Controlled-Release Fertilizer Effect on Potato and Groundwater Nitrogen in Sandy Soil

Controlled-release fertilizer, specifically polymer coated urea (PCU), may reduce nitrate-nitrogen (NO3-N) leaching to ground water; however, few if any field scale studies have been performed in Wisconsin on sandy soils to validate these assertions. A 2-yr field experiment was conducted at the Hancock Agricultural Research Station using potato (Solanum tuberosum L.) planted in Plain field sand. Experimental design was a randomized complete block with three replicates and four N fertilizer treatments: (i) no N, (ii) 224 kg N ha(-1) as PCU, (iii) 280 kg N ha(-1) as PCU, and (iv) 280 kg N ha(-1) split-applied as ammonium sulfate [(NH4)(2)SO4] and ammonium nitrate (NH4NO3) fertilizer. Three groundwater monitoring wells were installed in each plot, sampled weekly during the growing season, and analyzed for NO3-N, ammonium (NH4-N), and dissolved organic N. Potato N uptake and yields were similar among conventional fertilizer and the PCU treatments; PCU may only be economically viable when applied at lower than recommended rates. Significant increases in the partial nutrient balance (PNB) were calculated for 224 kg N ha(-1) of PCU compared to 280 kg ha(-1) of PCU or conventional fertilizer. These large gains in N use efficiency were not reflected in the groundwater quality. The plot-to-plot variation in groundwater nitrate concentration was too large to statistically determine differences among fertilizer treatments. Therefore, relying on measurements of N uptake and N use efficiency are the clearest way to assess impacts of N fertilizer on groundwater quality.