Is carryover nitrogen still there from last fall?
How far has carryover nitrogen moved in our soils with the moisture we have received mostly this spring. Soil nitrate-N is water soluble, so it can move with the infiltration of the precipitation. However, movement of water and nitrate-N within the soil is complex and all the nitrate-N doesn’t move exactly with the water wetting front.
In a nitrogen study on corn last year at the Southeast Farm near Beresford, after the extremely dry summer, we measured carryover nitrate-N in late August after the corn was cut for silage. The nitrate-N levels were 28, 20, and 16 lbs/a in the 0-6, 6-12 and 12-24 inch depths, respectively. The total two foot nitrate-N sum was 64 lb/a after 100 lb/a of N was applied last spring. The rainfall or snow equivalent since last August was 9.1 inches, with 3.3 inches of that in April. All of this precipitation is not present in the soil as some evaporation would have occurred last fall as well as potential runoff from snow. We again measured nitrate-N the last of April and found 6, 14, 52, and 28 lb/a in the 0-6, 6-12, 12-18 and 18-24 inch depths for a total of 100 lb/a. There was a gain of 36 lbs per acre, presumably from organic matter mineralization last fall. The highest concentration of nitrate last fall was in the top 6 inches (45% of the total), whereas this spring the highest concentration is in the 12-18 inch depth (52% of the total). So with this level of precipitation, the nitrate-N moved about 12 inches in this silty clay loam soil. Even with coarse textured soils, the nitrate-N should be well within the root zone (typically considered 4 foot) with this kind of precipitation levels.
Urea or ammonium fertilizers applied late last fall or earlier this spring would have moved little as these would be in ammonium form which does not move with water.
Of course we still can receive additional rainfall this spring before plants begin to take up significant water. However, larger rainfalls typically don’t move soil nitrate-N as much as smaller, gentler rainfalls. Runoff is much more prevalent with heavier rainfalls, therefore infiltration is less. In addition, when the soil becomes saturated, water tends to enter the soil and flow in macro pores. These large pores allow water to bypass much of the soil nitrate that is stored in medium and fine textured pores where water tends to move much more slowly.