The role of phosphorus in nitrogen fixation by young pea plants ( Pisum sativum )

The influence of P on N 2 fixation and dry matter production of young pea ( Pisum sativum L. cv. Bodil) plants grown in a soil‐sand mixture was investigated in growth cabinet experiments. Nodule dry weight, specific C 2 H 2 reduction and P concentration in shoots responded to P addition before any growth response could be observed. The P concentration in nodules responded only slightly to P addition. A supply of P to P‐deficient plants increased both the nodule dry weight, specific C 2 H 2 reduction and P concentration in shoots relatively faster than it increased shoot dry weight and P concentration in nodules. Combined N applied to plants when N 2 fixation had commenced, increased shoot dry weight only at the highest P levels. This indicates that the smaller plant growth at the low P levels did not result from N deficiency. The reduced nodulation and N 2 fixation in P‐deficient plants seem to be caused by impaired shoot metabolism and not by a direct effect of P deficiency of the nodules.