I've seen it said before that annuals prefer nitrate and perennial plants prefer ammonium (I assumed that's the way around they had it). Seemingly doesn't hold true for conifers at least....
I also found this in a paper about rice nutrition:
"When rice plants in solution culture were fed with a mixture of NH4 and NO3 compared with either of the nitrogen sources applied alone at the same concentration, yield increase of 40-70% were observed (Heberer and Below, 1989; Qian et al., 2004). The growth and the nitrogen acquisition of rice were significantly improved by the addition of NO3 to nutrition solution with NH+4 alone (Cox and Reisenauer, 1972; Duan et al., 2005)."
From:
http://www.cibtech.org/J-Plant...
