Tuesday, November 6, 2007 - 10:15 AM
179-2

Soil Structure:A History from Tilth to Habitat.

Benno P. Warkentin, Oregon State University, 3126 NW Buttercup Dr, Corvallis, OR 97330

Soil structure of surface horizons was perceived for millennia as tilth of the seed bed and plowing to achieve it. This was the concept until about 1850 while mostly practitioners wrote about soils, then the soil scientists stepped in and defined it as aggregation. The early social history of tilth was largely about the plow—how to plow well.

With the advent of soil science laboratories the dominant concern soon became measurement of static properties expected to be related to tilth and stability of structure: aggregate-size distribution, bulk density to calculate porosity, grain-size distribution, shape and size of aggregates, and stability of aggregates. It was a concept based on the solid particles. The importance of organic matter and clay content in soil structure was generally recognized. All these properties were related to cultivation, and cultivation to tilth and yield, but not close enough to allow prediction of tilth from measured properties.

By 1950 soil structure research was in the doldrums. Two major changes occurred after 1970 to reinvigorate it. The concept of hierarchical arrangement of different aggregate sizes, and the bonds responsible for stability, drew attention to different void sizes and to the soil functions of each. And then the recognition of the unique role of soil in ecosystems led to considering soil structure as defining the habitat for soil bio-physico-chemical functions such as decomposition, water routing, etc. Structure now became a concept centered on voids, and the term soil architecture became more appropriate; the spaces and surfaces of the spaces were more important than the solids of walls and roof. This is the venue for studies begun in the last decades. Today's soil structure research is again productive in concepts and applications.