Millions of acres went unseeded in the region last year. Some fields had massive weed pressure without any control until late in the season while other fields had good weed control. Some fields were tilled several times through the summer, yet other fields had little or no tillage. Rainfall was more than excessive in some areas, while other areas had just a little too much rain to get seeded. All of these variables caused tremendous variability in residual soil nitrate in unseeded fields last fall.
Soil testing went at a record pace last fall. Early on it became obvious that soil nitrate levels were extremely variable from field to field. The best way to show people this variability is to use soil test data to tell the story. AGVISE Laboratories in Northwood, ND tested over 185,000 soil samples last year from all over the region. The nitrogen test results from these samples show that there is more variability than we have seen for many years. This includes cropped fields and fields that were not seeded this year (7-10 million acres in the region). Everyone knows the reasons why fields have different levels of soil nitrate in the fall (even fields within a mile or two). The list includes, different crop yields, different N fertilizer rates, more or less N lost to leaching or denitrification, more or less N gained from soil OM mineralization, differences in weed control, differences in tillage, differences in previous crop, manure applications etc. etc. The list of reasons is very long. Each region experienced different environmental conditions, so the data is broken down by zip code or postal code area. The data in the table is from areas that had the highest amount of unseeded acres in 2011. The first table reflects the nitrogen soil test data from over 30,000 wheat fields tested last fall. The second table reflects the nitrogen soil test data from over 8000 fields which were not seeded in 2011 (fallow/unseeded). In the “fallow” table it is obvious that the soil N test levels vary a lot from field to field. There is also a large percentage of fields testing higher than 60 lb/a, which reflects nitrogen that has accumulated in the soil profile due to fallow practices such as controlling weed growth through the season, and nitrogen released from summer tillage. The value in each column is the percentage of fields testing in each range of soil nitrogen (Example: 18% of wheat fields tested from 0-20 lb/a for the 582 zip code area).
Making a nitrogen fertilizer plan on the unseeded fields is not an easy task. Fields that tested very high in nitrogen (150-200 lb/a) will need some level of nitrogen fertilizer to account for field variability. Even though the field average nitrate may be very high, some parts of the field will need a base rate of 20-40 lb/a to prevent yield loss in those areas that test lower than the field average. It is important to remind growers that fields testing high in nitrogen will likely have some lodging issues, so variety selection will be important as well. Unfortunately, many of the unseeded fields were not soil tested last fall. This means you will be asked to help make an educated guess on the rate of N fertilizer to apply, without the aid of a soil test. You will need to talk with the grower about how each unseeded field was treated last year and make a somewhat informed decision with the grower. Hopefully Mother Nature will cooperate and there will be time to get more of these fields tested this spring before the seed goes in the ground.