Penetrometer CALIBRATION AMERICAN JOURNAL 2.docx


Kasali, M.Y1, O.O. Olla1, D. James1 and M.B. Makanjuola1 and I.E. Ahaneku2

ABSTRACT
Cone penetrometers standardized by American Standards for Agricultural Engineering (ASAE)

have been major instruments for investigating and quantifying soil compaction. However, the

continuous use of penetrometers often introduces errors to the Cone Index (CI) readings obtained

from the instruments over time. An ASAE calibrated digital cone penetrometer and a manual

penetrometer were tested on a sandy clay loamy soil at the National Centre for Agricultural

Mechanization (NCAM), Ilorin, Nigeria; using the ASAE small cone. The CI readings obtained

for the two penetrometers were subjected to a statistical t-test. The results showed that the

sensitivity of the manual cone penetrometer is comparable to that of the digital penetrologger at

depths greater than 12cm as the mean difference showed no significant difference at this depth.

However, at depths 0-11cm, there was significant difference in their mean difference; this shows

the relative insensitivity of the manual cone penetrometer. The calibration results also showed

that the continuous use of the manual cone penetrometer will require a multiplying factor of

1.165 to get a CI reading close to ideal. Improvement on the manual cone penetrometer will also

assist in obtaining CI readings after tillage operations.

Keywords: cone penetrometer, soil compaction, ASAE small cone, calibration.

INTRODUCTION

Soil compaction is the process by which the soil grains are re-arranged to decrease void space 

and bring them into closer contact with one another, thereby decreasing the bulk density (SSSA, 

1996). The major contributor to forming soil compaction is various loads applied to the surface 

of unsaturated soils.

Soil compaction is one of the paramount information required for effective soil and crop 

management, and for terrain trafficability. It affects crop production by limiting potential yield 

and trafficability by limiting the potential traction. Regions of high mechanical resistance in the 

soil may result from natural soil features, heavy agricultural machinery traffic, or the formation

3. National Centre for Agricultural Mechanization, P.M.B. 1525, Ilorin.
4. Dept. of Agric. And Bioresources Engineering, Federal University of Tech., Minna

 American Journal of Agricultural Science, Engineering and Technology 

1. National Centre for Agricultural Mechanization, P.M.B. 1525, Ilorin.
2. Dept. of Agric. And Bioresources Engineering, Federal University of Tech., Minna.

CALIBRATION OF A MANUAL CONE PENETROMETER USING 
A DIGITAL PENETROLOGGER



of tillage implement pans. Thus soil quality must be determined and analyzed in order to increase 

crop productivity and trafficability (Boon et al., 2005).

Bradford (1986) in his work asserted that soil mechanical strength is an important soil parameter 

that affects root growth and water movement, and controls nutrient and contaminant transport 

below the rooting zone. The most common way to assess soil strength is by using a soil 

penetrometer, which characterizes the force needed to drive a cone of specific size into the soil

Moreover, quantitative assessment of soil compaction is necessary to determine its severity and 

to identify suitable mechanical, chemical or biological methods of intervention recommended for 

ameliorating or controlling soil compaction. Many studies have been conducted to understand 

the influence of bulk density (ρ) and water content (θ) on PR in the laboratory (Taylor and 

Gardner, 1963; Mirreh and Ketcheson, 1972; Ayers and Perumpal, 1982; Ayers and Bowen, 

1987; Ohu et al. 1988) and field (Simmons and Cassel, 1989; Vasquez et al. 1991, Busscher et al. 

1997, cited in Carlos, M.P.Z (2003), from which both empirical and theoretical relationships 

were obtained. Several indirect methods exist to measure soil compaction that rely on either 

increase in soil strength (i.e. mechanical impedance to penetrating objects) or reduction in 

interconnected pore spaces (Hemmat and Adamchuk, 2008, cited in Mojtaba et al, 2009).

Penetration resistance of soil can be measured with penetrometer. The penetrometer consists of a 

cone mounted on a rod that is pushed into the soil; penetration force is measured and recorded. 

For manual penetrometer, the metal rod pushed into the soil contains spring and an analogue 

readout that shows the corresponding resistance to motion in milliPaska (mPa). The rod is 

graduated with 0-7, 7-14 and 14-21 marks to indicate depth of penetration with corresponding 

resistance to motion of instrument. On the other hand, the digital penetrometer consists of the 

long rod pushed into the ground and a digital scale that displays both the penetration depth and 

the penetration force to give the Cone Index (CI) of the soil, which is calculated by dividing the 

force by the cross-sectional area of the cone. Penetration resistance is widely measured because 

it provides an easy and rapid method of assessing soil strength.

Calibration is a measurement process that assigns values to the property of an artifact or to the 

response of an instrument relative to reference standards or to a designated measurement process.

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Instrument calibration is intended to eliminate or reduce bias in an instrument’s reading over a

range for all continuous values. A functional relationship is then established between the values

of the standards and the corresponding measurement.

Two standard cone sizes have been recommended by ASAE (ASAE Standard S313.3, 2006) that

the large size cone is often used in tractor-mounted penetrometers, and the small size in

hand-pushed penetrometers. Amount of CI can be affected by the cone size and geometry. The

problem of providing metrological centers with standards (standard specimens) of a prescribed

class of accuracy is one of current interest. As was mentioned in Ermishin, Izmerit (2000), most

of the equipment we use, both in the field and in the laboratory might be having some degree of

errors over some long period of use without calibration. Therefore, calibration enables the

determination of the degree of error in the instrument, i.e. Cone Penetrometer.

Therefore, the specific objectives of this research include (i) calibration of the manual cone

penetrometer with the digital penetrologger, (ii) determination of the error, if any in the manual

cone penetrometer and finding ways of improving the manual cone penetrometer.

MATERIALS AND METHODS

FIG. 1: Digital Cone penetrometer Manual Cone penetrometer in use 

A hand-pushed penetrometer with ASAE small cone and an Eijkelkamp (P1.52) digital 

penetrologger with the same cone type were used for this study. The two equipment were used 

on an untilled soil plot of length 50m and width 25m. Tillage practices were carried out on the 

plot of land in the direction of previous operation (ASAE Standard EP542). Other soil properties 

such as soil type, moisture content, bulk density were also determined under each condition. Soil 

type was determined using sieve method of soil classification, while the gravimetric method of 

soil moisture content determination was employed. The bulk density was determined using the 

core volume of the core sampler. In each of these processes, samples were collected in 

transparent nylon and duly labeled. Five points were randomly selected on the experimental plot,

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5 insertions each was carried out for both untilled and tilled soil plot, respectively. An 85

Horsepower 2-wheel tractor was used to carry out ploughing and harrowing tillage operations,

respectively and all soil properties earlier determined for the control was also carried out after

the tillage operations. Penetration curves for the two penetrometers were plotted and standard

statistical packages such as mean and standard deviation was used to determine the extend of

deviation from actual reading of the penetrometer. Calibration of the manual penetrometer was

done using the sensitive Eijkelkamp penetrologger; this will assist in ameliorating the defects of

the penetrometer, if any.

Bulk Density Determination

Soil Bulk Density was calculated using the equation below:

( 1)𝐵𝑢𝑙𝑘 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝐵𝐷( ) =  𝑀𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 (𝑔)

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑖𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 (𝑐𝑚3)

Soil Moisture Determination

Moisture content of soil samples were determined gravimetrically using the equation below:

(2)𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 %( ) =  𝑀𝑎𝑠𝑠 𝑜𝑓 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙  𝑥 100

RESULTS AND DISCUSSION

Table 1: Soil Bulk density (g/cm3) with depth for both tilled and untilled soil.

SOIL DEPTH

Before

Ploughing

operation

After Ploughing

Operation

Before

Harrowing

Operation

After Harrowing

Operation

0-7 cm 1.452 1.131 1.344 1.262

7-14 cm 1.532 1.410 1.332 1.440

14-21 cm 1.151 1.620 1.410 1.630

The soil type test revealed a sandy clay loam soil type for the plot used in this experiment. Bulk 

density of the soil was determined gravimetrically and the behavior of the soil bulk density with 

depth is as shown in Table 1. The figure indicated a slight increase in bulk density from 

1.45g/cm3 to 1.53 g/cm3 and further decrease to 1.51g/cm3.

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Fig. 3:Moisture percentage with depth for both tilled and untilled soil.

Moisture content was taken for soil depths, 0-20cm; five replications of this were taken. The

variation of the soil moisture relative to depth is as shown in Figure 3. The figure indicated that

the average moisture for the different depths was highest for untilled soil and increased with

depth. The moisture content of the soil after ploughing operation initially decreased to a depth of

14cm and rose with depth to 21 cm depth. Moisture before harrowing operations was similar to

that before ploughing operations as it increased with depth, though more steady and with

relatively lower values, while after harrowing operation, moisture percentage rose steadily with

depth, though relatively with much lower values.

Fig. 4: Cone Index of penetrometers relative to depth before ploughing operation

Penetration (in mPa) recorded for the two penetrometers (Fig.4) shows that the digital 

penetrometer gives higher values for every depth of penetration taken. For most of the readings 

taken for the manual cone penetrometer, initial CI values usually for depths 0-3cm could not be

obtained like that of the sensitive digital penetrologger. Fig. 4 also shows that the CI obtained for 

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the digital penetrologger increased initially from 0.96 mPa at 4cm soil depth to 1.35 mPa at 6cm

depth and started falling until it reached 1.25 mPa at 10cm depth. It increased sharply to 1.51

mPa at 12cm depth and later fell to 1.13 mPa at 16 cm depth. It later increased to 1.23 and later

to 1.28 at depth 20 cm. This behavior may not be unconnected with the soil bulk density data

earlier obtained for the soil depths; the bulk density increased from 1.45 to 1.53 g/cm3, therefore

required greater force for penetration for depths 0-14 cm (for untilled soil). The latter increase

from about 16cm depth may not be unconnected with an increase in the moisture content at this

depth (fig. 3).

Table 2: Descriptive Statistics of the two penetrometers before tillage operations
Soil Depth Device N Mean Std. Error Mean

0-4cm Digital Reading 5 0.9620 0.16865

Manual Reading 5 0.3460 0.13534

5-6cm Digital Reading 5 1.3500 0.08820

Manual Reading 5 0.6040 0.11232

7-8cm Digital Reading 5 1.2760 0.04366

Manual Reading 5 0.7440 0.09130

9-10cm Digital Reading 5 1.2540 0.04739

Manual Reading 5 0.9500 0.07071

11-12cm Digital Reading 5 1.5120 0.09505

Manual Reading 5 1.2000 0.07583

13-14cm Digital Reading 5 1.2380 0.04705

Manual Reading 5 1.2760 0.11369

15-16cm Digital Reading 3 1.1267 0.06360

Manual Reading 3 1.2833 0.04410

17-18cm Digital Reading 3 1.2267 0.04631

Manual Reading 3 1.1833 0.06009

19- 20cm Digital Reading 2 1.2800 0.04000

Manual Reading 2 1.2750 0.12500

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Table 3: Independent t test for the penetrometer

soil

depth T Df

Sig.

(2-tailed)

Mean

Difference

Std. Error

Difference

95% C. I of mean

difference

Lower Upper

0-4cm 2.849 8 0.022 0.61600 0.21624 0.11735 1.11465

5-6cm 5.224 8 0.001 0.74600 0.14281 0.41667 1.07533

7-8cm 5.257 8 0.001 0.53200 0.10120 0.29863 0.76537

9-10cm 3.571 8 0.007 0.30400 0.08512 0.10770 0.50030

11-12cm 2.566 8 0.033 0.31200 0.12159 0.03161 0.59239

13-14cm -0.309 8 0.765 -0.03800 0.12304 -0.32174 0.24574

15-16cm -2.024 4 0.113 -0.15667 0.07739 -0.37153 0.05820

17-18cm 0.571 4 0.598 0.04333 0.07587 -0.16730 0.25397

19 20cm 0.038 2 0.973 0.00500 0.13124 -0.55970 0.56970

Statistically, the t-test and descriptive statistics in Tables 2 and 3 show that there was significant

differences between the CI means for the digital penetrologger and the manual penetrometer with

respect to depths 0-12cm; but thereafter, there was no significant difference in their means. This

indicates that the sensitivity of the manual penetrometer is comparable to that of the

penetrologger at depths greater than 12cm.

Fig. 5: Penetrometers’ sensitivity after ploughing operation

CI values could not be obtained for most of the insertions after tillage operations for the manual

cone penetrometer; this may not be unconnected to its relative insensitivity (see fig.5). On the 

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other hand, the manual cone penetrometer reading showed no response initially, but had a

continual increase in CI value of 0.35 mPa at depth 4cm to a CI value of 1.28 mPa at depth 16

cm. It later fell slightly to 1.18 mPa at depth 18cm and rose again to 1.28 mPa at depth 20 cm.

Instrument Calibration

Fig. 6: Calibration curve

Due to the inconsistency in the CI obtained from

the manual cone penetrometer after tillage

operations; the CI obtained before tillage

operations was utilized in the calibration

exercise.

A quadratic fit to the CI data obtained for the penetrometer produces the calibration curve given

as

DP = 0.71358 + 1.1249Mp – 0.050381Mp2 R = 60.8%
(0.12801) (0.31844) (0.18520)

where DP = Cone Index (CI) for digital penetrologger.

The correlation coefficient (R) shows a strong significant relationship between the calibrated

curves. For a future measurement of CI using the manual cone penetrometer, for every Manual

penetrometer reading as 1.15, the reference digital penetrologger will read 1.34, i.e.

for Mp = 1.15, Dp = 1.34 (see fig. 6)

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Conclusion

The results showed that the manual cone penetrometer has lost some degree of sensitivity and

may need to be repaired. However, the t-test result indicates that the sensitivity of the manual

penetrometer is comparable to that of the penetrologger at depths greater than 12cm.

The continuous use of the penetrometer will require a multiplying factor of 1.165 to get a reading

close to ideal. Improvement on the manual cone penetrometer will also assist in obtaining CI

reading after tillage operations.

Acknowledgement
The authors gratefully acknowledge the sponsorship of the research work by the Executive
Director and Management of the National Centre for Agricultural Mechanization (NCAM). We
are also grateful to Prof. Adewumi, A.O. for his helpful comments, suggestions and guidance at
various stages of this study which made this research work a huge success. The collaborative
efforts of the statistician, Mr Aduba Joseph are duly recognized. Unparallel dedication of the
Industrial Attachment students, especially Mr. Sunday Fagbemi is also commended. Tractor
drivers and tractor mechanics, especially Mr. Fatai Ogundero’s effort to see this work done is
highly recognized.

REFERENCES
ASAE Standards. (2006). Soil cone penetrometer, ASAE S313.3ASAE st. Joseph, MI 49805.

ASAE Standards. (2006). Procedure for using and reporting data obtained with the cone 
penetrometer ASAE Standard EP542 FEB99. ASABE St. Joseph, MI 449805.

Boon, N.E., Yahya, A., Kheiralla, A.F., Wee, B.S. and Gew, S.K. (2005). A tractor 
mounted, automated soil penetrometer-shearometer unit for mapping soil mechanical 
properties. Biosystems Engineering 90(4): 381-396.

Bradford,J.M. (1986). The penetration resistance in a soil with well defined structural units. 
Soil Science Society America Journal 44(3): 601-606.

Carlos, Manoel Pedro Vaz. (2003, March 3-21). Use of a combined penetrometer-TDR 
moisture probe for soil compaction studies. Retrieved 13/07/2012 from
http://users.ictp.it/~pub_off/lectures/ ins018/37vaz1.pdf.

Ermishin S., Izmerit, (2000). M,. Tekh., 2 (11).

Hemmat, A. and Adamchuk, V.I. (2008). Sensor systems for measuring soil compaction: 
Review and analysis. Computers and Electronics in Agriculture 63: 89-103.

SSSA (Soil Science Society of America). (1996). Glossary of soil science terms, SSSA, Madson, 
WI, USA.

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http://users.ictp.it/~pub_off/lectures/