Pathways of soil genesis in the Coast Range of Oregon, USA

Background and Aims

Soil chronosequences on marine terraces along the Pacific Coast of California and Oregon show evidence of podzolization, though soils ultimately evolve to Ultisols. It is not clear if this pathway of soil evolution can be extended to the humid, inland Oregon Coast Range.

Methods

We analyzed soil properties for a fluvial terrace chronosequence sampled along the Siuslaw River (Oregon, USA) about 50 km from the Pacific coast. The seven terraces ranged in age from <3.5 ky to nearly 1,000 ky.

Results

There was no evidence of early podsolization. Instead, evidence was found that andisolization starts early and occurs even in older soils when pedogenic iron accumulation and clay synthesis and illuviation dominate. Soils develop the morphology characteristic of Ultisols sometime between 20 and 70 ky, but high levels of oxalate extractable iron and aluminum satisfy criteria of an andic subgroup. Alfisols are not formed as an intermediary stage.

Conclusions

The lack of Spodosols inland is due to the inland shift from udic to ustic or xeric moisture regime, which favors summer drying and ripening of short-range order minerals rather than deep leaching or translocation. Other factors are higher pH, different organic chemistry and faster calcium cycling under the Douglas fir inland when compared to the Sitka spruce of the coastal terraces.