Frequency of Nitrogen Fertilizer Carryover in the Humid Midwest
- Matias B. Vanotti and
- Larry G. Bundy
Soil tests for residual NO-3 can improve prbdiction of crop N fertilizer requirements in the humid Midwest, but their adoption as a standard procedure in fertilizer N recommendation programs depends on the frequency of significant profile NO-3-N carryover. Data from a 25-yr crop sequence experiment (1967 to 1991) on a Rozetta silt loam soil (finesilty, mixed, mesic Typic Hapludalf) at Lancaster, WI, were used to estimate the frequency and extent of N fertilizer carryover. Sequences studied included unfertilized oat (Avena sativa L.) grown after corn (Zea mays L.) that received a range of N rates. Oat yields were influenced by the residual effects of the previous year's N application in 21 of the 25 yr, with yield responses varying from 500 to 1500 kg ha−1. Soil profile measurements (0 to 90 cm) taken before seeding oat in spring 1987 through 1991 showed substantial NO-3-N carryover that was usually found below the 30-cm depth. Significant increases in profile NO-3-N content occurred at low N rates (56 to 112 kg N ha−1) in years with limited rainfall during the preceding corn growing season, but only at higher N rates under optimum corn growing conditions. Total N uptake by oat was also significantly affected by corn N treatments, and it was well correlated with profile NO-3-N. A nutrient efficiency index that characterized the yield response of oat to profile NO-3-N was used to estimate carryover NO-3-N from 1967 to 1986 oat yield data. Expected N fertilizer carryover values (P = 80%) ranged from 32 to 106 for a 168 kg N ha−1 rate, and from 63 to 151 kg NO-3-N for a 224 kg N ha−1 rate. The wide variation of expected amounts of N carryover reflects typical year-to-year changes in conditions affecting N recovery by corn and retention by soil. Results of this study indicate that significant residual profile NO-3-N occurs in most years on well-drained silt loam soils in Wisconsin, and that year-specific tests for profile NO-3-N are needed to adjust N recommendations for varying amounts of NO-3-N carryover.
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