Phosphorus fertilizer is frequently fall-applied for corn (Zea mays L.). Diammonium-(DAP) or monoammonium-phosphates (MAP) are the preferred P fertilizers and the N in MAP and DAP is assumed to be available for corn. Our objective was to evalu- ate the effect of application time on availability of ammoniated-phosphate N to the crop in two different environments. In a laboratory study, MAP and DAP at rates of 70 and 140 mg N kg-1soil were incubated at either 80 or 120% field capacity (FC). Nitrification proceeded rapidly in all treatments, but nitrate levels fell to near zero by 8 wk at 120% FC while remaining above 50% after 16 wk at 80% FC. In a 3-yr study at Waseca, MN and Urbana, IL with a factorial combination of three N sources [DAP, MAP, ammonium-sulfate (AMS)], two application times (fall, spring), and two N rates (45, 90 kg N ha-1), N rate and application time, but not N source, affected grain yield and inorganic soil-N concentrations. Yields were 0.71 and 0.37 Mg ha-1 greater for spring than fall applications at Urbana and Waseca, respectively. Late May soil-N recovery ranged from 31 to 35 and 90 to 100% for fall and spring applications, respectively. It thus appears that only about one-third of N applied in the fall as ammoniated phosphates at typical rates is available to the next year's corn crop in Corn Belt mollisols. Colder, drier fall to spring conditions may increase this proportion, while warmer, wetter conditions would be expected to lower availability.
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