THE TOXICITY OF BANDED UREA TO CORN GROWT H AND YIELD AS INFLUENCED BY TRIPLE SUPERPHOSPHATE

Banding of urea in corn (Zea mays L.) production is restricted by toxi c conditions from urea hydrolysis, but higher urea rates could reduce application cost. A growth chamber experiment and a field experiment w ere conducted to study toxicity of banded urea when applied with tripl e super-phosphate (TSP). In the growth chamber experiment, surface sam ples from a Ste. Rosalie clay (Humic Gleysol), a Chicot sandy clay loa m (Grey-brown Podzol), and an Uplands sandy soil (Humo Ferric, Podzol) , were used. The field experiment was conducted on a Ste. Rosalie clay , a St. Bernard loam (Eutric Brunisol), and a Chicot sandy clay loam s oil. In the growth chamber experiment, banded urea increased soil pH t o 7.2-7.5 at a distance of up to 4 cm from the fertilizer band, and NH 3 concentrations were as high as 2.1 mM at 2 cm from the fertilizer ba nd after 20 d. The high pH and NH3 concentrations from urea hydrolysis inhibited corn root and seedling growth. Banding TSP with urea reduce d soil pH increases from urea hydrolysis and reduced NH3 concentration s. Root weights increased with added TSP in urea bands. In the field e xperiment, banding urea at > 40 kg N ha(-1) reduced plant density and shoot dry weight in early growth stages. Banding TSP with urea increas ed seedling growth in all soils. Maximum rates of urea compatible with increased corn yield could be increased from 40 to 80 kg ha(-1) when banded with TSP.