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.
- February 2014