Sampling depth depends on the crop for which the soil sample is being used to make N recommendations. For corn and small grains, University of Minnesota recommends a soil sample to a depth of two ft. The recommendation for sugar beet is four ft. The depth reflects the improved ability to predict the N needs for each crop.
The time when a soil sample is taken in the fall is even more important. Soil samples should not be taken when the soil temperatures at the four to six in. depth are greater than 50 degrees. This normally occurs in mid-October in central Minnesota and the last week of October on the Iowa-Minnesota border.
The conversion of organic nitrogen to nitrate-N is still occurring at a significant rate when soil temperature is greater than 50 degrees. Studies indicate that the conversion process actually does not stop until the soil temperature is 43 degrees. This conversion is called mineralization.
The soil nitrate-N content from a sample taken in September will usually be less than the soil nitrate-N content from a soil sample taken in mid-October. The recommendation from the sample taken too early will be greater than what the crop needs. This creates an extra expense for the grower for unneeded N fertilizer for corn and small grains and in the case of sugar beet, a reduction in quality and a resultant reduction in income.
This fall, the conversion of organic N to nitrate-N is of more concern because of the drought in July, August, and part of September. The September rains, we have experienced, causes increased mineralization. This makes results from early soil samples less accurate.
In the drier climates of the Great Plains, substantial changes in soil nitrate-N over the summer and fall have been documented. Research in Nebraska and Colorado indicate that the changes can be reliably predicted and thus the soil sample results can be adjusted for an early sampling date. The reliability of these results is caused by the lower rainfall that occurs in the Great Plains and the lower organic matter contents of the soils. In more humid climates, the results cannot be adjusted reliably. Recently two soil sampling studies were conducted with the objective of understanding the effect of sampling date on soil nitrate-N.
Results from a study conducted in Northwestern Minnesota in cooperation with Minnesota Extension Service Educators where six different fields representing several tillage systems and previous crops were sampled to a depth of two feet between early August and late October shows there were no consistent trends in the nitrate-N content over time with some of the changes in nitrate-N between sampling dates as large as 75 pounds per acre. The soil samples were taken from the same location in the fields each time and analyzed for nitrate-N.
In view of this, a soil sample for nitrate-N should be taken after the soil temperatures are less than 50 degrees at the 4 to 6 inch depth. Results taken before that time cannot be reliably adjusted.