Nodulation is the ability of a plant to fix atmospheric nitrogen for growth and development. A pot experiment on the effect of population density on the nodulation of cowpea plants Vigna unguiculata L. Walp was carried out at the Institute for Agricultural Research/Faculty of Agriculture, Ahmadu Bello University, Samaru - Zaria, in Northern Nigeria's, Sudan Savanah Ecological zone, located (11° 11'N and 7° 40'E). Ten cowpea population levels (1, 2, 3, 4, 5, 6, 7, 8, 9 and 10) per pot, were laid out in a completely randomized block design replicated 3 times. The mean total nodules per plant according to the plant population per pot decreased with increase in number of plants per pot. Values of 81.0, 53.5, 63.2 26.7, 38.8, 25.0, 33.0, 28.8, 22.2 and 14.3 per pot were observed. The total effective nodules per plant also followed the same trend, 54.2, 47.7, 39.0, 30.9, 50.0, 40.5, 66.2, 35.8, 27.1, 32.2 for the ten treatments respectively. The populations at which nodulation and probably nitrogen - fixation could be maximized were between 1 and 3 plants per pot.
