In this third installment in the Everyday Soil Science series, we move our discussion from soil as a “living” ecosystem “fed” with life-giving organic matter to the abiotic “mineral fraction” comprised of sand, silt, and clay particles.
In the soil world, being small rules. The diameter of sand particles ranges from 2mm to 0.05mm, silt particles from 0.05mm to 0.002mm, and clay less than two microns (about the size of a bacterium). As the particles get smaller, they get more difficult to see, and in fact, clay particles are invisible to the naked eye. Unable to actually see the proportion of sand, silt, and clay in a soil, soil scientists have cleverly utilized the difference in how each particle “feels” to determine the relative proportion in a given sample of soil.
To understand how each particle feels, let’s think about clothing. Imagine a burlap jacket or wool sweater against your skin. How would that feel? How about rayon, silk, or satin? And Velcro? We can associate the feel of the clothing examples with the feel of sand, silt, and clay. Sand is rough or “gritty” like burlap. Silt is soft and “smooth” like silk, and clay is “sticky” like Velcro.
Why does knowing the feel of soil particles and their relative proportion matter? Well, as you might imagine, sand, silt, and clay in addition to feeling different, have different physical properties that influence how a soil functions. These include water holding capacity, porosity, bulk density, and other physical characteristics that will be addressed in future issues of Everyday Soil Science.
The arrangement of various combinations of sand, silt, and clay are represented in the Soil Textural Triangle (shown below). The triangle features an axis for each soil separate (sand, silt, and clay), and where the three intersect is the textural class of the soil.
Looking at the triangle you’ll notice a textural class called loam. Now think about how sand, silt, and clay feel. Loam is gritty, smooth, and sticky all in one soil! Most gardeners, landscape professionals, and farmers consider loam to be the soil sweet spot. Why? Quite simply, loam represents the ideal blend of sand, silt, and clay. In round numbers loam is 40% sand, 40% silt, and 20% clay. This combination is easy to work (tilth), holds moisture (water holding capacity), drains well, and can be penetrated by roots and organisms with relative ease.
You can determine the feel of your home or worksite soil by utilizing the soil texture flowchart illustrated below. Go ahead, get the “feel” of your soil, and record the textural class. We’ll use this information next month when we discuss soil structure.
In the meantime, if you have a question about soil texture, email me at sandrews@berkeley.edu.
Get out there and put your hands to work feeling (texturing) some soil.
Stephen Andrews
~The Soil Sommelier
RESOURCES:
Flowchart for Determining Soil Texture by Feel
Modified from S.J. Thien, 1979. A flow diagram for teaching texture by feel analysis. Journal of Agronomic Education. 8:54-55.
http://soils.usda.gov/education/resources/lessons/texture/
