Crop Productivity and Nitrate Leaching in Coarse-Textured Soils as Affected by the Use of Polymer Coated Urea and Deep Tillage

Abstract

In Minnesota, cultural practices for crops that are commonly grown on coarse textured soils are inherently at risk for increased nitrate (NO3) leaching to groundwater. Polymer coated urea (PCU, Agrium U.S. Inc) was tested as a nitrogen (N) source for potato (Solanum tuberosum L.) and dry edible bean (Phaseolus vulgaris L.) production. PCU slowly releases N over time, attempting to match plant N uptake, and therefore should reduce NO3 leaching. PCU was compared with uncoated urea across equivalent N rates and application times for both crops. For dry beans, tillage management is also important to reduce disease severity. Previous studies have reported that deep tillage to break up restrictive soil layers can reduce the incidence of disease, so deep and shallow tillage treatments (chisel plowed to 47 and 29 cm, respectively) were also tested. Variables measured included yield, NO3 leaching, plant N accumulation, net monetary returns, tuber quality for potato, and disease severity for bean. Nitrogen release from PCU over time was also characterized for each crop. For potato, over 90% of applied N had been released from PCU by potato vine harvest. Emergence applied PCU resulted in similar potato yields and tuber quality to those produced by split applications of urea at emergence and post-hilling. At equivalent N rates, PCU applied at potato emergence significantly reduced NO3 leaching and improved fertilizer N recovery compared with split applied urea, even though conditions were dryer than normal. Net monetary returns for potato were similar when fertilized with urea or PCU. For dry bean production, tillage depth generally did not affect grain yield, NO3 leaching, plant N accumulation or disease severity. The N release rate study determined that over 90% of applied N had been released by bean harvest, but evidence suggests that emergence applied PCU had released less than 50% of N when maximum plant N accumulation began. Emergence applied PCU resulted in lower bean yields compared with split applications of urea at emergence and prebloom, and generally reduced net monetary returns. Under wet conditions, PCU applied at emergence significantly reduced NO3 leaching while plant N uptake was similar to split applied urea treatments. In dry conditions where leaching mainly occurred post-harvest, however, PCU applied at emergence tended to result in lower plant N accumulation and more NO3 leaching compared with split applications of urea. Planting applied PCU resulted in similar bean yields, net monetary returns and plant N uptake as split applied and planting urea, and generally reduced NO3 leaching regardless of leaching conditions, although differences were not always significant. This suggests that timing of PCU application is important to maximize yields, and varies depending on crop. Overall, when managed properly, PCU has the potential to reduce NO3 leaching while maintaining crop yields without a reduction in net monetary returns.