Answer: There are many types of nitrogen fertilizers; some are highly soluble, and some are more slowly released. Concentrated solutions of highly soluble nitrogen fertilizers are described as "burning" plant roots--this may be because the solutions are quite acidic, though I am not sure about that. You will probably want to consult a botany textbook for detailed information about the nitrogen cycle and its function in plant growth.
Even large amount of slow-release fertilizer should not harm growing plants, because the nutrient becomes available over time to the plants, usually through the process of decomposition by microorganisms. It would be an interesting experiment to apply similar amounts of actual nitrogen to two sets of plants--one a highly soluble fertilizer, one an organic, slow-release type, and compare the results.
Nitrogen is important in plant growth, and nitrogen fertilization usually results in lush, green growth. This succulent growth is often more susceptible to insect and disease attack. Also, an excess of nitrogen may cause such a flush of growth that other nutrients become deficient.
I hope this helps, and gets you interested in researching further the role of nitrogen in plants. It's really a fascinating subject--especially how some plants like legumes can "fix" nitrogen from the air, while others rely on other forms.
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