Manual de Plantas de Costa Rica | The physical environment| 77 
Soil structure, composition, and fertility are determined by numerous factors, in- 
cluding climate, relief, parent material (both inorganic and organic), organisms (e.g., 
earthworms, ants, termites, mycorrhizae, or plant cover), and time (Gémez P., 1986). 
High rainfall results in much alluvium, hence more fertile soils, but also leaches out sil- 
ica and soluble bases (calcium, magnesium, potassium, etc.), causing acidic soils. This 
is the situation, for example, in the General Valley. On the other hand, salts accumulate 
under conditions of high temperature and seasonal aridity, as in the Guanacaste region, 
characterized by more or less neutral soils. Soils tend to be thin on steep slopes (as most 
of Cocos Island), but deep on flats or plains. 
Costa Rica’s complex geology is partly responsible for its varied mosaic of soil 
types. The nature of the parent material affects both the texture and composition of de- 
rived soils as, for example, with lateritic soils, identified with the lowland tropics, 
which typically comprise reddish, porous clays high in oxides of aluminum and iron. 
These soils develop through a process called laterization, which involves disintegration 
of the underlying rock through percolation of water over time. Lateritic soils gradually 
become poorer in silica and bases, and consequently more acidic. Laterization takes 
place under conditions of good drainage, in moist or wet climates with moderate to 
high temperatures (GOmez P., 1986), and also where peridotite is present (Kruckeberg, 
1984: 18). In Costa Rica, lateritic soils are most frequent at elevations below 800 m, 
where temperatures are highest, as in the San Carlos region, on the Santa Elena Penin- 
sula, and in the General Valley. 
Termitaria have been shown to improve soil fertility in Amazonian Brazil, com- 
prising “rich nutrient patches that contrast significantly with the highly weathered soils 
of the region” (Salick et al., 1983: 1). Abandoned termitaria provide important micro- 
habitats for the establishment of tree seedlings. Locally elevated concentrations of min- 
eral nutrients have also been documented beneath refuse dumps of leaf-cutter ants on 
Barro Colorado Island, Panama (Haines, 1975). Epiphytic “histosols,” derived mainly 
from host-tree and epiphyte litter, have been discussed by Lesica & Antibus (1991) and 
Clark et al. (2000: 27). These occur on tree stems and branches in cloud-forest habitats. 
Epiphytic soils are exploited not only by epiphytes, but also apparently by some trees 
and even lianas (e.g., Marcgraviaceae) that exhibit adventitious “canopy roots” (see 
Merz, 1991). “Carton” nests of arboreal ants provide substrate for some epiphytes, e.g., 
species of Codonanthe (Gesneriaceae) and Peperomia (Piperaceae), resulting in so- 
called “ant gardens” (see, e.g., Longino, 1986); some epiphytes appear to have evolved 
special adaptations for this association. 
It seems scarcely necessary to belabor the close relationship between soil and veg- 
etation types. A conspicuous example is the restriction of particular plant taxa to beach 
sands: e.g., Canavalia rosea and Tephrosia cinerea (Fabaceae), Ipomoea pes-caprae 
(Convolvulaceae), Jouvea spp. and Uniola pittieri (Poaceae), Physalis minuta (Solana- 
ceae), and Sesuvium portulacastrum (Aizoaceae). Limestone and serpentine soils, both 
present in Costa Rica (see previous section), are also known to support distinctive plant 
