Some Recent Research Regarding Soil Physical Properties

Russell Yost, Ph.D.

Department of Tropical Plant and Soil Science

University of Hawai`i at Manoa

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

• 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

• 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

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

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

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

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

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

Weakly Aggregated

R2 > 0.96P < 0.001

Clay Maxima

Strongly Aggregated

R2 > 0.98P < 0.001

Approached Clay Maxima

Weakly Aggregated

R2 > 0.96P < 0.001

Clay Maxima

Limitations: Particle Damage

• Control soil (Salinas) showed significant decrease in sand-size particles with ultrasonication

• Scanning Electron Microscopy to investigate surface of sand

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

Limitations: Particle Damage

Hali‘i Honoka‘a

Ultrasonicated Sand Particles

• Concavity suggested damage from bubble collapse of cavitation process

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.

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.

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.

Problems with measurements of soil physical properties

Problems with measurements of soil physical properties

Data: Joshua Silva, 2013

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

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.