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Soil Health Tool Shows Nutrient Availability and Soil Respiration

Measuring soil respiration and its contribution to soil fertility is the purpose of a testing procedure called the Soil Health Tool.

The tests evaluate relationships between the soil’s physical properties and its biological processes. This, in turn, suggests the level of microbial activity and its effect on soil respiration. Estimating plant-available fertility is then possible.

“This is the first test that combines an analysis of the chemical, physical, and biological components of the soil,” says Gabe Brown, an early user of the testing process who farms and ranches near Bismarck, North Dakota. “This test uses that information to show nutrient availability for the next year’s crop.”

The Soil Health Tool measures the following eight indicators of soil health.

  • Microbial activity. “Microbes exist in soil in great abundance,” says Rick Haney, a soil scientist at the USDA Agricultural Research Service’s Grassland, Soil and Water Research Laboratory in Temple, Texas. “Their composition, adaptability, and structure are a result of the environment they inhabit. Since soil microbes take in oxygen and release CO2, we can couple this mechanism to their activity. It follows that soil microbial activity is a response to the level of soil quality, or fertility.”
  • Water-extractable organic C. This type of carbon provides energy to soil microbes. “The pool of water-extractable organic C in the soil is about 80 times smaller than the pool of total soil organic C [which is the percent organic matter],” says Haney. “The organic C in the soil water extract reflects the quality of the C in your soil that is highly related to the microbial activity.”
  • Water extractable organic N. This type and volume of N in the soil correlates to the amount of water-extractable carbon in the soil. Soil microbes readily break down the organic N and release it to the soil in inorganic forms of N that are readily plant available.
  • Ratio of organic C to 
organic N. This ratio measures a critical relationship affecting the nutrient cycle. It shows the degree to which N and P can be taken up by growing plants. A ratio indicating high amounts of carbon suggests that the N and P in the soil cannot be mineralized.
  • Soil health calculation. This number rates the overall health of a soil system. “The calculation looks at the balance of soil C and N and their relationship to microbial activity,” says Haney. “Keeping track of this number will allow you to gauge the effects of your management practices over the years.”
  • Levels of N, P, and K. Test results show the amount of inorganic forms of the nutrients in addition to the amount of N and P that soil microbes will provide.
  • Nutrient value per acre. “Current fertilizer prices are multiplied by the nutrients present in your soil,” says Haney. “This is the dollar value of these soil nutrients.”
  • Cover crop recommendations. Test results may include suggestions for cover crop mixes to increase soil health and fertility.

“I’ve been growing cover crops for years, and my soils are presently testing very high in microbial activity. As a result, they’re also high in inorganic nutrients,” says Brown.
He draws soil samples for testing from late May through early July, when the biological activity in the soil is strongest.

Woods End Laboratories jointly researched the Soil Health Tool with Rick Haney. Woods End is presently one of several commercial labs now offering this testing procedure.

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