SOIL SCIENCE
Figure 4 : A . Soil above the plastic limit can be rolled into threads . B . The thread crumbles as it dries below the plastic limit . C . Soil paste flows at the liquid limit .
and mineralogy are ignored . They are the true drivers of soil behavior , but this information is not really needed to decide whether a soil “ acts ” more like a silt or a clay . Civil engineers use lab tests to quantitatively distinguish “ clay-like ” behavior from “ silt-like ” behavior . These tests measure the soil ’ s response when water is added or subtracted . There are two critical water contents : the plastic limit and the liquid limit . Figure 4 shows a sample at these thresholds .
At the plastic limit , the soil is just moist enough to be molded into a new shape . At the liquid limit , it begins to flow as a paste . The difference between the two is the plasticity index : it is the range of water contents over which the soil can be shaped and molded . Clays generally accept more water before becoming liquefied . Therefore , they have larger plasticity indices than silts .
Figure 5 shows a plasticity chart . A given soil is considered clay if it plots above the A-line . It is classified as a silt if it plots below the A-line . Both clays and silts can be further described as having high or low plasticity . The position of a soil on this chart helps predict its physical properties such as drainage and stiffness . The “ clayeyness ” of a soil increases from lower left to upper right .
Prof . Arthur Casagrande developed this system during World War II . In 1942 the United States was desperate to build air bases in the Pacific isles . Casagrande ’ s system helped military engineers ensure the soil below their runways would support heavy bombers during takeoff and landing . The tests required little equipment , which meant they could be performed in makeshift laboratories right on the island . More importantly , the tests directly recorded how the soil behaved when it got wet .
Soils that behave as clays are excellent for skin areas , but they are not conducive to growing healthy grass . Clays impede drainage and root respiration when compacted . When dry , their high strength prevents roots from penetrating . Clays retain more moisture than silts , but much of the water is held too tightly for plants to use .
WHAT IS CLAY ? THERE IS NO SINGLE ANSWER . There are three ways to think about clay : as a particle size , as a family of minerals , and as a behavior . The first two are most useful to scientists , and the latter might better relate to sports surface performance . All three are important because the particle size and mineralogy are what ultimately dictate the soil behavior we observe and depend on .
Thinking on the micro scale is hard , but worthwhile . The performance of clay goes all the way back to atoms . That idea is both useful and fun ! SFM
Evan C . Mascitti is a Ph . D . candidate in soil science at Penn State University . He previously worked for several years as a grounds manager in professional baseball .
Figure 5 : Plasticity chart used to distinguish silts from clays . Soils become more “ clay-like ” from left to right and from bottom to top .
12 SportsField Management | October 2021 sportsfieldmanagementonline . com