Genesis, Mineralogy and Related Properties of West Indian Soils: I. Bauxitic Soils of Jamaica

N. AHMAD, ROBERT L. JONES AND A. H. BEAVERS2

ABSTRACTThe red and brown bauxite soils of Jamaica occurring over

hard, pure white limestone are strongly aggregated, freely drainedsoils of almost pure clay texture. They are of low fertility, nitro-gen and potassium being very deficient. The red bauxite isdeficient in available phosphate but not responsive to phosphorusfertilization. Phosphorus fractionation data show both soils to beadequately supplied with phosphorus. The brown bauxite ismuch higher than the red in total phosphate and in aluminum-and calcium-bonded forms. Both soils are very high in iron-bonded phosphate with the red bauxite having a greater per-centage of the total phosphate in reductant soluble form. Chemicalanalysis shows the soils to be very high in A12O3 content withlittle or no difference between the two soils. Silica content is verylow. Iron is high and virtually all of it is present in dithioniteextractable form. No minerals containing potassium seem to bepresent in the soils. Mineralogical study shows the A12O3 to bepresent as gibbsite and boehmite in a ratio of 2:1 for bothsoils. The soils had less than 10% kaolinite, the brown bauxitebeing higher. Traces of chlorite were identified in both soils, andtraces of montmorillonite were found in the deeper layers of thebrown bauxite. Titanium was present as rutile and anatase inthe brown bauxite but only as anatase in the red. The red bauxiteis richer in goethite. The low content of silicates in the bauxitesis associated with a low SiO2 content of the insoluble residue ofthe limestone and this is evidence for possible genesis of thebauxites from the underlying limestone.

BAUXITE SOILS make up about five-eighths of Jamaica andoccur over hard Oligocence and Lower Miocene lime-

stones in the central parts of the island. They were recognizedas bauxite deposits in 1942 as a result of soil fertility investiga-tions but actual mining and processing of the ore followedmuch later. The nature and extent of occurrence'of the orehas been described by Hili (3) and Hose (4). Hill alsoexamined the mineralogy of ore bodies and concluded thatgibbsite, boehmite, iron oxide, kaolinite, and quartz were themost abundant minerals. The mineable deposits representonly a small fraction of the total area occupied by bauxitesoils. Most deposits are too thin for mining or even foragriculture.

The origin or these superficial ore deposits is still an

unsettled question. Hill (3) concluded that the limestoneresidue was the source of the bauxite. Zans (9) and Burns (1)thought that volcanic material weathering over limestone wasthe main source. Based on their low silica content, Hose (4)concluded that in any event, volcanic ash could only haveaccounted for a small amount of the bauxite, the bulk comingfrom limestone residues.

Two kinds of bauxite soils are recognized, chiefly by color.The more widely distributed is red and mapped as "redbauxite" or St. Anns clay loam; the other is brownish yellowand mapped as "brown bauxite" or Chudleigh clay loam.Both soils are called terra rossa locally. Both ore bodies seemto have about the same content of aluminum and the differencein color has been attributed to a difference in content andstate of hydration of iron oxide. Agriculturally these soilsare different in important ways. The red bauxite has anextremely stable structure throughout the profile with littlewater retention, so that moisture becomes severely depletedin the dry season. The soil is very responsive to N and Kfertilization. Phosphate deficiency is observed but responseto fertilization is not attained due to high phosphate-fixingcapacities.

In contrast, the brown bauxite is well-structured in thesurface horizon but becomes massive with depth especiallyif the profile is deep. The soil has better moisture-supplyingproperties and deeper penetration of organic matter. Thebrown bauxite is as deficient in K but not as responsive as itsred equivalent to N fertilization. Phosphorus is available inadequate amounts.

The soils support a wide range of agriculture on the island.The various bauxite mining companies have large areas ofboth soils under well-developed pastures for beef cattle as wellas citrus. Other crops of importance are pimenta (Pimento,dioica), bananas (Musa paradisiaca) and food crops. Under na-tive occupation a mixed culture of food crops and fruitsis characteristic.

The aim of this investigation is to determine more of thefundamental properties of the Jamaican bauxites as agriculturalsoils. The mineralogy of these soils is studied in detailand inorganic P is fractionated into aluminum-bonded,iron-bonded, reductant-soluble and calcium-bonded forms.Amounts of other nutrients such as K, Ca, and Mg have alsobeen assessed.

SOILS STUDIED AND METHODSProfile samples were taken from typical red and brown bauxite

soils, the profiles having the following descriptions:

720 SOIL SCI. SOC. AMEH. PROC., VOL. 30, 1966

Table 1—Mechanical and chemical analyses of Jamaican bauxite soils

Mechanical analysis

Coarse sand2-0.2 mm

Fine sand0.2-0.02 mm

Silt.02-. 002 mm

————————— pH CClay

< .002 mm

Total

ablebases

Na

inches

0-66-12

12-2424-36

-% oven dry soil-

201319

76

9793

% % ppm

Red Bauxite Soil

6.16.16.37.1

4.22.2

0.500.28

12.06.94.02.3

12.37.03.82.4

meq/lOOg soil

8.44.82.53.0

3.33.3

0.800.40

0.310.190.030.02

0.510.320.180.39

Brown Bauxite Soil

0--66-1.212-2424-3636-48

00000

11000

1916666

7587979798

6.66.66.66.56.7

4.31.9——

~

0.360.26———

269304344

13.46.42.23.13.1

13.36.42.22.93.0

12.85.81.92.52.4

1.500.500.300.600.60

0.190.080.020.010.01

0.400.120.150.120.11

Red Bauxite SoilRough Guinea grass pasture, (Panicurn maximum jacq.)

Walclers Wood, Parish of St. Anns.

Depth,indies

0-6 Dusky red (7.5 R 3/3) clay with extensive pro-liferation of roots; very strong granular structure;few small pieces of limestone randomly distributed.

6-12 Dusky red (7.5 R 3/4) clay; fine granular structure;with fewer roots.

12-24 Dark red (7.5 R 3/6) clay; strong blocky structure;few thin roots.

24-36 Dark red (7.5 R 3/6) clay with large blocky structureon drying, some evidence of clay movement into thislayer; very few roots; horizon separated with sharpboundary from White Limestone formation below.

Brown Bauxite SoilRough Guinea grass pasture, Greenfield, Parish of St. Anns.

Depth,inches0-6

6-12

12-24

24-36

36-43

Very dark grey brown (10 YR 3/2) clay; greatproliferation of roots; very strong crumb structure.Dark yellow brown (10 YR 4/4) clay; very strongcrumb structure; fewer roots.Yellow brown (lO YR 5/4) clay; fine granularstructure, roots not present below 20 inches.Changing to dense yellowish brown (10 YR 5/8)clay with nodules of up to 1 cm diameter, clearlydefined but generally not indurated.Same. Horizon separated with sharp boundaryfrom White Limestone formation below.

Mechanical analysis was done by the hydrometer method (6)Determinations of cation-exchange capacity (CEC) and ex-changeable cations were based on the ammonium acetate leachingmethod (6), carbon was determined by the Walkley-Black wetcombustion method, and nitrogen by the micro-Kjeldahl method(6). Available phosphate was estimated by the Truog method(5), free iron oxide by reduction with sodium dithionite (5), andsoil phosphorus was fractionated by the method of Jackson (5)except that extraction of occluded phosphorus was done withO.LVNaOH solution.

The soil was separated into sand, silt, and clay fractions bydispersion with sodium hexametaphosphate, sieving, and repeatedcentrifugation. An ultrasonic vibrator was used to aid redisper-sion after each centrifugation.

X-ray spectrographic analyses were carried out on the clayand silt fractions. Both clay and silt were fused and prepared inthe manner described by Rose et al. (7). X-ray diffraction analyseswere performed on sodium- and strontium-saturated samples inethylene glycol-solvated condition using Cu radiation.

RESULTS AND DISCUSSIONGeneral Soil Properties

Both bauxite soils are very fine-textured with clay contentof 75 to 98% (Table 1). The reported values of silt and sandmay be misleading since subsequent ultrasonic treatment ofsuspensions resulted in complete breakdown of the whole soilto clay-size material. Some aggregated clay particles well-stabilized by iron oxide films were not dispersed by routinetreatment in mechanical analysis and were estimated as sandor silt. St. Ann's clay loam for the red bauxite and Chudleighclay loam for the brown are therefore misleading terms. Theoutstanding feature of these soils is the low cation-exchangecapacity values for soils composed almost entirely of clay.Values for surface horizons are somewhat higher and this maybe explained by higher organic matter content. The two soilsdiffer in Mg, K, and P status. The red bauxite has higherlevels of exchangeable Mg and K but no Truog phosphorus.Exchangeable K is extremely low for both soils.

Phosphate StatusHill (3) showed that total P205 for bauxite ore in Jamaica

was between 0.1 % to 5.0%, the higher values being associatedwith brown bauxites. This might indicate that the agricul-tural soils on these materials may also be high in total P206.However, field experience has shown that P-availability toplants grown on red bauxite is inadequate. This is confirmedby Weir (8) who demonstrated that the brown bauxite

Table 2—Fractionation of soil phosphorous for red and brownbauxite soils

Depthinches

0-612-2424-36

Iron-P

145286385

Reductant Alumi- Occludedsoluble-P num-P Al-P &

Pe-P

Red Bauxite

1,000 28 701,262 26 881,562 11 131

Cal-cium-P

768755

Total-P

1,9722,4252,750

0-612-2436-48

8201,3101,750

1,5501,1501,800

Brown Bauxite

41140200

223198256

190286152

4,8505,0006,037

AHMAD ET AL: PROPERTIES OF WEST INDIAN SOILS; JAMAICA

Table 3—Chemical analysis of the clay fractions of the bauxite soils

721

Depth

inches

0-612-2424-36

Loss onignition

28.627.024.2

SiOz

4.424.393.17

AUOs

43.6644.6545.71

FeiO3

19.5320.5322.04

FezO«reducible

/v

Red Bauxite Soil

17.5817.3718.87

CaO K2O

0.20 0.100.25 0.110.25 0.08

Ti02

1.481.501.47

CEC

7.46.64.0

SiOi/AhO

0.160.160.11

Brown Bauxite Soil

0-012-2436-48

20.422.421.2

0.12' 0.94

7.60

37.4244.5145.45

14.6419.1818.84

14.0117.2317.23

0.510.620.54

0.180.140.14

1.321.511.51

7.65.06.0

0.270.260.28

supplies much more P to plants than the red. It is recognizedin Jamaica that P availability in the brown bauxites isadequate but plants may show severe phosphate deficienciesin the red bauxites. Neither soil responds to phosphatefertilization and consequently phosphate fertilizers are notused. Results of fractionation of the total soil P are given inTable 2. The brown bauxite had twice as much total P asthe red and is higher in all inorganic P fractions. Ironphosphates (sodium hydroxide-extractable and reductant-soluble forms) were the most important source of inorganic Pfor both soils and accounted for 49% of total P in the surface,60% deeper in the profiles. In the red bauxite profile,reductant-soluble P was more than four times as much asNaOH-extractable but in the brown bauxite only twice asmuch in the surface horizon, any difference disappearing withdepth. Aluminum phosphate was about 0.05% of the total Pin the red bauxite and 0.05 to 0.07% of total P in the brownbauxite. The low amount of aluminum phosphate is anunexpected feature of such highly aluminous soils. Calciumphosphate decreased with depth in both profiles. The higherlevel of this fraction in the brown bauxite may have resultedin the higher Truog values reported in Table 1. The greaterinherent P-supplying capacity of the brown bauxite soilcompared to the red can be partly attributed to higher total P,almost half of which is in the NaOH-extractable or surface-held iron phosphate and larger amounts of calcium-bonded Pand organic P.

MineralogyResults of the chemical analyses of the clay and silt fractions

are given in Tables 3 and 4. The SiO2 content of the clayfraction is very low for both soils. Aluminum content, asindicated by SiO2/Al203 ratios, is very high for agriculturalsoils although no toxicity to plant life was observed. Thetotal Fe content is abnormally high for soils with the redbauxite only slightly higher than the brown. Virtually all ofthe Fe can be extracted by sodium dithionite treatment whichbleaches both soils, leaving a chalk-white residue. Raresilt-size magnetite crystals were conspicuous in the bleachediron-extracted samples of both soils. The Ca content of allfractions of the soils is very low, indicating the extent to whichleaching and weathering of the parent limestone has takenplace, the red bauxite being more affected. Both soils arepractically devoid of any reserves of K for agriculturalpurposes.

Table 4—Partial chemical analysis of silt fraction and weath-ering ratios

Depth

inches

0-612-2424-36

FesOa

6.156.306.87

CaO

0.380.280.35

K2O

Ri

<0.01<0.01<0.01

TiOi

nf

'.d Bauxite

1 .421 . 56

, 2.00

ZrOj

0.020.030.03

CaO

ZrOz

422120

TiOs

ZrOj

1.110.811.04

0-612-2436-48

5.426.026.39

0.780.530.46

lirown liauxitfi

0.03 1.500.04 2.220.04 3.42

0.030.060.10

1910

0.780.570.53

Table 5 shows the estimated amounts of the variousminerals present in the soils. Calculations were made on thebasis of X-ray diffraction and chemical analysis. Gibbsitewas estimated by the amount of A1(OH)3 made soluble byboiling the untreated clay sample in 0.5./V NaOH for 2.5 min.(2). Kaolinite was determined by assigning all of the Si02obtained by chemical analysis assuming 46.5% Si02 for themineral. The remainder of the A1203 after allowing forgibbsite and kaolinite, was estimated as boehmite assumingan Al20s content of 85%. Chlorite was present in amounts ofapproximately 1 % on the basis of X-ray diffraction intensities.Kaolinite was identified in the presence of chlorite by referringto the (002) reflection of kaolinite and the (003) and (004)reflections of chlorite. Since no other forms of crystalline ironoxides were identified and iron chlorite was negligible, thetotal Fe20s content is expressed as goethite. Anatase andrutile were estimated on the basis of identification by X-raydiffraction and on total Ti02 found. Trace quantities ofmontmorillonite and quartz were identified by diffractionanalysis. The amounts of the various minerals obtained thus

.showed good agreement with thermogravimetric analysis.The clay fractions of both red and brown bauxites contain

goethite, gibbsite, boehmite, kaolinite, and iron-rich chlorite.The red bauxite has more gibbsite and less kaolinite andgoethite than the brown. A difference in color of the twobauxites has always been attributed to differences in Feextraction and alkali treatment. Montmorillonite wasdetected in the deepest horizon of the brown bauxite sample(Fig. 1). The occurrence of traces of montmorillonite typemineral is thought to be a response to lower permeability ]ofthe limestone which affords accumulation of bases adjacent to

722 SOIL SCI. SOC. AMER. PHOC., VOL. 30, 1906

Table 5—Mineral content of the bauxite soils

Depth

inches

0-66-12

12-2424-36

Gibbsite

40455050

Boehmite

34272122

Kaolinite

55b3

Chlorite Montmoritlonite

/y

Red Bauxitei _1 —1 —1 —

Goethite Rutile &anatase

18 —20 —22 —99 __

Anatase

2222

Quart/.

TrTr

Brown Bauxite

0-66-1212-2424-3636-48

3843474747

3529242221

88999

11111

_——_1

1616181618

9 _2 _2 —9 __9 __

TrTr_—

Fig. 1—Diffractograms of 24- to 36-inch horizon of the redbauxite (RB) and 36- to 48-inch horizon of the brown bauxite(BB). Samples treated by dithionite and alkali extractiontechniques and thus concentrated giving much increaseddiffraction effects for kaolinite, chlorite and montmorillonite.Analyzed in sodium-saturated and glycol-solvated condition.Figures in Angstrom units. Recording is linear.

the bedrock. These soils differ principally in the occurrenceof anatase and trace amounts of montmorillonite in the brownbauxite soils. Gibbsite and boehmite, in that order, judgingfrom volatile loss data, make up the principal Ca-bearingsuite. .

Titanium and zirconium contents of the silt fraction(Table 3) increase with depth in both soils but to a muchgreater extent in the brown bauxite. Titanium is presentprimarily as crystals of anatase and rutile, mostly of greaterthan. 2 ju size. No rutile was found in the red bauxite. TheCaO content decreased with depth in both soils but muchmore so for the brown bauxite. CaO/Zr02 and TiCVZrOsratios are calculated for each soil (Table 4). The ratios arenot particularly informative with respect to weathering

• within the profile because of the differences in content of thethree elements between the surface horizons. Some overwashmay be present as indicated below.

The parent White Limestone Formation is a very pure

CaCOs having a CaC03 equivalent of 99.5% as determined bygravimetric means. Insufficient insoluble residue was re-covered for analysis or mineral identification. If the insolubleportion of the limestone is the source of the bauxite overlyingit, enormous thicknesses of the limestone must have beenweathered to produce the aluminous residue. Assumingimpurities in the limestone of 0.5% and an average AlsOacontent of the residue at 23% (1), the specific gravity oflimestone of 2.67, and a bulk density of the ore body of 1.8,it can be calculated that 21 feet of limestone will give rise to1 inch of bauxite soil. Weathering in situ of the limestonecould hardly account for the great depths of more than 50 feetof bauxite in some places. Therefore, subsequent redistribu-tion of the bauxite by erosion and sedimentation processes canexplain the thickness variability of the soil in most of thelimestone area.

The very small amount of silica in the insoluble residue oflimestone may account for the minute amount of kaolinitedetected in the red bauxite as well as the montmorillonitewhich forms under favorable conditions in the brown bauxitesoil. In conclusion our results support the hypothesis that theinsoluble residue of the limestone is the source of the bauxite.