Effectiveness of Split Nitrogen Applications in the Midwestern U.S.

Rationale and Objectives

Nitrogen (N) fertilizers are used to supplement the soil’s natural N supply to increase corn grain yield and maximize economic profit.
However, the use of N fertilizer has been associated with increased nitrate levels of ground and surface waters. High nitrate levels can cause negative environmental, human, and animal health effects.
Splitting N fertilizer across multiple application timings during the growing season is a suggested method to lower the risk of N loss by applying N fertilizer in amounts and at timings that more closely match corn N uptake.
The objectives of this study were:
- Determine the effects of split N application on soil nitrate (NO₃-N) at VT, N uptake at maturity, and corn grain yield.
- Classify environmental conditions in which split N application is likely to increase corn yield.
- Compare the effect of split N application when a low or high N rate is applied near planting on soil N concentrations, corn N uptake, and corn yield.

Study Description

Research was targeted to two sites (representing higher- and lower-yielding environments) in each of eight Midwestern states over three years (2014-2016). A total of 49 site-years were included in the study (Figure 1).

Map showing research locations across eight states included in a 3-year split corn nitrogen application study (2014-2016). — **Figure 1**. Research locations across eight states included in the 3-year split nitrogen application study (2014-2016).

Study Factors:

Total Nitrogen Rates:

160 lbs/acre (near economic optimum rate)

240 lbs/acre (above economic optimum rate)

Nitrogen Application Timings:

S - Single application near planting

40+SD - 40 lbs at planting + remainder at V9 side-dress

80+SD - 80 lbs at planting + remainder at V9 side-dress

Data Collection

Soil Sampling: Percent sand, silt, and clay, cation exchange capacity (CEC), organic matter, organic C, total N, pH, and bulk density (0-1 ft). Soil NO₃-N at VT development stage only at 160 lbs/acre (0-2 ft).
Plant Sampling: N content at physiological maturity and grain yield.
Weather Measurements:
- Daily: Minimum and maximum temperature and precipitation.
- Calculations: Growing degree-days, cumulative precipitation, and Shannon diversity index (SDI) (SDI = 1 implies complete evenness (i.e., equal amounts of rainfall in each day of the period); SDI = 0 implies complete unevenness (i.e., all rain in one day)).

Results - Soil NO₃-N at VT

Nitrogen fertilizer application timing did not affect soil NO3-N at VT (65-77%) in the majority of the sites (Figure 2).
In the sites where application timing affected soil NO3-N at VT, single N applications were always less than split N applications. Thus, splitting up the application of N only increased the amount of soil NO3-N available at VT in a small percentage of sites.
The amount of N fertilizer applied at planting (40 vs. 80 lbs/acre within the split N applications did not alter the amount of NO3-N available for the corn crop at the VT development stage in 98% of the sites.
These results demonstrate that applying a low or high rate of N at planting with a split-N application did not significantly alter the amount of soil NO3-N available for the corn at VT.

Chart showing the percent of sites where soil nitrate at tasseling was affected by N application timing at 160 lbs/acre total N application rate. — **Figure 2.** Percent of sites where soil nitrate at tasseling was affected by N application timing (S, 40+SD, and 80+SD) only at the 160 lbs/acre total N application rate.

Results - Nitrogen Uptake at Maturity

Nitrogen timing had no effect on corn N uptake at physiological maturity 77-84% of the time regardless of N rate (Figure 3).
When differences occurred, N uptake was greater 9-10% of the time using split N applications and 6-15% of the time using a single N application.
For split N applications, there were no differences in N uptake at physiological maturity when applying 40 or 80 lbs/acre at planting 98% of the time regardless of the N rate.
When there was a difference, applying less N at planting and more at sidedress (40+SD) led to greater N uptake at physiological maturity.

Chart showing the percent of sites where corn nitrogen uptake at maturity was affected by N application timing at total N application rates of 160 and 240 lbs/acre. — **Figure 3.** Percent of sites where nitrogen uptake at maturity was affected by N application timing (S, 40+SD, and 80+SD) at a total N application rates of 160 and 240 lbs/acre.

Results - Corn Yield

Corn yields ranged widely across the 49 site-years of the study, although yield ranges were similar among nitrogen rates and application timings (Table 1).

Table 1. Corn yield ranges across all site-years of the study associated with N rates and application timings.

Table showing corn yield ranges across all site-years of a study associated with corn nitrogen rates and application timings. — Nitrogen timing had no affect on corn yield in 80-84% of the sites regardless of N rate (Figure 4). When differences occurred, corn yields were greater 14-15% of the time using split N applications and 1-6% of the time using single N applications.

Chart showing percent of sites where corn grain yield was affected by N application timing at total N application rates of 160 and 240 lbs/acre. — **Figure 4.** Percent of sites where grain yield was affected by N application timing (S, 40+SD, and 80+SD) at a total N application rates of 160 and 240 lbs/acre.

Split N applications increased corn yield over single N applications as sand content, bulk density and evenness of rainfall over the season increased greater than the critical values shown in Table 2.

Table 2. Critical soil or weather values where larger values were associated with greater corn yield for split-N applications (40+SD or 80+SD) and smaller values were associated with greater corn yield for single N applications.

Table showing critical soil or weather values where larger values were associated with greater corn yield for split-N applications and smaller values were associated with greater corn yield for single N applications. — † SDI, Shannon diversity index (measured from 30 d before to 30 d after sidedress)

Single N applications increased grain yield over split N applications as clay, silt, CEC, total N, and early season temperatures increased above the critical values shown in Table 3.
Corn yields associated with 40+SD and 80+SD applications were similar 97% of the time. In the remaining 3%, corn yield was less with 40+SD than 80+SD.

Table 3. Critical soil or weather values where larger values were associated with greater corn yield for single N applications and smaller values were associated with greater corn yield for split-N applications (40+SD or 80+SD).

Table showing critical soil or weather values where larger values were associated with greater corn yield for single N applications and smaller values were associated with greater corn yield for split-N applications. — † CEC, Cation exchange capacity.

‡ Mean temp. was measured from planting to V5 development stage.

Conclusions

No differences in corn yield among 40+SD, 80+SD, and single N applications were found in the majority of site-years (77-84%) regardless of N rate.
Overall, split N applications had greater corn yield in areas with consistent rainfall around the time of sidedress application that incorporated the fertilizer, and in soils with greater potential for N loss early in the growing season (i.e. sandy soils that have greater leaching potential).
In general, single N applications had greater corn yield in soils with greater potential for mineralization throughout the season (i.e. greater total N content) and better nutrient and water retention as indicated by greater CEC and silt and clay content.
No differences between 40+SD and 80+SD were found in 97-98% of the sites, indicating that applying 40 lbs/acre near planting is all that may be needed when using split N applications.