Meet Claude the Clod: a Scale Model of a Soil Aggregate and Its Resident Microorganisms.

 At Penn State’s agricultural and environmental field days, we use a model called “Claude the Clod” to help audiences visualize how microorganisms help improve soil structure and make nutrients more available to plants. Constructed mainly from chicken wire and papier mache, Claude is a one-meter-wide scale model of a soil aggregate and its associated organisms. At 4000X magnification, Claude portrays an aggregate that is just one-fourth of a millimeter in diameter.  Although Claude’s “actual” size is on the small end of the range for soil macroaggregates, the scale makes the model portable while conveying the idea that macroaggregate formation results in improved soil tilth, aeration, and infiltration. The model’s scale also allows us to use rice grains to depict soil bacteria, which in reality are only about half a micron in width.  On the model, “bacteria” are located exclusively on patches of “decaying plant residues” to demonstrate how microbial abundance and distribution in soils are not random but dependent on amount and availability of organic matter.  The role of fungi in macroaggregate formation is shown with a “fungal hypha” (i.e., a large-sized pipe cleaner) wrapped around the outside of the model to convey how fungi help bind minerals with organic matter. During discussions of the model, audiences learn that visible soil organisms like earthworms have massive dimensions at this scale. Thus, the model is an excellent introduction to soil biota unseen by the naked eye and to microorganisms’ constant but unobserved contributions to soil productivity and environmental quality.