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Mineral N effects on cowpea and soybean crops in a Nigerian soil

II. Amounts of N fixed and accrual to the soil


The amounts of N fixed in the field in Nigeria by four cowpea and two soybean cultivars were determined at different levels of available N by the difference and the ‘AN’-value methods using non-nodulating soybean, maize and celosia as non-fixing controls. No significant differences were found between the two methods or with different controls. Grain removal and return of residues to the soil were measured and net soil-N changes calculated. N-benefits accrued from two of the cowpea cultivars when grown at low mineral-N levels, but with higher mineral-N availability, no cultivar contributed N because of inhibition of nodulation and lowered N2-fixation. Although the soybeans fixed more N than the cowpeas they caused greater net N-depletions because they sequestered a greater fraction of N in the grain.

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Eaglesham, A.R.J., Ayanaba, A., Ranga Rao, V. et al. Mineral N effects on cowpea and soybean crops in a Nigerian soil. Plant Soil 68, 183–192 (1982).

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Key words

  • ‘AN’-value
  • Cowpea
  • Difference method
  • Fertilizer-N
  • Glycine max
  • Nitrogen-15
  • N-balance
  • N2-fixation
  • Soybean
  • Vigna unguiculata