Effect of Salinization, Phosphorus Fertilization and Their Interaction on the Symbiosis between Rhizobium trifolii and Berseem (Trifolium alexandrinum L.)

The studies revealed that salinization of soil had a decisive adverse effect on the symbiosis showing therby a significant reduction in biological yield, nodulation and nitrogen Uptake of berseem crop even at the lowest level of salinity, i.e. ECe 5.0 mmhos/cm. About 50 and 90% yeild reductions were observed at electrical conductivity of 10.0 and 17. 0 mmhos/cm (saturation extract) respectively. The salinity equivalent to ECe 20.0 mmhos/cm proved too high for the crop to survive. It was also evident that highest salinity bearing crop growth (ECe 15.0 mmhos/cm) delayed nodulation and nodules formed were observed to be ineffective in appearance. Phosphorus fertilization, in contrast to salinization, significantly increased the number and dry weight of nodules, yield and nitrogen uptake up to 150 kg P₂O₅/ha during 1971–72 and up to 100 kg P₂O₅/ha during 1972–73. Phosphorus application like salinization increased nitrogen concentration in plants. Salinity × phosphorus interaction with regard to all parameters of symbiosis was found significant showing thereby that responses to phosphorus application were maximum in normal soil and decreased with increasing salinity. At moderate salinity equivalent to ECe 5.0 mmhos/cm, 100 kg P₂O₅ gave the similar symbiotic responses as obtained at phosphorus dose of 50 kg P₂O₅/ha in normal soil. Plant mortality and ineffective symbiosis were specific to salinity and could not be corrected by phosphorus addition.