nitrogen/phosphorous ratio plays a role. too high a ratio (with ammoniacal nitrogen) and we have stretch. Too low a ratio can produce crazy tight internodes with tiny leaves.
Sort of true, but only because ammonical nitrogen fertilizers have higher concentrates of phosphates in them. Higher ratios of phosphorus adds to the stretch as well.
http://www.gpnmag.com/what-really-causes-stretch
http://www.gpnmag.com/sites/default/files/GPN Jan '02 -NC State.pdf From an NC State study....
[h=2]"Conclusions[/h]
When all of these results are put together, it is clear that
the form of nitrogen did not govern plant size. Repeatedly, the differences in
plant growth were a consequence of the amount of phosphorus supplied to the
plants, not the form of nitrogen. Going back to the original question,
“Does ammonium-nitrogen really cause plant stretch,” the answer
would have to be no. Differences in plant height among the many fertilizers
varying in ammonium-nitrate proportion are controlled by the phosphate —
low phosphate levels result in compact plants, high phosphate levels result in
tall plants.
Fertilizers with high proportions of their nitrogen in the
nitrate form typically contain little or no phosphate, resulting in compact
plants and leading to the incorrect assumption that nitrate nitrogen causes
compactness. Fertilizers with high proportions of nitrogen in the ammonium form
(33 percent or more) invariably contain high levels of phosphate. These
fertilizers yield the more luxuriant growth to which we are accustomed and
result in the belief that ammoniacal nitrogen causes stretch.
Thus, if compact plants are your goal, you should limit the
amount of phosphorus applied to the plants; conversely, if full plants are your
goal, you should apply fertilizers containing the appropriate levels of
phosphorus."