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Give Your Soil a Physical Exam

February 9, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
p18 Give Your Soil a Physical Exam 1

New tools and tests provide benchmarks to measure progress as you improve the health of your soil

It’s a good idea to pay a visit to your doctor every year—even if you’re feeling fine. A complete physical exam can give you the piece of mind to keep doing what you’re doing, or it could uncover areas of concern.

Soil is no different. "By evaluating the health of soil, we can get an idea of what’s good; what’s bad; learn where to start making improvements; and set benchmarks to measure progress," explains Farm Journal Field Agronomist Ken Ferrie. "The process can tell you whether a piece of land you’re thinking about renting or purchasing will be a sound investment."

Roots have difficulty penetrating resistance above 300 psi

There’s no time like the present to add a soil physical exam to your arsenal of management tools. "The sicker soil gets, the harder it is to bring it back to health," Ferrie says. "New tools and laboratories offering soil quality analysis enable us to give soil a physical exam."

You can’t expect all soils to attain the same level of health, Ferrie points out. "That would be like expecting an 83-year-old person to perform like a 17-year-old," he says. "When you

understand your soil, you can make the most of what you have."

p20 Give Your Soil a Physical Exam 2

A penetrometer measures soil resistance to penetration in pounds per square inch.

What to assess. A soil health evaluation involves assessing chemical, biological and physical aspects. You’re already familiar with the chemical aspect, which is your soil test. The biological aspect includes soil microorganisms that break down old crop residue and make nutrients available to plants.

Physical aspects include soil texture, aggregate stability, available water capacity, surface and subsurface hardness and infiltration rate. Texture and aggregate stability are key factors

that influence the other three traits.

"Soil texture—the amount of sand, silt and clay particles—affects nutrient- and water-holding capacity," Ferrie says. "Farmers know water percolates easily into sand because of its large particles and pore spaces, but it tends to move right on through. It’s harder for water to infiltrate into a silt loam soil, but it tends to stay there after it enters. So the silt loam has more water-holding capacity."

Think of aggregate stability as how well the soil maintains a crumb-like structure, in which sand, silt and clay particles are held together by organic matter and glues given off by mycorrhizal fungi.

"A healthy, crumb-like structure provides a stable structure for infiltrating and storing water," says Robert Schindelbeck, Cornell University soil scientist. "The large macropores

between crumbs allow for rapid water intake and air exchange."

A crumb-like structure allows better water infiltration, and downward and upward movement through the soil profile (via capillary action). "If we destroy that crumb-like structure (with abrasive tillage, for example), the soil surface will seal up, and water will run off," Ferrie continues. "The surface will crust over and plants will have a tough time emerging."

How to test. You can determine the percentage of sand, silt or clay by squeezing moist soil in your hand and trying to make a ribbon. "If it won’t make a ribbon, it’s sand," Ferrie says. "How long a ribbon you can make determines whether it is silt loam, clay loam or some other texture."

USDA’s Natural Resources Conservation Service (NRCS) offers an online soil survey to help determine soil texture. You can also have it analyzed in a commercial laboratory.

"The online soil survey is an excellent resource," Ferrie says. "Just choose your state and location, and it will tell you your soil types. From that, you can figure out the physical properties, such as organic matter content, bulk density, percent sand, silt, clay and water-holding capacity."

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FEATURED IN: Farm Journal - Mid February 2013



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