V. O. Chude, W. B. Malgwi, I. V. Amapu and O. A. Ano, 2004. Manual on Soil Fertilit Assessment, Federal Fertilizer Department (FFD) Incollaboration with FAO/National Special Programme for Food Security. Abuja, Nigeria.
 R.M. Gifford and L.T. Evans, 1981. Photosynthesis, carbon partitioning, and yield. Ann Rev Plant Physio, 32, 485-509.
 B. Hirel, J. Le Gouis, B. Ney and A. Gallais, 2007. The challenge of improving nitrogen use efficiency in crop plants: towards a more central rule for the genetic variability and quantitative genetics within integrated approaches. J Exp Bot, 58, 2369-2387.
 T.R. Sinclair and T. Horie, 1989. Leaf nitrogen, photosynthesis, and crop radiation use efficiency: a review. Crop Sci, 29, 90-98.
 T.R. Sinclair and P.D. Jamieson, 2006. Grain number, wheat yield, and bottling beer: an analysis. Field Crop Res, 98, 60-67.
 J.L. Rotundo, L. Borras, J. De Bruin and P. Pedersen, 2014. Soybean nitrogen uptake and utilization in Argentina and united states cultivars. Crop Sci 54: 1153-1165.
 G. Santachiara, L. Borras and J.L. Rotundo, 2017. Physiological processes leading to similar yield in contrasting soybean maturity groups. Agron J, 109, 158-167.
 F. Salvagiotti, K.G. Cassman, J.E. Specht, D.T. Walters, A. Weiss and A. Dobermann, 2008. Nitrogen uptake, fixation and response to fertilizer N in soybeans: a review. Field Crop Res 108: 1-13.
 L.C. Purcell, R. Serraj, T.R. Sinclair and A. De, 2003. Soybean N2 fixation estimates, ureide concentration, and yield responses to drought. Crop Sci, 44, 484-492.
 T.R. Sinclair, C.D. Messina, A. Beatty and M. Samples, 2010. Assessment across the United States of the benefits of altered soybean drought traits. Agron J, 102, 475-482.
 T. George, P.W. Singleton and B. Bohlool, 1988. Yield, soilnitrogen uptake, and nitrogen-fixation by soybean from 4 maturity groups grown at 3 elevations. Agron J, 80, 563–567.
 F. Salvagiotti, K.G. Cassman, J.E. Specht, D.T. Walters, A. Weiss and A. Dobermann, 2008. Nitrogen uptake, fixation and response to fertilizer N in soybeans: a review. Field Crop Res, 108, 1–13.
 F. Salvagiotti, J.E. Specht, K.G. Cassman, D.T. Walters, A. Weiss and A. Dobermann, 2009. Growth and nitrogen fixation in highyielding soybean: impact of nitrogen fertilization. Agron J, 101, 958–970.
 K. M. Kakar, M. Tariq, F. H. Taj, and K. Nawab, 2002. Phosphorous use efficiency of soybean as affected by phosphorous application and inoculation. Pakistan Journal of Agronomy, 1(1), 49–50.
 C.S. Snyder, 2000. Raise soybean yields and profit potential with phosphorus and potassium fertilization. PPI.
 R. George and S. Michael, 2002. Potassium for crop production. Minnesota crops. University of Minnesota.
 I. S. M. Farhad, M. N. Islam, S. Hoque, and M. S. I. Bhuiy, 2010. Role of potassium and sulphur on the growth, yield and oil content of soybean (Glycine max L.). An Academic Journal of Plant Sciences, 3(2), 99–103.
 F. Zapata, S.K.A. Danso, G. Hardarson and M. Fried, 1987. Time course of nitrogen fixation in field-grown soybean using nitrogen–15 methodology. Agron. J., 79, 172–176.
 T. George and P.W. Singleton, 1992. Nitrogen assimilation traits and dinitrogen fixation in soybean and common bean. Agron. J., 84, 1020–1028.
 Y. Jefing, D.F. Herridge, M.B. Peoples and B. Rerkasem, 1992. Effects of N fertilization on N2 fixation and N balances of soybean grown after lowland rice. Plant Soil, 147, 235–242.
 M.J. Goss, A. de Varennes, P.S. Smith and J.A. Ferguson, 2002. N2 fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop Canadian Journal of Soil Science, 82(2), 139-145.
 A.A. Bloem and R.O. Barnard, 2001. Effect of annual legumes on soil nitrogen and on the subsequent yield of maize and grain sorghum. South African Journal of Plant and Soil, 18:2, 56-61.
 J. Streeter, 1988. Inhibition of legume nodule formation and N2 fixation by nitrate. CRC Crit. Rev. Plant Sci., 7, 1–23.
 L.C. Purcell, R. Serraj, T.R. Sinclair and A. De, 2004. Soybean N2 fixation estimates, ureide concentration, and yield responses to drought. Crop Sci., 44, 484–492.
 M.B. Parker and H.B. Harris, 1977. Yield and leaf nitrogen of nodulating and non-nodulating soybeans as affected by nitrogen and molybdenum. Agron. J., 69, 551–554.
 M.C. Soares Novo, R.T. Tanaka, H.A.A. Mascarenhas, N. Bortoletto, P.B. Gallo, J.C.V.N. Alves Pereira and A.A. Teixeira Vargas, 1999. Nitrogênio e potassio na fixação simbiotica de N2 por soja cultivada no inverno, Scientia Agricola, 56(1), 143–156.
 M.S. Aulakh, N.S. Pasrich and A.S. Azad, 2002. Phosphorus-solphur interrelationships for soybean on phosphorus and sulphur deficient. Soil Sci., 150, 705-709.
 S.L. Tisdale, J.L. Halvin, W.N. Nelson and J.D. Beaten, 2005. Soil fertility and fertilizers. An introduction to nutrient management (7th eds). Pearson education. USA.
 N.C. Brandy and R.P. Weil, 1999. The nature and properties of soils (12th eds). Prentice Hall, New Jersey.
 E.L. Gene, L.J. John and J.M. Donald, 2010. Impact of potassium nutrition on food quality of fruits and vegetables. Dept. of Agric. and Res. Service. USA.
 A. Venter, 2003. Comparing plant yield and composition with soil properties using classical and geostatistical techniques. M.Sc. Thesis. Dept. of Plant Prod. and Soil Sci. UP.
 F. Rooyani and B. Badamchian, 1986. General soil science. L.A.C. Lesotho.
 F.R. Cox, F. Adams and B.B. Tucker, 1982. Liming, fertilization and mineral nutrition. In: H.E. Pattee and C.T. Young (eds). Peanuts Sci. and Tech. American Peanut Research and Education Society, Inc., Yoakum, Texas, U.S.A.
 B. Yankov, Zh. Terziev, H. Yancheva, R. Ivanova, I. Yanchev, T. Georgieva, T. Kolev, N. Tahsin, V. Delibaltova and H. Kirchev, 2013. Plant production. AU – Plovdiv.