Evaluating Soil Compaction
The end of the growing season is a good time to take stock of your most important input for next year’s crop— your soil.
Time and weather allowing, fall is a good time for soil sampling. While potassium (K) values from samples taken in frozen soil may test high compared to other times of the year, Sulfur (S) and chloride (Cl) are mobile in the soil, so sampling in the fall or spring is recommended, according to the North Dakota State University Extension Service.
Fall soil sampling results for nitrate-nitrogen (NO3-N) and sulfur (S) are similar in most years to spring sampling, and soil samples to be analyzed for soil pH, salt content, zinc (Zn) and phosphorus (P) can be taken nearly any time of year.
For more how-to information on soil sampling, see the NDSU extension publication, “Soil Sampling as a Basis for Fertilizer Application,” (Bulletin SF-990, revised in August, 1998) by soil scientists David Franzen and Larry Cihacek. The publication can be found on the Internet at http://www.ext.nodak.edu/extpubs/plantsci/soilfert/sf-990.htm
As long as you’re sampling, you may want to scout for soil compaction as well, especially on irrigated ground, or dryland fields that have had a lot of machinery traffic in wet soil conditions. Fields where crops have seemed more stressed than others, or had deformed roots, are also good candidates for scouting.
Fall is the easiest time to detect compaction, while the soil is relatively dry and has a high degree of strength, according to the Colorado State University Extension Service.
All soils are compacted to some degree. Some amount of soil compaction is necessary to support the plant, avoid overdrying, and provide seed-to-soil contact required for germination. However, excessive soil compaction can result in poor internal drainage, the potential for increased runoff, inhibited root development, and decreased yields, according to CSU.
Soil compaction can also inhibit root development in the subsoil. The stressed plant cannot take full advantage of subsoil moisture and nutrients, and becomes more susceptible to other stresses from adverse environmental conditions.
Soil compaction may not reduce yields every year. It is most likely to decrease yields in years when other stresses, such as excessive heat, insect infestations and diseases, are present.
Crops with deep tap roots provide more channels for subsequent crop roots to follow and allow water to percolate more deeply into the soil. Rotations that include alfalfa, clover and sunflower usually leave a less compacted soil than fields without these deeply rooted crops.
A visible sign of compaction is soil structure altered from granular to plate-like. The plate-like structure appears as sheets of paper stacked one on top of the other. Surface crusting may be apparent after rain-soaked soils dry. Still, it can be difficult to tell whether soil is overly compacted.
“There are a lot of misconceptions on how to evaluate soil compaction, and we really don’t do a good job of testing and measuring for it, like fertility testing,” says Joseph Benjamin, soil scientist at the USDA Central Great Plains Research Station, Akron, Colo.
Easy tool to evaluate compaction
Penetrometers can be useful tools for evaluating compaction, Benjamin says, and can be constructed inexpensively with easily obtained materials and average welding skills. You will need a ¼” to ½” diameter steel rod that is three to four feet long, a 4”x4” or 6”x6”steel plate (don’t use wood or plastic materials, because they won’t be strong enough).and a bathroom scale.
Sharpen the tip of the rod to a point with a 30-degree cone angle. Mark the probe at 6-inch increments along the shaft. Then weld the other end of the probe at the center of the steel plate. This serves as a support for the bathroom scale and as a handle for removing the penetrometer following soil measurements.
To take penetrometer readings, place the tip of the penetrometer on the soil surface and put the bathroom scale on the base. Push on the top of the scale to force the pentrometer into the soil. Make sure to push vertically on the scale to minimize sidewall friction on the penetrometer probe. Note the maximum reading on the scale for the first 6” of soil. Continue pushing the probe down into the soil, taking measurements and noting the maximum scale reading for each 6” interval to give a penetrometer resistance profile for the soil. It’s helpful to have another person on hand to monitor when each 6” interval of the probe is pushed into the ground, Benjamin notes.
Penetrometer resistance that is restrictive to crops depends on the crop species. As a rule of thumb, soil penetrometer resistance above 300 psi starts to restrict root growth. To convert the scale readings to psi, use the chart below. Multiply the scale reading by the probe factor to get psi. Thus, if the rod of your penetrometer has a ¼” diameter and the scale reads 9 lbs after you’ve pushed the probe into the first half-foot of soil, 11 lbs in the second half-foot, and 16 lbs in the third half foot, psi would be 180, 220, and 320 for each half-foot measurement, respectively.
Probe diameter 1/4” 3/8” 1/2” 3/4” 1”
Probe factor 20 10 5 2.25 1.25
Just as a number of samples are needed to accurately evaluate available nutrients that can vary within a field, you need to know the extreme variations of penetrometer resistance for your soil, by probing different areas in a field to evaluate compaction.
One area that is not normally compacted is soil along a fence row or tree line, while soil in a field entrance is more likely to be compacted. Measurements in these areas along with other probes within a field will give a range of penetrometer resistance readings to help analyze overall soil compaction.
No-till fields will have higher readings than cultivated fields. Bear in mind too that moisture content can influence penetrometer readings. Expect lower readings in wetter areas of a field compared to drier areas. Thus, try to take readings when and where moisture content within a field is fairly uniform. “It’s more important to have consistent soil conditions to evaluate soil compaction than the precision of the measurement equipment,” Benjamin points out.
The decision is up to the producer on whether deep tillage is needed to alleviate soil compaction, and must be weighed against soil erosion and moisture concerns. If the ground is tilled, take penetrometer measurements afterward to determine the change in soil penetrometer resistance caused by the tillage operation. Repeat the measurements 3 to 6 months after the tillage operation to determine the longevity of deep tillage.
Soil has a tendency to reconsolidate over time, Benjamin notes, and you will want to know how long the effects of your tillage operation will last. —Tracy Sayler
For information on managing soil compaction online, visit the Colorado State University website http://www.colostate.edu/Depts/CoopExt/PUBS/CROPS/00519.html or the University of Minnesota, http://www.extension.umn.edu/distribution/cropsystems/DC3115.html#contents
For more information on scouting for compaction, contact soil scientist Joseph Benjamin, ph. 970-345-0518 or by email, firstname.lastname@example.org
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