What Really Causes Stretch?

Does ammoniacal nitrogen cause stretch in plants, or conversely, does the alternative nitrogen source, nitrate, cause compactness? What does your experience indicate? Do you have all of the facts?

Because the application of high-nitrate fertilizers often results in compact plants, growers’ experiences seem to confirm that high nitrate fertilizers result in compact plants. But we need to take a look at what their fertilizer is composed of before making this decision.

A Little Background

Fertilizers with a high proportion of nitrogen in the nitrate form include 13-2-13, 15-0-15, 14-0-14 and 17-0-17. The percentages of nitrogen in the nitrate form in these fertilizers are 11, 13, 8 and 20 percent, respectively. Growers who formulate their own fertilizer most often make a 15-0-15 fertilizer when they want compact plants. They do this by combining calcium nitrate and potassium nitrate in a 2:1 weight ratio. A typical formula would be as follows: 6 oz of calcium nitrate plus 3 oz of potassium nitrate suspended in 100 gallons of water yields a concentration of 100 ppm nitrogen.

When applied, this formulation will indeed result in compact plants; therefore, it would be easy to conclude that the nitrate form of nitrogen contained in this formula is the cause of the resultant compact plants. One could also then conclude that the converse of this formula must be true, namely that ammoniacal nitrogen causes stretch. In fact, growers often draw this conclusion about ammoniacal nitrogen because they use the above-mentioned, high-nitrate fertilizers to produce compact plants and because when they want more luxuriant growth, they use fertilizers with one-third or more of their Á nitrogen in the ammoniacal and urea forms. Formulations with high concentrations of ammoniacal nitrogen include 20-10-20 and 15-16-17.

This conclusion is, however, quite inaccurate because there is a confounding factor involved that has not been considered. Each of the high-nitrate fertilizers is low in or missing a key component for vigorous growth — phosphate. Unless we understand how each component in the fertilizer, or left out of the fertilizer, affects the plants, we can not accurately attribute the results to any one component. Thus, what we should be asking is if a high-nitrate proportion or a low phosphate level causes plant compactness in the above-referenced, high-nitrate fertilizers?

Throughout the years, we have formulated an extensive range of nutrient combinations in our nutritional research program at North Carolina State University. Our results from experimenting with these formulations have not indicated any relationship between form of nitrogen and rate of growth. To resolve this disparity between grower practice and research observation, we conducted the following experiments.

Experiments

We recently conducted four experiments that shed light on this phosphate/nitrate question.

Experiment #1. In the first experiment, plug seedlings of impatiens, marigold and tomato were produced receiving a series of nutrient solutions, all containing 91 ppm each of nitrogen and potassium (K2O) but different amounts of phosphorus (P). Phosphorus levels used were 2, 4, 6 and 20 ppm. The responses from all three plants were similar and are typified by the tomato plants shown on page 24. Although each fertilizer contained the same ratio of 60 percent nitrate to 40 percent ammonium proportion of nitrogen, growth was very different in each treatment. The results of the first experiment can be summarized as follows: plant size increased with increasing phosphorus concentrations.

Experiment #2. In the second experiment, several fertilizer formulations varying in phosphorous content and proportion of nitrate for ammonium were applied to impatiens, gomphrena and petunia plug seedlings. The level of phosphorous in each plant shoot was then measured. The relationship between plant height and concentration of phosphorous taken up into the plant shoots can be seen in the graph on page 26. The results show that height in the four plants increased with increasing phosphorus concentration in the shoot tissue. The results were surprising because the general consensus among nutritionists has been that phosphorous could only provide a certain degree of stimulus, after which growth would level off. The consensus has been that growth increases would occur up to a tissue phosphorus concentration of about 0.25 percent of dry matter and then level off, regardless of higher phosphorus concentrations. In this study, growth increased up to tissue phosphorus concentrations of 0.8-1.0 percent. Clearly, phosphorus supply is a stronger driving force behind the  growth of young plants than has formerly been realized.

Experiment #3. In our third experiment, we formulated various series of fertilizer treatments and applied them to seedlings of gomphrena, impatiens, marigold, petunia and tomato. In the first series, all fertilizers contained 100 ppm nitrogen and potassium (K2O) and 50 ppm (P2O5) phosphate. The percentages of nitrogen in the ammonium form in these treatments were 40, 13, 7 and 0. The results with gomphrena are typical of all crops and are shown in the top picture on page 28. Growth of all plants was similar. The fertilizers we used in the study ranged from a low-nitrate fertilizer, with 40 percent ammonium and 60 percent nitrate similar to 20-10-20, to a high-nitrate fertilizer, with 100 percent nitrate and no ammonium. This first series of fertilizers showed that form of nitrogen had no effect on growth.

A second series of fertilizers in this experiment had a fixed nitrogen proportion of 60 percent nitrate to 40 percent ammonium but varied in phosphate concentration, with levels of 50, 15, 7.5 and 0 ppm (P2O5). With this series, as in the first experiment, plant size increased with increasing phosphate concentration.

Experiment #4. We conducted the fourth experiment to see if something about the self-mixed fertilizer was affecting plant growth. In this experiment, a 15-0-15 commercial fertilizer was applied to seedlings of gomphrena, impatiens, marigold, petunia and tomato. Our results are represented by the marigold plants in the bottom picture on page 28. Plants that received the 15-0-15 fertilizer alone were compact. Plants that received the 15-0-15 fertilizer plus sufficient monobasic calcium phosphate to yield the equivalent to a 15-7.5-15 fertilizer were larger, indicating that in both self-mixed fertilizers and in commercial fertilizers the phosphorous component is necessary for full, luxuriant plant growth.  Á

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.

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.

The authors wish to thank The Fred C. Gloeckner Foundation, Inc. for financial support of this study, Ball Seed Co. for providing the seeds used in this study and Nancy C. Mingis for analysis of substrate and tissue samples.