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

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 () 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 Plevel mainplots. Phosphorus treatment ranged from 0.0015 (unamended soil) to 0.08 (1,880 kg P/ha) µg P/ml in 0.01M CaCl 2 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 900/. 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 Nsupplied 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.

Influence of <i>Bradyrhizobium japonicum</i> and Phosphorus on Micronutrient Uptake in Cowpea. A Case Study of Zinc (Zn), Iron (Fe), Copper (Cu) and Manganese (Mn)

“…Its concentration in the soil is usually very low for plant uptake [29]. Supplementation of phosphorus in known to increase its concentration in the soil [30], plant tissues [31] and availability of other nutrients in the rhizosphere [32,33]. From the current study, phosphorus supplementation at different levels significantly improved the uptake of micronutrients in different tissues of cowpea grown under screen house and field condition.…”

Seeds enriched with phosphorus and molybdenum improve the contribution of biological nitrogen fixation to common bean as estimated by 15n isotope dilution

“…Averaged across seed treatments, the highest soil P level increased shoot mass by 100 and 89 % in control and inoculated plants, respectively, whereas the highest P level increased shoot mass only by 16 % in plants with mineral N. It confirms that plants relying on symbiotic N had a larger demand for P to obtain an optimal growth than plants supplied with mineral N (Cassman et al, 1981;Israel, 1987). Plants under mineral N had greater shoot mass than the control and inoculated plants at both soil P levels, but the improved growth associated to mineral N was more intense at low soil P levels (Figure 1a).…”

P Metabolism in the Bean-Rhizobium tropici Symbiosis

“…P requirements for growth (Keyser and Munns, 1979;Cassman et al, 1981aCassman et al, , 1981b, P transport rates, and P storage (Cassman et al, 1981b;Beck and Munns, 1984) vary between strains. Many rhizobia are similar to other gramnegative bacteria (Torriani, 1990;Wanner, 1993) in that P stress induces phosphatases and leads to significantly increased P transport rates (Smart et al, 1984a).…”