SOIL SCIENCE

Clay appears so simple . It ’ s dirt , dust and mud . It is the Earth itself . What could be more ordinary ?

If we look closer , clay becomes complex and fascinating . Why does it form hardpan when dry but a soupy mess when wet ? What makes some clays act so differently from others ? To answer these , we first must wrestle with another riddle : what exactly is clay ?

Faced with this question , many would offer something like the famous utterance by Justice Potter Stewart : “ I can ’ t define it , but I know it when I see it .” This way of thinking about clay is perfectly suitable for practical , day-to-day workings with soil . For a more penetrating analysis , we must do a bit better .

WHAT ’ S IN A NAME ? How you define clay depends on who you are . Here is a long-winded , official statement maintained by the Clay Minerals Society :

“ The term ‘ clay ’ refers to a naturally occurring material composed primarily of fine-grained minerals , which is generally plastic at appropriate water contents and will harden when dried or fired . Although clay usually contains phyllosilicates , it may contain other materials that impart plasticity and harden when dried or fired . Associated phases in clay may include materials that do not impart plasticity and organic matter .” ( Guggenheim and Martin , 1995 )

This is quite a mouthful , and it characterizes the banter typical among academic scientists . This kind of debate is hardly unique to soils . Biologists often have trouble defining life , and some astronomers disagree on what constitutes a planet . Detailed definitions could seem like useless squabbling , but there is a good reason for it : our human desire to put things into categories doesn ’ t always jive with nature ’ s complexity . We have to draw lines somewhere , and definitions ensure everyone is on the same page .

The long passage above captures three ways to think about the word “ clay :” as a particle size , as a mineral , or as a behavior . Each is useful in its own way .

CLAY AS A PARTICLE SIZE Particle size is probably the most familiar definition of clay . Clay-size particles are 2 microns ( 0.002 mm ) and smaller . Even this is not clear-cut – what is “ size ?” Does it mean height ? Width ? Volume ? For example , which has the greater size : a Frisbee or an apple ? The Frisbee is wider , but the apple is taller . Actually , I would

Figure 1 : Clay particles are much , much smaller than sand grains .

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