Soil particle analysis is more complicated than it looks Accurate soil texture information is critical for understanding experimental results or modeling—and if you’re just guessing—you’ll be in trouble when it comes time for publication. Soil particle analysis is hard. You need to know what to watch out for, or your accuracy can be off by orders of magnitude. And that’s a problem—get it wrong, and your models and assumptions will be incorrect and ultimately you’ll reach bad conclusions. What you need to know Measuring soil texture can be tedious, complex, and prone to human error. In this 30-minute webinar, researcher and application expert Leo Rivera teaches best practices for higher accuracy and how to choose the right method for your unique application. Learn: - How soil texture measurement has evolved over time - Fundamentals behind the measurement - Comparison of different measurement methods (including - Stokes law-based and optics-based) - Pros and cons of each method - Best practices: making an accurate measurement regardless of the methodology

2. 5 REASONS YOUR SOIL
TEXTURE ANALYSIS ISN’T
ACCURATE ENOUGH
Leo Rivera, M.S.
METER Group, Inc. USA

3. PARTICLE SIZE DISTRIBUTION
OF SOILS
SOIL TEXTURE
Particle Size Distribution of Soil (PSD) is a
fundamental soil physical property
• Soil Water Retention
• Hydraulic Conductivity
• Leaching
• Erosion Potential
• Plant Nutrient Storage
• Organic Matter Dynamics
• Carbon-Sequestration Capacity
US Soil Taxonomy

4. HOW WE HAVE TRADITIONALLY
MEASURED SOIL TEXTURE
• Hand texturing
• Sieving
• Sedimentation Methods (Stokes Law)
• Hydrometer
• Pipette
• Optical Methods
• X-ray attenuation
• Laser diffraction
• VisNIR Spectroscopy

5. CORE MEASUREMENT
FUNDAMENTALS TO THINK
ABOUT
• Fractional quantification of each particle-
size class
• Several systems of classification
• What is the application?

6. CORE MEASUREMENT
FUNDAMENTALS TO THINK
ABOUT
• How is the data portrayed
• Soil Handling and Prep
• Dispersing soil into individual primary
particles
• Organic matter removal
• Iron Oxide removal
• Carbonate removal

7. METHODS

8. SEDIMENTATION (STOKES’
LAW) BASED METHODS
Aqueous suspension in a sedimentation
cylinder
After homogenization particles settle
Settling velocity depends strongly on particle
size
Stokes Law relates the settling time to the
size of the particles remaining suspended

9. SEDIMENTATION METHODS
COMMON TECHNIQUES
Hydrometer
Pipette
Integral Suspension Pressure

10. SEDIMENTATION METHODS
HYDROMETER
Based on hydrometer settling depth (𝛳) over
time (t)
• Solution density changes as particles settle
• Requires a correction in a blank cylinder for
temperature and dispersant solution
Sand fraction separated and quantified
separately
Typical measurement time 24 hrs
𝑋 = 𝜃𝑡 ⁄
'( )

11. SEDIMENTATION METHODS
HYDROMETER
CHALLENGES
• Manual readings are error prone
• Fixed measurement times
• Disturbance of the sedimentation process
when inserting hydrometer
• Typical accuracy +/- 3%

12. SEDIMENTATION METHODS
HYDROMETER
CHALLENGES
The dreaded 24-Hour Readings

13. SEDIMENTATION METHODS
PIPETTE
Gold standard method
Direct sampling technique
• Pipette sub-sample at set depth, h, and
time, t
• H and t represent specific particle sizes
(2, 5, & 20 µm)
Sand fraction separated and quantified
separately
Typical measurement time 6 hrs

14. SEDIMENTATION METHODS
PIPETTE
CHALLENGES
• Manual readings are error prone
• Fixed measurement times
• Disturbance of the sedimentation process
from inserting pipette
• Typical accuracy +/- 3%

15. SEDIMENTATION METHODS
INTEGRAL SUSPENSION
High precision pressure transducer
measures density change as particles settle
PSD determined by inverse modeling
Sand fraction separated and quantified
separately
Typical measurement time 8-12 hrs.

16. SEDIMENTATION METHODS
INTEGRAL SUSPENSION
Pressure Measurement Cumulative Particle Sized Distribution

17. SEDIMENTATION METHODS
INTEGRAL SUSPENSION
CHALLENGES
Dependent on precision electronics
Potential sources of error
• User error in dry mass calculations
• Lack of temperature equilibrium
• Errors in sand fraction
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
2 20 200 2,000
Cumulative
relative
mass
(-)
Particle diameter (µm)
Cumulative Particle Size Distribution

18. SEDIMENTATION METHODS
INTEGRAL SUSPENSION+
Extension of original ISP Method
At a measured time, part of suspension is drained
and oven dried
Improved accuracy integration of drained
suspension
ISP+ method decrease measurement time to
2.5hrs with an accuracy of +/- 0.5%

19. OPTICAL METHODS
X-Ray Attenuation
Laser Light Scattering (Diffraction)
VisNIR Spectroscopy

20. OPTICAL METHODS
LASER DIFFRACTION
Particles diffract light at an angle relative to
their size
Light passed through a suspension of soil
Diffracted light related to particle size
distribution (Mie theory)
Typical range is 0.04 to 2000 µm

21. OPTICAL METHODS
LASER DIFFRACTION
CHALLENGES
• Less representative due to small sample
size
• Very expensive
• Particle orientation can impact
measurement

22. OPTICAL METHODS
VISNIR SPECTROSCOPY
Laboratory and portable spectrometers allow
for fast measurements
Visible (350-760 nm) and Near infrared light
(760-2500 nm)
Spectral response analyzed using
multivariate calibration models

23. OPTICAL METHODS
VISNIR SPECTROSCOPY
CHALLENGES
Performance is dependent on the model
Other factors (i.e. soil moisture) can impact
performance
Sample prep in the lab may still be needed
Jason P. Ackerson, C.L.S. Morgan, Y. Ge, Penetrometer-mounted VisNIR spectroscopy:
Application of EPO-PLS to in situ VisNIR spectra, Geoderma, Volume 286, 2017, Pages
131-138, ISSN 0016-7061,

24. SUMMARY
There are many methods available to measure particle size distribution, all with their
advantages and disadvantages.
You can present soil texture data in many ways. Take into account your audience.

25. QUESTIONS?
Leo Rivera, M.S.
Director of Scientific Outreach
METER Group, Inc.
2365 NE Hopkins Ct, Pullman, WA 99163
T 509.332.5600
E leo.rivera@metergroup.com
W www.metergroup.com

26. THANK YOU!