Most plant fertilizers usually include the three main plant nutrients: N (Nitrogen), P (Phosphorus), and K (Potassium). This journal entry is about the first nutrient, Nitrogen. Nitrogen is available in different forms, and these different forms may lead to different results, where your plants are concerned. Since the different forms of Nitrogen may have different effects upon your soil's chemistry, your plants' abilities to take-up various nutrients (not just Nitrogen!) may be adversely effected.
For more info on the different forms of Nitrogen, and how these different forms may react in your soil, see: www.greenhouse.cornell.edu/crops/factsheets/nitrogen_form.pdf