Effects of fertilizer timing and placement on grain yield and leaf nutrients of sole‐crop cowpeas (V. unguiculata) in Sierra Leone

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Abstract
Effects of fertilizer timing and placement on the grain yield and leaf nutrients of uninoculated sole-orop cowpeas in 2 experiments during the minor cropping season on Njala upland soil of Sierra Leone [Africa] were studied. Timing, placement and their interaction had no significant effects on grain yield. Placement affected lamina Mg, Cu and Co highly significantly and lamina N and Ca. Placement and timing .times. placement interaction affected highly significantly petiole N and petiole P, respectively. Timing affected highly significantly petiole N, which decreased as fertilizer application was delayed. The spectrum of nutrient distribution in both lamina and petiole was decidedly different. Multiple regression studies indicated that variability in only petiole Mg, Cu and Co significantly contributed to grain yield variability. By the introduction of 15 nutrients (6 lamina and 9 petiole) after examining the linear regression coefficients, into a composite multiple regression study, lamina Ca, petiole Fe and petiole Co were selected as significant contributors to grain yield variability with marked improvement in R2. Removal of the effects of the correlated variables produced significance in the effects of fertilizer placement and timing .times. placement interaction on grain yields.