Effects of Inoculum Densities of Rhizobium Meliloti and Different Rates of Nitrogen Fertilizers on Alfalfa Plants Grown in Calcareous Soil

The influence of Rhizobium meliloti inoculum density and application of different rate of nitrogen fertilizers, on nodulation, N2-fixation, dry matter and N or P nutrients uptake by alfalfa plants (Medicago sativa L.) grown in calcareous soil of Saudi Arabia was studied in pot experiment. Application of rhizobial inoculum densities up to (9 X 109 cell pot-1 and N-fertilizers (urea or diamonium phosphate denoted as DAP) up to 90 mg N pot-1 significantly enhanced nodulation, nitrogenase activity, dry matter, nitrogen and phosphorus contents in alfalfa plants grown on calcarous sandy soil. Increasing in dry mattcr due 10 application of different inoculum densities (9 X 107 X 108 and X 109 cell pot-1) in soil received N-fertilizer in rate of 90 mg N pot-1, ranged from (60 to 110%) over noninoculated treatment that fertilized with the same rale of N-fertilizer. Highest nitrogen contents were found in plants inoculated with 9 X 109 rhizobial cell pot-1 and fertilized with DAP in rate of 120 or 150 mg N pot-1. The peak of phosphorus concentration and contents in alfalfa plants was found when soil treated with (OAP) in the rate of 150 mg N pot-1and inoculated with Rhizobium with density of (9 X 109) rhizobial cell pot-1. Residual nitrogen and phosphorus concentrations in soil after harvesting of alfalfa crop, tend to increase significantly by ii1creasing both rhizobial inoculum density and N-fertilizer rate. From the economic point of view, this work recommends the usage of (DAP) as a nitrogen fertilizer at the rate of 90 Kg N h-1 along with the rhizobial inoculant at the density level of (5 X 107) . cell seed-1 which equal (72 X 1012) cell Kg-1 seeds