Tracking Soil Nitrogen Fertility across Minnesota.

Sufficient in-season soil nitrogen (N) availability is essential to produce high yielding corn (Zea maysL.). Minnesota receives much of its yearly rainfall in the early spring between the time of planting and rapid plant N uptake. Depending on the form of N, soil texture, and the rainfall patterns, N loss may be significant resulting in poor corn yields. Our study examined the effects of multiple fertilizer rates, sources, and application timings on in-season soil fertility dynamics across fifteen site years in Minnesota. We also measured plant development and yields to calculate N use efficiencies (NUE).

Predictive modeling indicated that soil (0-30 cm) nitrate at the V4 development stage predicted yield as well or better than any other combination of N form, depth, or sampling times. The N rate and soil nitrate concentration required to produce optimal yields varied by site and ranged from 135-250 kg N ha^-1 with soil nitrate concentrations of 17-37 mg kg^-1 (0-30 cm). Sites that required larger N rates and soil nitrate concentrations experienced substantial N loss.

Irrigated sands had significant leaching losses, especially with urea based products. We found that pre-plant urea should be avoided in favor of other products such as anhydrous ammonia (AA) or enhanced efficiency products such as polymer coated urea (PCU) which improved yields by 157%. Split-applying urea was even more effective with yield improvements of 197% compared to a single pre-plant application.

Several sites with fine-textured poorly-drained soils experienced denitrification losses following heavy spring rains. Polymer coated urea and PCU-urea blends reduced N losses and improved yields by 120% over pre-plant urea. When rainfall did not result in ponding, there were no significant differences in yields between N sources. Regardless of spring precipitation, there were no differences in yields between fertilizer application timings across all fine-textured soils. These findings may suggest that fine-textured soils are capable of supplying sufficient in-season soil N until supplemental N is supplied, without taking a significant reduction in yield.