The Evolving Classification of Acid Sulfate Soils

The process of classification helps us organize knowledge and it helps us to better appreciate relationships and connections. Classification also facilitates communication, and good classification systems will be (to some degree) practical and utilitarian. While the problematic nature of acid sulfate (AS) soils has been recognized (by some) at least since the 18th century, much of what we understand about AS soils only began to be recognized around the time of the 1st International AS Soils Conference in 1972. As our modern soil classification systems emerged during the last half century, many of their architects knew to incorporate these challenging and unique soils. Thus, as our experience and understanding of these soils has expanded, we have also seen corresponding changes or adaptations in the classification systems to accommodate the new knowledge. In this paper we examine revised principles, perspectives and structures for the classification of AS soils through three systems used broadly around the world: US Soil Taxonomy, the World Reference Base for Soil Resources and the Australian Soil Classification. All of these classification systems have accommodated AS soils throughout their history and each has demonstrated distinctive changes during particular periods. As our concepts and understanding of AS soils have developed, these have become encoded (quickly or slowly) in our classifications. This paper will explore how various AS soil concepts have been addressed within these three classification systems, how these concepts have changed through time, and how changes in each system have preceded, followed or paralleled the other systems. We highlight differences between systems that have existed and that may remain, and offer perspectives on the rationale for these distinctives. We will also demonstrate the ways in which collaborations and shared knowledge have drawn parts of these systems more closely together.