Effect of phosphorus and pH on dry matter production, dinitrogen fixation and critical phosphorus concentration in pigeon pea (Cajanus cajan(L) millsp.)

A pot experiment was conducted under greenhouse conditions during the summer to determine the effect of the P content and pH of the culture medium on dry matter production, dinitrogen fixation, and critical P concentration in pigeon pea (Cajanus cajan (L) Millsp.) 1. Dry weight in plant parts increased with increasing P application rates. Whole plant weight did not increase significantly above the 0.50 g/pot P application at 92 days while it increased rapidly up to 5.0 g/pot P application at 132 days. At low P levels (0–0.5 g/pot) differences in the whole plant weight with respect to the pH were not significant but at high P levels (2.5 and 5.0 g/pot) the whole plant weight of the plants cultured at pH 6 was slightly higher than that of the plants culture at pH 4. The highest pod weight was achieved at pH 4.2. P application significantly increased the nodule weight and dinitrogen fixation. At a given P level, the acetylene reduction activity tended to be higher in the pH 4 plants than in the pH 6.3. The critical P concentrations on a dry weight basis at 92 days in plant tops, leaves, pods, stems, and roots were 0.15, 0.13, 0.27, 0.05, and 0.09%, respectively. Values were almost similar at pH 4 and 6. However, critical P concentrations were not clearly observed at 132 days.4. At the green maturity stage, P absorbed was mainly distributed to pods. The amount of P translocated from the leaves to the growing pods and grains was higher at low P levels than at high P levels after anthesis.