Farm Journal Corn College attendees learned how to do a soil respiration test to measure the amount of carbon dioxide exhaled by soil microorganisms. The more carbon dioxide there is, the more microbes and the healthier the soil.
Simple tools can measure soil health in the field
How many times have you looked at a yield map and wondered why one part of a field outyielded another? The answer might be a difference in soil health. New tools make it easier to give your soil a physical examination to discover yield-limiting problems and start to remedy them.
A soil physical exam can also help you decide how much to pay for a farm and how to make all of the land you operate more productive.
"Getting a handle on soil health and then improving it is a trend that’s here to stay," says Farm Journal Field Agronomist Ken Ferrie. "We now understand that healthy soil produces higher yields, uses inputs more efficiently and protects the environment from pollution."
A comprehensive soil physical exam involves tests you can do in the field and some that are done in a laboratory. For now, we’re going to focus on the in-field tests. In 2013 at Corn College Advanced, participants had the opportunity to practice each of these tests in the field.
An Internet search will lead you to sources for soil health test kits, such as Gempler’s Inc., which contain the tools you need.
Infiltration and soil respiration tests. You can conduct infiltration and soil respiration tests at the same time using the same equipment. Both involve a canister that you drive 5" into the ground. Choose locations outside wheel tracks; the soil there might have been compacted by machinery.
To determine infiltration, pour a measured amount of water into the canister and see how long it takes to soak into premoistened soil. Then cap the canister and conduct the soil respiration test, which evaluates soil microbial activity. The more microbial organisms you have, the healthier the soil—and the more carbon dioxide those tiny critters respire as they breathe.
In addition to the canister, this test requires using a syringe, a hose and a Draeger tube, which changes color to indicate carbon dioxide content. Following the infiltration test, cap the tube so the canister collects carbon dioxide. Return later and use the syringe to draw air through the Draeger tube. (If the time lapse is short or microbial activity is low, you might have to use an alternate procedure that involves drawing air five times.) Do the test in four or five places and average the results.
A color scale on the Draeger tube shows the carbon dioxide content.
If you just want to compare two areas of your field, that’s all you need to do. But if you want to follow up in future years to see if your corrective measures are working, you can make the test repeatable by calculating bulk density and water-filled pore space. That requires collecting a certain volume of soil, drying it and weighing it.
There are two other ways to measure soil respiration, explains soils technician Thomas Zerebny, who works for Ferrie. "You can put soil in a sealed container, insert the wand of a carbon dioxide meter [available at www.vaisala.com] and measure the amount of carbon dioxide respired over a period of time," he says.
"Or you can use the Solvita Soil CO2 Burst Test Kit [www.solvita.com]. Dry, weigh and moisten samples of soil and seal them in a plastic jar. Read a color-sensitive tab to determine the amount of carbon dioxide given off."
Slake test. The slake test reveals how likely soil is to crust. Collect a tablespoon of surface soil, drop it into a 1"-diameter cylinder with a screen at the bottom and immerse the cylinder in water several times.
"If soil is poorly aggregated, the onrush of water as you immerse the cylinder will blow the structure apart," Ferrie says. "Small silt and clay particles fall through the screen. The more the aggregate holds together, the healthier the soil and the more resistant it is to crusting."
There are other ways to measure a soil’s potential to crust. You can purchase a Cornell University Sprinkle Infiltrometer, which simulates rainfall. (It can also be used to measure water infiltration.) Using the Infiltrometer, you apply a given amount of "rainfall" and measure how much soil falls through a screen.
Another method is to air-dry clods from the same soil type but with different management systems, drop them into a jar of water and compare how well the aggregates hold together.
To get an accurate reading across a field, it’s best to do a soil respiration test in four or five spots and average the results.
Soil pH. Nothing is more important to healthy soil than a proper pH level because acidity or alkalinity reduces microbial activity. There are several ways to test for acidity. You can send soil to a laboratory, or you can use a pH meter sold by lawn and garden stores and swimming pool supply shops.
Finally, you can test soil by using a soil probe to draw a 6" core. Thoroughly mix the soil, then drop a portion of it into a jar and fill the jar the rest of the way with distilled water. (Distilled water is essential because it’s pH neutral, so it won’t affect the pH reading of the soil.) Mix the solution, insert a paper test strip and compare the strip to a color chart.
Soil hardness or compaction. A soil penetrometer or tile probe helps identify density layers that restrict root growth. "A tile probe will identify layers," Ferrie says. "But a soil penetrometer will give you numerical readings.
"A logging penetrometer made by Spectrum Technologies measures resistance for each inch of soil. Connected to a GPS unit, it can create a map identifying density layers throughout a field. It also lets you return to the same spot in future years to measure your progress in removing them."
In conjunction with your soil penetrometer or tile probe, dig some corn root balls during the growing season and visually evaluate them. "Roots should grow downward at a 35° or 40° angle from the center of the crown," Ferrie says. "If roots are being turned by density layers and growing horizontally, start using vertical tillage and possibly cover crops such as radishes to correct the problem."
If acidity is one of the limiting factors on your lower yielding soil, fix that problem first, by applying lime, Ferrie advises. Other measures to consider are cover crops and diversified crop rotations (if possible) to increase and vary microbial populations. Grasses, whether cover crops or cash crops, can help improve aggregate stability. Reducing tillage, switching to vertical tillage and applying organic matter such as manure can also help improve soil structure.
Soil health testing is becoming a common management practice. If you don’t have time to conduct a physical exam yourself, consider finding a consultant who offers the service. Or, you can send soil samples to commercial soil health testing laboratories or to the Cornell University soil health testing service (http://soilhealth.cals.cornell.edu). You can also get more information about soil health testing from your Natural Resources Conservation Service staff.
Giving your soil a yearly checkup will help answer those postharvest yield map discrepancies.
You can e-mail Darrell Smith at firstname.lastname@example.org.
Tests Influence Cover Crop Choice
Among the things you can glean from performing soil physical tests is guidance on cover crop selection. "If your tests reveal poor aggregate stability, consider a grass cover crop such as barley, annual ryegrass, wheat or cereal rye," advises Farm Journal Field Agronomist Ken Ferrie. "The roots of grass crops produce glue-like substances that help soil particles form aggregates."
If there’s a pH problem, fix it before you introduce cover crops. "Acid soil has poor microbial activity, which causes a low rate of nitrogen mineralization," Ferrie explains. "In that case, a cover crop will throw a lot of organic material at a sick soil, and you won’t have enough microbial activity to decompose it. You’ll wind up with tough-looking corn."
If soil has turned acid and aggregate stability has been degraded by too much aggressive tillage, it can take 10 to 15 years to bring it back to health, Ferrie adds. But conducting a soil physical examination will show you how to start the process.
Building on the Systems Approach, the Soil Health series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance.www.FarmJournal.com/soil_health
- February 2014