CALCIUM STIMULATION OF AMMONIUM ABSORPTION IN ONION

The inefficiency of N use in soils has been studied widely, producing varied suggestions for increased effectiveness. The object of this research was to examine the use of Ca2+ as a means to increase NH4+ absorption, using onion (Allium cepa L.) as a test crop. Ammonium absorption in the presence of variable Ca2+ concentrations was measured in short-term greenhouse studies as well as longterm greenhouse studies and field-production trials. The greenhouse (NH4+)-N-15 absorption study was conducted in 4-L pots containing 3.5 kg of calcareous Gila sandy loam (coarse-loamy, mixed, calcareous, thermic Typic Torrifluvents). The long-term growth study was conducted in calcareous quartz sand. Irrigation solutions consisted of half-strength nutrient solutions (5 mol m-3 in NH4+) with Ca2+:NH4+ molar proportions of 0, 0.25, 0.50, 1.00, and 2.00. The (NH4+)-N-15 absorption study was conducted during a 30-h period on 60-d onion. The long-term growth experiment received the same nutrient solutions until mature onion bulbs were produced (about 5 cm in diameter). A 2-yr field production trial used single rows on 1-m wide beds. The soil was a calcareous Saneli silty clay loam [clayey over sandy or sandy-skeletal, montmorillonitic (calcareous), thermic Vertic Torrifluvent]. In (NH4+)-N-15 studies, NH4+ absorption increased with added Ca by as much as 189% in onion bulbs and by 67% in onion tops relative to absorption with no Ca added. These maximum increases were in association with solution Ca2+:NH4+ molar proportions (MP) of 0.5 in onion bulbs and 1.0 in onion tops. Total plant dry weights with Ca added increased as much as 100% in bulbs and 25% in tops in 30-h (NH4+)-N-15 absorption tests over the 0 Ca2+ controls. In greenhouse growth tests, vegetative matter increased as much as 40% with increasing Ca2+ concentrations at constant NH4+ levels compared to the 0 Ca2+ control. This translated to increased total N absorption. In 2 yr of field testing with a MP of 0.25, mature bulbs from both years increased 64% over yields with urea alone. Thus the addition of Ca2+ with urea produced significantly improved plant N-use efficiencies (greater plant yield at the same level of N application). These results were suggested from both the short-term greenhouse (NH4+)-N-15 absorption experiment and from the long-term greenhouse growth trial.