly, yields increased from less than500 kg/ha at pH levels of about 4.0 to around 1,500 kg atpH levels approaching 5.0 (r = .68**). Yields increasedwith percent base saturation from less than 500 kg/ha at22 to 28% saturation to over 1,300 kg in the 50 to 60%base saturation range (Table 5). Base saturation had no ap-parent effect on the N, P, K, Ca, or Mg content of thetobacco leaves but Mn content decreased slightly with in-creasing soil base saturation.

All Soils CombinedRelative yields produced by tobacco, considering the

80

S|3txIs

S60

40

2Or=.72** ,Y*-446-6+l87-8X-l7-OX':

—CIALITOS CLAYi-COTO CLAYO-COROZAL CLAYX- COROZAL CLAY SUBSOIL

3.* 4-0 4-5 5-OpH

5-5 6-O

Fig. 4—Relationship between pH of four typical soils of thehumid tropics (Ultisols and Oxisols) and percent of maxi-mum yields produced by tobacco.

ABRUNA-RO0RIGUEZ ET AL.: CROP RESPONSE TO SOIL ACIDITY: TOBACCO 633

three soils that had significant amounts of exchangeable Al,decreased from around 80% when Al saturation was 0, toonly 10% when Al saturation level was above 40% (Fig. 3).

The combined relative yields for all soils were closelyrelated to pH as shown on Fig. 4. Yields increased fromessentially no crop at pH 3.5 to nearly 80% at pH 5.5.

A similar correlation was obtained for percent base satu-ration and cured tobacco yields. The regression analysisshown in Fig. 5 showed that yields increased sharply as thepercent base saturation increased from 20 to 60% andfrom there on at a slower rate.

EFFECTS OF ACIDITY ON TOBACCO ROOTS

Experiment 1—The results of Table 6 show clearly thatsimply correcting soil acidity to a point where Al toxicitywould not be a factor was inadequate for maximum growthof tobacco roots on the Humatas clay. When acidity wasneutralized with MgCO3, Ca concentration in the soil solu-tion was very low and the Ca/total cation ratio was farbelow the 0.2 level previously shown to be required fornormal cotton root elongation (Howard and Adams,1965). As a result, only 5% relative root growth occurred.When the same soil pH was brought about by addition ofCaCO3, the level of Mg in the soil solution was extremelylow, and both top and root growth were severely restricted.In fact, little more than 50% as much root or top yieldresulted as when both CaCO3 and MgCO3 were addedto raise soil pH to 5.6. Thus, both Ca and Mg were limitingas plant nutrients for overall tobacco plant growth in thissoil. However, it is not possible to differentiate betweendirect effects of deficiencies on the roots and indirect effectscaused by poor top growth in this experiment. The nearconstant root/top ratio observed over such a wide rangein plant growth indicates a very strong interaction of rootand top growth.

The next three experiments were designed to measuredirect effect of soil acidity on tobacco root development.All utilized types of the split-root technique.

Table 6—Response of tobacco to Ca and Mg in potexperiments with Humatas clay

80

LJ li.

§°40

20Y*-l7-07+2-l5X-O-l3X'

—CIAUTOS CLAYfl-COTO CLAYO-COROZAL CLAYX-COROZAL CLAY, SUBSOIL

20 40 60PERCENT BASE SATURATION

80 100

Cation content of soilsolution meq/llter

Treatment*MgCOiCaCOjCaCO5 Ht MgCOj

Ca2

3013

Mg N a - f K21

2 216 2

Ca:total

cations0.080.900.42

Rel.top

yield7

65100

Rel.rootyield

553

100

Root-top

ratio554

' Soil pH adjusted to 5.6 by additions of materials indicated. Untreated soil pH was3.7.

Table 7—Relative tobacco root yield in Humatas clay used asthe subsoil of model profiles in a greenhouse experiment

in relation to certain soil properties

Soil pH

4.24.85.7

ExchangeableAl

meq/100 g6.953.600.10

Activity ofsoil solution

AlpM

80.02.90.0

Relativeroot yield

%3265

100

Fig. 5—Relationship between percent base saturation of fourtypical soils of the humid tropics (Oxisols and Ultisols) andpercent of maximum yields produced by tobacco.

Experiment 2—The results of this experiment, presentedin Table 7, show that tobacco root growth into acid sub-soil in a greenhouse experiment using a model profile inpots was progressively increased as level of acidity, ex-changeable Al, and soil solution Al activity decreased. Rela-tive root yield in the subsoil at pH 4.2 was only 32%, andmolar activity of Al in the displaced soil solution was 80ftM, which is far above levels known to be toxic for cottonroots (Adams and Lund, 1966). Even at pH 4.8 Al activityin the soil solution was 3.0 /jM, and root yield was only65% of that made in the absence of Al. Thus, root growthin the acid soil horizon decreased sharply with increasingamounts of exchangeable Al, and the molar activity of Alin the displaced soil solution was above the expectedtoxic level.

Experiment 3—This experiment was performed in orderto check the direct effect of acidity on tobacco rootgrowth under field conditions using small quantities oftreated soil implanted in mesh bags. The results confirmedthe short-term pot experiment results presented above andindicate that either Al toxicity or Ca deficiency, or both,were directly restricting root growth of plants grown tomaturity. Relative root weights at pH 4.2 and 4.8 were33% and 62%, respectively. Since in this test only a verysmall fraction of the root system of any one plant was sub-jected to treatment, it is highly unlikely that overall plantgrowth could have been affected. Thus, observed effectsare assumed to have been a direct response of the roots toeither Al toxicity, Ca deficiency, or both.

Experiment 4—In the final experiment using the glass-front box technique, treatments of the subsoil materialwere devised to differentiate between Al toxicity and Cadeficiency for tobacco root growth in the Humatas B2material. The B2 horizon material was used in this experi-ment because of its somewhat lower level of exchangeableCa (0.4 meq/100 g) than occurred in the surface soil atsimilar pH values. The results presented in Fig. 6 showthat root elongation stopped soon after roots entered thesoil layer at pH 4.0 and 4% Ca saturation. When Al wasprecipitated by liming to 5.6 with MgCO3, maintainingCa at its previous level, root growth continued at a nearlyconstant, but relatively low, rate for about 60 hr, afterwhich elongation essentially ceased. However, when Al wasneutralized and Ca level raised to about 4 meq/100 g at

634 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

24 48

(HOURS)

Fig. 6—Effect of soil pH and degree of Ca saturation ontobacco taproot elongation in Humatas clay used as sub-surface horizon in a glass-front box experiment.

the same time, root elongation continued at a rapid ratethroughout the experiment.

It seems clear from this series of experiments thattobacco roots were subject to direct injury by both Altoxicity and Ca deficiency in the strongly acid Humatassoil. It is further likely that low subsoil pH levels, evenwhere surface soils have been adequately limed, may influ-ence tobacco yields through restriction of rooting depthand reduced ability to exploit subsoil moisture duringperiods of moisture stress.

SUMMARYThe relationship between soil characteristics related to

acidity and yields and foliar composition of tobacco wasdetermined on four typical soils of the humid tropics.

Yield of cured tobacco on Humatas clay was increasedfrom about 230 kg to over 1,600 kg/ha by liming the soilfrom approximately 20 to about 60% base saturation.Yields increased as pH went up from 4.0 to 5.5, and de-creased with increasing exchangeable aluminum fromabout 10 to 40% saturation. Foliar composition of thetobacco was not affected by liming.

Yields of low-yielding chewing tobacco on Goto clayincreased from about 260 to over 600 kg/ha when soilbase saturation was increased from about 35% to around60%. This soil had very small amounts of exchangeableAl. Yields tended to increase with decreasing Mn contentof the tobacco leaves (from about 2,000 to 500 ppm) sug-gesting that Mn toxicity was a factor affecting tobaccoyields on this high-manganese soil. Liming did not other-wise affect foliar composition of the tobacco.

Tobacco yields on Corozal clay increased from over1,400 kg/ha at around 30% base saturation to approxi-mately 1,900 kg at about 55% base saturation. Tobaccoyields decreased as Al saturation increased from less than10 to nearly 30% and increased as pH went up from about4.5 to 5.5. Liming had no marked effect on foliar com-position of tobacco on this soil.

Tobacco yields on Corozal subsoil increased fromaround 480 to nearly 1,400 kg/ha when base saturationwas increased from 25 to 55%. Yields increased sharplyas pH increased from nearly 4.0 to about 5.0 and decreasedas exchangeable Al content increased from less than 10 tonearly 40% saturation. Liming did not appreciably affectfoliar composition of the tobacco.

Data for all soils combined show that yields of tobaccoincreased sharply with liming up to about 60% base satu-ration or about pH 5.0, and decreased with increasingexchangeable aluminum content above about 10% satura-tion of the soils exchange capacity.

Tobacco root growth increased as pH increased up toaround 5.8 in the Humatas clay. As soil pH decreased fromabout 5.0, the molar activity of Al in the soil solutionincreased rapidly to levels far above those known to betoxic to some plant roots. Neutralization of the Al withMgCO3 improved tobacco root growth but did not resultin maximum growth in Humatas B2 material until Ca wasadded. It thus appeared that in the Humatas soil as pHlevels well below 5.0 both Al toxicity and Ca deficiencywere limiting for tobacco root growth. It therefore seemslikely that restricted root development was an importantcontributing factor to the tobacco yield reductions observedin these strongly acid soils.

BARBER AND OZANNE: EFFECT OF FOUR PLANT SPECIES IN ALTERING THE CALCIUM CONTENT OF SOIL 635