Production of commercial nitrogen (N) fertilizers depends on fixing atmospheric N, a process which consumes natural gas. As world energy prices soar, the cost of fertilizer N must also increase, making efficient crop uptake of applied N ever more important. It may be possible to increase the efficiency of crop N utilization by splitting the total crop requirement among two or more applications. In some situations, application of the entire crop N requirement at one time may lead to significant losses to volatilization, leaching, or runoff. In addition to the economic loss, N fertilizer contamination of ground or surface water is considered an environmental pollutant. The following experiment was designed to determine whether splitting N applications would increase the yield of sweet corn relative to applying all N at planting.
Methods
The experiment was first carried out during the 1979 growing season and was repeated in 1980. The variety `Jubilee` was used both years. Planting date was May 14 in 1979 and May 16 in 1980. In both years the plot area received a broadcast application of 200 pounds/acre of 0-0-52; atrazine (1.25 pounds/acre) and alachlor (2.0 pounds/acre) were used for weed control. Total irrigation plus precipitation during crop growth was 16 inches in 1979 and 15 inches in 1980. Between-row spacing was 30 inches; plant population was about 25,000/acre in 1979 and 20,000/acre in 1980. Individual plots consisted of 6 rows x 30 feet. All plots received 300 pounds/acre of 13-39-0 banded two inches to the side and two inches beneath the seed row at planting.
The following nitrogen treatments were applied in randomized block design with six replications. (1) All N at planting; 360 pounds/acre of ammonium nitrate (34-0-0) was broadcast and irrigated in immediately after seeding. (2) Two-way split; the 360 pounds/acre of ammonium nitrate was side-dressed when corn plants were about one foot tall (early July). (3) Three-way split; side-dressed with 180 pounds/acre of ammonium nitrate at the one foot stage and again just prior to tasseling. (4) Four-way split; side-dressed with 120 pounds/acre of ammonium nitrate at the one foot stage, at tasseling, and again at first silk. Thus for each treatment, total N applied was 161 pounds/ acre. Plots were once-over harvested on August 20, 1979, and September 10, 1980. Ears were weighed with husks on and graded as follows: Grade 1, completely filled ears with fully mature kernels; Grade 2, incomplete fill or immature; Grade 3, culls.
Results
In both 1979 and 1980, the differences in total ear weight as a result of treatment were small and not statistically significant. But the yield of Grade 1 ears did respond to treatment in both years (Table 9). In 1979, the yield of Grade 1 ears increased by 49 percent from 7.6 to 11.3 T/A when application of the bulk of the N was delayed until early July. Further splitting of N application did not increase yield. Similarly, a 57 percent yield increase occurred with the two-way split in 1980 and, again, further splitting did not affect yield. The overall lower yields in 1980 were caused by lower plant population and poorer growing conditions.
These results, obtained on a Willamette Sandy shot loam, are in contrast to those of Spencer Apple (personal communication) who found no yield differences caused by timing of N application with 'Golden Cross Bantam' grown on Chehalis silty clay loam. Heavier soils with higher clay content may absorb and retain fertilizer and water more effectively than lighter, sandier soils. The form of N applied may also influence the loss to leaching and volatilization. Application of all N in the ammonium form or as urea may slow leaching loss.
In situations where significant N loss is likely, the economic gain of splitting the application to insure more efficient N utilization must be weighed against the higher costs for an extra application. Applying the additional N through the irrigation system should result in an increased net return provided that the higher cost of liquid N fertilizer solutions does not exceed expected increase in gross because of higher yield.
Table 9. Effect of Timing of N Application on Sweet Corn Yield Yield of Grade 1 ears Total yield (Grades 1 plus 2) Treatment 1979 1980 1979 1980 -----------tons/acre fresh weightZ----------- All N at Planting 7.6a 5.3a 14.3a 9.6a Two Applications 11.3b 8.3b 15.8a 10.4a Three Applications 10.7b 8.4b 15.2a 10.6a Four Applications 10.9b 7.2b 15.0a 10.4a ZMeans within a column which are followed by different letters are significantly different at 99% confidence level.