With several million acres not seeded in the plains this year, there has been a lot of interest in the soil nitrogen status of fields that did not get seeded. To give some insight on how the soil nitrogen levels changed during the growing season on these fields, AGVISE did a demonstration project on two fields in the Northwood ND area. A site in each field was sampled each week, starting in early July. Even though these fields were only 2 miles apart, you can see in the figure, that the soil nitrate levels through the season were quite different. The grower practices are the reason the soil nitrate level in these fields are so different. Field one had chemical weed control in early June and was tilled in July and August. As you can see, having good weed control and tillage resulted in quite a bit of soil nitrate in the soil profile. Controlling the weeds reduced the amount of soil N removed from the soil, and each tillage created a flush of microbial activity and nitrogen mineralization, adding to the soil nitrate in the soil profile.
Field two did not have chemical weed control until mid-July and the weed growth was mowed in August. There was no tillage on this field all summer and fall. Weed growth used all of the soil nitrogen in the profile and there was no tillage to stimulate nitrogen mineralization. The result was not very much soil nitrate in the profile at the end of the season.
This demonstration project is a good example of why unseeded fields in the same area can have soil nitrate levels that are very different. The management practices will have a large effect on the amount of soil nitrate left in the soil profile this fall.