deposition of ash and other particles from volcanic
eruptions periodically contribute to "new," unweath-
ered parent material to the soils of Monteverde.
Undisturbed soils developed on the Aguacate and
Monteverde Formations are likely to be similar to udic
Andisols at other montane sites in Costa Rica and have
low to moderate clay contents. For example, subsoil
clay contents along an altitudinal transect on Volcan
Barva in the Cordillera Central decreased from 80%
at 100 m elevation to less than 10% at 2000 m (Grieve
et al. 1990). This pattern is not definitive evidence for
decreasing intensity of weathering with altitude; it
may also be that younger parent materials occur at
higher altitudes on Volcan Barva, a pattern that also
occurs in Monteverde.
Soils are typically deep on low-angle slopes and
benches of the Monteverde Formation. The upper soil
horizons are characterized by high porosity and low
bulk density (<0.9 g/cm^3 ), which result in high volu-
metric moisture contents and hydraulic conduc-
tivities when they are saturated. Textures of the A
horizon range from silt to sandy loams, with variable
amounts of clay. Moist soil is friable, that is, it readily
breaks apart when handled. The color of the A hori-
zon is light to dark brown, primarily due to the accu-
mulation of organic matter.
Where it is well differentiated, the B horizon gen-
erally has a higher bulk density and lower hydraulic
conductivity when compared to the A horizon, re-
flecting both differences in soil texture and smaller
amounts of organic matter. For example, carbon in
organic matter decreased from 12% in the A horizon
to 5-7% in the B horizon at 1500 m on Volcan Barva
(Grieve et al. 1990). In Monteverde, textures of the
B horizon range from gravelly to sandy loams. Clay
contents typically decrease with depth in the profile,
which reflects reduced intensity of weathering re-
actions. The color of the B horizon is variable, and
these differences form the basis for recognizing un-
differentiated humic Andisols, and differentiated, or
chromic, Andisols. Undifferentiated humic Andisols
have no clearly defined soil horizons and are charac-
terized by large amounts of organic matter. For ex-
ample, studies of leeward cloud forest soils indicated
the O and Aa horizons (ca. 0-20 cm depth) were highly
organic. The A 2 and B horizons (ca. 20-180 cm depth)
were poorly differentiated, indicating that this soil is
a humic Andisol (Vance and Nadkarni 1990). Differ-
entiated or chromic Andisols are characterized by a
B horizon that is distinct from the A horizon. In the
presence of lesser amounts of acidic organic matter,
the relatively slow crystallization of iron oxides pro-
duces an ochre color.
The type and amount of electrical charge on soil
particles are a function of the clay minerology and the
organic matter content. The surface of allophane and
iron sequioxides carries a net negative charge and has
an affinity for positively charged ions. Organic mat-
ter also has a net negative charge and substantially
contributes to the overall electrical charge of a soil.
The cumulative negative charge per unit mass of a soil
is referred to as its cation exchange capacity (CEC).
Humic Andisols and Histosols typically have high
CECs, but due to weathering reactions and the pro-
duction of organic acids by organic matter, which
results in the leaching of exchangeable bases, a ma-
jority of the exchange sites are occupied by hydrogen
ions. For example, although the CECs ranged between
590 and 1007 meq/kg in surface soils from 1000 to
2000 m on Volcan Barva, the concentrations of ex-
changeable bases ranged between 9.5 and 22.1 meq/
kg (Grieve et al. 1990). The relatively high concentra-
tions of hydrogen ions on exchange sites also result
in moderate to high levels of soil acidity and there-
fore low pH (see Clark and Nadkarni, "Epiphytic His-
tosols," pp 34-35). For example, soil pH in the upper
soil horizons ranged from 5.4 to 3.0 pH units at other
tropical montane sites (Tanner 1977, Zinck 1986,
Grieve et al. 1990, Bruijnzeel et al. 1993, Kitayama
1993). Soil pH levels typically increase with depth in
the profile.
There is a general increase with altitude in soil or-
ganic matter (and therefore soil C and N) in the A and
B horizons of tropical montane forests throughout the
tropics (e.g., Grubb 1977, Marrs et al. 1988, Grieve
et al. 1990, Kitayama 1993). For example, organic
matter in soil samples taken at 0-15 cm depth in-
creased from 19% at 100 m to 47% at 2000 m along an
altitudinal transect on Volcan Barva (Grieve et al.
1990). At 1500 m and 2000 m, thick amorphous organic
material formed a histic horizon to a depth of 50 cm.
On a smaller scale, microclimate also affects soil
properties. Physical changes in soils are coincident
with exposure to cloud water and precipitation inputs
in Monteverde and include persistent waterlogging
and increased amounts of organic matter in the A and
B horizons.
In many soils, soil organic matter content is posi-
tively correlated with clay content, because allo-
phane and other clay particles stabilize organic col-
loids through physical and chemical interactions
(Motavalli et al. 1994,1995, Ritter et al. 1995). How-
ever, the high organic matter content of soils in tropi-
cal montane forests is also due to relatively low rates
of organic matter decomposition because of lower
temperatures and hydric (moist) soil conditions
(Jenny 1980), and because of poor litter quality (i.e.,
high lignin and polyphenol contents, and relatively
low N:C and P:C ratios). At the highest elevations in
Monteverde, litter from bryophytes (mosses and liver-28 The Physical Environment