The Toxicity of Banded Urea or Diammonium Phosphate to Corn as Influenced by Soil Temperature, Moisture, and pH

Abstract

AbstractA 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.