The problem of spacing is worldwide and concerns all crops. The usual plant density for soybeans is 25-65 plants/m2 (Scott and Aldrich, 1970). Nowadays there are efforts to put more soybeans on the field. For example , Gogerty (1976) pointed out that the yield increases as the row distance narrows.
Gretzmacher, Ralph F.
"Research Notes: Spacing and soybean breeding,"
Soybean Genetics Newsletter: Vol. 4
, Article 14.
Available at: http://lib.dr.iastate.edu/soybeangenetics/vol4/iss1/14