DIAS report

Lars J. Munkholm, Jørgen Berntsen & Flemming Steffensen

Plant Production no. 83 ◆ December 2002

A fully automated micropenetrometerfor sampling in a three-dimensional grid

Ministry of Food, Agriculture and Fisheries

Danish Institute of Agricultural Sciences

Lars J. Munkholm, Jørgen Berntsen & Flemming SteffensenDepartment of Crop Physiology and Soil ScienceP. O. Box 50DK-8830 Tjele

DIAS report Plant Production no. 83 • December 2002

DIAS reports primarily contain research results andtrial statements aimed at Danish conditions. Also,the reports describe larger completed research pro-jects or acts as an appendix at meetings and con-ferences. DIAS reports are published in the series:Plant Production, Animal Husbandry and Horticulture.

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ISSN 1397-9884

A fully automated micropenetrometerfor sampling in a three-dimensional grid

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Preface A fully automated micropenetrometer for sampling in a three-dimensional grid was developed at the Danish Institute of Agricultural Sciences (DIAS). The micropenetrometer was developed for the purpose of studying spatial effects of different direct drilling methods on soil structure in the vicinity of the seed. However the system may be used in many other studies where small-scale information on soil strength is needed. The micropenetrometer was built to enable tests to be performed in a square grid of variable size at a high depth resolution and for variable sample sizes. Thanks to Per Schjønning, DIAS, for fruitful discussions during the design phase of the micropenetrometer. Also thanks to Tim Grønne Müller, DIAS, for technical assistance in developing the instrument. Lastly, thanks to Bodil B. Christensen, DIAS, for doing most of the measurements in the laboratory and to Anne Sehested, DIAS, for working with the layout of this document.

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Contents Summary.....................................................................................................................................5 Sammendrag ...............................................................................................................................7 1. Introduction ............................................................................................................................9 2. State of the art.........................................................................................................................9

2.1 Micropenetration in soil ....................................................................................................9 2.2 Data analysis ...................................................................................................................10

3. The micropenetrometer system ............................................................................................11 3.1 Drive system and x-y table..............................................................................................11 3.2 Sensor ..............................................................................................................................12 3.3 Control and data acquisition............................................................................................13 3.4 Calibration.......................................................................................................................14

4. Measurements.......................................................................................................................14 4.1 Soil core sampling...........................................................................................................14 4.2 Procedures when running analysis ..................................................................................15 4.3 Layering ..........................................................................................................................16

5. Dataanalysis..........................................................................................................................16 5.1 Handling stones ...............................................................................................................16 5.2 Piecing layers together ....................................................................................................16

6. Results and discussion ..........................................................................................................18 6.1 An example .....................................................................................................................18 6.2 Performance ....................................................................................................................20 6.3 Applications ....................................................................................................................21

7. Conclusions ..........................................................................................................................21 8. References ............................................................................................................................22

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Summary Spatial variability in soil mechanical strength influences the conditions for root growth. Micropenetrometers have been used to study tillage and traffic effects on soil rootability. However, only few appear to have used them to analyse small scale spatial effects based on a grid point sampling system. The present study was initiated to measure effects of different direct seeding techniques on the soil structure in the near seed environment. The objective of this research was to design a micropenetrometer for measuring penetration resistance at high spatial resolution. The aim was to enable measurements at a spacing between points down to 10 mm in a square grid in the x-y direction and to approximately 100 mm depth. The system was requested to be fully automated to yield a low labour requirement. The micropenetrometer was constructed by modifying an Alwetron T100 mechanical press that operates with a steady loading rate. An x-y table placed on top of the mechanical press served as the base on which to place cylindrical soil samples to be moved around in a selected square grid. Samples with a maximum of 200 mm in diameter could be used. The precision of finding a random position was <0.1 mm in both directions. The penetrometer probe consisted of a 30° semi-angle stainless steel cone (diameter 3.0 mm) placed on a shaft made of tungsten (diameter: 1.7 mm, length: approximately 120 mm). A micro-controller circuit was developed to control the position of the x-y table. A LabView program was developed to communicate with the x-y table controller, control the movement of the probe, calibration and collection of raw force data. The micropenetrometer was used for testing columns of undisturbed soil (180 mm diameter, 200 mm height) taken from the 0-200 mm layer of a sandy loam soil used in a field experiment on technology for row cropping in ploughless soil tillage systems. Direct drilling systems using either a single disc coulter (DD-D) or a chisel coulter (DD-C) were applied and a traditional ploughing-harrowing tillage system (PL) was used as reference. The samples were tested in a 10*10 mm x-y grid. The probe was pressed into the soil at a constant loading rate of 10 mm min-1. The samples were divided into three layers (0-60 mm, 55-115 mm and 110-180 mm). For a 180 mm diameter soil sample with sampling in 10*10 mm grid, one series of measurements to 60-70 mm depth lasted between 20 and 24 hours. The soil samples were sealed with aluminium foil to minimize evaporation during measurement. In the dataanalysis the penetration resistance data from the different layers were pieced together. However, recordings from the first 15 mm were excluded for the middle and bottom layer due to unreliable measurements. The micropenetrometer proved to be a powerful tool in detecting variations in mechanical resistance with a high spatial resolution. The micropenetration results showed that both direct drilling techniques produced potentially root-restricting compaction in the lower part of the arable layer. This was in accordance with field penetration resistance results (Munkholm et al., 2003). However much better spatial resolution was obtained from the micropenetrometer

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data than from the field data employing an automated cone penetrometer using a 20.27 mm diameter / 60o semi-angle. The micropenetration data suggested that the compaction effect of DD-D might be more important than the field tests indicate. Median values were calculated for each x position (perpendicular to the seeding row). More than 50% of these values at 40 to c. 160 mm depth exceeded 2 MPa for the DD-D treatment in comparison with 25 and 7% for DD-C and PL, respectively. The micropenetrometer was easy to operate and required a low input of manpower. The water loss during testing was acceptable. For the tested samples, on average 0.3% w/w total soil weight was lost during testing (standard deviation=0.3% w/w). As the soil water content of the samples were 12% w/w in average this corresponded to a loss of approximately 3% of the initial soil water content during one series of measurements.

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Sammendrag Rumlig variabilitet af jordstyrken har stor betydning for rodvækst i jorden. Mikropenetrometre er blevet brugt til at bestemme effekter af jordbearbejdning og trafikbelastning på betingelserne for rodvækst i jorden. Der er dog kun få, der har målt penetreringsmodstanden med veldefineret afstand imellem målepunkter med henblik på at undersøge rumlige effekter på en lille skala. Arbejdet med at opbygge et mikropenetrometer blev igangsat med henblik på at måle effekten af forskellig såteknik til direkte såning på jordens struktur i nærheden af de udsåede frø. Målet var at opbygge udstyr til at måle penetreringsmodstand med høj rumlig opløselighed. Vi ønskede at kunne måle ned til 10 mm mellem målepunkterne og til omkring 100 mm dybde. Det var endvidere et krav, at mikropenetrometeret skulle være fuldautomatisk, let at betjene og derved kræve et lille forbrug af arbejdskraft. Mikropenetrometeret består af en ombygget Alwetron T100 mekanisk presse, der kører med en meget stabil hastighed. Et x-y bord blev monteret på pressen. Cylindriske jordprøver kunne placeres på dette og bevæges rundt i et fastsat kvadratisk mønster. Prøver op til 200 mm i diameter kunne anvendes. Et tilfældigt punkt i x-y mønstret kunne genfindes med under 0.1 mm nøjagtighed i begge retninger. Penetrometer sonden bestod af en 30° rustfri stål konus med en diameter på 3.0 mm. Sonden var sat på et 1.7 mm tykt og 120 mm langt skaft af wolfram. Der blev udviklet en micro-controller enhed til styring af x-y positionen og et LabView program til at kommunikere med micro-controlleren, bevæge sonden op og ned, kalibrering samt opsamling af data. Til at teste systemet blev der udtaget uforstyrrede jordprøver (180 mm diameter, 200 mm højde) fra 0-200 mm laget i et markforsøg med forskellig teknik til rækkedyrkning under pløjefri forhold beliggende på en sandblandet lerjord. Direkte såning med henholdsvis skive- og tandskær (DD-D, DD-C) blev sammenlignet med konventionel pløjning, såbedstilberedning og såning (PL). Jordprøverne blev penetreret i et 10*10 mm kvadratisk gitter. Sonden blev trykket ned i jorden med en konstant hastighed af 10 mm min-1. Prøverne blev inddelt i tre lag (0-60 mm, 55-115 mm og 110-118 mm). En måleserie til 60-70 mm tog mellem 20 og 24 timer. For at minimere fordampningstabet under kørslen blev prøverne dækket med et lag aluminiumfolie. Ved data-analysen blev penetreringsmodstanden for profilerne sammensat udfra målingerne fra de enkelte lag. Dog blev målingerne fra de øverste 15 mm af 55-115 og 110-180 mm lagene udelukket pga. upålidelige resultater. Mikropenetrometeret var et godt værktøj til måling af variationer i mekanisk modstand med høj rumlig opløselighed. Resultaterne viste, at begge metoder til direkte såning gav potentielt rodhæmmende pakning under sådybden. Dette var i overensstemmelse med feltresultater. Mikropenetreringsmålingerne tydede dog på, at pakningseffekten af DD-D var større end feltresultaterne angav. Median værdier blev beregnet for hver x-position (dvs. for hver målt

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afstand til sårækken). Mere end 50% af median værdierne for 40-160 mm laget var større end 2 MPa for DD-D til sammenligning med 25 og 7% for henholdsvis DD-C og PL. Mikropenetrometeret var let at betjene og krævede kun en lille indsats af arbejdskraft. Vi brugte ca. 30 minutter mellem hver kørsel til vejning, afskæring af nyt lag, etc. Fordampningen af vand under kørsel var på et acceptabelt niveau. For hver måleserie var tabet i gennemsnit 0.3% af totalvægten (standardafvigelse 0.3%). Prøverne indeholdt i gennemsnit 12% w/w vand og det betyder at fordampningstabet var på omkring 3% af det oprindelige vandindhold.

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1. Introduction Spatial variability in soil mechanical strength influences the conditions for root growth (e.g. Dexter, 1987). Many researchers have found that soil management affects the soil mechanical strength with respect to both average bulk soil properties and the spatial variability of these physical properties (e.g. Gronevelt et al., 1984, Schjønning and Rasmussen, 1989, 2000; Hadas and Shmulewich, 1990). Micropenetrometers have been used to study tillage and traffic effects on soil penetration resistance to yield information on soil rootability (Baker and Mai, 1982a,b; Schjønning and Rasmussen, 1989, 2000; Hadas and Shmulewich, 1990; Lipiec and Usowicz, 1997). The present study was initiated to enable measurements of effects of different direct seeding techniques on soil structure in the near seed environment (Munkholm et al., 2003). The objective was to design a micropenetrometer for measuring penetration resistance at high spatial resolution. We wanted to be able to measure at a minimum of 10 mm spacing between grid points in a square grid in the x-y directions and to approximately 100 mm depth. The system should be fully automated and thereby result in a low labour requirement. 2. State of the art 2.1 Micropenetration in soil Micropenetration has been used to evaluate soil resistance to root growth even though there is fundamental difference between the penetration of a fine probe and the growth of roots. Mechanical penetration with a metal probe cannot totally simulate the complex process of root growth in soil. However, earlier studies have shown that penetration resistance of fine probes gives a good indication of a soil resistance to root growth (Bradford et al. 1971; Voorhees et al., 1975; Bradford, 1980). In general, a curvilinear decrease in root elongation with increased penetration resistance has been found (Bennie, 1996). The critical penetration resistance to root growth depends on texture and macrostructure as well as plant species (Bennie, 1996). In a review Bengough and Mullins (1990) reported that root elongation stopped in soil with penetrometer resistance between 0.8 and 5.0 MPa. Boone et al. (1986) proposed penetration resistances of 1.5 or 3.0 MPa as critical lower and upper limits, respectively. There is a general lack of standardization for micropenetrometers with very little consistency regarding probe size and design, and loading rate used by researchers in the past. This reflects that micropenetrometers have been designed for many different purposes. The probe size has varied in diameter from as little as 0.150 mm (Groenevelt et al., 1984, Grant et al., 1985) to 5 mm (e.g. Voorhees et al., 1975). Most researchers have used probe diameters in the range 1-5 mm (Chi and Tessier, 1995). Groenevelt et al. (1984) argued that a small probe size approaching the diameter of a growing root was desirable. However, finding a material that is

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able to withstand soil stresses without breaking or bending when penetrating to several cm depth may be a problem. In the range 2-5 mm in diameter Voorhees et al. (1975) showed that probe diameter had no significant effect on penetration resistance. Many researchers have used blunt probes (60-90º angle) (Chi and Tessier, 1995) but a 30º angle has also been used in a number of studies (e.g. Whiteley et al.1981; Schjønning and Rasmussen, 1989; Lipiec and Usowicz, 1997). A reason for using an angle of 30º is that minimum penetration resistance has been found for cone angles of 20-40º (Koolen and Vaandrager, 1984). However, larger cone angles may be preferable as a blunt cone may to a greater extent simulate the penetration of a growing root (Farrell and Greacen, 1966). The difference between cone and shaft diameter is also of particular importance as shaft friction can contribute significantly to the recorded resistance (Campbell and O’Sullivan, 1991). In general, a significant difference between shaft and cone diameter is preferred. However, even when this is the case, shaft friction may be of significance when penetrating to large depths using a probe with a flexible shaft. For probes with no difference between shaft and cone diameter, it is recommended to record penetration resistance at a specific depth. Whiteley et al. (1981) recommended that penetration resistance should be read at 4x cone diameter depth for such cones. Some have recorded penetration resistance over a larger depth interval for probes with similar cone and shaft diameter and used the readings at probe withdrawal to correct for shaft friction (e.g. Groenevelt et al. 1984). There is also a significant inconsistency regarding loading rates in the literature. Very low loading rates have been used in some studies to simulate the behaviour of a growing root that elongates at a rate of approximately 1 mm h-1 (Waldron and Constantin, 1970). However, this is an impractical time-consuming loading rate and therefore most researchers have used loading rates that are orders of magnitudes larger (Chi and Tessier, 1995). Bradford (1986) recommended 0.5 mm min-1 for laboratory studies and 50 mm min-1 for field studies. Whiteley et al. (1981) used a loading rate of 3 mm min-1 for a laboratory study and Baker and Mai (1982a) used a loading rate of 20 mm min-1. The difference in loading rate applied in the studies in the past makes it difficult to compare results from different investigations. Moreover, past studies have reported conflicting results on the effect of loading rate as reviewed by Chi and Tessier (1995). 2.2 Data analysis Highly valuable considerations on interpretation and presentation on penetration resistance data have been presented by O’Sullivan et al. (1987) and Campbell and O’Sullivan (1991). For stone-rich soils systematic procedures are needed to take account of missing data and unrealistically high values. O’Sullivan et al. (1987) and Glasbey and O’Sullivan (1988) discuss different procedures. Unreliably low penetration resistances are usually recorded at the top of a profile (Bradford, 1980). This is due to lack of full contact between cone and that soil compacts increasingly ahead of the probe until a certain depth (Gill, 1968). For soil

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penetrated in consecutive layers, standard procedures are needed to deal with this problem (see chapter 5.2). Penetration resistance recorded in a grid or along a transect allow data analysis of the spatial variability. Data analysis methods on such datasets have varied from using classical statistics and presenting data in e.g. contour diagrams (Tessier et al., 1991; Chi and Tessier, 1995) to much more sophisticated data analysis methods using geostatistics (O’Sullivan et al., 1987; Lipiec and Usowicz, 1997) or applying spectral analysis (Grant et al., 1985; Hadas and Shmulewich, 1990). The former is a powerful tool when the objective is to present easily understandable effects of management. Geostatistical dataanalysis allows detailed analysis on spatial dependency of penetration resistance (O’Sullivan et al., 1987; Lipiec and Usowicz, 1997). Spectral analysis may provide information on the spatial distribution of clods in the soil as demonstrated by Grant et al. (1985) and Hadas and Shmulewich (1990). 3. The micropenetrometer system 3.1 Drive system and x-y table The micropenetrometer was constructed by modifying an Alwetron T100 mechanical press that operates with a steady loading rate (Figure 1). A x-y table was placed on the mechanical press. Cylindrical soil samples were placed on top of the x-y table and moved around in a selected grid. The force transducer and the penetrometer probe were mounted on a steel arm connected to the mechanical press.

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Figure 1. Overview of the micropenetrometer. A: Computer screen, B: Mechanical press, C: Arm with pressure transducer mounted at the end, D: Soil sample, E: x-y table. The x-y table consisted of a plate that was moveable in the x and y directions within a steel frame. Two actuators (LA30.1, LINAK) were used to move the plate in respectively the x and y direction. The precision of finding a random position was better than 0.1 mm in both directions. This was achieved by always moving in the same direction towards the random position from an offset point fixed at one of the corner co-ordinates within the frame. The soil sample was fixed on the moveable plate by the use of adjustable clamps. Samples with maximum 200 mm in diameter could be used. 3.2 Sensor A force transducer (Hottinger Baldwin Messtechnik S2, 100 N) was mounted on the arm connected to the mechanical press. The HBM S2 transducer was selected because of its high transverse force stability (effect of transverse force 10% of nominal force) and high accuracy (accuracy class 0.05). A penetrometer probe was screwed onto the force transducer. The penetrometer probe consisted of a 30° semi-angle stainless steel cone (diameter 3.0 mm) placed on a shaft made of tungsten (diameter: 1.7 mm, length: approximately 120 mm) (Figure 2).

A B

CA

D

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Figure 2. Penetrometer probe. Left A: HBM S2 pressure transducer, B: penetrometer probe. Right Close up photo of the 3.0 mm 30º semiangle stainless steel cone placed on a 1.7 mm shaft. 3.3 Control and data acquisition A LabView program was developed with the purpose of controlling the position of the x-y table, the movement of the probe, calibration and collection of raw force data. The x-y data grid points were arranged in a square grid. The spacing, soil core diameter, and the distance between cylinder wall and the nearest sampling points were user controlled (Table 1). When starting a series of measurements the probe was initially moved to the “upper” left corner of the table (reference point). The probe was then moved to the first sampling point, which was the “upper” right point within the sample. After finishing penetration at the first grid point the Table 1. Adjustable settings in the LabView programme.

Parameter Range Used values

Radius of cylinder 0-100 mm 90/100 mm Penetration depth 0-100 mm 60/70 mm Grid spacing 5-50 mm 10 mm Band1 0-50 mm 20 mm Overload 0- 4.5 kg (=6.25 MPa penetration resistance) Samplings per mm 0- 2 Gear2 1-3 3 Speed 1-4 1 1band = smallest distance between grid points and cylinder wall 2The rate of loading is set by selecting main gear (3 levels) and speed level (4 options) within main gear.

A

B

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probe was set to move back to the reference point and from there to the next grid point in the y-direction beside the first grid point. After finishing one row in the y-direction the instrument started measuring in the adjacent row. The software controlled both the speed of the probe and whether the probe was moved up- or downward. When a profile is made the probe moves according to the following pattern: • Fast downward (100 mm min-1) to a position controlled by a mechanical switch. • Slowly downward (adjustable loading rate) into the soil to a given depth specified by the

user (10 mm min-1 used). • Fast upward (100 mm min-1) to mechanical switch. • Move further 1 second at high speed. Data collection was only performed during the slow movement. The loading rate during the slow movement was arbitrarily fixed at 10 mm min-1 to allow many profiles to be penetrated within a reasonable time period. If the force is greater than a user specified overload limit (e.g. when hitting a stone) during the slow movement, the rest of the step is skipped and the probe is withdrawn. Thereafter, the penetration continues at the next grid point. The user specifies the number of samples per mm. Data from each profile is stored in a datafile for later processing. The depth (z) of penetration was determined indirectly from the time elapsed since the start of recording at a known z-value. Pilot studies showed that a constant loading rate could be assumed during measurements. 3.4 Calibration Loading with a fixed load performed the calibration of the probe and a linear calibration line was determined between the measured input from the transducer in volt and the known loading. Known loadings were subsequently used to validate the calibration curve. The linear regression was used to convert raw signals in volt to a calibrated force during the data collection. 4. Measurements 4.1 Soil core sampling Soil cores were sampled in spring 2000 in a field trial on technology for row growing in ploughless soil tillage systems (Jørgensen and Rasmussen, 2000). The experiment was established in 1999 at Bygholm Research Centre on a sandy loam that is classified as a Glossic Phaeozem (WRB 1998). The tillage treatments were applied to 6m x 20 m plots in a randomised block design with four replicates. Direct drilling systems using either a single disc coulter (DD-D) or a chisel coulter (DD-C) were applied and referenced by a traditional ploughing-harrowing tillage system (PL) (Jørgensen and Rasmussen, 2000). Undisturbed soil columns (180 mm diameter, 200 mm height) were taken from the 0-200 mm depth. The

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cylinders were held in position by a special flange ensuring vertical sampling and retrieved by gently hammering. One column was collected in each plot (i.e., four per treatment). All soil columns were sealed with plastic caps and stored at 2° C until analysis. 4.2 Procedures when running analysis The field-moist samples were tested within a 2-3-month period from sampling. The samples from the different seeding treatments were placed on the x-y table such that the seeding row was parallel to the y-axis. A mark was set on the column to enable placing the column in the same position when the next layer was to be tested. The penetrometer probe was centred in relation to the sample in order to identify the base position (0, 0). In all cases the instrument was set to make penetrations in a 10*10 mm x-y grid. Measurements were not made within 20 mm of the column wall. The penetration cone was pressed into the soil at a constant loading rate of 10 mm min-1 and for a given time until the determined depth of penetration was reached. The overload stop was set at 4.5 kg (=6.25 MPa penetration resistance). For a 200 mm diameter soil sample with sampling in a 10*10 mm grid, one series of measurements to 60-70 mm depth lasted between 20 and 24 hours. To minimize evaporation during measurement, the soil samples were sealed with aluminium foil (Figure 3). To determine the weightloss during testing, the samples were weighed before and after each series of measurements. The position of the seeding row was drawn onto the aluminium foil to identify the exact position of the points of measurement in relation to the seeding row. The aluminium foil also allowed a determination of the z=0 depth of the column. I.e. a peak in penetration resistance was found when the probe passed the foil. This was especially useful for top layers with a rough surface located 0-40 mm from the top of the column. The aluminium foil was stored afterwards. If problems had occurred during testing, the penetrated aluminium foil was used to locate where the problems had started.

Figure 3. Soil sample sealed with a sheet of aluminium foil and seed row marked with black ink. Left: Before testing, Right: After testing.

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4.3 Layering The soil columns were divided into three layers to reduce the influence of shaft friction and reduce problems with shaft breaking or bending. First penetration resistance was measured to 60 mm depth in the intact sample. Then the soil sample was pushed up in the cylinder by the use of 15-25 mm thick plastic discs with a diameter slightly smaller than the inner diameter of the cylinder. The top 55 mm of the sample was cut off with a knife and a sub-sample was taken from the removed layer to determine the gravimetric water content (oven-drying at 105° C for 24 h). Afterwards penetration resistance was measured in the 55-115 mm layer (middle layer). A lower height was chosen for the cut-off layer than for the penetrated layer to avoid disturbance of the surface of the new layer and to secure full contact between the cone and the soil during testing. After testing the middle layer, 55 mm were cut off and penetration resistance determined in the 110-180 mm layer (bottom layer). Some problems with bending and breaking of the shaft were encountered for stony samples. However, in these situations the measurements were stopped shortly after the problems had started. The soil sample was then turned 180° such that the new series of measurements started from the other side of the sample. I.e. the former lower left corner in the grid was now the upper right corner. The turning of the soil sample was done to obtain a dataset that was as complete as possible. The programme only allowed starting penetrations in the “upper” right corner of the grid. 5. Dataanalysis 5.1 Handling stones The tested soil was rather rich in stones, which resulted in both missing data (penetration stopped due to overload) and stone-induced peaks and high values. To reduce the effect of stones values >4.0 MPa were excluded and median values were calculated for each x-position (i.e. at different distance to the seeding row) (Campbell and O’Sullivan,1991). The median values were used in the following dataanalysis. 5.2 Piecing layers together In many cases the penetration curves pieced together for the three layers were offset at the interface between the layers (Figure 4). It appeared that measurements were not reliable until a penetration resistance equal to that of the bottom of the previous layer had been reached at >15 mm depth. For the first 5 mm this was because the zone had been penetrated before (i.e. only soil 5 mm above penetration depth of the former series of measurements was removed). The next 10 mm was considered the distance for soil compaction to reach an equilibrium with depth (Gill, 1968). I.e. the depth where the soil-tool friction results in no further increase in soil compaction hardening (Gill, 1968). From that depth the soil-tool friction may be assumed constant when the penetrometer probe is pushed further downwards in a homogenous soil. Gill (1968) showed that the depth of attaining compaction hardening was about four times the cone diameter for steel probes. This corresponds to 12 mm depth for the 3 mm steel cone used in this study. Other reasons for increasing penetration resistance with depth when other

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properties are similar could be the increasing overburden pressure (Bradford et al., 1971) and shaft friction.

Penetration resistance, kPa

0 500 1000 1500 2000 2500 3000 3500

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Figure 4. Example of 3 penetration resistance curves pieced together from data from the three layers. Each curve shows median data from a specific x-position. Left: Before deleting recordings from the first 15 mm for the middle and bottom layer. Right: After exclusion and interpolation of data. Significant shaft friction is especially found in wet clay and when using cones with a small difference between cone and shaft diameter (Campbell and O’Sullivan, 1991). In the present study a small or insignificant effect of soil-shaft interaction was expected. I.e. a moist sandy loam was used and the difference between cone and shaft diameter was relatively high. In addition, initial testing indicated that soil-shaft friction was of minor importance when the probe was inserted to maximum 70 mm. The importance of shaft friction was studied using 20 field sampled cores (diameter: 100 mm, height 80 mm). The probe was inserted from both ends of the sample with >20 mm distance between the points. An example on such a pair of penetration resistance recordings is shown in Figure 5. Table 2. Soil water content in the tested field-moist soil cores.

Treatment

Replicate PL DD-D DD-C % w/w

1 12.7 11.6 12.7 2 - 9.2 10.4 3 13.5 12.1 13.5 4 12.3 11.7 13.5

Layer interface

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Penetration resistance, kPa

0 1000 2000

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Figure 5. Example of penetration resistance recorded in intact soil cores (diameter 100 mm, height 80 mm) used for evaluation of soil shaft friction. Penetration started from either top or bottom. In the dataanalysis the penetration resistance curves were pieced together by excluding recordings from the first 15 mm for the middle and bottom layer. This means that data from 60-70 and 110-120 mm are missing. These data were interpolated when constructing the 2D contour diagrams shown in Figure 6. 6. Results and discussion 6.1 An example The potential of the micropenetration method is illustrated in Figure 6 that shows 2-D contour diagrams representing single samples from three tillage treatments. The diagrams are based on median values at different combinations of depth and lateral positions from the seed row. The average soil water content of the columns are shown in Table 2. For all treatments, penetration resistance was higher below seeding depth. In the PL soil a weak 40-50 mm layer overlies a stronger lower part of the plough layer. In the direct drilled soil a weak 30-50 mm layer overlies a strong 50-100 mm layer. At the bottom, the penetration resistance is lower, although still potentially root growth-limiting (i.e. many parts with penetration resistance > 2.0 MPa). The seed furrow is hardly observable for the DD-D treatment. Apparently the press wheel has successfully closed the furrow opened by the single disk coulter. Contrary, an approximately 40 mm broad and approximately 50 mm deep furrow is observable for the DD-C treatment.

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PL DD-D DD-C R

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Distance from seed furrow, mm-60 -40 -20 0 20 40 60

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Distance from seed furrow, mm-60 -40 -20 0 20 40 60

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Figure 6. Contour diagrams based on penetration resistance determined at different positions from the seed row at 0 to c. 150 mm depth. Each diagram is based on measurements from a single sample. PL: conventional tillage, DD-D: direct drilled with single disc coulters, DD-C: direct drilled with chisel coulters.

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The micropenetration results showed that both direct drilling techniques produced potentially root-restricting compaction in the lower part of the arable layer. This was in accordance with field penetration resistance results (Munkholm et al., 2003). However, much better spatial resolution was obtained from the micropenetrometer data than from the field data employing an automated cone penetrometer using a 20.27 mm diameter / 60o semi-angle (Olsen, 1988). The micropenetration data suggested that the compaction effect of DD-D might be more important than the field tests indicate. More than 50% of the median values for penetration resistance at 40 to approximately 160 mm depth exceeded 2 MPa for the DD-D treatment in comparison with 25 and 7% for DD-C and PL, respectively (Table 3). Using 1.5 MPa as the critical limit resulted in 79, 64 and 30% of the median values with critically high penetration resistance for DD-D, DD-C and PL, respectively. The micropenetration results revealed no clear effect of distance from the direct drilling seed furrows. This was not surprising, as a large part of the area has been directly affected by the direct drilling at the time of sampling (i.e. the seed row at sampling was established in the inter-row space from last year). Table 3. Percentage of median penetration resistance values exceeding critically high levels. Based on all the micropenetration measurements recorded within the 40 to approximately 160 mm depth.

>1.5 MPa >2.0 MPa %

Mean SE1 Mean SE PL 30 a2 13 7 a 3

DD-D 79 c 8 52 b 10 DD-C 64 b 9 24 ab 7

1 SE: Standard error (n=4) 2 Figures followed by the same lettering with each column are not significantly different at the P=0.05 level. 6.2 Performance The presented micropenetrometer system was a powerful tool in detecting variations in mechanical resistance with a high spatial resolution. Laterally, the system allowed a sampling distance of minimum 5 mm between sampling points in a square grid. The precision of repeating a random x-y grid point was under 0.1 mm in both directions. Vertically it was possible to detect variations with very high spatial resolution (i.e. measurements were recorded for each 0.5 mm). In this study a spacing of 10 mm between grid points was used for the 3 mm diameter cone. For this probe a closer spacing between grid points is not recommended due to interference from neighbouring penetrated grid points. The influence of neighbouring grid points was not evaluated with the presented setting (10 mm spacing). Chi and Tessier (1995) reported no significant effect of spacing when evaluating spacings of 10, 20 and 40 mm for a 2.5 mm diameter cone.

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The micropenetrometer was easy to operate and required a low input of manpower. We used around 30 minutes between each series of measurements for excavating the new layer, weighing, taking samples for water content determination, etc. The water loss during testing was acceptable. For the 36 layers tested on average 0.3% of the initial total soil weight was lost during testing (standard deviation=0.3% w/w). As the soil water content of the samples were 12% w/w in average this corresponded to a loss of approximately 3% of the initial soil water content during one series of measurements. We conclude that it would be difficult to reduce the loading rate during measurement without achieving unacceptably high water losses. A solution to this problem could be to keep the water potential constant during measurement by placing the samples on a tension table. 6.3 Applications The micropenetrometer was a useful tool for studying small-scale compaction/loosening effects of different types of coulters for direct seeding. The method may be used for many other purposes where information on small-scale variability in soil structure is needed. This is especially relevant if geostatistics and spectral analysis can be applied to yield information on the location and distribution of clods in the soil. For such purposes a cone with a smaller diameter would be needed. 7. Conclusions We conclude that the presented fully automated micropenetrometer performed well and required a low input of manpower. The micropenetrometer proved to be a powerful tool in detecting small-scale effects of different direct drilling coulters. The following needs to be highlighted: • High spatial resolution and precision in finding x-y grid points. Random x-y positions

could be repeated with a precision under 0.1 mm. The spacing between grid points could be set as low as 5 mm in a square grid. The depth resolution was also high, i.e. measurements for each 0.5 mm were performed.

• A division of the 200 mm high soil samples into 3 layers was needed to reduce problems

with shaft friction and breaking or bending of the shaft (i.e. max. penetration depth was 70 mm). Problems with unreliable measurements at the surface of each profile were solved by excluding recordings from the first 15 mm for the middle and bottom layer.

• Acceptable water loss during measurement. Sealing of the sample with aluminium foil

secured an acceptable low water loss (0.3% w/w on average for each series of measurements).

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8. References Baker, C.J. & Mai, T.V., 1982a. Physical effects of direct drilling equipment on undisturbed

soils. IV. Techniques for measuring soil compaction in the vicinity of drilled grooves. N. Z. Journal of Agricultural Research 25, 43-49.

Baker, C.J. & Mai, T.V., 1982b. Physical effects of direct drilling equipment on undisturbed soils. V. Groove compaction and seedling root development. N. Z. Journal of Agricultural Research 25, 51-60.

Bengough, A.G. & Mullins, C.E., 1990. Mechanical impedance to root growth: A review of experimental techniques and root growth responses. Journal of Soil Sciences 41, 341-358.

Bennie, A.T.P., 1996. Growth and Mechanical Impedance. In: Waisel, Y., Eshel, A. & Kafkafi, U. (Eds.). Plant Roots: the hidden half, 2nd edn. Marcel Dekker Inc., New York. Pp 453-470.

Boone, F.R., Vermeulen, C.D. & Kroesbergen, B., 1994. The effect of mechanical impedance and soil aeration as affected by surface loading on the growth of peas. Soil & Tillage Research 32, 237-251.

Bradford, J.M., 1980. The penetration resistance in a soil with well-defined structural units. Soil Science Society of America Journal 44, 601-606.

Bradford, J.M., 1986. Penetrability. In: Klute, A. (Ed.). Methods of soil analysis, Part 1. Physical and mineralogical methods, 2nd edn. American Society of Agronomy-Soil Science Society of America, Madison, WI, USA, pp. 463-478.

Bradford, J.M., Farrell, D.A. & Larson, W.E., 1971. Effect of soil overburden pressure on penetration of fine metal probes. Soil Science Society America Proceedings 35, 12-15.

Dexter, A.R., 1987. Mechanics of root growth. Plant and Soil 98, 303-312. Campbell, D.J. & O'Sullivan, M.F., 1991. The cone penetrometer in relation to trafficability,

compaction and tillage. In: Smith, K.A. & Mullins, C.E. (Eds.). Soil analysis - physical methods. Marcel Dekker, New York. Pp. 399-429.

Chi, L. & Tessier, S., 1995. A portable micro-penetrometer for measuring seed row compaction. Soil & Tillage Research 34, 27-39.

Farrell, D.A. & Greacen, E.L., 1966. Resistance to penetration of fine probes in compressible soil. Australian Journal of Soil Research 4, 1-17.

Gill, W.R., 1968. Influence of compaction hardening of soil on penetration resistance. Transactions of the ASAE American Society of Agricultural Engineers 11, 741-745.

Glasbey, C.A. & O'Sullivan, M.F., 1988. Analysis of cone resistance data with missing observations below stones. Journal of Soil Science 39, 587-592.

Grant, C.D., Kay, B.D., Groenevelt, P., Kidd, G.E. & Thurtell, G.W., 1985. Spectral analysis of micropenetrometer data to characterize soil structure. Canadian Journal of Soil Science 65, 789-805.

Groenevelt, P., Kay, B.D. & Grant, C.D., 1984. Physical assessment of a soil with respect to rooting potential. Geoderma 34, 101-114.

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Hadas, A. & Shmulewich, I., 1990. Spectral analysis of cone penetrometer data for detecting spatial arrangement of soil clods. Soil & Tillage Research 18, 47-62.

Jørgensen, M.H. & Rasmussen, K.J., 2000. Technology for row crop growing in ploughless soil tillage systems. In: Proceedings of the AgEng2000 Warwick conference, University of Warwick, UK, 2-7 July 2000, paper 00-PM-004, 8 pp.

Koolen, A.J. & Vaandrager, P., 1984. Relationships between soil mechanical properties. Journal of Agricultural Engineering Research 29, 313-319.

Lipiec, J. & Usowicz, B., 1997. Spatial variability of penetration resistance of soil at different compaction level. In: Proceedings of the 14th. ISTRO Conference July 27-August 1, 1997, Fragmenta agronomica 2B, pp. 423-426.

Munkholm, L.J., Schjønning, P., Rasmussen, K.J. & Tanderup, K., 2003. Spatial and temporal effects of direct drilling on topsoil structure for a sandy loam. Soil & Tillage Research (accepted).

O'Sullivan, M.F., Dickson, J.W. & Campbell, D.J., 1987. Interpretation and presentation of cone resistance data in tillage and traffic studies. Journal of Soil Science 38, 137-148.

Olsen, H.J., 1988. Electronic penetrometer for field tests. Journal of Terramechanics 25, 287-293.

Schjønning, P. & Rasmussen, K.J., 1989. Long-term reduced cultivation. I. Soil strength and stability. Soil & Tillage Research 15, 79-90.

Schjønning, P. & Rasmussen, K.J., 2000. Soil strength and soil pore characteristics for direct drilled and ploughed soils. Soil & Tillage Research 57, 69-82.

Tessier, S., Saxton, K.E., Papendick, R.I. & Hyde, G.M., 1991. Zero-tillage furrow opener effects on seed environment and wheat emergence. Soil & Tillage Research 21, 347-360.

Voorhees, W.B., Farrell, D.A. & Larson, W.E., 1975. Soil strength and aeration effects on root elongation. Soil Science Society America Proceedings 39, 948-953.

Waldron, L.J. & Constantin, G.K., 1970. Soil resistance to a slowly moving penetrometer. Soil Science 109, 221-226.

Whiteley, G.M., Utomo, W.H. & Dexter, A.R., 1981. A comparison of penetrometer pressures and the pressure exerted by roots. Plant and Soil 61, 351-364.

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