Measuring Soil Bulk Density by Using Vibration-induced Conductivity Fluctuation

A. Sz. Kishné, C. L.S. Morgan Dept. of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA L. B. Kish Dept. of Electrical and Computer Engineering, Texas A&M University, College Station, TX, USA

Objectives

Materials and Methods

Results

References

1. Introduce a new, non-destructive method for measuring soil bulk density by exposing soils to a periodic vibration and measuring the vibration-induced conductivity fluctuation (VICOF)

2. Test the measurement principles on packed soil samples

3. Test the effect of salinity and moisture on VICOF at current measurement setup

Kish,L.B., C.L.S. Morgan and A.Sz. Kishné (2006). Vibration-induced Conductivity Fluctuation Testing of Soils. Fluctuations and Noise Letters. SubmittedTexas A&M University patent disclosure (November 2005)

Appreciation is expressed to Dr. K. McInnes and Dr. T. Hallmark (TAMU) for their valuable

suggestions for compacting and salinizing the soil samples, respectively.

The research was supported by Texas Agricultural Experiment Station.

dRs =2R1

U2,2

U1 - U2,1

(1+U2,1

U1 - U2,1

)

Resistance fluctuation (dRs) Normalized resistance fluctuation (dRs/Rs)

1,21

1,2

1,2

2,2 12 UU

U

U

U

R

dR

s

s

Measurement circuitry:

Ground

To the lock-in amplifier

Soil resistance(modulated by f2)

R1

U1 [f1 =1kHz]

Rs

U2,1 [f1]

U2,2 [f1+2f2 and f1-2f2]

AC Voltage generator

Driving resistor

Vibration by f2

~

Justification

1. Soils Texture Class

ClayFine sand

Particle Size Distribution

ClaySiltSand--------------%--------------

11.297.6

36.7 0.9

52.1 1.5

Elec.Cond.(sat.paste)

dS m-1

0.60.3

Discussion and Conclusions

A curvilinear, inverse relationship exists between normalized vibration-induced resistance fluctuation and soil bulk density that is specific for soil texture.

Soil moisture and salinity effects this relationship at 60 Hz vibration frequency, but these effects may be eliminated by taking measurements at multiple frequencies.

Instrument noise increases with increasing bulk density. Error ratio and soil-electrode contact is expected to improve with the use of a blade-shaped compared to the current needle-shaped electrode.

According to preliminary result, salinity (2 dS m-1) has a minimal effect on the measurement

Acknowledgements

Measurement Errors (Normalized to Signal)

0

0.6

1.2

SD

/ A

vera

ged

dR

s/R

s

Clay Fine Sand

Comp1 Comp2 Comp3 Comp4 Comp1 Comp2 Comp3 Comp4

D W D WD WD W D W D WD WD W

Instrument noise

Within sample

Within treatment

Quantification of soil bulk density under field conditions is time consuming and difficult. A novel method for quantifying soil bulk density in the field involves using recent advances in fluctuation and noise research. Theoretically this method can be made independent of soil salinity and moisture. Possible application of this method is explored.

Principles

3. Experimental setup for the electrical conductivity measurements

4. Lock-in amplifier

3. Periodic vibrator, 60 Hz

2. Antivibration table

1. Electrode (0.3cm diam. & 7.1 cm length)

5. Software for averaging signal over time

1

2

3

4average

signal

Time period:30 sec: U1 & U2,1

90 sec: U2,2,0 & U2,2,vibr

5

2. Soil sample treatments

1 kg

3 kg

Weights compacting the soils by layers

• Wet Soil: -1000 J kg-1 (D) and -100 J kg-1 (W) (clay: 0.20-0.35 cm3 cm-3, sand: 0.03-0.09 cm3 cm-3)• Compacted Soil: used a 1 kg or 3 kg weight

Comp 1Comp 2Comp 3Comp 4

• Salinizied Fine Sand: 2 dS m-1 EC in saturated paste

Clay (g cm-3)

0.80.8-0.90.9-1.0

1.1

1.1-1.21.21.31.5

Fine Sand (g cm-3)

Normalized Resistance Fluctuation vs. Soil Bulk Density

0.6 0.8 1.0 1.2 1.4 1.6

dR

s/R

s

2 10-4

1.5 10-4

1 10-4

5 10-5

0 100

Bulk Density (g cm-3)

Normalized Resistance Fluctuation vs. Soil Bulk Density

Clay, 1000 J kg-1

Clay, 100 J kg-1

Fine sand, 1000 J kg-1

Salinized sand, 100 J kg-1

* Electrical contact problem (left out of fitting the model)

** * *

**

Fine sand, 100 J kg-1

0.6 0.8 1.0 1.2 1.4 1.6

dR

s/R

s

2 10-4

1.5 10-4

1 10-4

5 10-5

0 100

Bulk Density (g cm-3)

* *

Clay, 1000 J kg-1

Clay, 100 J kg-1

Fine sand, 1000 J kg-1

Salinized sand, 100 J kg-1

* Electrical contact problem (left out of fitting the model)

**

Fine sand, 100 J kg-1**

-

-

-

-

-