Chief', a very tall Maturity Group IV variety, was used as a donor parent in backcrossing to 'Clark' to transfer Np (a gene for high phosphorus tolerance). In the field in 1963, I grew progenies from 40 selected Np F2 plants from Clark BC5 and was surprised to see 2 of the progenies uniformly very tall and 3 of them segregating approximately 1/4 tall plants. The Np gene appears to have no effect on field-grown plants in normal soils.
Bernard, R. L.; Cremeens, C. R.; Rode, M. W.; and Wax, L. M.
"Research Notes: U.S. Regional Soybean Laboratory and University
of Illinois, Urbana-Champaign,"
Soybean Genetics Newsletter: Vol. 2
, Article 13.
Available at: http://lib.dr.iastate.edu/soybeangenetics/vol2/iss1/13