Effects of phosphorus supply on the growth and nodulation of cowpeas.
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The effects of P supply (0, 20, 40, 80, 160 and 320 kg/ha) on growth and nodulation of cowpeas (Vigna unguiculata) cv. Vita 4 and Ife Brown grown in a podzolic soil (Haplustult) was studied in a greenhouse trial. The seeds were inoculated with Bradyrhyzobium CB 756. Plants were harvested at the beginning of flowering. The number of trifoliate leaves and DM yield in both cv. increased with increasing P application. The yield of the tops of Vita 4 was maximum at 160 kg P/ha and that of Ife Brown at 320 kg P/Ha. Extractable P related linearly to the rate of applied P, and DM yield increased with increase in extractable P. Plants grown without P had fewer and smaller nodules. An increase in P from 1 to 160 kg/ha increased the number of nodules/plant from 16 to 113 in Vita 4 and from 14 to 70 in Ife Brown. Similarly, increasing P increased nodule dry weight. The P conc. of the youngest fully expanded leaf (YFEL) and in the whole plant top increased with increasing P supply in both cv. and was generally higher in the YFEL than in the whole top. A critical P conc. of 0.3 and 0.25% was found in the YFEL and the whole top, resp., for cv. Vita 4.