3.3 Soil and Topographical Considerations
3.3.1 Soil Moisture Calibration
There are four calibration curves depending on the texture of the soil. The calibrations curves are
polynomials that include the real dielectric permittivity and several coefficients (Topp 1980,
Seyfried and Murdock 2004). The four user selectable soil texture settings are Sand, Silt, Clay,
and Loam. The texture is determined by the percentage of sand silt and clay there is in the soil.
Figure 3.1 shows the corresponding percentages to the different textures. If you are unsure of
your soil texture, determining which soil setting that is best for your soil is easy and there are a
number of different ways to make the determination (Birkeland 1999).
a)
A detailed soil survey for your area can be downloaded for free from the US Department of
Agriculture at
http://soildatamart.nrcs.usda.gov/
b)
Many times, it will be obvious. Sand looks different from clay.
c)
Grab a baseball size portion of the soil in your hands. Wet the soil with water and work the
moist soil with your hands. The stickier it is, the more clay there is. The “soapier” the soil
feels the higher the silt content. Grittiness is indicative of sand.
Figure 3.1 Soil Texture Triangle
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