Chemistry of Várzea and Igapó Soils and Nutrient Inventory of Their Floodplain Forests

Abstract

There is general agreement that the alluvial soils of the várzea, which are derived from the settled suspended load of the Amazon River, are rich in nutrients sustaining a high natural productivity (Sioli 1954a, 1969, 1975; Irion 1978; Cochrane and Sanchez 1982; Nascimento and Homma 1984; Sombroek 1984; Fearnside 1985; Meggers 1985; Lima 1986; Furch and Klinge 1989; Junk et al. 1989; Martinelli et al. 1989). Sediments originate in the Andes and the pre-Andean zone, they contain clay minerals, i.e., montmorillonite, illite and chlorite, with relatively high cation exchange capacities (Gibbs 1967; Irion 1976b, 1984a; Sect. 2.4). In contrast, the soils of the black-water floodplains, the igapó, are considered to be poor in nutrients and of low production potential (Sioli 1954a, 1969, 1975; Irion 1978; Fearnside 1985; Singer and Aguiar 1986; Furch and Klinge 1989). These soils partly originate from erosional processes of strongly weathered and lixiviated tertiary sediments containing mainly kaolinite which has a low cation exchange capacity (Irion 1976b, 1984a; Irion and Adis 1979).