SOIL SCIENCE argue that they are about the same size because their volumes are nearly equal . Only a perfect sphere can be described by a single “ size ” because all its dimensions are identical . Clay particles tend to be flat and wide ( for reasons discussed below ), so the “ size ” of a clay particle depends on how it is measured . To standardize the definition of size , soil scientists measure the settling of particles in a column of water . We assume the particles are spherical ( even though they aren ’ t ) and calculate their settling rates based on a mathematical principle called Stokes ’ Law . Clay-size particles sink at the same rate as a sphere having a diameter of 2 microns . This is called the “ equivalent spherical diameter ,” or ESD .
Clay particles are not just a little smaller than sand or silt . They are so much smaller that it is almost impossible to grasp . It takes serious mental effort visualize their size – even if you are a soil scientist who thinks about this every day . Figure 1 shows a scaled representation of some soil particles . The “ coarse ” clay is barely visible , and the “ fine ” clay (< 0.2 microns ESD ) cannot be drawn at this scale . The differences are magnified even further by comparing volume , which is the truest definition of “ size .” By volume , a fine clay particle is 1 trillion times smaller than a sand grain . The word “ trillion ” is used casually these days by politicians and journalists , but I try hard to remember that one trillion is not just more than one billion , it is a thousand billion – a number that feels impossibly large .
CLAY AS A MINERAL Soil particles are minerals . Minerals are made of atoms . This is the key to understanding the different types of clay .
Clay minerals are officially named phyllosilicates . Phyllo is a Greek root meaning “ sheet-like ,” and silicate refers to the element silicon . Clay minerals are sheet-like because their atomic bonds are stronger in the two horizontal directions than in the vertical direction ( Figure 2 ). The particles tend to grow in the x and y directions and separate more easily in the z-dimension . Most clay mineral particles have an aspect ratio of at least 40:1 , with some well over 100:1 ( Dixon and Weed 1989 ). Figure 3 shows some platy kaolinite particles under an electron microscope .
The atomic arrangement also determines how a clay interacts with water . Some clay minerals have a net negative charge at their surfaces . These charged surfaces attract water molecules . High-charge clays like vermiculite and smectite hold water very tightly . Illite also has a high charge , but it has less surface area , so its overall affinity for water is intermediate . Kaolinite has no permanent charge .
Clay minerals usually occur as very small crystals , so
Figure 2 : Clay minerals are strongly bonded in the x and y dimensions and weakly in the z-dimension , leading to their platy shape . Adapted from Wang et al . ( 2020 ).
Figure 3 : Kaolinite particles agglomerated into stacks . Image reproduced from the “ Images of Clay Archive ” of the Mineralogical Society of Great Britain and Ireland and The Clay Minerals Society ( https :// www . minersoc . org / images-of-clay . html )
the particle size and mineralogy definitions are easy to conflate . Soil scientists adopted the 2-micron silt / clay boundary in the 1930s because it is the smallest size , on average , at which non-clay minerals occur . There are still plenty of clay mineral particles larger than 2 microns , and plenty of non-clay minerals smaller than 2 microns . Before 1938 , the clay size boundary was variously defined as 1 , 5 or 15 microns ( Simonson 1999 ). Therefore , the 2-micron cutoff is not as black and white as it is often considered .
CLAY AS A BEHAVIOR I believe thinking of clay as a behavior is the most useful definition for sports field managers . This definition is based on plasticity . Plasticity is the soil ’ s ability to be molded into a new shape . Plasticity is what allows potters to shape clay to their liking , and it is what allows a sports field manager to scratch and pack a pitcher ’ s mound .
In the behavioral definition of clay , particle size sportsfieldmanagementonline . com October 2021 | SportsField Management
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