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Abstract

Effects of phosphorus supply on nodulation and growth, and temporal variation in critical phosphorus concentration of cowpea, mung bean, and pigeon pea were examined in glasshouse experiments and in the field for cowpea. Initially a glasshouse experiment was conducted to examine the effects of phosphorus on dry matter yield and nodulation of two cowpea cultivars, and to establish critical phosphorus concentrations. The two cultivars responded differently to phosphorus application. Vita 4 responded more strongly than Ife Brown. The limited response in Ife Brown was attributed to boron deficiency. Phosphorus application substantially increased the nodule number and weight in both cultivars. Critical phosphorus concentrations of 0.25% and 0.30% in the whole tops and in the youngest fully expanded leaf (YFEL) respectively of cv. Vita 4 were established at the commencement of flowering. A further glasshouse experiment was used to examine temporal variation in critical phosphorus concentration in cowpea cv. Vita 4, using the (YFEL), two older leaf fractions, and the whole tops as index tissues. Critical phosphorus concentration in all tissues declined with plant age when expressed on the basis of fresh weight, dry weight, or tissue water. Selection of tissues of the same physiological age did not prevent the decline in critical phosphorus concentration with time. The YFEL was identified as the best index tissue for cowpea. A subsequent pot experiment investigated the effects of phosphorus supply on plant growth of cowpea, mung bean, and pigeon pea. Pigeon pea had a lower external phosphorus requirement than cowpea or mung bean. In all species, the phosphorus requirements ranked nodulation > dry matter production > seed yield. Phosphorus deficiency delayed maturity in all species. In cowpea and mung bean, critical phosphorus concentration in the YFEL declined with time; in pigeon pea, no clear trend could be established. In a further glasshouse trial with three soils, critical phosphorus concentration in the YFEL of cowpea declined linearly with time. Critical tissue concentrations were lower in the Wallum sand than in the other two soils. In all three soils, the soil solutionconcentration for 90 % dry matter yield was much higher in the Wallum sand than in the grey podzolic soil or the krasnozem.