Calcium Stimulation of Ammonium Absorption in Onion

Abstract

AbstractThe 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 NH+4 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 15NH+4 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 NH+4) with Ca2+:NH+4 molar proportions of 0, 0.25, 0.50, 1.00, and 2.00. The 15NH+4 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 15NH+4 studies, NH+4 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+:NH+4 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 15NH+4 absorption tests over the 0 Ca2+ controls. In greenhouse growth tests, vegetative matter increased as much as 40% with increasing Ca2+ concentrations at constant NH+4 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 15NH+4 absorption experiment and from the long‐term greenhouse growth trial.