Soil health and drainage: Haney test results from northwest Minnesota don’t differentiate by drainage

By: Anna Cates, Extension soil health specialist

Soil health can be measured in many ways. Often, tests estimate the capacity of soil biology, and the Haney test is one way of estimating how much food, in the form of organic carbon (C) and nitrogen (N), is available to microbes in the soil.

Recently, UMN colleagues and I applied the Haney Soil Health Test to silty clay loam soils with and without tile drainage at the Northwest Research and Outreach Center in Crookston. We compared drained and undrained plots to see if changing the soil water regime with drainage would alter microbial activity enough to detect differences in the Haney Soil Health Test. No differences with drainage were found, but measurements of organic C and N were greater in April or May than summer or fall sampling.

Seasonal variability and N supply

Organic C and N pools measured by the test are mostly decomposed, soluble in water, and ready for microbes to take up as food. Other research in Wisconsin and Ohio also found higher values early in the year. When soils warm up in the spring, lots of residue is rapidly converted into the soluble organic C and N measured by the Haney Soil Health Test. This can be a nice supply of nutrients to plants growing early in the season, like small grains or forage crops.

The Haney Soil Health Test calculates the expected N release from soil organic matter over the course of the year based on spring microbial activity and organic N pools. We calculated N credits between 1 and 166 lb/ac across our plots, with an average of 75 lb/ac in 2020 and 36 lb/ac in 2021. There was no difference between drained and undrained plots. N mineralization is always different from year to year, as microbes mineralize N in response to good temperature and moisture conditions. Lower expected N credits in 2021 are likely due to drier conditions, but both credits suggest substantial contribution of the soil organic N to crop N needs. Recent UMN research showing minimal response to N fertilizer in northwest Minnesota backs this up.

The N credit variability found across the plots showcases that relying on a specific quantity of N provided from soil is a bit risky. Growers looking to reduce N could start by applying strips with modest reductions (~25%) from current rates. Do this in multiple spots across the field to make sure you account for in-field variability, and see how yields and profits work out for the different strips.

Ongoing research

We were surprised that changing soil hydrology didn’t affect microbial activity enough to change organic C and N pools. Microbes need water, but too much water in the soil slows them down. Water-logged soils are expected to have slower microbial activity and more organic C and N, while well-aerated soils should have more rapid microbial activity, depleting pools of organic C and N. But 2021 was historically dry, so drainage didn’t have much of an effect on soil water. Plus, soils may respond more over time, as it recovers from drainage installation in fall 2019 and as the soil pore network connects to drain tile.

The research is ongoing, and next we’ll look to see how drainage changed total soil C stocks, and losses of N and phosphorus (P) through various pathways.

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