AGENCY FOR INTERNATIONAL DEVELOPML;4 FOR AID USE ONLY WASHINGTON. 0. C. 20523 BiBL;OGRAPHIC INPUT SHEET 16IIA.-C~ i

A. PRIMARY l.SUBJECT Agriculture AF22-0000-G339 CLASSI- FCASI- SECONDARY FICATON IS. chemistry and physics--Puerto Rico 2. TITLE AND SUBTITLE -highly weathered,red of the tropics

3. AUTHOR(S) Beinroth,F.H.

4. DOCUMENT DATE S.NUMBER OF PAGES 8. ARC NUMBER 1973I 5p. ARC 7. REFERENCE ORGANIZATION NAME AND ADDRESS Puerto Rico

8. SUPPLEMENTARY NOTES (Sponcoring Organization, Publiahers, Availability) (InSoils of the southern States and Puerto Rico,ed.by S.W.Buol,p.87-91)

9. ABSTRACT

10. CONTROL NUMBER 11. PRICE OF DOCUMENT PN-RAB-104

12. DESCRIPTORS 13. PROJECT NUMBER Puerto Rico 14. CONTRACT NUMBER CSD-2857 211(d) 15. TYPE OF DOCUMENT

AID 590.1 (4-74) Chapter 12 OXISOLS-IIGIILY WEATIIERED, RED SOILS OF TIlE TROPICS

F. H. Beinroth Introduction and General Setting of a western spur of the Cordillera Central, there Although the term "" readily springs to ranging in altitude from 200 to 500 m. (600 to mind when the tipic of red tropical soils is raised, 1,500 feet), and consisting of ultrabasic plutonic it is but one of many names that have been pro- rocks (serpentinite) of Early Cretaceous age. For posed to characterize these soils. , Ferra- the most part this area is strongly dissected and lities, and Terra Roxa are some other of these only in a few places have older erosion surfaces vaguely defined and often synonymously used been preserved. As it is on those remnants where terms. Inthe,USDA Soil Taxonomy many of the Oxisols occur, they are of minor extent (some soils so named are included inthe order of the 600 ha. or 1,500 acres) and are included in map­ Oxisols. ping unit T 5. SatThirdly, Oxisols are found in the central up­ Oxisols are confined to tropical areas. Puerto lands of the Island. There they are developed in Rico and the U.S. Virgin Islands are, therefore, andesitic-basaltic volcanic rocks of Cretaceous age the only places within the Southern Region where (Beinroth, 1969). These Oxisols occupy moderate­ such soils are to be expected. However, it is only ly sloping, bench-like positions that are strikingly on the island of Puerto Rico where they are ac- contrasting to the very dissected adjacent moun­ tually encountered and where three distinct areas tains. These isolated areas are mainly found at of occurrence can be distinguished, altitudes between 600 and 750 m. (1,800 to 2,250 First, there is a discontinuous band of varying feet) in the headwaters of streams north of the width extending from the northwest corner of the main water divide of the Island and are considered Island to the periphery of the metropolitan area part of an old peneplain that is now greatly dis­ of San Juan. This area comprises some 31,000 ha. sected (Daniels et al., 1972). Consequently the (77,000 acres) and corresponds with mapping preservation of the ancient surfaces is the excep­ units 01 and 02. Here the Oxisols are developed in tion rather than the rule. Since Oxisols are only clayey to sandy sediments of largely Quaternary found on these remnants, they are not the pre­ age known as "blanket deposits" (Briggs, 1966). dominant of the uplands (Beinroth, The eastern part of this zone of Oxisols lies be- 1971). They are ,stimated to comprise approxi­ tween a belt of Tertiary limestones to the south mately 17,000 ha (40,000 acres) and are included and the Recent floodplains, marshes and lagoons in mapping units 119, U 70, and U 29. to the north. The topography is subdued and hum- The climate throughout the area of mocky and characteristically contains karst fea- Puerto Rico is maritime humid tropical. Mean tures in the form of .ione-shaped limestone hills annual air temperatures in the coastal areas range and some sinkholes. These almost level areas lie at from 24.5 to 25.5'C (76-78°F) ; those of the up­ altitudes between 10 and 50 m. (30 to 150 feet), land from 23.5 to 24.5°C (74-76"F). The tempera­ and are probably Pleistocene marine terraces. In ture differences between the coolest and the warm­ the western part these plains assume a plateau- est months are small in both cases and amount to like aspect and the littoral alluvials diminish to a 3 to 4.5°C (5-8'F). narrow band. The broad undulating plain3 range Mean annual precipitation along the coast aver­ in elevation from 50 to 100 m. (150 to 300 feet) ages 150 to 175 cm. (60-70"), and 200 to 300 cm. and are bordered to the south by limnestone hills (80-90") in the interior. The distribution of rain­ and by the escarpment that marks the locally cliff- fall during the year is uneven. While dry spells are like break to the Atlantic Ocean. not infrequent, seasonability is not strongly re­ The second area of Oxisols is located southeast flected by longterm averages. of Mayaquez in western Puerto Rico. It forms part The native climax vegetation of Puerto Rico's 87 Oxisol area has been characterized as Humid slightly sticky and slightly plastic but in a few Tropical Forest (Little and Wadsworth, 1964), cases nonsticky and nonplastic. but today's plant cover is almost entirely con- Colors vary from dark reddish brown (2.5YR trolled by man. The original forest along the 3/4) to dark brown (7.5YR 4/4) in the topsoil coastal area was mostly evergreen and was corn- and are dusky red (7.5YR 3/6 and 1OR 3/4), red posed of species like Corozo palm (Hymenaea (2.5YR 4/6) or dark reddish brown (2.5YR 3/6) courbaril), black olive (Bucida buceras), West In- in the subsoil. dian locust (Acrocmia media), and Spanish cedar Among the prominent chemical properties of (Cedrella odorata). Prominent species of the for- these soils is a reaction that it very strongly acid mations of the uplands included trumpet tree (pH 4.2 to 5.0) in the subsoil. Cation exchange (Cecropiapeltata), angelin tree (Andira inermis), capacity, of the sum total of exchangeable cations candlewood (Dacryodes excelsa), and tree fern a soil can absorb, ranges from 11.4 to 25.4 meq. (Cyatheaarborea). per 100 g. of soil in the A horizons but drops markedly to 4.1 to 7.6 in the subsoil ( MORPHOLOGY OF OXISOLS Staff, 1967). This decrease is conditioned by the vertical distribution of organic matter which con­ tributes significantly to the cation exchange ca­ other areas, have relatively featureless profiles. pacity. Contrary to general belief, the topsoil of They are reddish brown or dark red soils of pct.Cnrr ognrlbletetpolo cneraleredeph thatn ocupy smohr lanscapso Oxisols contains usually a high amount of organic considerable depth that occupy smooth landscapes matter although this is not reflected in the soil and exhibit characteristics associated with ex- color. The organic carbon levels in the Oxisols of treme weathering. The soil mass shows little dis- Puerto Rico range from 2.5 to 4.5 percent or more tinct horizon differentiation and has no strong macrostructure but is highly porous and perme- in the surface horizon but decreases to 0.2 to 0.5 able. Being exceedingly weathered soils, they have percent in the subsoil. As regards plantwhich nutrients, in some a low natural asoilsoilo fertility.fetiltythesenaura profilessoils amounts are deficient only toin calciumtraces inl the subsoil. The sola of the Oxisols of Puerto Rico are com- Pofi ous on ly o e e subsil. monly from 100 to 150 cm. (40 to 60 inches) thick. Potassium occurs in similarly low levels varying The horizonation pattern in characterized by an from 0.4 to 0.6 meq. per 100 g of soil. Correspond­ A-132-C sequence. The A horizon, usually the plow ingly the values for base saturation percentage, layer, is underlain by a series of B2 horizons con- i.e. the extent to which the adsorption complex is sisting of a residual concentration of sesquioxides saturated with exchangeable cations other than and silicate clays, and C horizons composed of hydrogen, are low and commonly less than 10. very highy weathered rocks (saprolite) or clays. Only the soils where limestone occurs at shallow Some of the Oxisols along the north coast of depths have higher values, likely due to secondary Puerto Rico rest abruptly on limestone and are of recharge by capillarity. shallow depth. Although 5 or more horizons can i Mineralogically, the clay fraction is character­ be differentiated, the soil profile has a uniform ap- ized by and halloysite, minerals belong­ pearance since the horizon separation is based on ing to the group of aluminosilicates of the 1:1 minor differences in color and structure. crystal lattice type. Kaolinite and halloysite are The texture of the Oxisols of Puerto Rico is the predominant mineral in the clay fraction f invariably clay, and the proportion of the clay all Puerto Rican Oxisols except those in mapping fraction (particles smaller than 0.002 mm.) fre- unit T5 where the aluminum mineral gibbsite, Al quently exceeds 60 percent. In spite of this high (0nd)3, exceeds the amount of kaolinite. Oxides clay content there is little or no exidence (such as and hydi'ous oxides of iron (goethite, hematite, clay skins) that clay is moving in the profile. magnotite) are further characteristic and impor­ Oxisols do not, therefore, have a zone of clay ac- tant constituents because they control any soil cumulation (argillic horizon). Due to the fine tex- properties. It is also peculiar to the clays of Oxi­ ture and the high porosity, the water holding sols that they have a low activity and therefore capacity of these soi:; is relatively high and field exhibit the properties commonly associated with capacity may exceed 40 percent. It needs to be. high clay content to a minor degree only. emphasized, however, that only about 1/3 or less FACTORS AND PROCESSES OF of this water is in a form available to plants. OXlSOL FORMATION Structure is weakly developed in Oxisols and is fine granular or fine subangular blocky at the sur- Although factual knowledge about tropical soils face and weak fine subangular blocky to almoit is gradually replacing the broad generalizations there massive in the subsoil. The consistence is mostly that have been made with great frequency,

88 remains much to be learned about their formation, along the northern coast of Puerto Rico (mapping As a place of soil formation, the lower altitudes units 01 and 02). It has been shown that the f the tropics are characterized by environments parent material of these soils are sediments de­ which are markedly different from those of tem- rived from old and presumeably laterized pene­ perate regions in that temperatures are always plains that in Tertiary times were extensive in high and that there is little variation from season central Puerto Rico (Briggs, 1960 and 1966). In to season. While these conditions exist in Puerto these already preweathered sediments, Oxisols Rico, the soils that are customarily associated could form relatively fast whei'e drainage condi­ with such an environment-"" or Oxisols tions were favorable. Laterization, furthermore, -are of limited extend in Puerto Rico as well as proceeds at a faster pace in certain rock types in the tropics as a whole. This points to the fact such as the ultrabasic serpentinite which is the that a tropical climate, although a necessary con- parent rock of the Oxisols in mapping unit T5. ditlon, cannot be the only decisive environmental This rock, essentially consisting of hydrated iron­ factor for the formation of Oxisols. The great age magnesium-silicates, weathers readily and because of the geomorphic surfaces which these soils fre- of its composition lends itself to rapid laterization quently occupy emphasizes that, apart from the under stable geomorphic conditions. nature of the parent material, time and landscape evolution are equally important factors. Geomorphic evidence from many tropical coun- CLASSIFICATION AND tries sti-ongly suggests that the formation of Oxisols requires, in most cases, geologic times. The Oxisols of Puerto Rico comprise a relatively This is substantiated by recent studies in Puerto narrow taxonomic range and are all classified into Rico which indicate that the Oxisols of the in- the Suborder Orthox. The Orthox are those tropi­ terior began to form some 20 million years ago on cal soils that have a subsurface horizon (oxic hori­ surfaces that have been essentially stable over this zon) consisting of a mixture of iron and aluminum period (Daniels et al., 1972). The latter condition oxides and clay minerals of the non-expanding is intrinsic because only then can the soil-forming type, that have relatively low organic matter con­ processes continuously act upon the same soil ma- tents (less than 20 kg. org. C. per surface cubic­ terial and eventually produce Oxisols. Such geo- meter), and that are usually moist in the subsoil. morphic conditions are, however, rare in Puerto In terms of the next lower level of classification, Rico (Beinroth, 1971). The Oxisols of the uplands the Great Group category, the Orthox encountered are, therefore, confined to the few remnants of old in Puerto Rico are differentiated as Acrorthox, surfaces that have escaped erosion. Eutrorthox and Ilaplorthox. The Acrorthox are In general, it may be postulated that smooth deep, highly weathered and leached soils with landforms stable over a considerable length of weak or massive stricture; the Euthorthox are time under a humid tropical climate favor the less leached as indicated by their base saturation formation of Oxisols on parent materials which of more than 35 percent; and the liaplorthox have contain some iron and/or aluminum. It is further a lower base saturation than the Eutrorthox but essential that conditions of free drainage prevail are higher in *,ascs than the Acrorthox. at least in the upper part of the solum. Two associations with Oxisols are recognized This is the type of environment that is conduc- along the north coast of Puerto Rico. In the west­ tive to what is collectively known as laterization. ern part this is the Ilaplorthox-Eutrorthox- Laterization is a weathering process that results Tropudalfs association. In this unit (02) the in residuum whose aluminum and iron contents Haplorthox have thick sola whereas the Euthor­ are higher than that of the parent rock, and that thox are relatively shallow Oxisols that Lverlie leads to the formation of new compounds of low limestone. The Tropudalfs occupy the footslopes mobility, ultimately oxides. It is further charac- and sideslopes of the limestone hills that outcrop terized by decreases of silicon, calcium, magne- in this area. sium and potassium. The high degree of weather- The association to the east consists of Eutror­ ing accounts for the absence, or near absence, of thox-Paleudults-Limestone Rockland (unit 01). weatherable minerals that could replenish the im- The Eutrorthox of this unit have: thicker sola poverished plant nutrient resources of the soil. than those of unit 02, but also rest on limestone. Although Oxisols occur for the most part on They adjoin Paleudults which occupy lower posi­ ancient surfac,.6 there are certain exceptions to tions in the coastal plain and are more than 1.5 m. this general rule. A case in point are the Oxisols (60 inches) thick, reddish brown, strongly acid that are found on the largely Pleistocene surfaces clay soils. Steep-sided limestone hills, essentially

89 devoid of soil cover account for the third member Apart of this association, from the adverse water regime, Oxiso1 The have excellent physical properties. Oxisol units of the north coast of They are well Rico Puerto drained, easy to work are geographically associated with and they occupy a relief the shal. that is suitable for agriculture. low, moderately leached soils. (Tropudalfs) Provided they are and properly managed and planted shallow, humus-rich, dark colored to adapted crop sils (Rendolls) they can be very of the limestone productive. belt to the south. To the north they juxtapose various alluvial and organic soils (Tropaquepts, The Trophemists). Oxisols of limited areal extent encountered in the mountainous LITERATURE AND interior of the island are in- cluded in the Tropohumults-Eutropepts.Haplor. SELECTED REFERENCES Beinroth, F. H., thox, Eutropepts-Tropohumults-Haplor-thox, 1969, An Outline of the Geology of and Puerto Rico. Tropudults - Eutropepts - Haplorthox Univ. P. R., Agr. Exp. i3ta., Bull. 213. associations •,1971, The ,(units U7, 119, U29). These Haplorthox General Pattern of the Soils of occur in Puerto Rico. n intricate pattern with shallow, Trans. 5th Carib. Geol. Conf., Geol. grayish brown, Bull. No. 5, silightly acid soils (Eutropepts) Queens College Press, N.Y., pp. 225-230. that occupy the Briggs, moderate and R. P., 1960, Laterization in East-Central very strongly steep sideslopes and the thick, red, ver stngyciclaacid clay ,Poils (Tropohumults) Puerto Rico. Trans. 2nd Carib. Geol. for e y os ro pohuultes ) P.R., Mayaguez, Conf., Univ. -, 1966, Thepp. Blanket103-19. Sands t~hich are typicai for the more rolling sites. of North Puerto Rico. Trans. 3rd Carib. Geol. Conf., Kingston, i In the Serpentine Rockland-Haplorthox.Acror. Ja­ maica, thox association (mapping unit pp. 60-68. T5) the soils oc. -_, and cur in a toposequence where the Acorthox Akers, J. P., 1965, Hydrogeologic Map occupy of Puerto Rico and Adjacent the stable mesa-like surfaces Islands, U.S. Geol. Sur­ and the Haplorthox vey, Hydrol, the more steeply sloping sites. Serpentinite Inv. Atlas HA-197. Ih exposed in the strongly dissected rock Daniels, R. B., Beinroth, Setyceptisols part where Landscape Relations F. H., and Rivera, L. H. 1972, of Oxisols, and In­ erosion has removed most of in East-Central Puerto Rico. USDA, the soil cover. Cons. Serv. Soil in coop. with Univ. P.R. Agr. Exp. Sta., LAND Soil Surv. Inv. Rep. (IN USE Jeffries, PRESS). C. P., Bonnet, J. A., and Abruna, F., The Oxisols of Puerto Rico The Contituent 1953, are for the most Rico. J. Agr. Minerals of Some Soils of Puerto part in agricultural use. The Haplorthox Univ. P.R. 87: 114-39. in the in- Little, E. L., and terior are dominantly cultivated Wadsworth, F. H., 1964, Common to tropical sub- Trees of Puerto sistence crops such as plantains, Rico and the Virgin Islands. USDA, bananas, yams, Forest Service, and taniers. To a minor degree Agr. Handbook No. 249. coffee, sugar cane Mitchell,Puerto Rico. Univ. P. R., and tobacco are R. C., 1954, A SurveyAgr. ofEgr. the Exp., GeologyTechn. grown; small areas are of The in pasture. Paper Haplorthox and the Eutrorthox Pico, 13. Puerto Rico of northwest R., 1950, The Geographic are mainly planted to sugar cane, Relations of Puerto truck crops Rico. Univ. P.R. Press, Rio Piedras. or are in pasture. Along the central part of the Roberts, R. C., and party, 1942, Soil northern coast pineapples are pro- Survey, Iuerto duced at Rico, USDA, Bur. Plant Ind. in a commercial scale. Most of the Oxisols coop. with Univ. P.R. Exp. Sta., Series 1936, No. in western Puerto Rico (mapping 8, Washihgton, forested, unit T5) are D. C. in native pasture or are lying idle, but Samuels, G., 1966, A urbanizations are Soils Survey of the pH-Status of the rapidly infringing onto the level of Puerto Rico. Univ. P.R., Agr. Exp. hites where Acrorthox occur. Techn. Paper 42. Sta., Smith, R. M., The agricultural use and Abruna, F., 1955, Soil and Water of the Oxisols of Puerto Conservation Ico is not in Puerto Rico, 1938-1947. Univ. P.R. without problems. Owing to the men- tibned Agr. Exp. Sta. Bull. 124. condition.,, annual plants with shallow root Soil Survey Staff, 1965, Soil Survey systems are apt to suffer from even Lajas Valley Area. Ahort USDA, Soil Cons. Serv. in coop. W~bst dry periods. Particularly in the • with Univ. P.R. tte of the Island eflicient crop productiondrier north- ,Agr. Exp. 1967,Sta., Soil re- Dsrpin o SeriesoeSisoSurvey 1961, No.Laboratory 23. Descriptions for Some uroRcDatan andh therefore,".ires, supplementary irrigation, Soils of Puerto Rico and the the Virgin Islands. .he low inherent fertility USDA, Soil Cons. Serv. in coop. makes for a second with Univ. .1.blem. P.R. Agr. Exp. Sta., Soil Surv. Inv. Rep. However, research into soil fertility Mfbwn that most of has No. 12. the chemical deficiencies of Thorp, tfib Oxisols of J., 1941, Climate and Settlemeit Puerto Rico can be corrected ef- in Puerto Rico and the Hawaiian Islands: In Climate fectively. 1941 Yearbook of Agriculture, and Man, D. C. pp. 217-26. USDA, Washington, 0 Table .12.1 Composition and Characteristics of Subunits *Dominated by"Oxisols

MAP MAP UNIT COMPOSITION PHYSICAL CHARACTER UNIT NAME CLIMATE GENERAL USE SYMB. AND SUITABILITY Acres Relief -Parent Elev. Ann Mean Mean Frost Material ppt Jan. July Free Temp Temp Days (xl000) 01 Eutrorthox % (ft.) (in.) (F-) 35 50 Coastal Quaternary 100 (F*) Paleudults 62 73 80 365 Pineapple dominant, some 31 45 terraces clayey to to sugarcane Limestone and pasture. with pro- blanket 400 79 Rockland 3 5 truding deposits and limestone Tertiary 69 hills, limestone level to undulating and very steep 02 Haplorthox 25 50 Plateau- Quaternary 20) Eutrorthox 51 73 80 365 Pasture and sugarcane, 18 35 like clay clayey blan- to to to some vege­ Tropudalfs 7 tables and subsistence crops. 15 plain; ket deposits 606 66 74 level to and Tertiary 50 undulating limestone

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