Phosphorus Requirements of Soybean and Cowpea as Affected by Mode of N Nutrition1

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Abstract
A field experiment was conducted on a tropical soil (Humoxic Tropohumult) with a high P sorption capacity to compare the critical external and internal P requirements of soybean (Glycine max (L.) Merr.) and cowpea (Vigna unguiculata (L.) Walp.) as affected by the predominant mode of N nutrition during crop growth. The experiment had a split‐plot design with two N‐level subplots established within each of six P‐level mainplots. Phosphorus treatment ranged from 0.0015 (unamended soil) to 0.08 (1,880 kg P/ha) µg P/ml in 0.01M CaCl2 solutions equilibrated with soil for 6 days. Nitrogen levels were either deficient (plants primarily dependent on N fixation) or sufficient (N fertilizer supplied at rates sufficient to satisfy the crop N requirement). Nitrogen‐fixing soybeans required 750 kg P/ha to obtain a 90% relative yield which was 320 kg P/ha more than that required by N‐supplied plants to obtain a comparable relative yield. The P concentration of N‐fixing soybean plants was significantly lower than that of N‐supplied plants at all levels of applied P fertilizer. The external P requirement and tissue P concentration of cowpea were unaffected by soil N level. The data show that cowpea was more tolerant of P stress than soybean, especially when dependent on N fixation. The cowpea cultivar grown without P or N fertilizer yielded 72% of the maximum yield obtained at optimum P levels while the comparable relative yield for the soybean cultivar was 28%. We conclude that (i) some N‐fixing grain legumes can make respectable yields with little or no P fertilizer while others might not and, (ii) screening N‐fixing grain legumes for tolerance to nutrient stress should be conducted on N‐deficient soil to Insure that nutritional requirements are assessed for the N‐fixing plant, especially on the highly weathered soils of the tropics.