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Serving: WI

What is the value of soil organic matter?

TAGS: Crops
stevanovicigor/Getty Images Close up of a farmer holding soil in both hands
SOIL ORGANIC MATTER: Farmland in the Upper Midwest typically ranges from less than 1% to 5% organic matter, with much of the intensely farmed land on the lower end of this range.
Field Fodder: Take time to evaluate what worked well in your 2020 cropping system.

With another cropping season wrapped up, now is the time to evaluate what worked well in 2020 and plan for the 2021 growing season. A farmer’s work is never done. That said, the rush is over, for the most part, and one can actually take some time to consider such questions as, what is the value of soil organic matter, anyway?

To address this question, first we need to define what organic matter is. Soil organic matter (OM) is the portion of the soil derived from plants or animals in various stages of breakdown and decomposition. Farmland in the Upper Midwest typically ranges from less than 1% to 5% OM, with much of the intensely farmed land on the lower end of this range.

Organic matter benefits

What are the benefits of organic matter? OM in the soil leads to improved water infiltration and drainage, reducing runoff and the soil and nutrient losses that go along with it. At the same time, OM increases water-holding capacity, improving resiliency during dry periods and drought. Increasing soil OM leads to better soil aeration from improved aggregation and, in turn, improved root growth and less crusting. OM in the soil increases the cation exchange capacity. Generally, the higher the soil OM content, the higher the CEC, which increases the soil capacity to retain nutrients, reducing leaching.

OM is also loaded with nutrients. As the OM breaks down, these nutrients will be consumed by soil organisms and released into the soil solution, making them available for plant uptake or lost to leaching and volatilization. In other words, soil OM serves as slow-release nutrient supply.

Nutrient availability can be tracked through routine soil testing for phosphorus, potassium and sulfur. For estimates of plant-available nitrogen, use the pre-sidedress soil nitrate tests (PSNT). Soil nitrate testing estimates the amount of plant-available nitrate-nitrogen in the root zone. This allows nitrogen fertilizer recommendations to be adjusted for field-specific conditions that can influence crop N needs. PSNT consists of soil sampling to a depth of 1 foot when corn is 6 to 12 inches tall.

This test is intended to predict the amount of plant-available N that will be released from organic sources during the growing season. It is most useful for confirming legume, manure and cover crop N credits, and for providing a site-specific estimate of soil N availability. Note: Soil nitrate testing is not reliable on coarse-textured sand or loamy sand soils because their nitrate content can change rapidly.

For more details on the PSNT procedures, see the University of Wisconsin-Madison Division of Extension publication A2809, Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin, pages 46-50.

In conclusion, soil organic matter has considerable value and is worth working to build up and preserve — not only for your benefit but for future generations, as well. For more in-depth coverage of this topic, see Caley Gasch’s and Jodi DeJong-Hughes’ publication Soil Organic Matter Matters! 

Schroeder is the Extension agriculture agent in Portage County, Wis.

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