ANDISOL FORMATION IN A HOLOCENE BEACH RIDGE PLAIN UNDER THE HUMID TROPICAL CLIMATE OF THE ATLANTIC COAST OF COSTA-RICA

Soil formation has been studied in relation with time in a 5000-year o ld chronosequence on volcaniclastic beach ridges of the perhumid tropi cal Atlantic coast of Costa Rica. All soils are under tropical rainfor est. Drainage conditions change by subsidence from excessively drained in the two youngest soils to imperfectly drained in the two oldest so ils. Parent material is rather homogeneous andesitic sand with a volca nic glass component of less than 10%. It has been found that under the se conditions Andisols form within 2000 years. Imperfect drainage caus ed mottling and accumulation of iron-coatings, as well as the formatio n of a thin O-horizon in the oldest profiles. Sand content of the soil s decreases regularly with soil age, while the amount of fine material increases concurrently. The increase in fine material and the accumul ation of organic matter cause an increase of CEC and andic properties, and a decrease in bulk density and pH with soil age. Depth of biologi cal influence increases with soil age, but soil faunal activity is ham pered in the oldest three profiles, probably by imperfect drainage. Du e 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, notwi thstanding the presence of a considerable amount of weatherable primar y minerals.