A system for measuring soil physical properties in the field

A system for measuring soil physical properties in the field

Soil & Tillage Research, 26 (1993) 30 t - 3 2 5 301 Elsevier Science Publishers B.V. All rights reserved A system for measuring soil physical prope...

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Soil & Tillage Research, 26 (1993) 30 t - 3 2 5

301

Elsevier Science Publishers B.V. All rights reserved

A system for measuring soil physical properties in the fieldl Mark T. Morgan*, Robert G. Holmes, Randall K. Wood Department of Agricultural Engineering, The Ohio State University, Columbus, OH 43210, USA (Accepted 30 March 1993)

Abstract

An automated soil sampler and a system for measuring the physical properties of soil cores in the field were developed for the study of soil compaction and tillage effects on soil physical properties. The system measures the bulk density, air-filled porosity, intrinsic air permeability, and moisture content on 47.6 mm diameter soil cores as well as measuring cone penetration resistance in the field. Results of a laboratory calibration study illustrated that the system measures the soil properties with acceptable accuracy. A main benefit of the system was the fact that it could measure all of the above properties on ten core samples down to a depth of 0.5 m, typically within 15 min. A field compaction study showed that the system was capable of detecting the effects ofa 15.2 tonne axle load on the soil down to a depth of 0.4 m, 1 year after compaction.

Introduction Precise and rapid in-situ characterization of soil physical properties is a major constraint toward development and adoption of sustainable systems of soil surface management. Presently, the measurement of most soil physical properties is a time-consuming process. Compounding the problem of quantifying the properties is the large variation among soil samples. For this reason, many soil compaction studies focus on changes in only one property such as bulk density, cone index or saturated hydraulic conductivity. A review by Soane et al. (1981) indicates that the literature describing studies on soil compaction by agricultural vehicles reveals that a wide and confusing variety of methods, units and expressions are used, making it difficult to gain a coherent understanding of the subject. The authors also identify the need to *Corresponding author at: i 146 Agricultural Engineering, Purdue University, West Lafayette, IN 47907, USA. ~Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Manuscript No. 84-92.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-1987/93/$06.00

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collect data as rapidly as possible to avoid changes owing to weather, etc. Collection of data within the course of 1 or 2 days is suggested. Soane (1974) describes a soil test vehicle on which instruments could be mounted for the measurement of bulk density, moisture content, cone penetration resistance, rut dimensions and air permeability. The concept significantly reduced the physical effort required by the operators and the time required to make measurements. This same concept of a portable system to measure soil physical properties rapidly, accurately and with minimal operator effort was the focus of the research described here. The purpose of this paper is to describe the design and operation of a system for rapidly measuring and analyzing soil physical properties in-situ. The system includes: (a) an automated core sampler for obtaining soil cores and cone penetrometer measurements; (b) a computer controlled Soil Physical Properties Measurement System (Sp2MS) for measuring physical properties and calculating the air-filled porosity, total porosity, bulk density, intrinsic air permeability and moisture content of the core samples as well as calculating cone index values from measurements on undisturbed soil in the field. The results of laboratory tests to demonstrate the accuracy of the system and a field compaction experiment to demonstrate its capability for detecting changes in soil physical conditions after machinery traffic are also reported.

Background Compaction of agricultural soils results in increased soil bulk density (Blake, 1965; Freitag, 1971; Erbach, 1982 ), and decreased porosity, hydraulic conductivity and air permeability (Kirkham, 1946; Freitag, 1971; Nau, 1987 ). However, bulk density is difficult to use as a measure of soil compaction because, for undisturbed soil, very high pressures applied to the soil surface usually result in relatively small changes in bulk density. Also, sampling variability requires a large number of samples for statistical significance. In addition, methods of measuring soil bulk density are usually tedious and time consuming. Many of the methods in use today are described in Blake and Hartge (1986). Cone index is an important method of measurement for identifying soil compaction. Perumpral (1987) gives a review of cone penetrometer applications. The author states that penetrometer measurements are very easily obtained in the field and, for immediate comparison within an experiment, can correlate well with compaction. May studies have used cone index values to compare soil conditions under different tillage practices and wheel loads, etc. (e.g. Voorhees et al., 1978; Soane et al., 1981; Voorhees et al., 1986). However, the comparison of these measurements is limited by the effect of moisture content. If moisture content is not uniform in the treatments of a

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single field experiment, the comparison of cone index among treatments can be difficult. Soil hydraulic conductivity is very sensitive to changes in soil porosity, pore size distribution and bulk density. A small increase in bulk density often causes a very large decrease in hydraulic conductivity (Blake et al., 1976). Thus, hydraulic conductivity has been recognized as a potentially sensitive measure of soil compaction (Freitag, 1971 ). However, hydraulic conductivity measurements are very time consuming and highly variable. Thus, a large number of samples and a relatively long time are required for these measurements. Air permeability has also been recognized as a soil property affected by a compactive stress (Freitag, 1971 ). Air permeability is very sensitive to changes in porosity and bulk density and can be measured quickly (Nau, 1987; Morgan, 1988 ). Therefore, the use of air permeability as a index of compaction has much promise. Materials and methods

Design and operation of equipment Soil core sampler The soil core sampler developed in the study is shown mounted on a highclearance tractor in Fig. 1. The core sampler was designed to automate the collection of the soil samples and to control the sample lengths. By mounting the sampler on the high-clearance tractor, it could be used to take samples from a corn field with a mature crop without considerable crop damage. The clearance height of the tractor was 1.52 m.

Fig. 1. Soil core sampler m o u n t e d on a high-clearance tractor.

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Figure 2 shows a diagram of the soil sampler. The main portion of the sampler consists of a 38 m m bore X 914 m m stroke hydraulic cylinder (Parkertron model D2HXKUS23, Parker Hannifin Corp., Cleveland, OH)t mounted vertically and guided by two stainless steel rods, 25.4 m m in diameter. Attached to the cylinder is a coring probe which holds sample tubes using an expandable collet mechanism similar to that used by Nau (1987). The sample tubes were made of seamless, heat-treated, 4130 carbon steel tubing 89 m m in length and 47.6 m m inner diameter as shown in Fig. 3. The Parkertron cylinder was equipped with a spirally grooved cylinder rod and two Hall-effect sensors to indicate movement of the rod. The sensors enabled the position of the rod to be determined within 1.3 mm. The vertically guided cylinder assembly moved horizontally along a slide rail on Garlock DX bearings (Fig. 2). The slide rail consisted of a 76.2 m m X 152.4 m m X 6.4 m m thick rectangular steel tube, 2.03 m long with two 25.4 m m diameter cold-rolled steel rods bolted to the top and bottom. Two adjustable-height legs at each end of the slide rail supported the sampler and the front end weight of the high clearance tractor during sampling. The sampler was powered by a Honda, model E 2200, generator and the WVPIDAI

II II~

P Y I Ikli-II~l~

CONTROL BOX & VEEDER IE

D2HXKU523

SLIDE RAIL HALL-EFFE( SENSORS CONT

LOl

CONE PENET~

EXPANDING COLLET

Fig. 2. Diagram of soil core sampler. IMention of product names is for information only and does not imply endorsement by The Ohio State University to the exclusion of other suitable products.

M.T. Morgan et al. / Soil & Tillage Research 26 (1993) 301-325

17.00 °I

~

30 5

i,65 ,80

4130 SEAMLESS STEEL TUBING HARDENED, HEAT TREATE~ AND ZINC OR PHOSPHOR PLATED DIMENSIONS IN MILLIMETERS

\

8 . 0 0 ° ---,..

~___( \

/

/

89

-

47,6

-':

I-

50,8

-

Fig. 3. Cross-section of soil sample tube.

hydraulic system of the high clearance tractor. The vertically guided hydraulic cylinder was controlled by a Parker model D 1VW8CY40, four way, three position, solenoid operated, directional control valve and a Parker, model FM2D, flow control valve. The flow control valve was used to set the vertical speed, of the cylinder for cone penetration measurements. The control system of the core sampler consisted of a Veeder Root Series 7910 Predetemining Counter, compatible with the Parkertron cylinder, three Potter & Bromfield TPDT relays, four D P D T push buttons, two manual switches and one limit switch. The Veeder Root controlled: ( 1 ) the solenoid valve on the high clearance tractor which supplied the circuit used by the sampler and (2) the solenoids of the directional valve for the hydraulic cylinder. The control system was designed to control automatically the cylinder stroke

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and therefore the length of the soil core samples at 50 m m each. The length of the soil cores was easily changed by reprogramming the limits on the Veeder Root counter. The m o v e m e n t of the coring probe was initiated by the use of push buttons located on a control panel of the core sampler and operated in two modes, automatic or manual. More details of the programming and operation of the Veeder Root counter - shown in the Veeder Root Operation Manual (Digital Products, 1985 ) and Morgan ( 1988 ). During operation, ten soil cores were removed, one at a time, from each sample hole using the soil core sampler. Thus, each core sample represented 50 m m of the 500 m m profile. In order to minimize drying and shrinking of the soil cores prior to testing, a plastic pipe cap was placed over the tapered end of the sample tube as soon as it was removed from the coring probe. At this time, any soil and extraneous material clinging to the sides of the sample tube was wiped off and any loose fines inside the tube were removed by lightly tapping the tube allowing the fines to fall out. The sample tubes were then placed in a tray which held all ten sample tubes in the order collected. If any of the sample tubes were bent or nicked from hitting a rock, the tubes were replaced in the set before the next use. The time required to collect a set of ten cores averaged 10-15 rain. Cone penetrometer measurements The core sampler was also designed to collect cone penetration data by replacing the coring probe with a cone penetrometer and load cell. The cone penetrometer consisted of a 30 ° circular stainless steel cone with a 12.7 m m diameter base attached to a 9.5 m m diameter by 0.6 m long stainless steel rod as described in ASAE Standard S 313.2 (American Society of Agricultural Engineers, 1991 ). The rod was attached to a Strainsert, model FL05U-2SGKT, 227 kg m a x i m u m load, flat load cell which was m o u n t e d on the probe assembly of the core sampler in place of the coring probe during penetration tests. Cone penetration tests were performed following the guidelines in ASAE Standard $313.2. A flow control valve on the core sampler regulated the speed of the penetrometer to the r e c o m m e n d e d rate of 30 m m s- 1 and the limits on the Veeder Root Counter were rcprogrammed to drive the cone clown continuously to a depth of 0.51 m A BASIC computer program, running on a Hewlett Packard, model 320 computer, monitored the depth of the penetrometer via the digital output of the Veeder Root Counter and the force on the cone via the analog signal from the force transducer (Morgan, 1992). The force signal and the depth data were transmitted to the computer via a multi-conductor interface cable through a Hewlett Packard, model 3852A, Data Acquisition and Control Unit ( D A C U ) and an RS-232 interface. These two variables, depth and cone force were stored in a data file by the computer for later analysis.

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Field portable measurement system The measurement portion of this system, the Soil Physical Properties Measurement System (Sp2MS), consisted of various components for measuring the volume, weight, air-filled porosity, moisture content and air permeability of each individual core sample. Each measurement was made with the core still in the sample tube. The measurement procedures, data collection and data storage were controlled via a BASIC language program running on the same computer system mentioned above (Morgan, 1992 ). The operators task consisted of inspecting samples and moving them between measurement stations as prompted by the control program. The H.P. 320 computer and the 3852A DACU were attached to a metal frame with an air permeability measurement apparatus, an electronic balance, an air-filled porosity measurement apparatus, a volume measurement apparatus, and a dielectric moisture probe each attached to the top of the frame (Fig. 4). This complete system was transported to and from the field in a passenger van or pick-up truck.

Weight measurement The weight of each core sample was measured using an electronic balance, Mettler model PM2000, with an RS-232 interface. The interface allowed the sample weight to be automatically recorded by the computer. The sample tube weights, previously stored in a data file, were automatically subtracted from the scale measurement to obtain the wet weight of the soil cores. This sample weight was stored by the computer and used for bulk density calculations. The resolution of the scale was 0.01 grams.

Fig. 4. Photograph of Soil Physical Properties Measurement System inside a pickup truck.

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Volume measurement

As the cores from the soil sampler were fractured on the ends, a method of measuring the volume of these irregular size cores was needed. Trimming the ends of the cores to a defined dimension was avoided because of the errors which could be introduced in the air permeability and air-filled porosity measurements. Therefore, the following apparatus was designed to determine the volume and average length of each core. These measurements were required for the calculation of air permeability, bulk density, and air-filled porosity. The design of the soil volume measurement device used the basic concepts of the balloon m e t h o d described by ASTM Designation: D 2167-63 T, ASTM (1964) and Blake ad Hartge ( 1986 ). The principle behind this device was to fill the air spaces at each end of the sample tube with a fluid contained in small balloons, (i.e. condoms). The fluid, water in this case, was forced under approximately 21.0 kPa of pressure into the balloons to fill the air spaces as shown in Fig. 5. A plastic collar was used to provide an extension of the sample tube at the tapered end to confine the expansion of the upper balloon. The tips of the balloons conformed under the pressure to the irregular shape of the

TO AIR SUPPLY 21.0 ' ~

WATER SOD

SOLENOI

WATER COLLAR MPLE S TUBE O-RINGS

21.0 kPa

Fig. 5. Diagram ofbaUoon-methodvolume measurement device.

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ends of the soil core. The pressure was selected such that the balloons filled the air gaps and followed the surface contour, but did not compress the sample. The volume of the core in the sample tube was measured by a differential pressure transducer (Validyne, model P305D, with a 0-140 cm of water pressure diaphragm) which measured the height of the water in a column. Two plastic slugs with volumes of 55 and 102 cm 3 were used to calibrate the volume device in the field. During calibration these slugs were fit into a sample tube and placed in the volume device, one at a time. The corresponding pressure readings were then linearly correlated to the known volumes of the slugs. This linear equation was then used to calculate the volumes of the soil cores from the pressure readings. Typically the calibration was checked in the field between every set of ten samples to ensure there was no water leakage in the system In the laboratory, the accuracy of the volume measurement device was tested using several additional plastic slugs whose volumes were 102, 90, 86, 78 and 55 cm 3. After calibration using the above procedure, with two slugs, the volumes of the remaining slugs were all measured to within _+0.2 cm 3.

Air-filled porosity measurement The main components of the porosity device included a 63.5 m m bore X 152.4 stroke, double acting, H u m p h r e y air cylinder; two solenoid-operated, Model M3E 1, H u m p h r e y air valves; a Validyne, model P305D, 0-690 kPa pressure transducer; and two air chambers machined out of aluminum. The apparatus was designed to measure the air-filled porosity of 47.6 m m d i a m e t e r x 50 m m long soil core samples contained in sample tubes. The basic principle of the design was based on Boyle's Law similar to the apparatus used by Rees et al. (1982) and the Gas Pycnometer Method described by Danielson and Sutherland ( 1986 ). Using Boyle's Law and air chambers of known volume, a sample tube containing a soil core was sealed in the lower chamber and pressurized to a constant pressure of approximately 550 kPa (Fig. 6 ). The pressure was automatically recorded after waiting approximately 10 s for stabilization. Then the sample chamber was connected to a second chamber via a solenoid valve, allowing the air to expand into both chambers. After equilibration, the pressure was again automatically recorded. As the volume of air trapped in the two chambers was constant, before and after the valve connecting them was opened, the difference in pressure change, with and without the soil sample in the chamber, was related to the change in total air volume owing to the presence of the soil core. Using the moisture content and bulk volume measurements for the core, the air-filled and total porosities were then calculated from the following expressions.

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Secondary Chamber V2 Pressure Transducer

Sotenoid Valves ~

~[0-690 kPa I Sample Tube J with soil core

Supp{y Pressure 552 kPa (80 Psi)

Sample Chamber - Vc

O-ring seat

Fig. 6. Schematic diagram of air-filled porosity measurement device.

Air-filled porosity (1)

qa = ( G / V s ) XlO0

Totalporosity q= [ ( VplV~) + w~,lPw ] X 100

(2)

where lip was found by

Vp=V~- Vc-G

(p,_p2)3

(3)

where ~a is air-filled porosity (%), r/percent total porosity (%), Vp is volume of the air-filled pore space in the soil sample (m 3), Vs is bulk volume of the soil sample (m3), Vc is volume of the sample chamber (m3), Vt is volume of the sample tube (m 3), V2 is volume of secondary chamber (m 3), P~ is initial gauge pressure (sample chamber pressurized) (kPa), P2 is final gauge pressure (two chambers connected) (kPa), Pb is dry bulk density of the soil sample (Mg m-3), Pw is density of water (Mg m-3), w is gravimetric moisture content of sample (kg kg- 1). The air-filled porosity device was calibrated in the field and laboratory using plastic slugs. These plastic slugs were all 47.6 m m in diameter by 50 mm in length with holes drilled through them to correspond to porosities of 5.3, 6.4, 12.8, 17.3, 21.1 and 28.8%. In the field, only the 6.4 and 21.1% porosity slugs were used for calibration. However, after calibrating with these two slugs, the porosities of the remaining slugs could all be measured within 2% of their values.

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Air permeability measurement Intrinsic air permeability of each soil core was measured using the apparatus shown in Fig. 7. The soil core, contained in the sample tube, was sealed in the soil core test cell. Air flow rate through the soil core was measured by a series of four Sierra, model 840, Mass Flow Controllers (MFC's) with flow ranges of 0.0 to 0.01, 0.10, 1.00 and 10.0 standard 1 min -t (SLPM). The automatic selection and flow control of the appropriate mass flow meter was under the supervision of the BASIC language program running on the H.P. 320 computer. The control algorithm selected the proper range MFC and then regulated the flow rate until the pressure drop across the sample was 2.5 kPa. The pressure was measured using a Validyne, model P305D, pressure transducer with a diaphragm range of 0-13.7 kPa. (A detailed description of the design and operation of the air permeability measurement system is in Morgan, 1988. ) Intrinsic air permeability was calculated, based on Darcy's Law, as shown below

ka- 21taQaLPext2 2

X 1012

(4)

A (Pe~t - Pex, ) where ka is the intrinsic air permeability of soil core (/~2), fla is absolute viscosity of air ( 1.81 × l0 -5 Pa s-~ ), Qa is air flow rate for sample (standard m 3 s - i ), L is effective length of soil core = Vs/A (m), A is cross sectional area of soil core ( 1.78X l0 -3 m2), Pent is absolute air entrance pressure (Pa), Pext is absolute air exit pressure ( 101 325 Pa) Sierra mass flow meters ,,~~mode[ 8 40 Fairchild pressure ~ regulator, model 10

o,) '~ - - ' ~

-~'

T oair supply

~

(~ ~airchild

pressure pressore

Aluminum__ Manifold WIIR

u-nng

sea[

/

/ ¢

Vatidyne pressure transducer, model P305D

552 RPa pressure Io porosily device

Fig.7.Schematicdiagramofairpermeabilitymeasurementapparatus.

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Accurate calibration of the Sierra MFC's could only be done by the manufacturer with the proper testing equipment. However, a method was needed in the field for detecting leaks in the system and for checking the calibration of the flow controllers. Capillary tubes ranging in size from 0.34 to 0.61 m m diameter and 236.0 to 1378.0 m m in length were used for this purpose. For all of the capillaries, the measured flow rates were within 1% of the theoreticaUy calculated flows at the tested pressure, nominally 2.5 kPa. An error analysis on Eq. (4), similar to one described in Morgan ( 1988 ), to allow for measurement inaccuracies and fluctuations in atmospheric pressure and air viscosity showed that intrinsic air permeabilities within + 6% error could be achieved. Moisture content m e a s u r e m e n t

The volumetric moisture content of the soil cores was measured using a Portable Dielectric Measurement System (DMS) manufactured by Applied Microwave Corporation. The DMS measured the real and imaginary parts of the complex permitivity of the soil which was then correlated to the volumetric moisture content (Brunfeldt, 1987; Jackson, 1990). The major components of the DMS included a radio frequency (RF) box, a control box, three sizes of probe tips, a Hewlett Packard, 41CV hand-held calculator, and an RS 232 serial interface (Fig. 8). The RF box contained two 50 ohm coaxial cables. One cable was open ended and terminated inside the RF box, and the other was fitted with a probe tip and terminated outside the RF box for contacting the soil. The control box, interfaced via the H.P. cal-

/.~PUSH BUTTON SOIL SAMPLE RF BOX

PROBE TIP

COAXIAL CABLE

1

Y ~CONTROL BOX

RS-232 INTERFACE m

HP 41CX CALCULATOR

Fig. 8. Schematic diagram of dielectric-constant-based moisture content measurement system.

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313

culator, measured the magnitude and phase differences in reflected waves propagating through the coaxial cables. The wave frequency was 1.25 GHz for the DMS model used in this study. During operation, the surface of a soil core, previously smoothed with a scraping tool, was placed in contact with the probe tip and the dielectric constant was measured. Combining the real part of the dielectric constant with the approximate percentages of sand and clay in the soil, the volumetric moisture content of the sample was calculated using equations by Hallikalnen et al. ( 1985 ). The dielectric constant and volumetric moisture content were then transferred to the H.P. 320 computer via an RS-232 interface. The equations and calibration procedures for the DMS are described in Morgan et al. ( 1991 ) and in Brunfeldt (1987). Owing to the variability and uncertainty in the dielectric-based moisture measurements at the time the data was collected for this study, oven-dry moisture contents were used for the analysis of the experimental results. However, later calibration data showed that under closely controlled conditions, the DMS could measure soil moisture content within _+0.015 m 3 m -3.

Property measurement procedures Once a set of core samples had been collected using the soil sampler, as described previously, it was taken to the SpEMS. The air flow through the soil core and pressure drop across the sample, sample weight, air-filled porosity, volume and moisture content were measured for each sample according to the following procedure. ( 1 ) The BASIC computer program (Morgan, 1992 ) was run on the H.P. 320 computer and any initial information including the hole description, time, date and sample tube set number were entered into the program. (2) Following the prompts from the computer, the properties of each sample were measured in the following order: (a) air flow through the sample at a pressure drop of 2.5 kPa for the air permeability measurement, (b) sample weight, (c) air-filled porosity, (d) sample volume, (e) dielectric constant for moisture content. (3) The above sequence was repeated until all ten of the core samples were tested. (4) The intrinsic air permeability, air-filled porosity, total porosity, gravimetric moisture content and bulk density of the soil cores were calculated and stored on a 3.5, floppy diskette along with the raw data corresponding to each sample.

Laboratory and field experiment descriptions The experiments described below were designed to: (a) determine the ability of the SP2MS to accurately measure the desired soil properties of bulk density, air-filled porosity, and intrinsic air permeability; (b) evaluate the capability of the soil sampling system to detect compacted areas in a field; (c) determine the amount of time required to collect the soil properties data.

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Laboratory testing The laboratory portion of the study entailed preparing laboratory samples using two Ohio soils. These prepared laboratory samples, referred to as 'parent' samples, were compressed to varying stress levels at several initial moisture contents. The properties of these parent samples were then compared with the properties of subsamples measured with the SpEMS. The two Ohio soils used in this study were Blount silt loam and Rossburg loam. The particle size distributions of these two soils, as measured using the hydrometer method, and the particle densities, measured using a water pycnometer, are shown in Table I. The stress levels used for the study were 86, 172 and 345 kPa. Moisture contents prior to loading included: 18, 20, 25 and 28%, dry weight basis, for Blount and 13, 15.5, 18 and 20% for Rossburg. The stress levels were selected to cover the range of values which could be applied in the field by traffic. The moisture content range was selected such that the samples of lowest moisture content held together after compression at the lowest stress and water was not squeezed out of the samples during compression at the highest moisture content. The soils were prepared by air drying, followed by crushing until the largest aggregates were approximately 6 m m in diameter. This soil was then frozen at - 5 ° C. The desired moisture contents were obtained by mixing approximately 18-20 kg of frozen soil with a predetermined amount of crushed ice inside a walk-in freezer. The ice was previously crushed to approximately the same particle size as the soil. The frozen soil and ice mixture was scooped into a compaction mold, covered with plastic, and then allowed to thaw for 48 h prior to compressing. This method of mixing produced a very uniform moisture content when thawed, and most importantly, did not alter the aggregate sizes owing to the mixing action. The compaction mold consisted of three stacked rings of 254 m m diameter, schedule 40 polyvinyl chloride (pvc) pipe. The rings were 127, 127 and 152 m m in length from bottom to top respectively. These rings were held together Table 1 Details of soils used in laboratory testing Soil type

Description

Particle density (Mg m -3)

Sand (%)

Silt (%)

Clay (%)

Blount

Fine, illitic, mesic Aeric Ochraqualfs Fine-loamy, mixed mesic Fluventic Hapludolls

2.65

20

56

24

2.66

49

36

15

Rossburg

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31 5

with two adjustable banding straps and attached to a base plate with a metal ring and four (8 m m diameter) threaded rods. When the core was thawed and ready for compression, it was inverted onto a 25 cm diameter plate so that the base plate could be removed and another metal ring attached to the end. These two metal rings, one on each end, held the core together so that the assembly could be placed on an Instron Universal Testing Machine ( I U T M ) in a floating-ring type of setup. A load cell below the soil core monitored the force while the crosshead of the I U T M compressed the soil with another 250 m m diameter plate. The soil was compressed at a rate of 0.083 m m s- 1. When the desired stress level was reached, the crosshead was stopped and the stress was allowed to relax until it approached a constant value. After the relaxation period, typically 5-20 min, the center ring was cut away using a fine wire. Bulk density of this parent sample was determined from its dimensions, weight and oven-dry moisture measurements. Air-filled porosity was calculated for the parent sample using the bulk density and particle density. Intrinsic air permeability, was measured on this parent sample using the air permeability apparatus connected to the parent sample using rubber tubing as shown in Fig. 9. After the properties of the parent sample were determined, eight, 47.6 m m diameterX 50 m m long, soil cores were removed using the core sampler. The core samples were taken in two layers evenly spaced around the parent core. Samples 1-4 were taken in the top layer and Samples 5-8 were taken in a second layer directly below each of the first four samples. The properties of

H

cn~ of w a t e r

1

Air Ftow

ttttt Parent Sarape

II

~=

Air flow

Fig. 9. Schematicdiagramof apparatus usedfor measuringair permeabilityof parent soilsamples.

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these cores were measured using the Sp2MS according to the procedures described earlier.

Field testing For the field experiment, an 18 m 3 capacity, single-axle grain cart was filled to apply a 15.2 t load to a Celina silt loam soil, (fine, mixed mesic-Aquic Hapludalfs; approx. 26% sand, 23% silt, 51% clay). The grain cart was equipped with 23.1-26 bias-ply tires inflated to 210 kPa. Trafficking was repeated to provide from one to four passes in the same track. The degree of saturation at the time of trafficking was 72% in the top 0.2 m. In the portion of the study presented here, shallow tillage with a chisel plow was performed prior to the compaction. One year after the compaction, the soil properties beneath the center of the tire tracks and in the untrafficked areas between tracks were measured using the SP2MS. Three replicated measurements at each depth were taken in each wheel track and between each wheel track. A total of 60 samples were analyzed using the Sp2MS, 30 samples in each of the trafficked and untrafficked areas. Three replicated cone penetration tests were also performed in the trafficked and untracked areas. Air permeability, air-filled porosity, bulk density, cone index and moisture content data for the top 0.5 m were analyzed to determine how the soil responded to the grain cart traffic. Results and discussion The following results and discussion are divided into two sections. The first section describes the laboratory results for the Blount soil for the purpose of validating the measurement procedures and their accuracies. Similar results for the Rossburg soil are presented in Morgan (1992). The second section describes the field experiment results. These results are discussed merely as an example of the kind of data which can be collected in the field. For further discussion of the field results see Wood et al. ( 1992 ).

Laboratory results Table 2 compares the means of the bulk densities of the core samples with the bulk densities of their respective parent samples. All of the means of the core samples were within 0.02 Mg m -3 of the bulk density of their parent samples for the Blount soil. This agreement illustrates that the measurements of the core sample weight and volume were within the desired accuracy. Table 2 also lists the standard deviations of the bulk densities for the eight core samples from each parent sample. From this table, the m a x i m u m standard deviation of the bulk density data is 0.019 Mg m - 3. These standard deviations include effects from variability within the parent sample and mea-

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317

Table 2 Bulk densities measured using Sp2MS (average of eight cores) compared with bulk density of parent sample for Blount soils Applied stress (kPa, peak)

86 172 345 86 172 345 86 172 345 86 172 345

Moisture content (kg kg-l )

0.184 0.179 0.184 0.209 0.195 0.207 0.248 0.252 0.248 0.277 0.290 0.278

Bulk density Core samples (Mgm -3) (Std.)

Parent sample (Mgm -3)

1.14 (0.011 ) 1.26 (0.013) 1.36 (0.013) 1.21 (0.013) 1.29 (0.017) 1.37 (0.009) 1.28 (0.013) 1.35 (0.011 ) 1.47 (0.017) 1.35 (0.012) 1.39 (0.017 ) 1.43 (0.019)

1.14 1.27 1.38 1.20 1.31 1.39 1.27 1.37 1.48 1.34 1.40 1.44

Table 3 Air-filled porosities measured using Sp2MS (average of eight cores ) compared with air-filled porosity of parent sample (calculated from bulk density and particle density) for Blount soils Applied stress (kPa, peak )

86 172 345 86 172 345 86 172 345 86 172 345

Moisture content (kg kg- ~)

0.184 0.179 0.184 0.209 0.195 0.207 0.248 0.252 0.248 0.277 0.290 0.278

Air-filled porosity Core samples (%, v/v) (Std.)

Parent sample (%, v/v)

35.9 (0.51) 28.7 (0.71) 22.5 (0.87) 29.1 ( 1.04) 25.6 (1.42) 19.2 (0.67) 19.8 (0.49) 13.9 (0.72) 6.4 (0.80) 10.9 (0.93) 6.1 (1.25) 4.7 (1.15)

36.0 29.3 22.4 29.6 25.1 19.0 20.5 13.4 7.1 12.0 6.3 5.8

surement errors. Thus, the magnitude o f these values substantiate that the parent samples were very uniform and the effect o f compounded measurement errors was small. Table 3 compares the means o f the air-filled porosities o f the eight core

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M.T. Morgan et al. / Soil & Tillage Research 26 (1993) 301-325

samples with the air-filled porosities of their respective parent samples. The air-filled porosities measured using the Sp2MS, were in close agreement with the values calculated for the parent samples. However, the air-filled porosity measurement requires accurate measures of both bulk volume and air-filled volume. Therefore, compounding errors from each of these measurements resulted in an accuracy of approximately _+ 1% (0.01 m 3 m -a) air-filled porosity. This corresponds to relatively high percentage errors, up to 19%, for samples with very low air-filled porosities. Table 3 also lists the standard deviations of the air-filled porosities for the eight cores from each parent sample. The low standard deviations, similarly as those for bulk density, indicate that the parent samples' air-filled porosities were fairly uniform and that the errors in the bulk volume and air-filled volume measurements were fairly low. However, the magnitude of each of these effects cannot be separated. From this table, 75% of the standard deviations were less than 1.0% air-filled porosity with a maximum of only 1.42% airfilled porosity. Fig. 10 shows the relationship between bulk density and total porosity for the soil cores from the Blount soil. This comparison is significant because each of the measurements were made independently, i.e. air-filled porosity was measured using the method described earlier. Theoretically, the relationship is linear with a slope and intercept equal to the particle density of the 1.55

1.51.45-

E

1.4-

ne

v

. # 1.35-

Regresso inline ~

{,/')

c D

1.3-

~

~ 1.251.2~ 1.15.

1.142

52

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TotaP l orosty i(%) Mosiu tre(w/w) •

18%

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)~ 2 5 %

[]

54

58

28% ]

Fig. 10. Relationshipbetweenbulk density and total porosity for Blount soil. Each symbolrepresents averageof eight cores.

M. T Morgan et al. / Soil & Tillage Research 26 (1993) 301-325

319

soil. Comparing the data with the theoretical lines shows that there is relatively good agreement. Although trends in the data show that the measured particle densities, slopes of the regression lines fit to the data, are lower than expected. The slope for the Blount data (Fig. 10) is 2.45 whereas, the average particle density, calculated using the measured bulk densities and total porosities of individual samples, was 2.61 (Mg m - 3 ) and the particle density measured using water pycnometer was 2.65 Mg m -3. The difference in the estimates of particle densities may also be due to the statistical nature of errors in the observations. Kempthorne and Allmaras (1986) discuss the fact that the slope from regressing two variables y on x will be less than the true slope by the ratio ax/2 (ax2 + a?2 ) where a~2 is the variance of the x's and a~ is the variance of the measurement errors ofx's. This phenomenon is referred to as attenuation. Another possible explanation for the differences between particle density estimates, although not experimentally verified, is the presence of a systematic error in the air-filled porosity measurement which increases for lower porosity samples. This error may be due to a temperature effect in the airfilled porosity device. As the air around a sample is pressurized to 550 kPa the air temperature rises slightly. This temperature rise increases as the porosity of the samples decreases. An air temperature difference between the measurements of P~ and P2, (Eq. ( 3 ) ) will make P2 artificially higher. As increasing P2 decreases Vp (pore volume), the air-filled porosity measurement will be lower than the actual air-filled porosity of the sample, thus, producing an error. During field testing, the particle density of each sample was calculated using the bulk density, air-filled porosity and moisture content. This check in the system helped to determine if any significant errors had occurred in either the sample weight, volume, air-filled porosity or moisture content measurements. Thus, problems with a particular sample or measurement device could quickly be identified reducing the possibility of erroneous data. Table 4 compares the logarithmic means of the air permeabilities of the core samples with that of their respective parent sample. The use of the logarithmic means for air permeability is discussed in more detail in Morgan (1992). Table 4 also lists the logarithmic standard deviations for the air permeability measurements. Extraneous values in the air permeability data were evaluated using the Q-test described by Dixon ( 1986 ). The outliers were omitted from calculation of the means based on 90% confidence. Most of the core sample means were within one order of magnitude of the parent values except for very low permeable samples. One explanation for the larger values for the core samples could be air leakage around the core in the sample tubes. This leakage would be more significant for the core samples than for the larger parent cores owing to the ratio of perimeter to cross sectional area and would become more significant for very low permeabilities.

320

M.T. Morgan et al. ~Soil & Tillage Research 26 (1993) 301-325

Table 4 Intrinsic air permeabilities measured using Sp2MS (log means of eight cores ) compared with intrinsic air permeabilities of parent samples for Blount soils Applied stress (kPa, peak)

Moisture content (kg kg -1 )

Intrinsic air permeability Core samples

86 172 345 86 172 345 86 172 345 86 172 345

0.184 0.179 0.184 0.209 0.195 0.207 0.248 0.252 0.248 0.277 0.290 0.278

Parent sample

(~2)

Log mean (~2)

Std. (In ~2 )

20.9 3.72 0.85 6.88 2.42 1.17 3.91 1.26 0.009 1.05 0.034 0.014

(0.31) (0.17) (0.35) (0.17 ) (0.31) (0.29) (0.35) (0.44) (0.80) (0.45) (0.81 ) (1.30)

8.14 4.00 0.62 6.92 1.95 1.25 3.90 0.63 0.007 1.07 0.039 0.005

Other aspects of the evaluation of the Sp2MS are the time and labor required to measure the soil properties using the procedures described above. For these laboratory tests, most of the time required for collecting the data was attributed to the preparation and measurement of the parent samples. For each parent sample, the amount of time required for this portion was approximately 2.5 h. The actual measurement of the core sample properties using the Soil Physical Properties Measurement System was typically performed in 10-15 rain. This can be thought of as approximately 90 s to measure all of the properties; volume, weight, air flow rate and pressure drop and air-filled porosity for each sample. This short measurement time illustrates the advantage of this system over more conventional laboratory techniques. Automating much of the sampling and measurement procedures significanlly reduced the labor requirements, operator biases and opportunities for gross errors in the data. Storage of the data on computer diskettes at the time of measurement also reduced the analysis time. Field results Figures 11-14 show the measured soil properties of bulk density, air-filled porosity, air permeability, and cone index, as affected by four passes of the 15.2 t grain cart. Each point on the curves in Figs. 11-13 represents the average of three replicates with error bars indicating _+SE. For the air permeabil-

321

M. 72 Morgan et al. /Soil & Tillage Research 26 (1993) 301-325

1.6 1.55~E ~

1.51.451.4-

Z uJ 1.35121

x-

1.3-

m 1.25............

,**'""

0.05

011

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I -~-

0.'15

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4 PASSES

0'.4

0.&5

0.5

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Fig. 1 1. Effects of wheel traffic, four passes, by a 15.2 t axle load grain cart on the bulk density of Celina soil, error b a r s = _+SE. 30 25-

200

15

ul

~ lO 5, 0

0

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0.i5

0'.2 0.~5 0'.3 0.35

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SOIL DEPTH (m)

[- i -

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--~-. UNTRAFFICKED ]

Fig. 12. Effects of wheel traffic, four passes, by a 15.2 t axle load grain cart on the air-filled porosity of Celina soil, error bars = _+SE.

322

M.T. Morgan et aL / Soil & Tillage Research 26 (1993) 301-325

80 70-

-::~ 60v

...J

~4o

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tr

w 30

o.,. n"

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~: 20

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0'.4

0.~,5

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Fig. 13. Effects of wheel traffic, four passes, by a 15.2 t axle load grain cart on the air permeability of Celina soil, error b a r s = _+SE. 3.0

2.5

45

~.L

'40

a. 2.0

.<

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--UN~.~F~KEO 1

Fig. 14. Effects of wheel traffic, four passes, by a 15.2 t axle load grain cart on the cone index of Celina soil, error b a r s = + SE.

M.T. Morgan et al. /Soil & Tillage Research 26 (1993) 301-325

32 3

ity curves (Fig. 13 ), each point represents the logarithmic mean of three replicates. For the cone index curves in Fig. 14, these points represent the average of approximately four readings per 25.4 m m depth averaged for three replicates. Comparing the two curves on each graph shows how the bulk density and cone index were increased and the air-filled porosity and air permeability decreased as a result of the wheel traffic. In each graph, the effect of the grain cart traffic is evident down to approximately 0.4 m. In general, the effects were statistically significant at a = 0.10 down to this depth, also. The trends in this data are very similar to results by Voorhees et al. (1986), and are discussed in more detail by Wood et al. ( 1992 ). The time required for the measurement of the properties using the Sp2MS in the field, averaged 15 min for ten samples. This measurement time was approximately the same as for the laboratory portion of this study which indicates that the system performed equally well using field samples. Conclusions The results of both the laboratory and field testing of the Soil Physical Properties Measurement System confirm that the system can rapidly measure the soil properties of bulk density, air-filled porosity, intrinsic air permeability, cone index and moisture content. The average time required in the field to collect ten samples, 47.6 m m in diameter by 50 m m in length, is approximately 10 min. The average time required for the system to measure the above properties on ten samples is approximately 15 min. From calibration results and laboratory testing, typical accuracies which can be expected for properties measured using the system are: bulk density within _+0.02 Mg m -a, air-filled porosity within _+2% v/v, intrinsic air permeability within 6% error, and moisture content within _+ 1.5% v/v. These accuracies take into account compounding sources of error from the measurements associated with each property. For example, the accuracy of _+0.02 Mg m-3 for bulk density takes into account possible errors from the weight, volume and moisture measurements. Similar accuracies were found when comparing the average bulk densities and air-filled porosities for each of the eight core samples with the properties for the parent samples from which they were taken. However, the differences in the intrinsic air permeabilities between the parent samples and the logarithmic means of the core samples were much greater. Core sample intrinsic air permeabilities were within an order of magnitude of the parent sample values. The results from the field compaction study showed that the system was capable of measuring the soil properties directly in the field. The bulk density, air-filled porosity and intrinsic air permeability data from this field experi-

324

M. 1~ Morgan et al. ~Soil & Tillage Research 26 (1993) 301-325

merit showed that one year after the application o f a 15.2 t axle load on the soil, significant effects were present down to a depth of approximately 0.4 m.

References American Society of Agricultural Engineers, 1991. Soil cone penetrometer. ASAE Standard 5313.2 ASAE Standards, St. Joseph, MI, p. 591. ASTM Committee D-18 on Soil and Rock for Engineering Purposes, 1964. Procedures for testing soils; nomenclature and tentative methods, proposed and suggested methods. Am. Soc. Testing and Materials, PA, 540 pp. Blake, G.R., 1965. Bulk Density. Paper No. 4433. Scientific Journal Series, Minnesota Agric. Exp. Stn., St. Paul, MN, pp. 374-390. Blake, G.R. and Hartge, K.H., 1986. Bulk Density. In: A. Klute (Editor), Methods of Soil Analysis. Am. Soc. Agron., 2nd Edn., Agron. No.9 (Part I), pp. 363-375. Blake, G.R., Nelson, W.W. and R.R., 1976. Persistence of subsoil compaction in a Mollisol. Soil Sci. Soc. Am. J., 40: 943-948. Brunfeldt, D.R, 1987. Theory and design of a field-portable dielectric measurement system. IEEE Int. Geosci. Remote Sensing Symp. Dig., 1: 559-563. Danielson, R.E. and Sutherland, P.L., 1986. Porosity. In: A. Klute (Editor), Methods of Soil Analysis, Am. Soc. Agron., 2nd Edn., Agron. No.9 (Part I), pp. 443-461. Digital Products, 1985. Veeder Root Installation, Operation and Prog Manual. Manual No. 576013-203. Hartford, CT, 21 pp. Dixon, W.J., 1986. Extraneous Values. In: A. Klute (Editor), Methods of Soil Analysis. Am. Soc. Agron., 2nd Edn., Agron. No.9 (Part I), pp. 83-90. Erbach, D.C., 1982. State of the art of soil density measurements. ASAE Paper No. 82-1541. Am. Soc. Agric. Eng., St. Joseph, MI, 11 pp. Freitag, D.R., 1971. Methods of measuring soil compaction. In: K.K. Barnes, W.M. Carleton, H.M. Taylor, R.I. Throckmorton and GE. Vandenberg (Editors), Compaction of Agricultural Soils. Am. Soc. Agric. Eng., St. Joseph, MJ, pp. 47-103. Hallikainen, M.T., Ulaby, F.T. and Dobson, M.C., 1985. Microwave dielectric behavior of wet soil. Part I: empirical models and experimental observations from 1.4 to 18 GHz. IBEE Trans. Geosci. Remote Sensing, GE-23: 25-34. Jackson, T.J., 1990. Laboratory evaluation of a field-portable dielectric, soil-moisture probe. IEEE Trans. Geosci. Remote Sensing, 28:241-245. Kempthorne, O. and Allmaras, R.R., 1986. Errors and variability of observations. In: A. Klute (Editor), Methods of Soil Analysis. Am. Soc. Agron., 2nd Edn., Agron. No.9 (Part I), pp. 1-31. Kirkham, D., 1946. Field method for determinations of air permeability of soil in its undisturbed state. Soil Sci. Soc. Am. Proc., 2: 93-99. Morgan, M.T., 1988. A soil compaction evaluation system for rapid field measurement of porosity, density, air permeability, and cone index. M.S. Thesis. The Ohio State University, Columbus, OH, (unpublished), 219 pp. Morgan, M.T., 1992. Evaluation of a Soil Physical Properties Measurement System. Ph.D. Thesis. The Ohio State University, Columbus, OH, (unpublished), 180 pp. Morgan, M.T., Wood, R.K. and Holmes, R.G., 1991. Dielectric moisture measurement of soil cores. ASAE Paper No. 91-1528. Am. Soc. Agric. Eng., St. Joseph, MI, 18 pp. Nau, K.R., 1987. Air Permeability: A measure of soil compaction. M.S. Thesis. The Ohio State University, Columbus, OH, (unpublished), 133 pp.

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32 5

Perumpral, I.V., 1987. Cone penetrometer applications - a review. Trans. ASAE, 30 (4): 939944. Rees, D.V.H., Audsley, E. and Neale, M.A., 1982. Apparatus for obtaining an undisturbed core of silage and for measuring the porosity and gas diffusion of the sample. Br. Soc. Res. Agric. Eng., 28: 107, 114. Soane, B.D., 1974. A test vehicle for the rapid determination of soil physical properties in the field. Trans. 10th Int. Cong. Int. Soil Sci. Soc., 1: 363-368. Soane, B.D., Blackwell, P.S., Dickson, J.W. and Painter, D.J., 1981. Compaction by agricultural vehicles: a review. I. Soil and wheel characteristics. Soil Tillage Res., 1: 207-237. Voorhees, W.B., Senst, C.G. and Nelson, W.W., 1978. Compaction and soil structure modifications by wheel traffic in the northern Corn Belt. Soil Sci. Soc. Am. J., 42: 344-349. Voorhees, W.B., Nelson, W.W. and Randall, G.W., 1986. Extent and persistence of subsoil compaction caused by heavy axle loads. Soil Sci. Soc. Am. J., 50: 428-433. Wood, R.K., Reeder, R.C., Morgan, M.T. and Holmes, R.G., 1992. Soil physical properties as affected by grain cart traffic. ASAE Paper No. 91-1089. Am. Soc. Agric. Eng., St. Joseph, MI, 12 pp.