SUBSOIL NITRATE UPTAKE BY GRAIN PEARL-MILLET

Pearl millet [Pennisetum glaucum (R.) Br.] is a high-quality grain cro p that can that can be grown without irrigation during hot, dry summer s in the southeastern United States. The N rate required for optimum p earl millet grain yield varies widely in published studies, possibly b ecause of variability in residual soil Iri from the subsoil. Field stu dies 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 Imp ermeable subsoils. Nitrate N (1 M KCI-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 held plots with two levels o f N (0 and 168 kg ha(-1)) and studied root development during the grow ing seasons in 1993 and 1994. Soil cores were taken 40 d after plantin g to a depth of 1.2 m, separated into 0.3-m depth segments, and root l ength density was measured. Roots were found in the deepest segments. Nitrogen fertilization increased the proportion of the root length den sity found in the surface soil in one year. We also injected N-15 as 6 0% atom N-15-enriched KNO3 at a depth of 1 m at 30 and 40 d after plan ting. At physiological maturity, atom % N-15 abundance in the plants r anged from 10.1 to 12.0 where N-15 was injected and was near natural a bundance where no N-15 was injected. These results indicate that pearl millet can extract N from 1 to 1.2 m in an acidic and impermeable sub soil.