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

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 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 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. Additional index words: N fixation, P deficiency, Critical external P requirement, Critical internal P requirement. N soil, plant P uptake and fertilizer P requirements are dependent on root growth Khasawneh and Copeland, 1973; Edwards and Barber, 1976). The inherent differences in root morphologies of N-fixing and Nsupplied legumes may affect their critical ex1 Journal Series No. 2489 of the Hawaii Agric. Exp. Stn. Based in part on the dissertation submitted by the senior author in partial fulfillment of the Ph.D. Degree, University of Hawaii. This research was supported under UH/USAID Contract to-C1207 (NifTAL Project, P. O. Box 0, Paia, Maui, Hawaii 96779). Received 23 Nov. 1979. 2 Research assistant, professor of agronomy and professor of soil science, respectively, NifTAL Project, Dep. of Agronomy and Soil Science, University of Hawaii, Honolulu, Hawaii 96822. ternal P requirement. A negative correlation between lateral root number and nodule number has been reported for some legume species (Nutman, 1948; Dart and Pate, 1959). Cassman et al. (1980) showed that soybean (Glycine max (L.) Men.) plants supplied with adequate N had a larger and more extensive root system than comparable N-fixing plants and this difference was greatest at suboptimal P levels. The more extensive root system of an N-supplied legume should increase its capacity to absorb P and lower the requirement for added P fertilizer. A field experiment was conducted to compare the critical external and internal P requirements of soybean and cow pea (Vigna unguiculata (L.) Walp.) as affected by the predominant mode of N nutrition during crop growth. MATERIALS AND METHODS The experiment was conducted at the University of Hawaii Kuiaha Experiment Site on the island of Maui, Hawaii. Elevation at the site is 320 m and mean average rainfall is 1,800 mm. The soil is classified as a Haiku clay (clayey, ferritic, isohyperthermic Humoxic Tropohumult) weathered from basic igneous rock and volcanic ash. Surface and subsoil pH was 4.8 before liming. Nine months before planting 2,100 kg/ha finely ground agricultural lime and 1,600 kg/ha dolomitic lime were incorporated into the surface soil which raised the pH to 5.9. Six P treatments were also established to provide 0.0015, 0.011, 0.024, 0.070, 0.170, and 0.500 μg P/ml in the soil solution as estimated by the method of Fox and Kamprath (1970), requiring P applications of 0, 400, 620, 960, 1,360, and 1,880 kg/ha. Phosphorus was surface broadcast as treble super phosphate and rotary-tilled to 18 cm depth. Plots were 5-m X 10-m and were arranged in a completely randomized block design with three replications. One week before planting soil N availability was modified by incorporating sugarcane bagasse into the entire field at a rate of 16,000 kg/ha (equivalent to 0.8% of the surface so il on a dry weight basis). Two N-level subplots, each 5-m X 5-m, were established within each of the six P mainplots. One subplot received no N fertilizer (-N subplot) while 700 kg urea-N/ha was incorporated in the other subplot (+N subplot). Supplemental applications of 200 and 100 kg urea-N/ha were side dressed on +N subplots 4 and 6 weeks after emergence, respectively. Before planting blanket fertilizer applications of 200 kg/ha K as K2SO4 15 kg/ha Zn as ZnSO 4 and 2 kg/ha Mo as MoO3 were applied. Seeds were coated with peat cultures containing highly effective strains of Rhizob ium using gum arabic as a sticking agent. Soybean inoculum contained equal numbers of Rhizob ium strains TAL 378 and TAL 379. Cowpea inoculum contained equal numbers of Rhizobium strains TAL 169 and TAL 309. Viable plate counts made from coated seeds indicated that there were 10 viable rhizobia on both soybean and cowpea seeds at planting. Three 5 m rows of both soybean and cowpea were planted in each subplot on 6 June 1978. Soybeans were Reprinted from Agronomy Journal Vol. 73, Jan: Feb. 1981,p. 17-22 AGRONOMY JOURNAL, VOL 73, JANUARY-FEBRUARY 1981 2 spaced 3 cm apart in rows 0.75 m apart (333,000 plants/ha). Cowpeas were spaced 7.5 cm apart in rows 1 m apart (133,000 plants/ha). The soybean variety used ('Clark 63') is a determinate cultivar of medium stature. The cowpea cultivar used (TVu 745-P3) is an indeterminate, viny entry. from the 1976 Internation Cowpea Disease Nursery from the International Institute of Tropical Agriculture, Ibadan, Nigeria. Overhead irrigation supplemented rainfall whenever necessary. The first fully-expanded leaf was sampled from 20 plants in the harvest row of each subplot 42 days after emergence, dried at 55 C, and ground. Nitrogen concentration in these index tissue samples was determined colorimetrically after Kjeldahl digestion (Mitchell, 1972) and P concentration was measured from the same digest using the ascorbic acid phosphomolybdateblue method developed by Murphy and Riley (1962) and shown to be effective for plant tissue P analysis by Throneberry (1974). Fine roots < 2 mm diameter were collected from three soybean and three cowpea plants randomly sampled from each subplot 50 days after emergence. After washing and staining as described by Phillips and Hayman (1970), mycorrhizal infection was rated on a scale of 0 to 10 by examination with a dissecting microscope. Soybean plants were harvested at maturity, 81 days after emergence in -N sub lots and 3 days later in the +N subplots. At harvest, soybean pants from 2 m of the middle row in each subplot were cut at the soil surface. Plants were dried at 55 C, seed and haulm yields determined, and subsamples ground for N and P analysis as previously described. Cowpea was harvested 90 days after emergence. Plants from 2 m of the middle row in each subplot were cut at the soil surface, fresh weight recorded, and the plant material chopped. Two subsamples were taken; one to determine moisture content and another for N and P analysis. At harvest there were mature dry pods, green pods, and flowers present on plants in all treatments. Surface soil (0 to 18 cm) samples were collected from each subplot 28 days after emergence. Soil samples were air dried and screened through a 2-mm sieve. Soil P was measured by three methods: modified Olsen's extraction (Banderis et al., 1976), Bray P1 extraction (Council on Soil Testing and Plant Analysis, 1974), and equilibration for 6 days in .0.01M CaCl, (Fox and Kamprath, 1970). All yield data were correlated with the values obtained from these soil samples. The critical soil P requirement was based upon the 900Jo maximum yield level. Yield response curves for both soybean and cowpea best fit a Mitscherlich equation with three estimated parameters of the form: