Which of the following would not usually be a good source of nitrogen?

Irrigation water typically does not serve as a reliable source of nitrogen for crops. While it may contain trace amounts of nutrients, including nitrogen, the concentration is generally minimal and insufficient to meet the nitrogen demands of most crops. It often lacks the necessary nutrient load and is primarily utilized for moisture rather than nutrient delivery.

On the other hand, urea, ammonium nitrate, and calcium nitrate are all well-known nitrogen sources. Urea is a widely used synthetic fertilizer that provides nitrogen in a form that plants can easily assimilate. Ammonium nitrate is another effective fertilizer that delivers nitrogen in both ammonium and nitrate forms, promoting quick plant uptake. Calcium nitrate not only supplies nitrogen but also contributes calcium, enhancing soil health and plant growth. Therefore, while irrigation water plays a crucial role in crop irrigation, it is not considered a substantial nitrogen source compared to the other options listed.