Soil constraints on Northwest Yucatán, Mexico: Pedoarchaeology and Maya subsistence at Chunchucmil

Geoarchaeology: An International Journal, Vol. 13, No. 8, 759–791 (1998)q 1998 John Wiley & Sons, Inc. CCC 0883-6353/98/080759-33

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Soil Constraints on Northwest Yucatan,

Mexico: Pedoarchaeology and Maya

Subsistence at Chunchucmil

Timothy Beach

Program in Science, Technology, and International Affairs, School of Foreign

Service, Georgetown University, 37th and O Streets, Washington, DC 20057

The soils and subsistence of ancient Maya Chunchucmil in northwestern Yucatan are thefocus of this paper. Today and historically, the population and crop yields here have beenvery low. Archaeological field work, however, has shown the Late Classic site to be highlypopulated with densely packed walled mound and field groups. It is enigmatic that this highancient Maya population existed in a region of meager crop and soil potential. This enigmais addressed by investigating contemporary Maya agriculture, geoarchaeological evidence,and soil potential for intensive agriculture. The local Maya soil classification of kancab andboxluum synthesizes the Alfisols, Inceptisols, and Mollisols described here. The major soillimitations are shallowness, broad areas with no soil, insufficient water holding capacity, andvariable deficiencies in phosphorous, potassium, and zinc. Evidence for intensive agricultureand alternative crops can be seen in widespread field walls compartmentalizing the landscape,sascaberas, and preliminary phosphate fractionation signatures. q 1998 John Wiley & Sons, Inc.

INTRODUCTION

Studying potential agricultural productivity and forms of intensification to ad-dress the riddle of high ancient Maya populations is keenly important to both ar-chaeology and tropical agricultural development. In the Maya Lowlands historicalpopulations have been very low and agricultural methods very extensive, but someprehistoric populations are estimated to have been very high for hundreds of years.The conditions for agriculture in these tropical lowlands present many challenges,and this poses the basic question of how did ancient Maya high populations feedthemselves. This article approaches this question by analyzing the soil potentialaround the current village of Chunchucmil, Yucatan (Figure 1), Mexico, which liesin the midst of a large site dating mainly from the Maya Late Classic (A.D. 550–830). Ironically, this site must have been both very densely populated and also oneof the Maya world’s most limited agricultural environments. This article first re-views the archaeological landscape and environmental and agricultural history ofthe region. Second, based on field and laboratory testing, the article describes soildiversity and fertility in the geoarchaeological context of each environmental zone.Lastly, it explores the potential for alternative crops and intensive agriculture.

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Figure 1. Map of Yucatan.

PEDOARCHAEOLOGY AND MAYA SUBSISTENCE IN NORTHWEST YUCATAN, MEXICO

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Background

Since at least 1909 a stream of research has studied the soil and agriculturalenvironment of the ancient Maya world (Cook, 1909, 1921; Emerson and Kempton,1935; Steggerda, 1941; Higbee, 1948; Hester, 1953; Cowgill, 1962; Olson, 1970, 1981;Turner, 1974; Flannery, 1982; Bloom et al., 1983; Muhs et al., 1985; Pohl, 1985;Wilken, 1987; Coultas et al., 1992; Wingard 1996; Dunning, 1992; Dunning andBeach, 1994; Jacob, 1995, 1996; Fedick, 1996; Beach and Dunning, 1995, 1997;Beach, in press). Some forms of intensive cultivation (i.e., agricultural terraces)were recognized early (Gann, 1925), but long-fallow milpa systems were seen asthe likely ancient Maya agricultural system, producing enough food for 40–70 per-sons in the central Peten of Guatemala (Cowgill, 1961). Several early studies22kmof soil fertility (Cook, 1909, 1921; Emerson and Kempton, 1935; Steggerda, 1941;Cowgill, 1961; Cowgill and Hutchinson, 1963) showed crop yield declines in ex-perimental plots, which they ascribed to both weed competition and fertility de-clines. Based on these declines, some even anticipated population growth leadingto a shortened fallow cycle and subsequently to further fertility declines and soilerosion. Despite recognition of forms of intensification and a need for soil enhance-ment, scholars did not come to reject the milpa with long fallows as the main sourceof agriculture in the Maya Lowlands till the early 1970s (Hammond, 1978). Tworesearch streams led to this conclusion: high population estimates and evidencefor varied and intensive forms of agriculture.

From the 1960s on, reasonable population estimates for the Late22(c. 200 km )Classic Maya Lowlands far exceeded the potential productivity of milpas (Culbertand Rice, 1990). Thus Maya scholars have investigated many forms of possibleintensification and subsistence (Rice, 1978). Some have investigated the region’scurrent agricultural potential (Farrell et al., 1996); some have studied contemporaryagricultural methods and indigenous soil knowledge (Carter, 1969; Bocco, 1991;Dunning, 1992); some have studied alternative crops to the usual maize, beans, andsquash such as ramon nuts (Puleston, 1968), root crops (Bronson, 1966), and house-hold gardens (Netting, 1977); some have studied potential for crop production (Co-wgill, 1961; Muhs et al., 1985), and many have studied the evidence for intensifi-cation methods such as terrace systems (Turner, 1974; Beach and Dunning, 1995)and wetland agriculture (Siemens and Puleston, 1972; Turner and Harrison, 1983;Pohl, 1990). Additionally, some have attempted to discern what ancient methodscould be wisely applied to current agriculture in the Maya Lowlands: Lambert etal. (1984) discussed this in terms of wetland agriculture, and Beach and Dunning(1995) did so in terms of terracing for soil and water conservation. Others havediscussed the utility of studying ancient agricultural soils for shedding light on theneed for long-term sustainable agriculture (Olson, 1981; Sandor and Eash, 1991).Lastly, studies of indigenous agriculture (Ewell and Merrill-Sands, 1987) can beused as general estimates of ancient agricultural potential. All of this researchshows that ancient Maya agriculture was multifarious and complex in the MayaLowland environments and that the study of Maya agriculture has more than justacademic applications today.

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The underlying enigma of much of this research has been how did the LateClassic Maya population (generally asserted to be very high) sustain itself in thisagriculturally limited tropical environment? Among the most limited ancient Mayasites must have been several sites in the northwest Yucatan that lie in semiaridclimates with very sparse soils: Tiho, Dzibilchaltun, Yaxcopoil, Tzeme, Siho, andChunchucmil (Figure 1). This article focuses on Chunchucmil, where high popu-lation density in the Maya Late Classic period (A.D. 550–830) coincided with severeenvironmental limits.

ENVIRONMENT

The northwest Yucatan (Figure 2) lies in a zone of crop destroying hurricanes,modest rainfall, high evapotranspiration, and rapid infiltration into the region’s po-rous limestone aquifer. This region lies in the AW climate of Koppen, near to theBS border. For the nearest climate station to Chunchucmil, the soil temperatureregime is termed isomegathermic with a mean annual soil temperatures of ca.29.47C, and the soil moisture regime is ustic, subdivision typic tropustic (Van Wam-beke, 1987). Total annual precipitation varies from about , but the700 to 1000 mmtotal annual water budget deficit induced by high evapotranspiration is

. Water deficits are further complicated by the disproportionate sea-600–800 mmsonality of rainfall (80–90% occurs from May through October, peaking in Augustand September, at the two nearest recording stations in this region), high year-to-year variability, and the highly localized nature of convectional rainfall (INEGI,1981). Since maize generally requires about of rainfall in a season (Well-500 mmhausen, 1957), rain-fed maize production is a difficult challenge here. Two factorsprovide some compensation to these high moisture deficits: a shallow groundwatertable, currently only below the land surface, and silty and clay loam soils2–3 mwhich preserve localized soil water in deeper limestone pockets.

It is not known if any climatic conditions in the Late Classic Period were moreconducive to agriculture than today. Paleoclimate studies from this region indicateclimate and vegetation were largely similar to contemporary climate and vegetationwith the exception of a possible phase of aridity that peaked from ca. 1200–95014Cyr B.P., coinciding with the Late Classic Maya Collapse (Hodell et al., 1995; Curtiset al., 1996; Whitmore et al., 1996). The drought could have caused agriculturalstress and may have contributed to the site’s collapse in this already marginal zone,but these studies indicate that the climate has been less arid in the last 950 yrperiod, and lake levels—and thus groundwater levels—have risen. A sediment-pollen core from cenote San Jose Chulchaca, north of Chunchucmil, indi-20 kmcates no period of increased soil erosion, at which time a deeper soil cover couldhave been eroded (Curtis et al., 1996). Moreover, the region is too flat (mostly wellbelow 17 slopes) for significant erosion by running water and the soils too clayeyfor much wind erosion. Hence, when Chunchucmil’s population peaked in the LateClassic, no evidence suggests that climate was more conducive to soil development,and soils should not have been deeper or more productive. An additional factor of



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Figure 2. Map of Chunchucmil Region and surrounding ecological zones.

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soil formation is sea-level and thus groundwater level change, and some evidenceexists for a lower sea-level during the Late Classic (Dahlin et al., 1998). It ispossible that a lower groundwater table during this time could have displaced theactively precipitating tzekel zone to the west and slightly expanded the Karst Plainand the deeper soils associated with it (see below).

The Yucatan Peninsula is a platform of Tertiary and Cretaceous car-275,000 kmbonates and evaporites. The region is well known for its “ring of cenotes,” an arcof sinkholes associated with and lying above the 65 My Chicxulub impact crater(thought to be the Cretaceous–Tertiary dividing event) (Pope et al., 1996). Chun-chucmil lies about south of the southernmost ring of this crater, which still15 kminfluences ground water flow (Perry et al., 1995) and some aspects of soil andgeomorphology. Pope et al. (1996) used remotely sensed images (Landsat TM andSIR-C/X-SAR images) and 1:250,000 scale soil and geological maps (INEGI, 1983,1984) to study the current geomorphic expression of the Chicxulub Crater, whichitself is buried by . They interpreted these images to indicate a group300–1000 mof concentric normal faults that rise outward from the center at Chicxulub andextend to the northern edge of the Chunchucmil region with the surface expressionof two rings of cenotes (sinkholes). Based on this interpretation and the INEGIsoil maps, soils from the center of the Chicxulub Crater to the outer edge form achronosequence of increasingly older soils. Pope et al. (1996) state that time is themajor factor controlling soil development differences in this region, and their workindicates the Chunchucmil soils should have started forming in the Late Mioceneto Pliocene.

Four distinct provinces surround the Chunchucmil site (Figure 2). The regionaround Chunchucmil (Figure 3) falls within the Northern Pitted Karst Plain phys-iographic region (Wilson, 1980). This area ranges in elevation from ca. above3 mmean sea level to ca. on the highest ancient Maya mounds, and it has very20 mlow, karst relief: karren forms (small pits, pans, and grooves), small kegelkarst hills(called huitzes in Mayan), limestone pavements and petrocalcic layers, and shallowswales and small sinks (cenotes), some of which appear to be further excavatedfor wells. In the study region “halokarst” (underground drained) limestone pave-ments cover perhaps one-half of the total surface. The karst plain begins at thenormal fault scarp of the Sierrita de Ticul, where local relief is close to .100 mCrossing the karst plain from east to west, the plain very gradually descends (about

) and small karst hills gradually decrease in elevation from over211 m 10 km 3 mto less ; around Chunchucmil these hills are about and were sometimes1 m 1 mused as the base for ancient Maya mounds. The karst plain gives way to a tzekelzone with savanna vegetation that is seasonally inundated with freshwater. Thiszone has little soil cover, and surface cementation is creating a surface aquitardtoday, though it is penetrated locally by petenes or springs surrounded by islandsof trees (Perry et al., 1989). This zone grades into a mangrove estuary5–15-km-widethat is separated from the Gulf of Mexico by the accreted beach deposits of theCelestun Peninsula (Dahlin et al., 1998) and a open water estuary2-km-widethat carries groundwater funneled by the “ring of cenotes.”



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Figure 3. Photo of limestone surface and a major ancient Maya Mound of Chunchucmil.

Karst Soils and Surface Cementation

A typical feature in the carbonate rock surface throughout northern Yucatan isthe sequence of an upper residual, indurated caliche or petrocalcic horizon varyingabout in thickness, underlain by a less dense, more friable carbonate rock1 mregionally called sascab (or sahcab) up to several meters thick, which is underlainby bedrock limestone. Stevens (1964:304) wrote that the sascab is both porous andretains water, making it a water source for vegetation in the dry season. Isphordingand Wilson (1973) describe the ongoing processes of weathering and precipitationthat form the top two layers. During wet periods, mildly acidic rainfall, undersa-turated with calcite and aragonite, infiltrates through the soil and upper capstone.The percolating water becomes saturated with less soluble low magnesium calcitebut remains undersaturated with the more soluble high magnesium calcite andaragonite. Preferential dissolution of the latter two minerals occurs next, and thusthey decrease in the upper capstone. The high magnesium minerals are precipitatedin the sascab layer while the low magnesium species remain in solution. Duringdry periods, solar pumping drags pore water that is saturated with CaCO3 up tothe capstone, where calcite is precipitated as H2O and CO2 escape into the atmo-sphere. Isphording and Wilson (1973) write that the sascab is dominated by theminerals dolomite, talc, and chlorite, with relatively less calcite. The decreasedcalcite may be explained by its greater solubility: Calcite is easily dissolved and

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carried downward during wet periods but remains in solution until it precipitatesat the surface. Moreover, the high magnesium minerals dissolved later and precip-itated earlier in the sascab zone.

Lack of soil throughout this region may be explained by the nearly pure calcite–limestone parent material (Kellman and Tackaberry 1997:86–87). These parentmaterials have few minerals that resist complete weathering and dissolution. Thecarbonate minerals weather into substances that are soluble and easily leached,and thus the land surface dissolves leaving only patchy, thin soils formed of suchclay minerals as kaolinite and montmorillonite and of such resistant trace mineralsas hematite and magnetite (see Table IV).

PREVIOUS RESEARCH

The major regional sources of soils information include general sources such asthe soil maps (1:250,000 scale) of the Instituto National de Estadistica Geographicae Informatica (INEGI, 1984) and works by Ortiz (1950), Stevens (1964), Wright(1967), and Flores Mata (1977), and the larger scale research in the Puuc by Dun-ning (1992). The INEGI Map uses the UNESCO FAO Classification (FAO, 1977),and is based on broad geomorphic divisions and soil descriptions and analyses ofsamples taken at approximately one per . It divides the region into seven2100 kmmajor soil-geomorphic units: (1) Regosols (Calcaric) on the outer beach ridges; (2)Solonchaks (Orthic) on the older beach ridges and outer estuary; (3) Gleysols (Mol-lic) in the inner; (4) Histosols (Eutric) in the farthest interior estuary; (5) Lithosolsin the tzekel zone; (6) Cambisols (Chromic) in the savanna; and (7) Rendzinas inthe Karst plain.

The most detailed soil research in this region has focused on the Puuc in theregion’s best soils, around Sayil by Dunning (1992) and at the Uxmal ExperimentalField Station. At the latter, soils are often 2 or more meters thick, and include threemajor types on its 100 ha (64 arable and 36 stony lands): kancab or k’aankab

(Rhodic Nitosols or Vertic Eutropepts), ak’alche (Vertisols), and stony soils (Rend-zic and Lithic Leptosols). Kancab soils here are red-colored, very thick and fertile,and produce maize yields of up to (INEGI, 1994). Dunning216.5 metric tons ha(1992) made an intensive investigation at Sayil, which lies more than to the50 kmsoutheast of Chunchucmil in a region of much greater rainfall, local relief, slopediversity, and general soil fertility. Thus soil research at these sites can serve onlyas a rough baselines for comparison with Chunchucmil. Dunning’s (1992) own soilinvestigation of the Puuc shows a wide diversity of soils and remarkable folk knowl-edge, including a three-level soil taxonomy and at least nine different soils classifiedin Mayan terms and the modern USDA and FAO taxonomies. Studies at Chun-chucmil, however, have uncovered a local knowledge of mainly two soil names(boxluum and kancab), which are applied by local farmers in at least two differentways each (e.g., boxluum-tzekel).

In northeastern Yucatan, Kepecs and Boucher (1996) note only two general typesof soil: chichluum or dark, organic soils mixed with limestone gravel (chich) and



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kancabs or fertile red soils. These soils parallel the major soils (boxluum and kan-cab) recognized by the current farmers of Chunchucmil, which are discussed be-low. Kepecs and Boucher (1996) also differentiate three other types of agriculturallands: tzekel, limestone bedrock near the coast, aputun lands, which are like tzekelbut with more variation and inland farther, and what are locally called bajadas,

low, flat areas with little soil accumulation. Analogous lands to all of these occuraround Chunchucmil (Figure 4), but the other karst-sink features important to in-tensive cultivation elsewhere in the Yucatan—aguadas and rejolladas—do not.

HISTORICAL AND CONTEMPORARY AGRICULTURE

Milpa systems are the traditional forms of agriculture in this region (Carter, 1969;Gallegos de Castill, 1981; Ewell and Merrill-Sands, 1987), and about 50 of the cur-rent families practice milpa agriculture. Contemporary milperos are disconnectedfrom the long tradition of the milpa due to modern innovations, government agri-cultural programs, and the henequen boom that dominated this region’s agriculturefrom the late 19th to mid-20th century. Indeed, some of the older farmers stillreceive henequen pensions, and much of the land is still dotted with old henequenplants.

Interviews with two senior milperos from Chunchucmil and many other farmersin their milpas and solares (kitchen gardens) provide valuable insight into localagriculture. Most of these contend that milpa production is too low (total yieldsare about ) and too fickle for the food and income needs210.25 to 1 metric tons haof most Chunchucmil families, which echoes previous conclusions about the Yu-catan region (Ewell and Merrill-Sands, 1987). For comparison average world yieldsfor maize alone are (Pinstrup-Andersen, 1994). Historical evi-214.6 metric tons hadence from the hacienda Chunchucmil, in the study area, indicates a record of lowyields in a succession of agricultural activities. Low maize yields compelled a shiftfrom commercial maize to cattle ranching, which after the late 19th century gaveway to henequen, which produced half the yields of plantations farther east (Vlceket al., 1978).

Elsewhere in the Maya Lowlands but representative of agriculture on these trop-ical limestone soils, Olson (1981) describes the typical yields of unfertilized maizefrom milpas around Tikal as in the first year after slash and burn211 metric ton haand decreasing 25% each year to in the fourth year. Early210.25 metric tons hastudies by Steggerda (1941) near Chichen Itza and by Cowgill and Hutchinson(1963) in the central Peten of Guatemala also showed overall yield declines. Thesestudies underscore the necessity of fertilization or fallow, especially under inten-sive agriculture.

Besides milpa agriculture, contemporary Maya also intensively manage the for-ests, selecting for useful species and thus maximizing forest production over time.Colonial documents indicate extensive orchards (Jones, 1982), and indeed deLanda commented about the great diversity of trees for many purposes (Tozzer,1941:196–200). Several scholars have discussed forest gardens within aguadas,

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cenotes, and sascaberas (sascab extraction pits), “tree gardens” (caanche) or raisedbeds filled with organic materials and soil to grow vegetables and seedling treesfor transplanting, and solares (Kepecs and Boucher, 1996; Beach and Dunning,1995; Killion, 1992; Gomez-Pompa, 1987). Many contemporary Chunchucmil housesare surrounded by a variety of orchard and other crops, but contemporary researchon solares has found that only about 11% of average caloric intake is derived fromthem (Caballero, 1992).

Agriculture and Population

Based on work by Vlcek et al. (1978) and Dahlin et al. (1996), a very conservativeestimate of population for the ancient central residential area of is 20,000–26 km30,000. The ancient suburban and rural population was also dense because a highdensity of mounds and walls lie beyond the site for many km. In contrast, thecurrent municipio (county) of Maxcanu wherein Chunchucmil lies has about17,000 people or about spread out in a region of . Thus22 212.9 persons km 1321 kmthe ancient population in the Chunchucmil site alone was probably higher than forthe entire current Maxcanu region.

Such a large population has high requirements for maize production, the usualstaple crop of the Maya, for another staple crop, or for large food subsidies fromelsewhere. Regarding maize production, a study at Komchen (Shuman, 1974) in aequally dry region (water deficit of ca. ) north of Chunchucmil21700 mm yr 30 kmprovides some general comparisons. Here maize production can support about 8persons based on a typical 10 year fallow cycle (Farrell et al., 1996).22 21km yrTransferring these numbers to Chunchucmil’s estimated minimal population of20,000, produces an estimated sustaining area of , which is the size of the22500 kmentire region from the coast through the karst plain and including large tracts ofnonarable saline, estuarine, and limestone pavement lands. Total arable lands thatcould be used for maize cultivation today in this region add up to no more than

, conservatively one-quarter of the necessary land required under tradi-2600 kmtional methods. It should be noted that for Sayil, Tourtellot et al. (1990) used muchhigher population density figures, but for a region with more precipitation300 mmand more productive soils. A caveat to these high population estimates can befound in Becquelin and Michelet (1994), who suggest Sayil and Puuc populationestimates must be reduced by at least half. Applying this reduction to the Chun-chucmil estimate, however, still makes the population so high as to make foodproduction highly problematic.

Prehispanic Agriculture and Architecture

Since the ancient Maya population is generally considered to have been quitehigh (far higher than current population) and the milpa system alone is insufficientto support these populations, it is logical to postulate that some forms of intensiveand alternative agriculture were practiced in this region. Since the region has onlya few meters of relative relief (though ancient Maya mounds reach 18 m), the forms

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of intensive agriculture may have included intensive fertilization, raised beds withpot irrigation, multiple cropping systems, and agro-silviculture (Nations and Nigh,1980; Turner and Harrison, 1983; Gomez-Pompa, 1987; Kepecs and Boucher, 1996).Moreover, maguey (Agave spp.) and cotton cultivation could have been practicedin these semiarid regions to provide food and fiber as they did elsewhere in pre-historic Mesoamerica (Nobel, 1988; Evans, 1992). The forms of evidence that maycast light on ancient agriculture are relict features such as terrace walls, solarewalls, field walls; historical sources from the post-contact period; and soil chemicaland physical deviations such as high or low phosphate fractions.

Most assessments of this region have assumed that it is not conducive to agri-culture, but Kepecs and Boucher (1996) argue that orchard crops are quite suitablehere and that the site of San Fernando was a large Maya town covered by orchards.They note several forms of evidence for ancient orchards: walled subtle depres-sions (bajadas) of rocky soils (aputun), common chich mounds of the same sizeas modern chich circles used around the roots of fruit trees for water and soilconservation and stability. Many native fruit trees that require year-around mois-ture are present in this region such as guaya, zapote, nance, and ramon (Talisia

olivaeformis, Manilkara zapote, Byrsonima crassifolia, and Brosimium alicas-

trum). The shallow groundwater table in the Chunchucmil region can be reachedby some tree roots and is easily reached for pot-irrigation. For Yucatan in general,de Landa’s famous quote from 1556 echoes all of these points and refers to com-ments elsewhere about orchard crops:

Yucatan is a land of less soil than any I know. . . .[I]t is a marvel how muchfertility exists in the soil on or between the stones . . . since on the earthyground where it is to be found, no trees grow, but only grass. But where theysow over the stony parts they secure crops, and all the trees grow, some of themmarvelously large and beautiful (de Landa, 1556).

Archaeological Landscape

The typical landscape of the albarrada regions (Figure 5) is an extensive, walledbarrio that radiates from the site center. This region is largely anthropogenic inorigin; thus its modern soils were either subject to intensive ancient Maya uses andin some cases largely developed (by natural and anthropogenic means) since thissite was abandoned a millennium ago. Soils that have developed since abandon-ment are those that cover the sides and tops of mounds and are sometimes calledkakab soils (Stevens, 1964:304). Mounds present the ideal place for the pedogenesisof fertile soils in this environment because the broken rocks and plaster providemuch more surface area for mechanical and chemical weathering to decomposerock and release nutrients. Empirical evidence confirms this in two ways: densevegetation in this region coincides with most mounds and modern milperos con-sistently seek to farm on even steep mounds, including the mounds at the site



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center. Thus soils on these mounds co-evolved with human uses since the LateClassic abandonment.

SOIL INVESTIGATIONS: METHODS

The driving question behind this study is to understand the current soil resourcesof the Chunchucmil region to shed light on both ancient and modern Maya sub-sistence. The strategy was therefore to investigate soils from each soil-geomorpho-logic zone in the region and to focus on those soils in likely intensive paleo crop-lands near the main sites. Soil variation in this low gradient plain results mostlyfrom gentle topographic differences that divide the landscape into the coastal zone,the swamp estuary zone, the savanna-tzekel zone, and the karst plain. Soil surveysthrough each of these zones included soil pits, extensive coring of surroundingsites, and sampling for laboratory analyses (Carter, 1993). Overall, the soil database is founded on at least 20 soil pit sites and many more probings in 1994 (Farrellet al., 1996) and at least 35 soil pits and at least 50 probings in 1995 and 1996. Themost intensive soil surveys covered an albarrada barrio (Figure 5) north1–2 kmof Chunchucmil’s site center, an area of possibly intensive ancient agriculture.

Investigating soil diversity in each soil geomorphic zone requires understandingeach zone’s distinctive factors of soil formation. Typologies of these factors serveto organize and model regional soil diversity into a few basic soil types mainlybased on microrelief, which determines local drainage conditions. Developingthese typologies depended on local knowledge, regional expertise, maps, and fieldstudies. The field studies included several extensive probing transects and intensivesurveys in each of the four main soil-geomorphic zones with regional and localexperts. Very extensive maps of the region (INEGI, 1984) also provided some back-ground information for elucidating soil catenas of each zone. Field study of soilsfollowed standard methods for description and sampling (Soil Survey Staff, 1993;Carter, 1993). Soil pits in each of the major landform types of each zone provideda coarse soil catena typology, which was refined with multiple probings and soilpits.

Soil characterizations in the field included color, texture, structure, HCl reaction,and other descriptive terms (see Tables II–V) as outlined by the Soil Survey Manual(Soil Survey Staff, 1993). Farrell et al. (1996) discuss most samples from the Ce-lestun Peninsula and the Swamp Estuary. Samples from the Karst Plain region wereanalyzed at the University of Wisconsin-Milwaukee Physical Geography and SoilsLaboratory and included the following (see also Tables II–V for methods): per-centage carbonate by the chittick method; pH; exchangeable K, Ca, Mg, and Na;cation concentration of Ca, Mg, Na, and K by atomic absorption (P and K used theBray 2 method because of high carbonate); Fe by diathionate-citrate method; par-ticle size by pipette method; base saturation; estimated CEC from exchangeablecations; organic carbon (Walkley–Black); heavy minerals optically by binocularand polarizing microscopes; soluble salts; P fractionation by methods as seen inTable VI; C:N ratio measured by dry combustion using a Carlo Erba Elemental



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Analyzer; Si measured using x-ray spectrometry; and DTPA extractable Fe, Cu, Mn,and Zn (Carter, 1993).

RESULTS: COASTAL AND SWAMP ESTUARY SOILS

The coastal zone has formed only during the past 4000–5000 years (Dahlin etal., 1998); thus soils here are recent and pedogenesis is often interrupted by geo-morphic processes and human disturbance. Soils develop in a catena of inactivebeach ridges and swales, both of which have been partly stabilized by the densecoastal dune communities. Agriculture potential is limited in this zone since onlya thin beach ridge and back swale exist along most of the shoreline. One exceptionis the Celestun Peninsula, which provides slightly more land and some more pro-ductive soils. The Peninsula is up to across, and the soils farthest from the2 kmsea are geomorphically more stable, have developed with more time, and thus havemore mature top soils (e.g., greater melanization). Field investigations each yearfrom 1994 through 1996 have found only one milpa, which further suggests severeagricultural limits here.

Soil classes on the Peninsula range from mostly Aquents, Psamments, andAquepts (Soil Survey Staff, 1994) on old beach ridges and swales to a few Histosolsin wetlands (Table I). The most severe limitations for crops on the coast are severestorms, high water tables, coarse textures (all textures are sand to fine sandyloams), very low phosphorous levels, and high salinity. Since this region has a netdeficit water balance, sandy soils with low water retention would require attentiveirrigation. Phosphorous is the most limiting macronutrient in this environment be-cause available P ranges only from and pH in most of these21, 1 to 18 mg kgcalcareous soils is 8.0 (Farrell et al., 1996), far above the optimum of 6.5 for Pavailability. Commonly here, salinity levels are around 14 dS21 (decisiemens permeter), which is high enough to severely limit all typical Mesoamerican crops(though cotton would be the least limited).

Histosols form at spring sites within the vast swamp estuary that run 5–15 kminland from the beach front. Most of the swamp has mangrove spp. rooted in a soft,organic and calcareous, light gray and light brownish gray (Munsell 10YR 7/1 and6/1), very fine sandy clay (a limnic Histosol known locally as lodo blanco). Thislodo fingers into mucky and peaty Histosols that dominate soils in the mangroveestuary a few hundred m inland from the shore. Ten cores drilled into these peatyHistosols indicate a range in depth over bedrock from at the inland extent10 cmof the estuary to about in the middle of the estuary. A typical example of the3 mdeep, mid-estuary Histosol (Table I ) sampled about landward from the coast1 kmis a Tropohemist (Soil Survey Staff, 1994). In this soil a line between the lowestsapric and the hemic horizons (about ) coincided with the water table during55 cman extreme dry season (March 1995). The limits to agriculture in these soils arelow available phosphorous and excess salinity with the addition of seasonal inun-dation and inaccessibility during the wet season. A few crops are farmed at thesesecluded spots (Dahlin et al., 1998), but local farmers are skeptical about crop

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774 VOL.13, NO.8

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Tab

leI.

Typ

ical

soils

ofth

eco

asta

land

swam

pes

tuar

y.

Soil

Cla

ssifi

cati

onG

eom

orph

olog

yH

oriz

onD

epth

(cm

)C

olor

(Mun

sell

OM

)%

loi

Tex

ture

1St

ruct

ure2

Con

sist

ence

3

Moi

stB

ound

ary4

Oth

er5

Fea

ture

s

1P

enin

sula

T.T

ropa

quep

tA A

B0

–14

14–

2010

YR

2/2

10Y

R5/

33.

32.

4fs

lls

lm,g

r1m

,m-g

rvf

rvf

rgs gs

Cha

rcoa

lfr

agm

ents

C20

110

YR

6/3

—ls

mvf

rfl

SAR

.13

2B

each

Ber

mT

.Psa

mm

aque

nts

A C0

–20

20–

2610

YR

5/2

10Y

R7/

31 —

ls s1m

,m-g

rm

vfr

lgs cw

fl,m

any

shel

lsA

b26

–29

10Y

R5/

2—

lslf

,gr

vfr

cw13

.7dS

/m5

2C29

110

YR

7/2

—ls

ml

flLa

rge

shel

ls3

Est

uary

pete

nO

al0

–25

10Y

R2/

1—

—lc

,gr

frcw

,2

cmlim

nic

Tro

pohe

mis

tO

a225

–55

10Y

R2/

141

—lc

,sbk

frai

Lam

inae

Oe2

55–

216

10Y

R2/

1—

—2c

,pfr

—fl

Abb

revi

atio

ns:

12

s5

sand

;ls

5lo

amy

sand

;sl

5sa

ndy

loam

;fs

l5fi

nesl

.1

5w

eak;

25

mod

erat

e;3

5st

rong

;f

5fi

ne;

m5

med

ium

;c

5co

arse

;bl

ocky

;bl

ocky

;3

abk

5an

gula

rsb

k5

suba

ngul

argr

5gr

anul

ar;

m5

mas

sive

,p

5pl

atey

.fi

5fi

rm;

fr5

fria

ble;

vfr

5ve

ryfr

iabl

e;l5

loos

e;fl

5la

min

atio

ns.

4sm

ooth

;sm

ooth

;w

avy;

irre

gula

r.5

abso

rpti

onra

tio;

5fi

necs

5cl

ear

gs5

grad

ual

cw5

clea

rai

5ab

rupt

SAR

5so

dium

dS/m

5m

easu

re(T

able

V).

salin

ity



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production because of salinity. Local farmers borrow organic matter from thesesites to augment the soils of their house gardens, a practice that may have its originsin ancient times.

Savanna/Tzekel Zone Soils

The swamp estuary Histosols merge into the seasonally wet savanna as theground rises gradually up to in elevation through swales and natural hillocks1–2 m(tzekeles) of weathered limestone, broken cobble, and petrocalcic pavements.From this zone seaward the freshwater Yucatan aquifer intersects the ground sur-face and is actively precipitating calcium carbonate as a dense calcrete or petro-calcic horizon (Soil Survey Staff, 1994). This continuously causes the surface cap-stone to become denser, with ca. 1% pore space in contrast to ca. 15% pore space

below the surface (Perry et al., 1989). This tzekel zone has very thin soils1.5 mand has been long regarded as unfit for crops (Garza and Kurjack, 1981). Threeseparate sampling transects with at least 10 soil pits and 20 probings form the database for savanna soils.

Thin, highly organic, wetland soils occupy the lower areas around tzekeles. Soilsin this zone are transitional; some nearly qualify as histic epipedons (nearly 12%OC) of a few centimeters on top of weathered limestone, but most have moretypical ochric epipedons. Typical soils in swales between hillocks have hydrophyticvegetation and aquic conditions indicated by the seasonal inundation due to nearsurface ground water are Lithic Tropaquepts (1 on Table II) (Soil Survey Staff,1994). Typical soils on tzekel knolls in low forest vegetation (e.g., Acacia spp.) aremoderately well drained and have thin A horizons that overlie fractured bedrockon Bk horizons (2 on Table II). Some of these soils have A horizons 10 or more cmthick on top of bedrock, and thus classified as Lithic Mollisolls.

As this zone graduates to a higher savanna, three surface types dominate: thickersoils (Haplustolls or boxluum or chichluum) occur associated with fragmentedbedrock highs and mounds; thinner soils (Paleustalfs or kancabs) occur associatedwith fernlike vegetation, grasses, and sedges and shallow depressions underlain bydenser bedrock; and dense limestone pavements occur at the surface associatedwith some cacti, agave, and grasses. Soil number 3 (Table II), from a low secondaryforest between milpas, is typical of the Paleustalfs in this zone. One atypical sa-vanna soil (4 on Table II) occurs at San Miguel (the westernmost suburb of ancientChunchucmil) on the footslope of a 2-m-high mound. Field work thus far has un-covered no natural soils (away from mounds) in the savanna zone that are as thickor as fertile as this Lithic Calciustoll. Abandoned mounds are clearly preferred sitesfor milpas and for cattle grazing as in this case. This site had an order of magnitudehigher available P than surrounding modern milpa and fallow soils. More commonsoils here are the thinner kancabs (Paleustalfs) that occupy natural hillocks (tzek-eles) and the flat lower swales underlain by denser, less fragmented bedrock. Localfarmers plant on the former, even though the soils are skeletal, but they avoid thelatter, even though soils are thicker, because thick grasses choke out crops and

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Tab

leII

.F

our

typi

calS

avan

natz

ekel

zone

soils

.

Soil

Cla

ssifi

cati

onG

eom

orph

olog

yH

oriz

onD

epth

(cm

)C

olor

(Mun

sell

OM

)%

loi

Tex

ture

1St

ruct

ure2

Con

sist

ence

3

Moi

stB

ound

ary4

Oth

er5

Fea

ture

s

1T

zeke

lsw

ale

A1

0–

810

YR

2/1

18cl

lf,g

rfr

gsLi

thic

A2

8–

1110

YR

3/2

—cl

lf,m

-gr

vfr

gsC

alci

tecr

.T

ropa

quep

tC

gk11

–18

10Y

R5/

2—

cl2m

,sbk

vfr

aiC

alci

tecr

.C

km18

110

YR

8/2

——

——

—P

etro

cacl

ic2

Tze

kelk

noll

A0

–8

10Y

R2/

116

cl2m

,sbk

frgs

Lim

esto

negr

L.U

stro

pept

Bk

8–

2210

YR

7/3

—cl

2m,s

bkfr

ai—

Crk

221

10Y

R8/

2—

——

——

3Sa

vann

asw

ale

A0

–7

2.5Y

R3/

49

cl2m

,gr

frgs

—P

aleu

stal

fB

t7

–21

2.5Y

R3/

6—

c2m

,sbk

frai

Cla

yfil

ms

Ckm

181

10Y

R8/

2—

——

——

Pet

roca

clic

4K

noll-

mou

ndA

10

–19

10Y

R2/

114

cl1m

,gr

frcs

Few

sher

dsP

aleu

stol

lA

219

–35

10Y

R3/

2cl

1f,s

bkvf

rai

Lim

esto

negr

Crk

221

10Y

R8/

2—

——

——

Abb

revi

atio

ns:1

loam

;sl

.2bl

ocky

;c

5cl

ay;c

l5

clay

fs1

5fi

ne1

5w

eal;

25

mod

erat

e;3

5st

rong

;f5

fine

;m5

med

ium

;c5

coar

se;a

bk5

angu

lar

bloc

ky;

3fr

iabl

e;4

smoo

th;

smoo

th;

sbk

5su

bang

ular

gr5

gran

ular

;m5

mas

sive

.fi

5fi

rm;f

r5

fria

ble;

vfr

5ve

ry1

5lo

ose.

cs5

clea

rgs

5gr

adua

lw

avy;

irre

gula

r;5

cw5

clea

rai

5ab

rupt

cr.5

crys

tals

;gr

5gr

avel

.cr

.5cr

ysta

l;gr

.5gr

avel

.



shortstandard


water stands for long periods after rains (though soil color indicates they are welldrained).

Soils of the Karst Plain

This study made both extensive sampling and description transects and a moredetailed investigation of an “albarrada” barrio of Chunchucmil, so called for itshoneycomb of walled mound groups and field-gardens (Figure 5). The extensivesoil survey includes soil descriptions and analyses systematically sampled from thedifferent geomorphic landscape types. The only existing soil map (INEGI, 1984)places the entire belt in the Rendzina FAO unit, which generalizes the soils as thinover limestone. Fieldwork in 1997 now recognizes that much of the entire site ismade up of these albarrada groups, spreading for many square km across the karstplain. The barrios exhibit the following typology: huitz-mounds, ancient Mayamounds, wall systems, open field areas that cover ca. 60% of the area, streets (cal-

lajuelas), large processional roads (sacbeob), sascaberas, field-gardens (solares),and limestone pavements (Figure 4). This region contains no real rejolladas andaguadas (solutional dolines) and only small cenotes. The soil data base includes23 soil pits and at least 50 probings in this zone of about (Figure 5).20.5 km

Over the whole region today, limestone or petrocalcic pavements and human-placed boulders cover approximately 50% of the surface. In terms of ancient Mayausage the largest expanse is probably field-gardens, areas with no structures orplatforms inside walls. Since these are walled fields with thin and in some casesno soil cover, they are analogous to walled bajadas described by Kepecs and Bou-cher (1996). Soils in this landscape form lithosequences based on whether the soilhas formed in fragmented limestone or on dense petrocalcic pavement (aputun).Soils that formed on the latter range from no soil (only the petrocalcic pavement)to skeletal soils, to deeper soils in swales and over broken limestone, and to dep-ositional soils in sascaberas and solution features (Table III).

As with the Savanna zone, local farmers lump all soils into kancab, and boxluum,which correspond to four main USDA soil types in this zone (Table III). The essen-tial difference shown by x-ray spectrometer analysis of major oxide elements (Ta-ble IV) is that kancabs are about 94% SiO2, Al2O3, and Fe2O3, and 2% CaO, whereasboxluum soils are about 47% SiO2, Al2O3, and Fe203, and 58% CaO (Arnold, 1971).Heavy mineral analyses of five samples showed the same basic pattern for bothmajor soil types, about 75% hematite, 10% magnetite, 10% garnet, and 5% apatite.No extensive clay mineral analysis is available for these soils, but preliminary anal-ysis and the general literature indicate the presence of both kaolinite and mont-morillonite. The kancabs are swale soils that are thin ( ), dark reddish5–35 cmbrown (5YR3/2) silty clays or silty clay loams with high base status, and petrocalcichorizons. These soils can be either Paleustalfs and Paleustolls (Soil Survey Staff,1994) (Figure 4; Table III, 2, 4, and 5). The former are deeper soils that have anargillic horizon and the latter are thinner soils with only mollic A horizons (ofgreater than ) that lie abruptly on petrocalcic layers. The Mayan term box-10 cm

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Tab

leII

I.F

ive

soils

repr

esen

tati

veof

the

kars

tpl

ain.

Soil

Cla

ssifi

cati

onG

eom

orph

olog

yH

oriz

onD

epth

(cm

)C

olor

(Mun

sell

OM

)%

loi

Tex

ture

1St

ruct

ure2

Con

sist

ence

3

Moi

stB

ound

ary4

Oth

er5

Fea

ture

s

1kn

oll-m

ound

A1

0–

1710

YR

2/2

11cl

2f,g

rfr

gs10

%gr

.L.

Hap

lust

oll

A2

17–

285Y

R3/

2—

cl1f

,gr

vfr

gsF

ewsh

erds

“Box

luu

m”

Ack

28–

4010

YR

4/3

—cl

lf,g

rvf

rai

.50

%gr

.C

rk40

110

YR

8/2

——

——

—2

flat

swal

eA

10

–9

10Y

R2/

19

sic

2m,g

rfr

gs,

10%

gr.

Pet

roca

lcic

A2

9–

215Y

R3/

2—

sic

2m,g

rfr

gsP

aleu

stal

fB

t21

–30

2.5Y

R3/

6—

c2f

,sbk

fr—

Cla

ysk

ins

“Ka

nca

b”

BC

rk30

–33

2.5Y

R4/

6—

cl2m

,sbk

frai

50%

cobb

leC

km33

110

YR

8/2

——

——

—P

etro

cacl

ic3

Sasc

aber

aA

10

–8

10Y

R2/

114

cl2f

,gr

frgs

10%

gr.

Lith

icC

alci

usto

llA

28

–21

10Y

R2/

114

cl2f

,gr

frgs

50%

cobb

le“B

ox

luu

m”

AC

21–

4210

YR

2/1

—cl

2f,s

bkfr

ai75

%co

bble

“Chi

chlu

um”

Cr

421

10Y

R8/

2—

——

——

—4

flat

swal

eA

10

–9

5YR

3/2

12si

cl2f

,gr

frcs

Pal

eust

oll

A2

9–

155Y

R3/

212

sicl

2f,g

rfr

ai—

“Ka

nca

b”

Ckm

151

10Y

R8/

2—

——

——

Pet

roca

clic

5ka

rst

knol

lA

0–

135Y

R3/

213

sicl

2f,g

rfr

gs10

%gr

L.H

aplu

stal

fB

t13

–24

5YR

4/4

—cl

2f,s

bkfr

gsC

lay

skin

s“K

an

ca

b”

BtC

24–

345Y

R4/

4—

cl2m

,sbk

frai

40%

cobb

le6

kars

tce

note

A0

–39

10Y

R2/

118

cl1f

,gr

frai

Man

yw

orm

sL.

Hap

lust

oll

Crk

391

10Y

R8/

2—

——

——

Bou

lder

plug

“Box

luu

m”

Abb

revi

atio

ns:

1lo

am;

sand

ylo

am.

2c

5cl

ay;

cl5

clay

fs1

5fi

ne1

5w

eak;

25

mod

erat

e;3

5st

rong

;f

5fi

ne;

m5

med

ium

;c

5co

arse

;ab

k5

bloc

ky;

bloc

ky;

3fr

iabl

e;4

smoo

th;

angu

lar

sbk

5su

bang

ular

gr5

gran

ular

;m

5m

assi

ve.

fi5

firm

;fr

5fr

iabl

e;vf

r5

very

15

loos

e.cs

5cl

ear

smoo

th;

wav

y;ir

regu

lar;

5gs

5gr

adua

lcw

5cl

ear

ai5

abru

ptcr

.5cr

ysta

ls;g

r5

grav

el.

cr.5

crys

tal;

gr.5

grav

el.



shortstandard


Table IV. Kancab vs. boxluum soils: elements and nonclay, oxide minerals.

Elementsa (N 5 5) Kancab Boxluum

SiO2 48.15 (n 5 3) 6 0.04% 24.6 (n 5 2) 6 0.06%Al2O3 34.70 (n 5 3) 6 0.05% 17.28 (n 5 2) 6 0.07%CaO 2.08 (n 5 3) 6 0.21% 58.31 (n 5 2) 6 0.06%Fe2O3 11.05 (n 5 3) 6 0.11% 5.59 (n 5 2) 6 0.2%MgO 1.19 (n 5 3) 6 20.6% 0.73 (n 5 3) 6 36.85%TiO2 1.09 (n 5 3) 6 3.39% 0.47 (n 5 3) 6 10.2%K2O 1.89 (n 5 3) 6 0.21% 0.84 (n 5 3) 6 0.33%

Heavy Mineralsb of Kancab and boxluum c(N 5 5)

$70–75% hematite,10–15% magnetite

,10% garnet,5–10% apatite

,Trace horneblende

Carbonates Kancab Boxluum

CaCO3 0.7% (N 5 7) 28.3% (N 5 10)CaMg(CO3)2 0.24% (N 5 7) 7.6% (N 5 10)

a XRF: X-ray spectrometer of major oxide elements (weight percents using alpha matrix correction).Total oxides in the boxluum soils are greater than 100% because the high calcium amounts interferewith calculation.b Heavy minerals identified in sand fraction separated using tetrabromoethane at specific gravity of 2.9,then using binocular microscope; some minerals verified by powdering grains and using a polarizingmicroscope.

luum refers to upland soils developed on fragmented limestone mostly after thesite’s abandonment (ca. 1000 B.P.) since they occur on plazas and mounds, as wellas on some huitzes (Figure 4; Table III, 1 and 3). A major difference from kancabsis that boxluums have formed during the last millennium on an abundance of lime-stone cobble and still have an abundance of elemental calcium carbonate. Theseboxluums are either Lithic Calciustoll or Paleustolls since they have mollic epipe-dons, ustic moisture regimes, are calcareous throughout, have petrocalcic horizons,no argillic horizon, and a lithic contact within 50 cm (Soil Survey Staff, 1994). Thesesoils are thicker ( ), have higher amounts of organic carbon, carbonate,35–50 cmhigher pHs, less clay, and have darker colors, often very dark brown (10YR2/2) inthe thick A horizons. Some kancabs are Paleustolls, though similar to Palcustalfsexcept for lacking an argillic horizon.

There are three apparent differences in soils within the albarrada group com-pared with soils of the karst plain. First, ancient Maya land-use density decreasesand thus the density of walls, mounds, and other architectural features decreasesaway from Chunchucmil. Secondly, the total surface covered by limestone pave-ment continues to be about half, and the amount of land covered by fragmentedlimestone decreases substantially; thus there is less deep Haplustoll (boxluum) soilacross the Plain due to the lack of mound structures, a prime area where these

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780 VOL.13, NO.8

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soils exist today. Third, considerably more recent disturbance occurs away fromthe Chunchucmil albarrada area thus far studied, which means more soils are al-tered by human activity. The region around the main Chunchucmil site and thealbarrada group have been covered in second growth forest for at least 25 yearsaccording to aerial photographs, whereas more of the karst plain is covered by oldhenequen fields, more intensive cattle ranching, and some other crop types (e.g.,aloe vera and papaya). The same soil suite occurs away from the main site exceptfor two differences: A few cumulic soils occur in internally drained dolines or wellsand more soils have denser and less organic topsoils probably owing to compactionand mineralization of organic carbon by removal of vegetative cover (Table III, 6).This research around Chunchucmil indicates that soil fertility is indeed greater nearthe site, but this is caused by recent soil formation on archaeological features andthe fact it occupies the edge of the Karst Plain next to the relatively infertile tzekel

zone.

RESULTS: SOIL NUTRIENTS OF THE SAVANNA AND KARST PLAIN

Overall, as Ortiz Monasterio (1950) found generally for Yucatan’s soils, Chun-chucmil soils are sufficient in most nutrients with some salient exceptions (TableV). All the soils have high organic matter, loamy to clayey textures (loams, clayloams, silty clay loams, silty clays, and clays), moderate alkalinity (pH ranges from7.3 to 7.6); have high amounts of calcium ( ); have high CECs21x 5 9497 mg kg( ); have high base saturation (x 5 77.1; 65–109 cmol(1)/kg or meq/100 g x 5

); have moderate amounts of magnesium ( ); and have vari-2198.3% x 5 210 mg kgable deficiencies in available phosphorus ( ), potassium (21x 5 94 mg kg x 5

), and zinc ( ). The soil C/N ratios average about 13/1 and21 2188 mg kg x 5 0.4 mg kgnone of the soils have too high a ratio that would limit N uptake. Soil salinities areall low ( ) where more than 4 dS21 (EC in decisiemens per meter)2lx 5 0.18 dS mis considered saline. The other soil factors that limit crop production locally in thisenvironment include: slow drainage in areas of limestone pavements and skeletalor no soil cover on much of the land surface. Poor drainage or inundation limitscrops in their early development; thin soils limit rooting depth and water and nu-trient supply; and a lack of soil cover limits crops to perhaps one-half of the land-scape.

The six essential macronutrients generally make up over 500 ppm in plantswhereas micronutrients have generally fewer than 100 ppm (Foth, 1990). Nitrogenis the most important soil nutrient, and normally ranges from 0.02% to 0.5% ofmineral topsoils (Brady, 1990). Tests for both water soluble nitrate (NO3) and car-bon–nitrogen (C/N) ratios assessed their sufficiency and the nitrogen availabilityfor plant growth in these soils. All samples tested showed healthy nitrate levels( ) and C/N ratios ( ) in the top soils (Table V). It should be notedx 5 0.5% x 5 12.8that available nitrogen (often ca. ) is much less than soluble NO3 and2150 mg kgthat an intensive maize crop could remove most this in a season. The upkeep of N,



shortstandard


Table V. Chemical and physical properties of savanna & karst plain soil A horizons.a

Attribute Low High x Distribution Methods and Comments

Soil Depth 0 52 cm 17–33 ca. 50% no soil Estimates from soil sur-vey

pH 7.3 7.8 7.5 SMP single-buffermethod

Fe % 0.3 4.4 2.8 Oxalate-extractableOC 4.42 7.91 5.9 Walkley–BlackP 23 284 94 62% , 85 (Aval) bicarbonate ex-

tract (Olsen et al.1954)

K 26 220 88 77% , 90 (Ex) mg 21kg 5 ppm <or c. kg21(lb acre

21ha )/2Mg 20 410 210 (Ex) mg 21kg 5 ppmCa 7393 13,090 9497 (Ex) mg 21kg 5 ppmNa 95 257 147 (Ex) mg 21kg 5 ppm

Salts 0.11 0.23 0.18 dS (EC in decisie-21mmens per meter)

N 0.39% 0.68% 0.5% NO3 water solubleC/N 9 20.1 12.8 CHN analyzerCEC 65 109 76 meq/100g (cmol/kg)Zn (N 5 16) 0.10 1.15 0.40 88% # 0.50 DTPA-extractable, mg

21kg 5 ppmCu (N 5 4) 0.65 1.05 0.78 DTPA-extractable, mg

21kg 5 ppmFe (N 5 4) 5.10 12.20 7.56 DTPA-extractable, mg

21kg 5 ppmMn (N 5 4) 6.40 20 15.38 DTPA-extractable, mg

21kg 5 ppmBS 97.8 99 98.3% Sum of cations

a unless noted otherwise; texture (USDA): L, SiL, SiCL, CL, C (University of Wisconsin-Milwau-N 5 13,kee Soils Laboratory).

therefore, through natural mineralization from organic matter, fixation, organicmatter preservation, and fertilization is crucial for sustainable production. Chun-chucmil soils are rich in organic matter, a dominant source of N, but these soilsare prone to organic matter decomposition without systematic maintenance (Troehand Thompson, 1993).

Phosphorous is one of the major limiting factors for crop production because itis a major macronutrient but has low solubility. Like N, crops remove large quan-tities of P ( per season) and thus require soil P maintenance for sus-215–15 mg kgtainable production. In its several forms, phosphorous varies widely around Chun-chucmil; its available form ranges from based on the method of2123 to 284 mg kgOlsen et al. (1954), which is preferred for calcareous soils. In previous work, valuesof P were underestimated by the Bray P-1 procedure, ranging from less than

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Table VI. Phosphate analyses (University of Wisconsin-Milwaukee Soils Laboratory).a

Sample

Phosphate Frationations from Chunchucmil

Frac 1a Frac 1bFrac 2

(mg/kg) Frac 3 ava PSumFrac

% of Total P

F1 F2 F3Ratiob

FII/FI

8812 0 13.6 7.2 127 — 147.9 9.2 4.9 85.9 0.5298822 2.2 10.7 50.1 34 — 97.1 13.3 51.6 35 3.8848822dup 2.1 7.2 53.6 33.5 — 96.3 9.7 55.6 34.7 5.7638910.1 0 17.5 30.7 311.1 — 359.3 4.9 8.5 86.6 1.7548911.1 1.5 38.4 63.2 141.1 103 244.1 16.3 25.9 57.8 1.79581014.1 0.4 2.5 40.4 41.6 43 85 3.4 47.6 49 13.9318112ab 0 60.2 106.7 590.7 18 757.6 7.9 14.1 78 1.7728112dup 0 61.7 113.3 588.7 7 63.7 8.1 14.8 77.1 1.836x 318.9 63 3.908

Also, Apatite occurs in all samples from 5–10% of heavy minerals; soil humus is ca. 1% phosphorousa Average value for sedimentary (Eidt, 1984). Fraction 1a (NaOH/NaCl extraction) mea-rocks 5 ca. 200sures nonoccluded A1 and Fe bound P. Fraction 1b (NaCl and citrate-bicarbonate (CB) measures Psorbed by carbonates during the 1a extraction. Fraction 2 (citrate-dithionite-bicarbonate (CDB) mea-sures P occluded within Fe oxides and hydrous oxides. Fraction 3 (HCl extraction) measures Ca-boundP.b Ratio (see Eidt [1984] and Lillios [1992]).

(Farrell et al., 1996). The typical average for total phosphorous is211–8 mg kgabout , though available forms are much lower (Troeh and Thompson,21500 mg kg1993). Several factors in the soils of Chunchucmil limit phosphate availability: 62%of the soils around Chunchucmil have available P of less than ; clay2185 mg kgsoils, which dominate the study area, reduce phosphate availability; and every soilsample in this region has a pH above 7.2, which leads to less available forms of soilphosphorous. The soil’s somewhat alkaline conditions have led to the dominanceof very low soluble calcium phosphates: 63% of total P in six P fractionations isCa-bound P. One mitigating factor is that organic matter of these soils is quite high( ) and organic P is more available in alkaline conditions. Therefore,x 5 5.9% O.C.since a large fraction of soil P resides in organic matter, its maintenance would beextremely important for crop production (Troeh and Thompson, 1993). In this re-gion these soils are currently cropped with little vegetative cover, and organicmatter, and with it N and P, decline rapidly.

Although there are few phosphate fractionation analyses ( ), five of the sixn 5 6show higher ratios of Fraction 2 to Fraction 1 ( ) (Table VI). Fraction 2x 5 3.9:1measures P occluded within Fe and hydrous oxides, and has been used to indicateancient P enrichment (Lillios, 1992; Eidt, 1984). The one sample with a lower frac-tion 2 is from a plaza soil, an unlikely agricultural site, whereas four other samplesare from open areas within albarrada groups and one is from a possible outfield.Since no historical evidence (from the hacienda era) exists for fertilization thatwould have increased P fraction 2 (Vlcek et al., 1978), it is possible these heightened



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levels are linked to ancient intensive fertilization. These findings are particularlycurious compared with P-fractionations ( ) from Sayil (Dunning, 1992),n 5 10where the mean total P is and the ratio of Fraction 2 to Fraction 121538.9 mg kgis 0.082. The same figures for Chunchucmil are a total P mean of ,21233.5 mg kgand the ratio of Fraction 2 to Fraction 1 is 2.133. High P variability can be ascribedto both past and present human uses and to lithology, since shell fossils, associatedwith higher P, are variable in the regional limestone.

Potassium is the third most important fertilizer element for crops, and modernintensive corn crops use it at rates (ca. ) that are nearly as high as the2150 mg kgaverage level ( ) of exchangeable potassium of regional soils (Troeh and2188 mg kgThompson, 1993). For a site in Belize, Coultas et al. (1992) report comparablepotassium levels as limiting quantities in subsoils, while at the Uxmal ExperimentStation K levels of very fertile soils are nearly an order of magnitude higher( ) in topsoils (INEGI, 1994). As with P, several factors in Chunchucmil21640 mg kgsoils limit available potassium: K is moderately low (77% of soil samples are below

compared with typical rates of ); leaching rates are also21 2190 mg kg 50–200 mg kghigh; minerals (e.g., feldspar, mica) that release potassium from chemical weath-ering are in short supply (Table IV); every soil sample in this region has a pH above7.2, which encourages K fixation toward less available forms; and high soil calciumand magnesium (calcium ) can reduce potassium uptake by21x 5 9497 mg kgplants (Troeh and Thompson, 1993). As with N and P, K must be maintained forsustainable production especially in this environment where K is limiting. The onlyother element tested that limits some crop production elsewhere in the Maya Low-lands (Olson and Puleston, 1972) is zinc. Zinc is very low in all regional soils, with88% of samples , whereas typical levels range between 10 and21# 0.50 mg kg

. Moreover, zinc availability is reduced under the conditions of the21300 mg kgstudy area where alkalinity leads to insolubility and high quantities of carbonateslead to high adsorption. A few other analyses found low copper levels (x 5

) and adequate levels of iron and manganese, though the availability210.82 mg kgof these nutrients could be limited for some crops by high levels of calcium (Troehand Thompson, 1993).

In sum, since the traditional intensive crops at Chunchucmil (maize, beans, andtropical fruit trees) have high requirements for limited nutrients like phosphorous,potassium, and zinc, these would need to be consistently maintained for intensivecrop production. One of the persisting enigmas and myths about the ancient Mayais how did such an advanced civilization with very high population densities existwithin what has traditionally been considered a hostile tropical environment. Theprevious paragraphs show that this environment had its limits but soil productivitycan be maintained with attentive cultivation, and thus soil productivity is equallythe result of ingenuity as it is of soil nutrients (Jacob, 1996). All of this evidenceas well as the meager yields of current milpas and historical haciendas indicate theneed for distinct ingenuity for sustained agriculture around Chunchucmil andnorthwest Yucatan.

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DISCUSSION: ANCIENT MAYA INTENSIVE AND ATYPICAL

AGRICULTURE

Several types of intensive cultivation were practiced by the ancient Maya, in-cluding wetland agriculture in low, seasonally inundated areas and terrace agri-culture on sloping lands. The goal of both of these is to maintain a planting surfaceand suitable soil moisture conditions. On the flat plains of Chunchucmil Maya farm-ers were presented with a similar situation to those working on steep slopes: littleor no soil cover. On slopes they built a variety of stone terraces and dams, and atChunchucmil, they may have built walls that act as terraces. A striking feature ofthe albarrada groups and the outfield sites around this site are the presence ofwalled subtle depressions with skeletal or no soil cover. These walls could be dikesto contain raised beds of imported soil and humus, and the closely associatedsascaberas were probably easy access points to groundwater for pot irrigation orspecialty agriculture areas, where a moister microclimate would be hospitable tosilviculture. Thus far, circumstantial evidence in the form of walls and sascaberasspeak to this hypothesis. Since imported materials still carted in today are highlyorganic (e.g., night soil and tsekel-zone organic top soils), rapid tropical decom-position would remove evidence of accumulated soil.

A volumetric estimate of the necessary quantity of imported organic matter in-dicates only that this would be possible. Each raised bed within the walls wouldhave been about , and each would have required a thickness of about22000 m

of transported organic matter mixed with the soil to build the beds. Thus10 cmeach raised bed would require about or about of trans-3200 m 100 metric tonnesported organic sediment. This would require about 250–500 trips carrying

to build the beds over a period of years and would require at least 2520–40 kgtrips a year to replenish decomposed organic matter. Since contemporary farmerstravel 5 or more kilometers with loads often this heavy (to this day for carryingorganic matter), it is possible that intensive ancient Maya production could havemaintained raised beds in this zone from imported organic matter. Over a periodof years, however, farmers would have to have traveled farther for these resources.

The walls could also be enclosures around atypical crops, fruit orchards, or thesascaberas, as is the case in the solares of contemporary Chunchucmil. Two ancientcrops native to the region (INEGI, 1984) and that have been little explored archaeo-logically are agave or maguey (Agave spp.) and cotton (Gossypium spp.). Landamentions cotton in the Post Conquest Era (Tozzer, 1941:200); it can withstand thesemiaridity and higher salinity levels of this region; and it can provide cottonseedoil and tradable cloth. Agave produces a caloric syrup (aguamiel) as well as fiber(Dahlin et al., in preparation; Evans, 1992). Certainly henequen (Agave fourcroy-

des), harvested for fiber, was the most valuable crop ever grown historically in thissemiarid region, and clearly maguey was exploited in prehispanic Mesoamerica,where aridity and soils limited other more typical crops like maize (Nobel, 1988;Evans, 1992). Thus far the argument for maguey production in the Classic periodawaits archaeological support, but this landscape holds no major drawbacks for



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its production: It is native here; maguey produces a large dietary supplement inother regions with half the rainfall of Chunchucmil; it was also mentioned by deLanda (Tozzer, 1941:195); and regional soils are much less limiting for agaves thanfor maize (Nobel, 1988).

Cacao (Theobroma cacao) is another crop that may bridge the gap between tradeand agriculture or simply testifies to Maya ingenuity (and the prestige of choco-late!). It may have been grown here and traded for maize, but this is unlikely be-cause the areas with maize surpluses would have also been more conducive tocacao cultivation because of their moister conditions and deeper soils. Cacao, how-ever, is cultivated by contemporary Maya in the wetter east side of the Yucatanaround Coba in sascaberas (Kepecs and Boucher, 1996) and at the equally dry siteof Emal on the north coast of Yucatan in depressions with moister microclimates,which Chunchucmil has in abundance (Gomez-Pompa et al., 1990). The major lim-itations for cacao at Chunchucmil are the same as at Emal: Minimal quantities ofK and P, thin soils, and moisture limitations (Muhs et al., 1985), but all can beamended by fertilizing, planting in depressions with added soil, and pot irrigation.Cacao is one of many species that comprise Maya silviculture, a complex foodproduction system usually associated with extensive populations. Studies of indig-enous silviculture in this region (Nations and Nigh, 1980; Gomez-Pompa, 1987) anddiscussion of orchards in the Post-Conquest period (Tozzer, 1941:196–200; Whit-more and Turner, 1992; Jones, 1982) implicate its importance for northwest Yu-catan (Kepecs and Boucher, 1996).

CONCLUSIONS

Historically, yields around Chunchucmil have been low through a succession ofcrops, and currently the region has a deficit in food production. Human populationshave also been low historically, but the Maya Late Classic population of this regionwas very high, with multiple clusters of densely packed walled groups. If the localpopulation fed itself with local cultivation, then some element of this landscape forfood production is missing. Evidence also suggests that climatic conditions wereno more suitable in the Late Classic, though one line of evidence may indicate awider zone of more productive Karst Plain soils surrounding Chunchucmil duringthis period if sea levels and groundwater levels were lower. This somewhat widerzone of better soils, nevertheless, provides little compensation for the large dis-parity between poor contemporary crop production and high ancient populations.Soil analysis provides one perspective on this question: It shows what the circum-stances are for current soil fertility. But soil analysis can show only a part of theequation for crop production because human ingenuity is at least as important asinherent soil quality. Ethnography provides some information on indigenous cropintensification methods because this indicates what knowledge is available duringhistorical times. Indigenous knowledge together with soil analyses and archaeo-logical evidence provides the best possible clues about prehistoric agriculturewhere botanical clues and writing are not available.

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Soil studies of the Chunchucmil region show large limitations for intensive ag-riculture. Perhaps the largest limitation is a simple lack of soil on about 50% of thesurface. Few crops and none of the traditional Mesoamerican ones prosper withoutsoils, though certainly some orchard crops can grow under these conditions. Wheresoils exist, many are thin, which limits the quantity of nutrients available to croproots and soil moisture storage for the regions’s frequent dry periods. Since thesesoils are often underlain by dense petrocalcic layers, they also experience inun-dation during wet periods that hampers the development of young crops. Thesethin, reddish brown, silty, and clayey soils that are locally called kancabs (Paleus-talfs, Paleustolls, and Haplustalfs in USDA terms) make up at least 30% of the restof the landscape. At most, 20% of the landscape is covered by thicker, more fertile,black, clay loam, and moisture conserving boxluums (Calciustolls and Haplustollsin USDA terms). More of these soils are located around the site than in the rest ofthe karst plain, due to recent boxluum formation on mounds and plazas.

The only major soil fertility losses in this landscape could be from natural weath-ering and removal by crops, because the landscape is too flat for soil erosion byrunning water and the soils too tightly aggregated by clays for wind erosion. Interms of the basic soil fertility parameters, organic matter, base saturation, cationexchange capacity, calcium, magnesium, sodium, iron, and nitrogen are generallyat healthy levels. The essential nutrients of phosphorous, potassium, and zinc, how-ever, are variably to generally low, which creates a challenge for intensive cropproduction. Intensive cropping would remove large amounts of these three nutri-ents and could initiate decomposition of organic matter, a storehouse of essentialnutrients. Maintenance of intensive cropping would require the constant recyclingof all organic wastes and the importation of additional organic matter and brokenor charred rock into the fields or raised beds. These beds would then need to beirrigated, and soil moisture would need to be conserved by mulching. Otherwise,the region’s basic soil circumstances could not support intensive cropping. Indeed,a few contemporary farmers here still import organic soils into their solares fromuninhabited sites in the savanna, though none currently compost organic wastes.

It is difficult to imagine sufficient crop production to support high ancient Mayapopulations based on current techniques. Two lines of evidence provide possiblelinks to past intensive land uses: phosphate fractionation and archaeological evi-dence. Since phosphate fraction 2 is occluded within iron and hydrous oxides andresists weathering, it persists for long periods of time. Although not ideal becausephosphate in various fractions could have been elevated during periods subsequentto the Late Classic, fraction 2 levels in all probable agricultural sites are intriguinglyhigh. The archaeological evidence is the albarrada groups themselves. In thesesites, walls surround small mound groups, wells, sascaberas, and small open, arableareas. All the elements of intensive cultivation exist here: easy access to waterthrough wells and sascaberas, access to sascab for fertilizer, and a dense popula-tion for compost, wastes, and labor. Additional soil could also be imported fromthe savanna and even swamp zones, where seasonal inundation limited habitation.The walls are clearly some kind of property boundaries, but they also may have



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served as the edges of or terraces for raised beds of highly organic soil, whichwould decompose after intensive management was abandoned with the site’s collapse.

Alternatively, Chunchucmil, an atypical site, could have been based on atypicalcrops or could have had an alternative economy based on trade for other com-modities. The latter possibility is out of the purview of this article and is addressedelsewhere (Dahlin et al., 1996). The possibility for alternative crops such as cacaoand maguey has parallels elsewhere in Mesoamerica but as yet has no scientificevidence. Firm evidence may be found in the ongoing field study of artifacts as-sociated with these crops or their preparation and from studies of pollen, phyto-liths, macrofossils, and additional soil testing started in the 1997 field season.

I would like to thank the following people who have contributed to this field research and providedmuch discussion for the development of this article: Sheryl Luzzadder-Beach, Bruce Dahlin, Pat Farrell,Aline Magnoni, George Vrtis, Clara Bezanilla, and the villagers of Chunchucmil. I would also like tothank the following reviewers who made excellent suggestions: Russell Almaraz, William Doolittle,Nicholas Dunning, John Jacob, Gene Perry, and Kevin Pope. Finally, the following organizations deservethanks for funding parts of this research: the Association of American Geographers Anne White Fund,the Georgetown University Graduate School, the Walsh School of Foreign Service, Howard University,the National Geographic Society, and the National Science Foundation.

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Received August 26, 1997

Accepted for publication May 28, 1998

Documents

Soil constraints on Northwest Yucatán, Mexico: Pedoarchaeology and Maya subsistence at Chunchucmil