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A REVIEW ON THE SOIL COMPACTION MEASUREMENT SYSTEMS
FatemehRahimi-Ajdadi, a,* and YousefAbbaspour-Gilandehb
a: Department of Mechanization Engineering,University of Guilan, Rasht, Iran; b: Department ofBiosystems Engineering, University of MohagheghArdabili, Ardabil, Iran;
*Corresponding author: [email protected]
ABSTRACT
Soil strength is an important soilmechanicalcharacteristic which is considered in the relationship of soil/plant and also in the studiedon the interaction effects of soil/machine. Applying a standard cone penetrometer is the conventional approach for measuring soil strength or compaction in agricultural fields which has more acceptances in the recent decades. However, this method takes much time and labor consuming. Thus, it isn't a practical approachin developing the map of soil compaction within an agricultural field. The motive of dominance over the limitations of manual cone penetrometers and also the appearance of new electronic sensors in the recent years, caused to developthe on-the-go approaches. The present study reviews and centralizes over trend of evolution of this process until now.
Keywords:Electronic sensors; hand pushed penetrometer; soil mechanical resistance; tractor mounted penetrometer
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INTRODUCTION
Soil compaction as one of the important negative parameters affecting crops yield, is a serious problem in crop production (Abbaspour-Gilandeh and Rahimi-Ajdadi 2012). In many areas, it was reported as the main factor effecting on the limitation of plant root growth (Lipiec and Stepniewski 1995). The degree of soil compaction is called compactness and traditionally determined through laboratory tests of soil samples and expressed as pore space, void ratio, or dry volume weight (Koolen and Kuipers 1983). Overall, the methods of soil compaction measurement can be divided into two parts nominally laboratory and in situ approaches. The laboratory approaches include direct shearing test and triaxial test. In situ approachesare faster and cheaper than laboratory approaches. In addition, those are not required to transfer or handlethe samples to the laboratory. The in situ approaches despite restriction in the number of measuring parameters are preferred than laboratory approach caused the removal of soil disturbance during sampling and transporting to the laboratory. There are six main in situ approaches to determine soil strength including: ring cutting plate, rectangular cutting plate, cutting plotter, cone penetrometer, cutting blades and pocket penetrometer. Among these methods, the cone penetrometer has overmuch assented among researchers in the worldwide and accepted as a standard and basic method for soil compaction measurements (Perumpral 1987).
Considering the appearance of the precision agriculture as well as the site specific crop management concept, there is a need to collect a large amounts data the entire field. While cone penetrometer readings require a stop-and-go procedure with collecting at discrete locations (Chung et al. 2004) that is very tedious and time consuming and its results are highly variable
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(Adamchuket al. 2004). Therefore, the next studies were concentrated to raise the data collecting rate. These attempts led in appear the tractor mounted cone penetrometer. Also with entrancing different suitable electronic sensors in the market, the on-the-go sensors were faced welcome.
Thus, the development of the sensors can lead to increase the effectiveness of precision agriculture. Considering the importance of the case, the objective of present publication is to review in situ methods used to measure the soil strength as well as investigate their progresses so far.
MATERIALS & METHODS
There are introduced various criteria, indices and methods to identify soil compaction used by different researchers such as, soil color index, bulk density, radar penetrating into the ground and cone index (Upadhyayaet al.1994). In one way, existing methods can be divided into two categories including the static (hand pushed and tractor mounted penetrometer) and on-the- go methods.
The static methods for measuring soil compaction
Idea of using a pushy bar into soil for measuring soil compactionhas been discussed by researchers for a long time. According to the reports, application of this device in the world has attributed since 1846 (Perumpral 1987). Penetrometer in today common form has applied since 1934. The criteria obtained by the penetrometer readings is called cone index (CI). Cone index is defined as the force required to insert a cone tip within the soil dividing by the base area of the cone (ASAE standard 2000). During measuring operation, the standard penetrometer is pushed into the soil at a uniform speed rate of approximately 30 mm sec−1. A disadvantage of the device is difficulty in supplying the constant rate of penetration. Consequently, toovercome this problem, reduce manpower and increase the precision of data registration, the tractor mounted penetrometer were developed with a special mechanism. Wilfordet al. (1972) developed a tractor mounted cone penetrometer in which a hydraulic cylinder was used to movement of penetrated bar. In this system, a transducer to measure penetrating pressure and a potentiometer to measure operating depth were applied. Force values were plotted against operating depth graphically by an x-y recorder. Similar penetrometer was designed by Raperet al. (1999) andAhaniet al.(2009).
TabatabaeeKoloor and Alimardani (2008) evaluated and compared cone index in different depths by manual cone penetrometer and tractor mounted cone penetrometer. In both systems,a transducer and a photocell sensor were used to measure the soil mechanical resistance and depth,respectively. They did not find a significant difference between cone indices obtained from two systems (P<0.05).
On-the-go methods for measuring soil compaction
Understanding the spatial variability of soil strength requires to collect extremely large amounts data which is probably not a cost-effective process at long scale (Clark 1999) by common standard cone penetrometer. This limitationled to application of electronic sensors in this field that allowed to collect the data with extremely precise spatial resolution. On-the-go mapping can be accomplished through continuous logging of geo-referenced measurements
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while a sensor travels across a field (Adamchuket al. 2006). In the most related studies were used from a set of load cells and strain gauges. A chisel with wedge cutting edge using a set of strain gauges attached to the blade was manufactured by Glanceyet al. (1989 and 1996). Another system designed by Stafford and Hendrich (1988) used from a narrow blade to exposure in hardpan layer. It worked on the basis of registering the horizontal and vertical soil cutting force and their interactions against continuously variability of operating depth. Alihamsiahet al. (1990) and Alihamsiah and Humphries (1991) were developed a horizontally on-the-go penetrometer and studied on it concerning the geometry tools. Their results showed that the use of horizontally prismatic tip represents the closer data to standard cone penetrometer than the conical tip. More Acceptable readings of the prismatic tip compared with the conical tip was described such that the prismatic tip encountered the soil strength only in sideways directions while a cone penetrometer was loaded radially in all directions (Chukwu and Bowers 2005). Adamchuket al.
(2001) were applied an array of four strain gauges fixed on a vertical blade to map of soil mechanical resistance. A disadvantage of this method was that the strain gauges detected the deformation or bending moments of the tool instead of soil strength. Therefore,calibrating and validating the effect of tool geometry faced to difficulty (Chung et al. 2003). Another system was instrumented by Andrade et al. (2001) which included eight cutting edges supported by independent load cells that measured the force on each cutting edge. A drawback of this system was the low force sensed by the sensor that occurred by reason of the interaction between discrete cutting edges. Chung et al. (2003) fabricated a sensor which measured soil strength on its every five tips by a load cell located in a main blade. Their results indicated that the mechanical resistance was measured higher at locations with greater bulk density, lower electrical conductivity and lower water content. The system designed by Khalilianet al. (2002) was able to determine the depth and thickness of the hardpan layers to generate site specific tillage maps. Influence of failure mode induced by a single-tip sensor was investigated by Hemmatet al. (2009). Their results indicated that at the bottom of critical depth, there was significant relationship between soil mechanical resistance obtained by the sensor and standard cone penetrometer. However, this relationship at shallower depths wasn't significant. They resulted that this occurred due to change in failure mode from brittle to compressive mode below the critical depth. Also, theyattributed the presence of more signal spikes in the top soil to interaction effect between the tip and clods produced by tillage operation. An evaluation on a horizontally on-the-go single blade sensor showed thatat the depths between 0 and20 cm, the relation between soil mechanical by sensor and the tractor-mounted cone penetrometer was marginal while at the depth of 20-30 and 30-40 cm were approximately good (Rahimi-Ajdadiet al.2011). Evaluating failure mode was emphasized the presence of a critical depth at the depth of 20 cm within soil.
RESULTS AND DISCUSSION
Among two methods of laboratory and in situ to measure soil compaction,in situ method has more agreeable due to faster and cheaper sampling. In addition, among the in situ methods, the cone penetrometer has higher level of acceptance than others. Consequently, further efforts have been performed to improve the penetrometer and reduce or eliminate its restrictions.
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Generally, the methods of soil mechanical resistance measurement which were developed based on penetrating a cone within soil can be categorized in three groups:
1. Static hand pushed cone penetrometer which were expanded today form since 1934.
2. Tractor mounted cone penetrometer 3. Horizontally on-the-go penetrometer
It is clear that applying the hand pushed penetrometer and tractor mounted penetrometer for mapping soil compaction are not effective. Therefore, subsequent studies should be specifically concentrated on developing on-the-go methods and improving existing system. The main challenge facing the most developed on-the-go single blade systems were the interaction between the sensors such that the measurements of lower tips affect the higher tips. Also excessive soil disturbance resulted in obtaining lower values for soil strength in higher depth compared to actual value. In order to reduce or eliminate this problem,Siefkin et al. (2005) constructed and tested a multiple blade soil mechanical resistance measurement system with three independent sensing blades. They obtained an acceptable correlation (r2= 0.76) between the sensor and standard cone penetrometer measurements. Also, Abbaspour-Gilandehand Rahimi- Ajdadi(2016) used from the idea of discrete blade for each depth with the aim of eliminating the interaction between main blade and sensing tip. Each instrumented shank consisted of an extended octagonal load cells. This system represented significantly lower variance of measurements compared with a hand pushed soil cone penetrometer. Low soil disturbance, high correlation with CI and low measurement fluctuations can be mentioned as some advantages of this type of soil compaction sensor.
In terms of application,a static cone penetrometer generally uses in small-scale level which requires to high accuracy. In addition, it is applied in the research samplings in which the soil mechanical resistance is considered implicitly. While on-the-go systems have the potential of significantly reductionin the cost of data collection. It should be mentioned thaton-the-go sensors are still in developing stages. More data under various soils conditions are needed to increase their potential application by producers and researchers. Perhaps, the vision of the future development for these systems depends on the progress in the areas of new electronic and ultrasonic technologies.
CONCLUSIONS (FACULTATIVE)
1. In situ approachesare faster and cheaper than laboratory and those are notrequired to transfer or handlethe samples to the laboratory.
2. Difficulty in mapping soil mechanical resistance using a cone penetrometer led to development of on-the-go technologies.
3. On-the-go sensors solve the problems of static methods including, time consuming, having various fluctuations and to be tedious.
4. The vision of the future development for on-the-go systems depends on the progress in electronic and ultrasonic technologies.
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