Agriculture Reference
In-Depth Information
The International Scheme
Clay
<0.002 mm
Silt
0.002-0.02 mm
Fine sand
0.02-0.2 mm
Coarse sand
0.2-2 mm
The United States Department of Agriculture Scheme
Clay
<0.002 mm
Silt
0.002-0.05 mm
Fine sand
0.05-0.1 mm
Medium sand
0.1-0.5 mm
Coarse sand
0.5-1 mm
Very coarse sand
1-2 mm
The amounts of clay, silt, fine sand, and so forth are calculated as a percentage of
the fine earth, and their proportions determine the soil's texture. When these pro-
portions are plotted on a triangular diagram, shown in figure 2.7, we can pinpoint
the textural class of the soil. For example, a soil with 35% clay, 30% silt, and 35%
sand is classed as a clay loam. Experienced surveyors can estimate the approximate
proportions of clay, silt, and sand by hand-texturing in the field.
Based on the measured clay contents of the sample set, an EM calibration
graph similar to that in figure 2.8 is drawn up. The line of best fit through the
points is used to convert EM readings to clay content for the whole survey area
and can also be applied to subsequent EM surveys done on this soil type.
Ground-truthing
Calibration of EM readings for clay content is an example of “ground-truthing,”
which is the general term describing the use of actual soil analyses to check the
accuracy of information provided by remote or proximal sensing. The better
the calibration, the more accurate the soil map will be in representing the soil
property and its variation. As described previously, the samples for analysis can
be obtained from soil pits or by augering or coring the soil. When an EM map
such as that shown in figure 2.6 is available, the placement of the sampling points
can be guided by the variation in EM signal to ensure the maximum range of soil
variation is sampled.
After sampling and calibration are complete, the map's precision can be
checked by a random sampling of the area to determine whether the location, for
