SME
Physical Geography
Soils: formation, horizons and texture
Use this for “A soil is 40 % sand, 40 % silt and 20 % clay. What is its texture class?”
Diagram
Clay = 20%, Sand = 40%, Silt = 40% of Clay + Sand + Silt.
This is an example. Type your own values to start a new problem.
sand
Sand
example · %
silt
Silt
example · %
clay
Clay
calculated · %
T
Texture class
worked out (not typed)
Formulas
sand + silt + clay = 100
40 + 40 + 20 = 100
T
from
sand,
silt
and
clay
on
the
USDA
texture
triangle
20 ≥ 7, 40 < 50 and 40 ≤ 52, so loam
Assumptions
•
Only the fine earth counts: particles under 2 mm, with gravel and stones sieved out first.
•
Sand is 2 to 0.05 mm, silt 0.05 to 0.002 mm and clay under 0.002 mm (USDA sizes).
•
The three shares are by mass and add to 100 %; loams are mixes where no one size dominates.
•
The 12 classes are the fields of the USDA texture triangle, each bounded by straight lines.
Step-by-step
WE KNOW
sand = 40%, silt = 40%
FIND
clay (clay), texture class (T)
Step 1 · Find clay (clay)
Use sand + silt + clay = 100
The three sizes make up the whole fine earth, so clay is what sand and silt leave.
clay = 100 − sand − silt
clay = 100 − 40 − 40
clay = 20%
Step 2 · Find texture class (T)
Use T from sand, silt and clay on the USDA texture triangle
No one size dominates: the even mix of sand, silt and clay that farmers prize.
20 ≥ 7, 40 < 50 and 40 ≤ 52: loam
T
=
the
field
of
the
USDA
triangle
that
holds
sand,
silt
and
clay
T
=
the
field
of
the
USDA
triangle
that
holds
40,
40
and
20
T
=
loam
Check
Put the numbers back in. Both sides should match.
40 + 40 + 20 = 100 ✓
20 ≥ 7, 40 < 50 and 40 ≤ 52, so loam ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
More problem types
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