USE OF COWPEA AS A GREEN MANURE TO REDUCE EXTERNAL NITROGEN INPUTS IN A FALL BROCCOLI CROP

Abstract

Cowpea [Vigna unguiculata (L.) Walp.] was grown as a green manure preceding a fall crop of broccoli [Brassica oleracea L. (Italica Group)] in 1992 and 1993. Urea was used to supply 0, 84, or 168 kg·ha-1 of supplemental nitrogen (N) to broccoli which followed cowpeas. Control broccoli plots were grown on fallowed ground and were supplied with 168 kg·ha-1 of N from urea. Cowpea incorporation added over 100 kg·ha-1 of N to the soil in both years. In 1992, treatments had no significant effect on yield of marketable broccoli heads, but average head weight was reduced in the absence of external N. In 1993, broccoli yield was reduced following cowpeas except when 168 kg·ha-1 of external N also was supplied. Broccoli which followed cowpeas was delayed in maturity by 5 to 9 days in 1993. We conclude that a preceding cowpea crop alone will not supply enough N to sustain acceptable broccoli production. Further studies will determine how much external N must be supplied when broccoli follows cowpeas, and the best timing for supplying external N. We also will Investigate possible detrimental effects of the preceding cowpea crop on the broccoli.