Land Laser

Alternative measuring device records tillage effects

Randy L. Raper and Tony E. Grift

     Choosing a tillage tool that will cultivate the best soil conditions takes experience, luck and a bit of science.
     Measuring tillage-caused soil disruption above and below ground can help farm managers compare equipment and understand the effects different tillage tools have on soil. Growers typically use a tillage implement to maximize soil disruption underground. This loosened soil improves plant growth, particularly in compacted soils.
     However, growers aim to minimize soil disruption above ground because loosening soil there can bury plant residue. Excessive surface soil mounding before planting can also cause poor seed-to-soil contact and warrant a second tillage operation.
     Engineers studying tillage equipment seek systems that provide loose soil below ground to optimize root growth, and little above-ground disruption that increases erosion or prompts more tillage.

Study method analysis
     In the past, researchers commonly used equally spaced rods on the soil to measure above-ground disturbance caused by tillage or traffic. Each rod’s vertical position was manually recorded in relation to a reference height to determine overall width and cross-sectional heaving area. Another approach was to photograph the equally spaced rods then digitize the photo to determine above-ground disruption. However, these methods involve a time-consuming, two-step process. The rods’ size and distance between them are also a concern because the soil aggregates and voids being measured may be much smaller.
     A series of soil cone penetrometer measurements has been used to determine underground disturbance. These measurements are obtained by pushing penetrometers into the soil at equally spaced distances across a tillage tool’s path. The penetrometer is composed of a steel cone at the end of a shaft that is hydraulically pushed into the ground. Sensors on the base of the shaft collect force data. The data are used to develop a contour map of depths and forces to show a cross-sectional area of loosened soil.
     Soil cone penetrometer measurements must be carefully spaced to prevent interference with each other. According to ASAE Standard EP542, these measurements should be spaced at least 5.9 inches (150 millimeters) from each other. A straight, rigid sensing device such as a soil cone penetrometer would also fail to detect potential root channels created by tillage around soil aggregates.

A new alternative
     Developing a quick, easy way to determine the distance from the soil surface to a reference height for measuring soil disruption posed a challenge for engineers. Research over the last 10 years by USDA-Agricultural Research Service (ARS) scientist Chi-hua Huang, of West Lafayette, Ind., and more recently by USDA-ARS scientists Larry Wagner and Fred Fox of Manhattan, Kan., led to developing a laser profile measurement system. Huang used laser technology to measure three-dimensional soil surface roughness. Wagner and Fox used laser technology to measure standing plant residue.
     Laser profile measurement system equipment — a laser profilometer — has recently been developed at the USDA-ARS national Soil Dynamics Laboratory in Auburn, Ala., by Randy L. Raper, Tony E. Grift and USDA-ARS engineer Bobby H. Washington. This system uses a laser-based position sensor with a range of 12 to 35 inches (30 to 90 centimeters) with ±0.7 percent accuracy. The laser-based position sensor is mounted on a power-driven linear positioning actuator with a 39-inch (1-meter) stroke and speed range of 2 to 16 inches (5 to 40 centimeters) per second. A portable aluminum frame holds the actuator above the soil surface and allows the laser-based position sensor to pass over the soil surface. The laser profilometer system can be AC or DC powered, making it portable for field use.
     After an area has been tilled, the laser profilometer’s frame is placed over the disturbed soil surface. The frame’s feet rest on undisturbed soil outside the tillage implement’s range. A two-dimensional scan is taken by moving the linear-transverse mechanism at a standard rate of speed across disturbed soil. One cross-sectional scan is obtained in less than a minute. The resulting profile shows peaks and depressions associated with the tillage treatment. Several cross-sectional measurements are made close to each other so they can be statistically analyzed.
     Obtaining underground data using the laser profilometer is more labor intensive. A small area of the tilled zone must be excavated by hand, removing only the loosened material. A two-dimensional scan of the disturbed zone is then taken and multiple measurements are made in close proximity for statistical comparisons.

Testing the system
     A recent experiment conducted in soil bins at the USDA-ARS National Soil Dynamics Laboratory in Auburn, Ala., illustrates the laser profilometer’s usefulness. The objective focused on determining how moisture content affects soil disruption from deep tillage — subsoiling. The study used a Norfolk sandy loam soil wetted to near field capacity. A subsoiler shank tilled the soil to a 13-inch (33- centimeter) depth. After the soil dried for one week, researchers repeated the experiment. Over four weeks, they studied four different soil moistures to obtain disruption and tillage tool force data for a range of experimental conditions.
     Results showed that soil moisture content affected above- and below-ground soil disruption. Figure 1 shows the averaged profiles of above- and below-ground disturbed soil for the shank tested at four soil moisture contents. The figure shows some trench area enlargement for the very dry soil condition, especially near the soil surface. These two-dimensional, cross-sectional scans can be analyzed to determine the amount of disturbed soil above and below the original soil surface. The data can then be statistically compared.
     Less soil moisture contributed to increased aboveground soil disruption (Figure 2). A very dry soil condition created the most above-ground disruption while a wet soil condition experienced the least above-ground disruption. Less soil moisture also caused trench area enlargement but with fewer statistical differences.
     The laser profilometer helped determine that decreased soil moisture increases above-ground disturbance, which could contribute to soil erosion, planting difficulties or the need for a second tillage before planting. This information tells growers they should operate this implement at the dry soil condition that disturbs the soil surface less than the very dry soil condition.
     The laser profilometer also helped determine that underground disturbance increases when moisture is at the very dry soil condition, which should benefit root growth. However, if the dry soil condition is chosen to reduce above-ground disturbance, below-ground soil disturbance decreases little.
     The laser profilometer allows users to quickly gather data and use scientific methods to determine soil disruption differences caused by tillage components. Profilometer use offers new opportunities for understanding the relationship between tillage forces and soil disruption. Designers and producers can use this technology to select the best tillage tool for their soil condition needs.

ASAE member Randy L. Raper is an agricultural engineer with the USDA-Agricultural Research Service, National Soil Dynamics Laboratory, 411 S. Donahue Drive, Auburn, AL 36832, USA; 334-844- 4654, fax 334-887-8597, rlraper@eng.auburn.edu, http://msa.ars .usda.gov/al/auburn/nsdl/.

ASAE member Tony E. Grift is an assistant professor with the biosystems engineering department, Auburn University, Auburn, AL 36849, USA; 334-844-3545, fax 334-844-3530, tegrift@eng .auburn.edu, www.eng.auburn.edu/department/an/.