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=kD 2 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 Oxisol High oxide Andisol High amorphous 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, 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 R 2 > 0.96 P < Clay Maxima

Strongly Aggregated R 2 > 0.98 P < Approached Clay Maxima

Weakly Aggregated R 2 > 0.96 P < 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 J mL -1 Salinas Sand Particles “ Etching” of ultrasonicated sand particle Suggested alteration, potential damage of surface

Limitations: Particle Damage Hali‘iHonoka‘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 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

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.