Source: University of Minnesota
Split applying N in contrast to a single preplant application has gained a fair amount of attention in the media and in some policy-making audiences in the last 10 years. Reasons for this include:
- Delaying the application of N until closer to the time of maximum N uptake by the crop lowers the potential for N loss from the soil
- Greater yields and N use efficiency are often claimed with split application
- Profitability can be improved when yield responses are greater than input costs
- Split application has been identified as a best management practice (BMP) on coarse-textured soils.
However, labor and time demand is greater and application costs are higher with split application.
Research results from a specific soil/climate scenario, such as irrigated sandy soils, have often been used to document the advantages of split application. As BMPs and nutrient management plans (NMPs) are developed to assist crop producers across the state, precise information is needed for a range of soil/climate conditions.
Because economic and environmental concerns with single preplant application of N exist, 32 field experiments were conducted in 17 counties from 1989 to 1992 to determine the likelihood of improved corn yields with split application of N compared to single preplant application and identify the soil and climatic conditions that lead to consistent responses and benefits to split application for use in developing BMPs. Thirty two field experiments were conducted in 17 counties from 1989 to 1992. Fourteen sites were located on fine-textured, glacial till primarily in south-central Minnesota. Eleven sites were on medium-textured, loess soils in southeastern Minnesota while sevem sites were on coarse-textured, sandy outwash soils primarily in central Minnesota. Normal annual rainfall for the area covered by the 32 sites ranges from 24 inches on the west to 32 inches on the east. However, during this 4-year period rainfall at the sites ranged from 36% below normal in 1989 to 59% above normal in 1991; 1990 and 1992 were also wetter than normal.
Nitrogen as urea was broadcast-applied preplant and incorporated at rates of 0, 30, 60, 90, 120, 150 and 180 pounds N/A at each site. In 1989 and 1990, the split applications consisted of 60 pounds N/A preplant plus either 30, 60, or 90 pounds N/A knifed-in when the corn was about 12 inches tall. In 1991 and 1992, the preplant rate was only 30 pounds N/A with sidedress rates of 30, 60, and 90 pounds N/A. The sidedress treatments were always knifed in about 4 inches deep midway between the rows. Four replications were always used. Measurements included grain yield, moisture, and N content; stover yield and N content; and preplant residual soil nitrate
Corn yield response to N occurred at 28 of the 32 sites. All 14 of the fine-textured sites responded to N while 2 loess sites and 2 outwash sites did not. Yields were not different between the preplant and split applications at 16 of the 28 or 58% of the responding sites. Of the 12 sites where a yield difference did occur, yields with split application were higher than with preplant application at 8 sites. Four of these sites were on glacial till soils, 1 on loess, and, as expected 3 on the sandy, outwash soils. Yields were higher with preplant application compared to split application on 3 glacial till sites and 1 loess site. When summarized according to soil characteristics, yields with split application were higher on 28% of the glacial till soils, 11% of the loess soils, and 60% of the outwash soils. In all cases, growing season rainfall ranged from 43 to 59% above normal and/or the soils were sandy. When preplant outyielded split application, rainfall was either below normal (-37%) or insufficient preplant N (30 pound/A) was accompanied by high rainfall (16 to 46% above normal). A relationship between previous crop and the performance of preplant vs. sidedress N application was not found.
In 1991, at a site in Waseca County where May-September rainfall was 56% above normal, yields were increased an average of 11 bu/A by the split-applied treatments. It is likely that a significant portion of the preplant-applied N was denitrified in this wet year. In 1992, at a site in Blue Earth County where rainfall was only 16% above normal, yields were decreased an average of 11 bu/A by the split-applied treatments. Under these conditions, some N-deficient corn was visable at the time of the sidedress application (12-inch tall corn). Apparently the initial 30-pound preplant rate was insufficient to sustain an adequate supply of N to the plant early in the season. The plants never seemed to recover completely. In addition, rainfall later in the season did not appear adequate to move N into the active root zone to overcome the early-season deficiency.
- Corn grain yields with preplant application of N were equal to those with split application at 58% of the 28 sites during this 4-year period.
- Split application was superior to preplant application at 28% of the sites. Excessive rainfall and/or sandy soils were the common characteristics at each site.
- Preplant application was superior to split application at 14% of the sites. Above-normal rainfall coupled with an insufficient early N rate (30 pounds N/A), below-normal growing season rainfall, and medium or fine-textured soils were the common characteristics.
- Split application of N outperformed preplant application at 3 of 5 outwash sites (60%), only 1 of 9 loess sites (11%), and 4 of 14 glacial till sites (28%) and only in wetter-than-normal years.
- Nitrogen timing BMPs must be tailored to soil and climatic conditions. Factors such as extra labor/time demand, equipment needed, carryover of unused N, potential for using a soil N test to determine sidedress rate of application, and input/output economics must be carefully considered on those soils where a yield response to split application is less likely.