some recent research regarding soil physical properties russell yost, ph.d. department of tropical...
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Some Recent Research Regarding Soil Physical Properties
Russell Yost, Ph.D.
Department of Tropical Plant and Soil Science
University of Hawai`i at Manoa
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Particle size determination in Soils of the Tropics
• Some 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
content
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• Clay particles < 2µm – High surface area per mass
• Clay controls:1) Water retention
2) Cation, phosphorus retention
3) Carbon, nitrogen storage
4) Microbial activity
• Quantifying clay is critical to understand, predict, manage soil behavior
Background- Clay Particles
Sand Clay
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• Pipet method– Based on Stoke’s Law: V=kD2
• Requires all soil components are discrete particles
• However, in nature, clays exist as heterogeneous aggregates in soil
Measuring Clay Content
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Aggregation in Tropical Soils
•Bonding Mechanisms– Positive, negative charged
oxides– Amorphous minerals– Organic matter complexes
with oxide surface
• Soils derived from volcanic ash particularly problematic– High concentrations of oxides,
amorphous minerals, and organic matter
+
+
+
–
–
–
Positive Negative
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Problem
Soil Order Oxide, Amorphous Content Clay (%) 1500kPa 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 method– Low clay content contrary to large reactive surface area
• Problematic soils comprise 17% global, 50% Hawai‘i land area
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Ultrasonication Experiment• Standard Pipette Method
(NRCS)– Remove organic matter, salts– Dispersant: Na-HMP
• Ultrasonication– High frequency sound waves
(>20 kHz)– Rapid technique– Dispersed aggregates,
increased clay contents in studies
– Limited research on tropical soils
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Ultrasonication Experiment• Treatments:
5 ultrasonic energy levels– 0 (standard shaking), 100,
200, 400, 1600 J mL-1
– 10 g soil: 100 mL water– Triplicates for one soil in
each mineralogy group, duplicates for other soils
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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 properties
• Soil properties– Total Carbon
• CHNSO Elemental Analyzer
– Iron, aluminum • Total Free: Dithionite-citrate (DC)• Amorphous: Hydroxylamine-hydrochloride (HH)
– ΔpH• =(pH in 1 M KCl) – (pH in deionized water) • Measure of negative surface charge
• Linear, nonlinear regression in SigmaPlot 10.0
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Weakly Aggregated
R2 > 0.96P < 0.001
Clay Maxima
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Strongly Aggregated
R2 > 0.98P < 0.001
Approached Clay Maxima
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Weakly Aggregated
R2 > 0.96P < 0.001
Clay Maxima
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Limitations: Particle Damage
• Control soil (Salinas) showed significant decrease in sand-size particles with ultrasonication
• Scanning Electron Microscopy to investigate surface of sand
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Limitations: Particle Damage
0 J mL-1 1600 J mL-1
Salinas Sand Particles
• “Etching” of ultrasonicated sand particle• Suggested alteration, potential damage of
surface
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Limitations: Particle Damage
Hali‘i Honoka‘a
Ultrasonicated Sand Particles
• Concavity suggested damage from bubble collapse of cavitation process
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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.
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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.
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Problems with measurements of soil physical properties
• Example instrument:– ASD
Fieldspec
Pro 4- Cost
$50,000 US
down from
$450,000 a
few years
ago.
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Problems with measurements of soil physical properties
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Problems with measurements of soil physical properties
Data: Joshua Silva, 2013
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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 water• Usually takes a week to two weeks per sample• Requires careful preparation of the soil• Usually requires an “undisturbed” soil sample
Data: Joshua Silva, 2013
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Conclusions
• Measurement 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.