Conclusions
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.