RM3
Well-Known Member
From the ProMix website
http://www.pthorticulture.com/en/training-center/relationship-between-fertilizer-plant-stretching/
Nitrogen Forms: It has been long thought that the form of nitrogen used to fertilize a plant influenced plant stretching and quality. Ammoniacal nitrogen has been thought to cause plant stretching, increase leaf size and produce soft growth; while nitrate nitrogen produces compact, firm growth with smaller leaves. Experientially this seems to be true. For example 20-10-20, which has 40% ammonium and 60% nitrate, can promote stretching, larger leaf size and softer overall growth compared to 13-2-13 which is 6% ammonium and 94% nitrate. However there is another variable; if both fertilizers are applied at the same nitrogen application rate; 20-10-20 provides more than three times as much phosphorus as 13-2-13.
Traditionally fertilizers that have a high ratio of ammoniacal nitrogen also have higher levels of phosphorus due to their formulation with ammonium phosphate. Fertilizers with higher ratios of nitrate usually have little to no phosphorus content since the calcium nitrate they are formulated with can form precipitates with phosphate. So is the nitrogen source the factor in plant stretching?
Dr. Paul V. Nelson, Professor Emeritus of NCSU and his team, conducted research with various bedding plants in which he applied fertilizer with different ratios of ammoniacal to nitrate nitrogen, while keeping all other nutrients the same. The results demonstrated that there was little difference in plant growth. Based on these results, the old idea that ammonium causes plant stretch and nitrate produces short, more tone plants was not true. Dr. Nelson concluded that the form of nitrogen has little influence on plant height and growth.
Phosphorus: Dr. Nelson then looked at phosphorus. In his research, he formulated fertilizers with different levels of phosphorus, but kept the nitrogen ratios and all other nutrients the same. The results showed a significant difference in the height of bedding plants in which those fertilized with a high phosphorus fertilizer were taller than those given a low phosphorus fertilizer. The conclusion was obvious; it is not the form of nitrogen in a fertilizer that influences stretching and plant growth, but it is the amount of phosphorus.
This conclusion makes sense as symptoms of phosphorus deficiency include plant stunting and darkening of the leaves as seen in the following picture. These symptoms are good quality attributes for most plants; however, prolonged phosphorus deficiency can progress into foliar necrosis and deterioration of crop quality. A water soluble fertilizer should provide a minimum phosphorus level (P2O5) of 10-15% of the nitrogen concentration (i.e. fertilizers with analyses such as 20-2-20 or 20-3-20). Some plug crops may prefer higher levels of phosphorus and may benefit from an analysis such as 20-5-20. Fertilizers such as 20-10-20 or 20-20-20 provide more phosphorus than crops need, but other than plants being taller; these fertilizers do not have a negative impact on crops.
“Notice the impact of phosphorus on tomato growth. The plant on the left is grown
with normal levels of phosphorus, while the one on the right is shorter and smaller
due to phosphorus deficiency.
Source: apps.cdfa.ca.gov/frep/docs/Tomato.html, Jim Richards, UC Davis.”
Conclusion: Overall, we can see manipulation of fertilizer can influence plant height and crop growth. The old adage that ammonium stretches plants and nitrate leads to compact growth is not as significant as once thought; in fact, these nutrients do not seem to manipulate crop growth much at all. Reducing fertilizer application rates can help, but often compromise crop quality and end user satisfaction. The one variable that can reduce plant stretching without compromising crop quality is reduction of phosphorus application rates.
