The Toxicity of Banded Urea or Diammonium Phosphate to Corn as Influenced by Soil Temperature, Moisture, and pH
1980
Creamer, F. L. | Fox, R. H.
A laboratory and greenhouse study, using a Hagerstown silt loam (Typic Hapludalf, fine, mixed, mesic) was designed to determine the effect of initial soil pH, soil temperature, and soil moisture on NH₄⁺ + NH₃-N movement and concentration, pH change, and NO₂⁻-N production and accumulation around urea or diammonium phosphate (DAP) banded at 80 or 100 kg N/ha (assuming 76 cm between bands). Observations were also made in the greenhouse on corn root growth around banded urea plus triple superphosphate (TSP), NH₄NO₃ plus DAP, urea plus DAP, urea plus DAP with 100 kg Mg/ha, and urea plus DAP with a nitrification inhibitor (nitrapyrin). Root growth around those treatments was compared to banded NH₄NO₃ plus TSP and no-starter controls. Nitrogen and phosphorus were applied at 100 kg/ha and 60 kg/ha in the band, respectively. Increasing the initial soil pH and decreasing soil moisture increased the proportion of total NH₄⁺ + NH₃-N present as free NH₃ around banded urea or DAP. Lowering soil temperature appeared to inhibit nitrification and help maintain high pH and NH₄⁺ + NH₃-N concentrations for a longer time. With both urea and DAP, NH₃ toxicity appeared to be the main cause for root growth inhibition around the band. Magnesium did not decrease toxicity symptoms nor did NO₂⁻ accumulate to toxic levels. Ammonia toxicity potential from DAP was lower than an equivalent amount of urea due to the smaller pH increase adjacent to the band of DAP. At a banded rate of 100 kg N/ha, urea increased the pH in Hagerstown silt loam from 6.8 to 9.1 whereas DAP increased pH to only 7.3.
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