Andisol formation in a holocene beach ridge plain under the humid tropical climate of the Atlantic Coast of Costa Rica

Abstract

Soil formation has been studied in relation with time in a 5000-year old chronosequence on volcaniclastic beach ridges of the perhumid tropical Atlantic coast of Costa Rica. All soils are under tropical rainforest. Drainage conditions change by subsidence from excessively drained in the two youngest soils to imperfectly drained in the two oldest soils. Parent material is rather homogeneous andesitic sand with a volcanic glass component of less than 10%. It has been found that under these conditions Andisols form within 2000 years. Imperfect drainage caused mottling and accumulation of iron-coatings, as well as the formation of a thin O-horizon in the oldest profiles. Sand content of the soils decreases regularly with soil age, while the amount of fine material increases concurrently. The increase in fine material and the accumulation of organic matter cause an increase of CEC and andic properties, and a decrease in bulk density and pH with soil age. Depth of biological influence increases with soil age, but soil faunal activity is hampered in the oldest three profiles, probably by imperfect drainage. Due to the extreme leaching conditions, the sum of exchangeable cations is less than 2 cmol + kg- 1 in the B-horizons of the older soils, notwithstanding the presence of a considerable amount of weatherable primary minerals.