The following page describes gradients in soil texture, color, moisture retention, and nutrient availability across topographic gradients at the Marajoara field site. Use the links below to jump to the section of interest.
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Soils of the study region are richly diverse. Relatively recent sedimentary formations contact Brazilian Shield bedrock as well as slowly weathering extrusions to create a mosaic of red-yellow latosols, eutrophic podzols, and yellow podzols (oxisols, ultisols) interspersed with patches of bedrock-derived nutrient-rich red latosols (alfisols) moving west and north towards the Xingu River. At Marajoara, soil color and texture varies predictably with topographic relief. Reddish-brown to red-yellow sandy clay soils are found on slightly higher ground relative to pale brown, gray, or white sandy soils associated with seasonal streambeds and drainage systems. Drainage on low ground may be excessively rapid or impeded, depending on soil texture and depth to the water table. Lateritic and iron concretions are common in reddish sandy clay soils on slopes. Lateritic gravel mixes into soil horizons at unpredictable depths in soil profiles, forming the ‘stone lines’ that Ab'Saber (1982) attributed to soil exposure under reduced vegetative cover during arid periods coinciding with glacial eras.
Our interest in soils at Marajoara was prompted by mahogany’s clear preference for low-ground areas near seasonal streams. Soils in these areas are very different in texture and color from soils on higher ground. Could mahogany’s spatial distribution pattern have something to do with these distinct differences?
The distribution of soils within the core research area of 1035 ha at Marajoara illustrates the principal dichotomy on this landscape, with brown, gray, or white sandy soils on low ground flanking streambeds and darker brown to red-yellow sandy clay or clay loam soils on high ground away from streambeds. Based on point sampling along 54 km of transects, sandy soils covered an estimated 79% of the mapped area. Soils with the highest clay content, the clay loams, were not widely distributed within the mapped area; more common at midslope and high-ground positions where slopes rose more than five meters above streambeds were sandy clays. Where slopes rose only 3–4 m between first-order streams, surface soils often did not take on a clay component at slope tops. Surficial lateritic gravel and cânga (lateritic concretions) were almost always associated with sandy midslopes. Cânga often cropped out where slopes lifted abruptly.
Textural differences between the two principal soil types at Marajoara were consistent at four paired sampling sites. Low-ground topographic positions adjacent to seasonal streambeds were sandy to 100 cm depth, with higher silt content in surface (0–10, 10–30 cm) horizons than below 30 cm. Horizons were gray, grayish brown, or brown above paler, often coarser soils below 30 cm (or deeper), with Munsell Color Chart chromas ≤ 3 indicating reduced, seasonally anaerobic, hydromorphic conditions. High-ground soils were also sandy at the surface, but at all sites clay content increased with depth, yielding sandy clay loams or sandy clays at depth (argillic horizons sensu Brady 1990). Soil colors were yellowish brown in surface horizons shifting to strong browns, reddish yellows, and reds below; all Munsell chromas were ≥ 4, indicating oxidized, aerobic soil conditions. Bulk densities calculated for the surface 10 cm of soil were 1.28 g/cm3 (sd = 0.147, n = 5) for high-ground sandy loams vs. 1.38 g/cm3 (sd = 0.079, n = 5) for low-ground sands.
Soil horizonation patterns were clearly visible in soil pits excavated to 190 cm depth at high- and low-ground topographic positions. In the high-ground pit, thin organic and dark brown sandy horizons overlay successively paler yellowish brown to reddish yellow horizons with increasing clay content. A 30–50 cm layer of soil mixed with lateritic and quartz gravel ranged between 60–110 cm below the surface across the pit’s face. Below this, soils turned deep red-yellow with red mottling. Fine roots were most evident in the surface horizon’s organic capa (cover), while larger roots were common to 190 cm depth, penetrating the gravel horizon.
At the first of two low-ground sites, a thin surficial organic layer topped paler sands to 35 cm depth, below which darker soil appeared above a deeper, thicker horizon of very pale coarse sand. A narrow band of quartz gravel appeared at 110 cm. Below this, dense, extremely pale blue-gray clay streaked with yellowish banding extended to depth, the proportion of yellowish clays declining below 135 cm. Soil in the second low-ground pit was darker throughout, with a nearly black horizon from 35–60 cm depth located below and above paler sands that, moving deeper, graded into a coarse horizon above quartz gravel, also at 110 cm. Below this, blue-gray clays mixed equally with red clays, shifting to nearly solid red at depth (nearing 190 cm), flecked with gray mottling. In both low-ground profiles fine roots were most abundant in the surface horizon. Large roots were common to ~50 cm, becoming rare below this depth to the quartz gravel horizon. Roots were present, if rare, to 190 cm in both low-ground pits.
MOISTURE STATUS & RETENTION CAPACITY
Though soil gravimetric moisture content differed little between high- and low-ground positions through the rainy season, low-ground sands dried very rapidly during the dry season at 10–15 and 35–40 cm depths compared to high-ground sandy clays, falling to 1–3% dry weight compared to 11–12% at the high-ground position. Oscillations in moisture content within seasons reflected response to rains and dry periods. Low-ground soils demonstrated consistently higher moisture content at 10–15 cm than at 35–40 cm depth except during periods of early wet season recharge. High-ground soils behaved somewhat differently, with higher surface moisture content during the wet season but the reverse from the mid dry season through the early wet season recharge period. Sustained dry season transpirational demands by surface roots may have contributed to this pattern.
Soil samples collected at 1-m intervals while augering wells to the water table at the end of the 1996 dry season showed that deep soils consist of fine clays that persist red in color to 10–12 m depth at high-ground positions, and off-white to pale gray to 5–6 m depth at low-ground positions. Gravimetric moisture content increased steadily with depth, peaking where the water table appeared. On average, low-ground deep-soil profiles appeared drier, that is, they retained less water at comparable depths to midslope and high-ground profiles, suggesting coarser texture. At all slope positions, moisture content ranged between 24–37% below 2 m.
Measures of soil moisture content suggest that topslope soils remain wet through the dry season compared to bottomslope soils, but soil matric forces secure more water against gravity and plant uptake in soils with higher silt and clay content. Soil matric potentials, or water retention capacities, sorted predictably according to texture by composited samples, with finest-textured profiles – 30–100 cm at high-ground positions—securing more water at the permanent wilting point (–15 bars) than progressively coarser profiles at low-ground positions. Predicted dry-down endpoints from these curves – to 7–13% dry weight by topslope soils, and to 2–6% by bottomslope soils – conformed well to observed field values.
pH values measured in water and in salt solution were acidic to mildly acidic, consistently higher at low-ground topographic positions, and increased (became less acidic) with depth at both slope positions. Soil % C and % N were low at all sites and positions, with low-ground soils on average showing higher levels of both in the surface horizon but lower levels below 10 cm. C:N ratios ranged from 9.1–13.6 on low ground and from 10.1–16.6 on high ground. Total and extractable P (in ppm) were extremely low at both positions, with total values again falling more precipitously at depth at low-ground positions though levels of extractable P were essentially identical between positions. Available cations (K+, Ca++, Mg++) were also low at both topographic positions, with levels generally falling at depth; but low-ground concentrations were higher than on high ground, especially for Ca++. Exchangeable acids were more abundant at high-ground positions, while % base saturation on low ground far exceeded levels on high ground.
Soil color and textural changes were consistent and in agreement with those reported from studies in similar ecosystems on Precambrian bedrock in South America and Africa. The vertical migration and lateral flow of water across slopes are likely the principal factors driving the evolution of soils in this region. At high-ground positions, argillic (transported clay) horizons below 30 cm may arise both from gravitational transport of fine materials by percolating water and from their export from surface horizons with lateral flow. Indistinct horizonation indicates low organic matter content and intense weathering. Increasing silt and clay content with depth slows infiltration, restricting lateral flow during rainstorms to surface horizons. Remembering that soils on low ground remain sandy to 1 m depth, rainstorms generate lateral flow across slopes by forcing water through permeable surface horizons that deepen moving downslope, as demonstrated by Poels (1987) in Suriname for rain events exceeding 30 mm, and as observed at Marajoara when streams swell rapidly during wet season storms in the absence of surface flow. On low ground where relief is slight, elevated silt content in surface (< 30 cm depth) horizons indicate illuvial deposition from higher ground. A combination of processes could explain declining silt content below 30 cm depth to the layer of quartz gravel at 110 cm: removal of fine materials by lateral flow (eluviation), clay decomposition as Fe is leached during alternating oxidative and reductive acid cycles, or podzolization as mobile organic matter forms Al- or Fe-complexes and either precipitates at lower depths or exits with drainage water. Differential removal of fine soil fractions across slopes deepens relief and accelerates the cycle.
The texture and nutrient status of low-ground soils may therefore depend on how much lateral flow they are subject to and how long anaerobic conditions persist during the wet season. That is, low-ground soils may differentiate predictably along successively larger streams. Because low-ground soils adjacent to first-order streams draining small areas will be subject to less lateral flow and shorter anaerobic periods than low-ground soils adjacent to higher-order streams (recalling that first-order streams sit above the streams they flow into), they will be, on average, finer textured and exhibit higher nutrient status because eluvial and leaching processes are weaker. For these reasons we predict that cabeceiras – low-ground areas where streams originate – will typically have the richest soils on this landscape. Soils there drain better because they are slightly elevated, they remain wet longer into the dry season due to finer texture, and they are nutrient-rich because nutrients transported down slopes with lateral flow tend to accumulate rather than to decompose and flush downstream. Textural differences between low-ground soils at two sites at Marajoara support this hypothesis, as do observations at Marajoara that cabeceiras are often characterized by darker soils and denser, more closed forests compared to soils and communities along higher-order streams like the Grota Vermelha.
Smallholder agriculturists with long experience farming soils of this region consider the two primary soil types – low-ground brown-gray sands vs. high-ground brown or red-yellow sandy clays or clay loams – to have profoundly different working properties. In their view, low-ground soils are richer in nutrients but only superficially so, in the surface 20 cm; the sands are muito lavado, ‘very washed’ by lateral flow that has a tendency to flush nutrients out of the system through large pore spaces, especially if soils are disturbed through forest clearing. By contrast, “barro segura, não deixa lavar”, high-ground sandy clay soils secure nutrients against export as water flows laterally out of the system, and these sites, though poorer initially for crops, maintain fertility longer and therefore may sustain agriculture through two to several growing seasons. Farmers say that differences in soil water availability arise from textural differences: low-ground sands remain moist longer into the dry season because of lateral flow across slopes, but high-ground sandy clays, because the clay component holds water longer than porous sands, supports perennial crops better because subsurface horizons retain available moisture long after low-ground sands have drained dry. For example, if bananas (Musa spp.) are planted across a slope from top to bottom, low-ground plants will grow faster and more luxuriantly through the first rainy season, but these are likely to turn yellow and possibly die by the end of the following dry season. High-ground plants continue green through the dry season, chupando agua, taking up water. Smallholder agriculturists tend to plant shallow rooting, quickly maturing, nutrient demanding crops like rice and sugarcane at slope bottoms, reserving more deeply rooting species sensitive to poorly aerated soil conditions – corn, beans, manioc, bananas – for high-ground sites.
Smallholders also distinguish low-ground sands adjacent to cabeceiras from those adjacent to second- and higher-order streams. The former, they say, offer farmers two advantages: an exceptionally nutrient-rich surface horizon (capa), and higher subsurface clay content that holds more water during the dry season. It may be no coincidence that woodsmen exploring unlogged forest for mahogany gravitate toward cabeceiras, knowing from experience that densities of large adults are higher there than anywhere else on this landscape.
Ab'Saber AN (1982) The palaeoclimate and palaeoecology of Brazilian Amazonia. In: Prance GT (ed.), Biological Diversification in the Tropics, pp. 41-59. Columbia University Press, New York, NY, USA.
Askew GP, Moffatt DJ, Montgomery RF & Searl PL (1971) Soils and soil moisture as factors influencing the distribution of the vegetation of the Serra do Roncador, Mato Grosso. In: Ferri MG (ed.), III Simpósio sôbre o Cerrado, pp. 150-160. Editôra de USP, São Paulo, SP, Brasil.
Brady NC (1990) The Nature and Properties of Soils (10th Ed). Macmillan Publishing Company, New York, NY, USA.
Grogan, JE (2001) Bigleaf mahogany (Swietenia macrophylla King) in southeast Pará, Brazil: a life history study with management guidelines for sustained production from natural forests. PhD dissertation, Yale University School of Forestry & Environmental Studies, New Haven, CT, USA.
Grogan J, Ashton MS & Galvão J (2003) Big-leaf mahogany (Swietenia macrophylla) seedling survival and growth across a topographic gradient in southeast Pará, Brazil. Forest Ecology and Management 186: 311-326.
Grogan J & Galvão J (2006) Physiographic and floristic gradients across topography in transitional seasonally dry evergreen forests of southeastern Amazonia, Brazil. Acta Amazonica 36: 483-496.
Poels RLH (1987) Soils, Water and Nutrients in a Forest Ecosystem in Suriname. Agricultural University, Wageningen, The Netherlands.
Sombroek WG (1984) Soils of the Amazon region. In: Sioli H (ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 521-535. Dr. WJ Junk Publishers, Boston, MA, USA.
Young A (1976) Tropical Soils and Soil Survey. Cambridge University Press, Cambridge, UK.