Some Recent Research Regarding Soil Physical Properties

1. Some Recent Research Regarding Soil Physical Properties Russell Yost, Ph.D. Department of Tropical Plant and Soil Science University of Hawai`i at Manoa

2. 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

3. Background- Clay Particles • Clay particles < 2µm • High surface area per mass • Clay controls: • Water retention • Cation, phosphorus retention • Carbon, nitrogen storage • Microbial activity • Quantifying clay is critical to understand, predict, manage soil behavior Clay Sand

4. Measuring Clay Content • 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

5. 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 – + – + – + Negative Positive

6. Problem • 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

7. 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

8. 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

9. Aggregation Mechanisms • Explain strength of aggregationand 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 MKCl) – (pH in deionized water) • Measure of negative surface charge • Linear, nonlinear regression in SigmaPlot 10.0

10. Weakly Aggregated Clay Maxima R2 > 0.96 P < 0.001

11. Strongly Aggregated Approached Clay Maxima R2 > 0.98 P < 0.001

12. Weakly Aggregated Clay Maxima R2 > 0.96 P < 0.001

13. Limitations: Particle Damage • Control soil (Salinas) showed significant decrease in sand-size particles with ultrasonication • Scanning Electron Microscopy to investigate surface of sand

14. Limitations: Particle Damage Salinas Sand Particles 0 J mL-1 1600 J mL-1 • “Etching” of ultrasonicated sand particle • Suggested alteration, potential damage of surface

15. Limitations: Particle Damage Ultrasonicated Sand Particles Hali‘i Honoka‘a • Concavity suggested damage from bubble collapse of cavitation process

16. 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.

17. 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.

18. 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.

19. Problems with measurements of soil physical properties

20. Problems with measurements of soil physical properties Data: Joshua Silva, 2013

21. 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

22. 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.