Russell Yost PhD Department of Tropical Plant and Soil Science University of Hawaii at Manoa Particle size determination in Soils of the Tropics Some weathered soils of the tropics those containing large amounts of hydrous ID: 564876
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Slide1
Some Recent Research Regarding Soil Physical Properties
Russell Yost, Ph.D.
Department of Tropical Plant and Soil Science
University of Hawai`i at
ManoaSlide2
Particle size determination in Soils of the TropicsSome weathered soils of the tropics – those containing large amounts of hydrous
sesquioxides
, it is difficult to obtain a particle size analysis that represents
behaviour
in the field –
Examples water content at 1500
kPa
Surface area usually a function of soil clay contentSlide3
Clay particles < 2µm High surface area per mass
Clay controls:
Water retention
Cation
, phosphorus retentionCarbon, nitrogen storageMicrobial activityQuantifying clay is critical to understand, predict, manage soil behavior
Background- Clay Particles
Sand
ClaySlide4
Pipet methodBased on
Stoke’s
Law:
V=kD2 Requires all soil components are discrete particlesHowever, in nature, clays exist as heterogeneous aggregates in soil
Measuring Clay ContentSlide5
Aggregation in Tropical Soils
Bonding Mechanisms
Positive
, negative charged
oxidesAmorphous mineralsOrganic matter complexes with oxide surfaceSoils derived from volcanic ash particularly problematicHigh concentrations of oxides, amorphous minerals, and organic matter
+
+
+
–
–
–
Positive
NegativeSlide6
Problem
Soil
Order
Oxide, Amorphous Content
Clay (%
)
1500
kPa
Water (%
)
Alfisol
Low
15
7
Vertisol
Low
62
25
Oxisol
High oxide
12
20
Andisol
High
amorphous
15
134
Clay
content underestimated
in
oxidic
and volcanic ash
soils
Resist dispersion of
pipette methodLow clay content contrary to large reactive surface areaProblematic soils comprise 17% global, 50% Hawai‘i land areaSlide7
Ultrasonication Experiment
Standard
Pipette
Method (NRCS)
Remove organic matter, saltsDispersant: Na-HMPUltrasonicationHigh frequency sound waves (>20 kHz)Rapid techniqueDispersed aggregates, increased clay contents in studies
Limited research on tropical soilsSlide8
Ultrasonication Experiment
Treatments
:
5 ultrasonic energy
levels0 (standard shaking), 100, 200, 400, 1600 J mL-110 g soil: 100 mL waterTriplicates for one soil in each mineralogy group, duplicates for other soilsSlide9
Aggregation Mechanisms
Explain strength of aggregation
and dispersion
Changes in measured clay at each energy increment (0-100, 100-200 J mL-1 etc.) regressed to soil propertiesSoil propertiesTotal CarbonCHNSO Elemental AnalyzerIron, aluminum Total Free: Dithionite-citrate (DC)
Amorphous: Hydroxylamine-hydrochloride (HH)ΔpH=(pH in 1
M KCl) – (pH in deionized water) Measure of negative surface chargeLinear, nonlinear regression in
SigmaPlot 10.0Slide10
Weakly Aggregated
R
2
> 0.96
P < 0.001
Clay MaximaSlide11
Strongly Aggregated
R
2
> 0.98
P < 0.001
Approached Clay MaximaSlide12
Weakly Aggregated
R
2
> 0.96
P < 0.001
Clay MaximaSlide13
Limitations: Particle Damage
Control soil (Salinas) showed significant decrease in sand-size particles with
ultrasonication
Scanning Electron Microscopy to investigate surface of sandSlide14
Limitations: Particle Damage
0
J mL
-1
1600
J mL
-1
Salinas Sand Particles
“
Etching” of
ultrasonicated
sand particle
Suggested alteration, potential damage of surfaceSlide15
Limitations: Particle Damage
Hali‘i
Honoka‘a
Ultrasonicated
Sand Particles
Concavity suggested damage from bubble collapse of cavitation processSlide16
Problems with measurements of soil physical properties
Conclusion:
It continues to be very difficult to accurately measure soil particle size, especially if the objective is to predict soil behavior
Alternative
Specify the precise application of particle size and explore methods to directly measure it
Example: Measuring 1500 kPa water content. Why: critical to estimation of plant available water.Slide17
Problems with measurements of soil physical properties
Possibility: Use of diffusive reflectance visible near infrared
spectroradiometry
This methods has long been used for very rapid (5 min or less) estimates forage quality: for the last 15-20 years.
Recently has attracted a lot of attention by soil scientists as a very rapid (~ 5 min/sample) method of measuring soil properties.
Method used by Mars rovers “Opportunity” & “Spirit”
Many challenges with calibration.Slide18
Problems with measurements of soil physical properties
Example instrument:
ASD
Fieldspec
Pro 4
Cost $50,000 USdown from
$450,000 afew yearsago.Slide19
Problems with measurements of soil physical propertiesSlide20
Problems with measurements of soil physical properties
Data: Joshua Silva, 2013Slide21
Problems with measurements of soil physical properties
Advantages of Diffuse Reflectance, visible near infrared
Rapid: A scan takes no more than a couple of minutes
Minimal sample preparation
Contrast these with the laboratory determination of 1500 kPa waterUsually takes a week to two weeks per sampleRequires careful preparation of the soilUsually requires an “undisturbed” soil sample
Data: Joshua Silva, 2013Slide22
ConclusionsMeasurement of soil particle size in soils with large amount of hydrous sesquioxides
continues to be problematic
Some suggestions: Try to directly measure the properties of interest and importance
Diffusive Reflectance Visible Near Infrared may hold promise in rapid measurement of selected physical properties.