Phosphorus Requirements of Soybean and Cowpea as Affected by Mode of N Nutrition1
- K. G. Cassman,
- A. S. Whitney and
- R. L. Fox2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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