Widespread Prevented Planting (PP) acres across eastern North Dakota and northwest Minnesota in 2020 has prompted more questions about soil testing and timing. The most common question, “When can I start soil testing for nitrate-nitrogen?” The short answer is that soil sampling on some Prevented Planting fields can get started in mid-August, while others should wait until later in fall. For immobile soil nutrients (e.g. soil test P, soil test K), you can start soil sampling anytime. This question centers on the soil nitrate-nitrogen test, nitrogen mineralization, and cover crop nitrogen uptake.
Start soil sampling Prevented Planting fields without cover crop (fallow) as soon as possible. It is important to prioritize soil sampling on the fallow fields while you can still drive across them. Since these fields have no plant growth to use excess soil water through fall, the field trafficability might be challenging if we get another fall with excess precipitation. To ensure you can collect high quality soil samples on fallow fields, start soil sampling in August and early September.
For Prevented Planting fields with cover crop, wait until plant growth and nitrogen uptake has slowed. A healthy cover crop can take up a lot of nitrogen through the fall. In some NDSU cover crop project trials, fall-planted cover crop mixes contained 100 to 150 lb/acre nitrogen in the plant biomass, which is a sizeable amount of nitrogen that would not be measured as soil nitrate-nitrogen. To best reflect the amount of residual soil nitrate-nitrogen available for next year, wait until cover crop nitrogen uptake has slowed in October. If more precipitation arrives in fall, the cover crop will continue to use excess soil water and also provide a nice plant residue surface to drive on.
- Do you have black fallow fields? Start soil sampling now.
- Do you have a cover crop planted? Wait until the cover crop has stopped growing to begin soil sampling.
There are also many questions about cover crop nitrogen credits. AGVISE Laboratories has started a cover crop nitrogen cycling project with a local farmer-cooperator with fallow and cover crop fields. To learn more about soil sample timing and the fate of soil nitrogen, we will measure soil nitrate-nitrogen and cover crop nitrogen uptake through the fall. Next spring, we plan to continue the project and measure cover crop nitrogen release back to soil. We are eager to share the results at the AGVISE Soil Fertility Seminars in January 2021.