Subsoil Nitrate Uptake by Grain Pearl Millet

Abstract

AbstractPearl millet [Pennisetum glaucum (R.) Br.] is a high‐quality grain crop that can that can be grown without irrigation during hot, dry summers in the southeastern United States. The N rate required for optimum pearl millet grain yield varies widely in published studies, possibly because of variability in residual soil N from the subsoil. Field studies were conducted in the southern Coastal Plain at Tifton, GA, on a Tifton loamy sand (fine‐loamy, siliceous, thermic Plinthic Kandiudult) with nitrate accumulations near subsoil plinthite. The objective was to determine if pearl millet can extract N from acidic, relatively impermeable subsoils. Nitrate N (1 M KCl‐extractable) was 4 mg kg−1 in the surface 0.6 m, 1 mg kg−1 in the next 0.6 m, and 6 mg kg−1 from 1.2 to 1.5 m. We fertilized pearl millet field plots with two levels of N (0 and 168 kg ha−1) and studied root development during the growing seasons in 1993 and 1994. Soil cores were taken 40 d after planting to a depth of 1.2 m, separated into 0.3‐m depth segments, and root length density was measured. Roots were found in the deepest segments. Nitrogen fertilization increased the proportion of the root length density found in the surface soil in one year. We also injected 15N as 60% atom l5N‐enriched KNO3 at a depth of 1 m at 30 and 40 d after planting. At physiological maturity, atom % 15N abundance in the plants ranged from 10.1 to 12.0 where 15N was injected and was near natural abundance where no 15N was injected. These results indicate that pearl millet can extract N from 1 to 1.2 m in an acidic and impermeable subsoil.