Soil deficiencies of essential elements can dramatically decrease legume nodulation, nitrogen fixation, nitrogen accumulation and yield. Twenty four different locations were selected in Cuddalore and Nagappattinam district for the nodulation patter of black gram in rainfed areas. The other three location namely C.Mutlur, Pinnathur and Nagapattinam that supported good nodulation and also supported fairly in high native rhizobial populations of 77x106, 85 x106, and 60 x106, cells g-1 of moisture free soil respectively. The cell count was minimum in the soil sample of Thirukkuvalai (10 x106 cells g-1 moisture free soil). The present study clearly established the existence of a parallelism between the native soil population of rhizobia and natural nodulation. All these twenty four locations viz., Pichavaram, Killai, C. Mutlur, Mudasal odai, Thillai vidangal, Singarakuppam, Vallam padugai, Pinathur, Kodiyampalayam, Parangipettai, Puthuchathiram, Petiyapattu, Pandaravadi, Thiruvadudurai, Sirkali, Senniyalore, Thirukkuvalai, Nagapattinam, Thiruvalangadu, Vedaranyam, Kuthalam, Tharangapadi, Palaiyur, Palayakoodalore. Were analysed for the pH, EC nitrogen content, potassium content and phosphorus content. These data were used to correlate the natural population pattern and native rhizobial populations of these locations the soil pH 7.52, EC 0.42 and NPK 48.82, 21.20, 210.84 kg/ha-1 recorded high nodule number and native rhizobial population the properties of this soil seems to be very congenital for the multiplication’s of rhizobia and for the nodulation and nitrogen fixation. When the pH, EC and nitrogen content were high and potassium and phosphous were low, native rhizobial population and natural nodulation were less.
