Small grain harvest is well underway across the region, and soil testing is progressing quickly. Crop yields have varied from below average to exceeding expectations across the region and often in the same area. Planting date, summer temperatures, and rainfall (too little or too much) were major factors this year.
The major factors influencing the amount of residual soil nitrate-N after crops are:
1. Nitrogen fertilizer rate: too high or too low
2. Crop yield achieved: much lower or higher than expected
3. Nitrogen losses: denitrification and leaching after too much rainfall
4. Nitrogen mineralization from organic matter: cool or warm growing season
Seasonal weather is a large driving factor in the amount of nitrate-N in the soil profile. This changes from field to field and year to year. Early spring weather conditions were very wet across much of the region. In June and July, some areas continued to receive adequate to excess rainfall. Meanwhile, other areas received very little rain in the late growing season.
AGVISE has tested over 10,000 soil samples from wheat fields across the region. The table below indicates the percentage of soil samples in each soil nitrate-nitrogen category in several areas of Manitoba, Minnesota, North Dakota, and South Dakota. The data should give you a general idea of how variable residual soil nitrate is from field to field in each region. With such variable crop yields, there is quite a bit of variability in residual nitrate following wheat in the region. In drought-affected areas of Minnesota, North Dakota, and South Dakota, over 10 to 20% of soil samples have more than 60 lb/acre nitrate-N (0-24 inch soil profile) remaining after wheat.
For Prevented Planting or unseeded acres, the factors above plus some additional factors will affect the amount of residual nitrate-nitrogen:
1. How long was water standing on the field?
2. Was weed growth controlled, early or late?
3. Was tillage used? How many times? How deep?
4. Was a cover crop planted? What amount of growth was achieved?
When submitting soil samples from fields that were not planted, please choose “Fallow” or “Cover Crop” as the previous crop. This will allow us to send additional information on soil nitrate trends for unseeded and cover crop fields once we get enough data.
As the fall soil testing season continues, we will keep you updated. If you have any questions, please call our experienced agronomic staff. We hope you have a safe harvest and soil testing season.