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A Deeper Understanding

March 8, 2014
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
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New laboratory testing procedures lay the groundwork for improving soil health

When you start thinking in terms of soil health, soil testing takes on a whole new significance. You move beyond an exclusive focus on nitrogen (N), phosphorus (P), potassium (K) and pH levels to an acute awareness of the overall well-being of your soil. Once you reach that point, you can start to correct the weaknesses that limit the soil’s productive potential.

"Improving soil health involves assessing all three aspects—chemical, biological and physical," says Farm Journal Field Agronomist Ken Ferrie. "Some aspects of your soil might be healthy while others have room for improvement. The first step is to establish baseline levels. Then you can monitor your progress as you take steps to correct yield-limiting factors."

You can conduct several soil health tests yourself—water infiltration, soil respiration, aggregate stability, pH, surface hardness, compaction and density. Other aspects require the help of a soil-testing laboratory, which offers some tests you might not have encountered before, such as aggregate stability, texture analysis, active organic matter and mineralizable nitrogen.

"Some of these new soil health tests are not widely available yet," Ferrie notes. "As more farmers integrate soil health into their management plan, they will become more common."

Soil chemistry. The basic soil test you’ve probably been using for years measures the chemical aspect of soil health. A soil health test is similar but more comprehensive.

"Besides testing for macronutrients—N, P and K—we now need to test for micronutrients," Ferrie says. "Overapplying or underapplying any macro- or micronutrient might be detrimental to soil health."

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Take soil health samples in the spring when soil moisture is close to capacity.

Misapplication of one nutrient can trigger a chain reaction. For example, if you apply excess nitrogen to a soil where pH is optimum, the extra nitrogen stimulates microbial populations, Ferrie explains. Those microbes release soil carbon, which might be lost as carbon dioxide.

"Down the road, you might need to apply more nitrogen fertilizer because you reduced the supply of nitrogen stored in the soil," he adds. "If you apply an ammoniacal source of nitrogen, which creates acidity, soil microbial activity might be reduced. Then you will need to apply more limestone to correct the acidity."

Farmers who irrigate or apply manure should also test for sodium. "Excess sodium prevents clay particles from flocculating (just like hydrogen in an acid soil), so the soil particles run together and structure is destroyed," Ferrie explains.

In other words, balanced fertility, which includes the right amount of each nutrient and proper pH, is a key component of healthy soil.

Water pH versus buffer pH. Since proper pH is essential to soil health, you need to understand how much lime you need to apply. That requires a more sophisticated soil test that reports not just the traditional water pH reading but also the buffer pH reading.

"The water pH reading (which is the only one reported on many soil tests) measures soil acidity as it affects plants and microbes," Ferrie says. "But you can’t use that reading to determine how much lime to apply because the amount needed to neutralize acidity varies with different soil types. The reading that tells you how much to apply is buffer pH, which takes into account the soil’s buffering ability."

It works like this: A light soil, with low organic matter and low cation exchange capacity, might have a water pH of 4.9. A heavier clay loam soil might have a water pH of 5.5. Because of the difference in the soils’ buffering ability, the light soil might only need 1 ton of lime per acre to correct its acidity, while the heavy soil might need 3 tons. Liming both soils at the same rate would be a mistake, Ferrie adds, because the wrong rate on one field would reduce microbial activity.

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FEATURED IN: Farm Journal - March 2014

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