Soil N Testing in Mississippi – Why We Can’t Have Good Things

We in Extension sing the praises of soil testing constantly because it provides a snapshot in time of the ability of a soil to provide nutrients to growing plants. The results, along with a research base, are the basis for soil fertility recommendations. This process is robust and healthy for phosphorus (P) and potassium (K) management, even as noted previously, undergoing refinement. Conversely soil testing for nitrogen (N) is challenging in what we call the warm, humid south.

The direct way to describe soil organic N activity in soils is to say it changes forms as it goes from plant or animal residue until it is taken up by plants in the nitrate form. This transformative work is done by suites of microbes simply going through their growth cycles, which happens faster in warmer conditions, and no so fast when it is cool. I have notes from way back that say N plays up and down the oxidation/reduction scale, but we are not going there.

Soil test attempts to characterize N deriving from soil mineralization have had niche calibration and validation successes, usually in lower rainfall environments. But again, the currently available soil N tests cannot capture all N now present, or that will become available. This is magnified from the constant interactive effect of crops, weather, and soil on the timing of N release.

Mid-February 2021 weather encapsulates the environmental conditions that impact N mineralization rates. Table 1 contains the air and four-inch soil temperature data from Delta Agricultural Weather Center for the station located at the R. R. Foil Plant Science Research Center, known by many as North Farm, on the edge of Starkville. (Also, home to some serious student sledding at the time of these measurements.) The daily high air temperature decreased from 73 to 25 over six days, while the soil temperature went from 57 to 35.

Air and 4″ Soil Temperatures at North Farm, 2/9-16/2021

Soil N mineralization rates increase exponentially with soil temperatures, with the rate at 55 degrees at 1.8 times the rate at 40 degrees (Sullivan et al. 2020 based on Gilmour, 1998). The 22 degree decrease within a few days put a shock into the microbial community. How long will this last? What will be the result? The climate conditions – temperature and rainfall – going forward will determine the outcome.