Nitrogen is nitrogen. However, the availability of nitrogen varies depending upon the nutrient source used. Natural sources (compost or manure) contain nitrogen bound in the organic matter, which is released upon breakdown of the material by microorganisms in the soil. This takes time and warm temperatures to occur, and may not correspond to the best time to apply fertilizer for a particular crop. When applying synthetic fertilizers, either agricultural or slow release, a plant color response (greening) is usually noted within a few days.
For a more detailed discussion of fertilizers and organic matter, visit the eXtension website: Soils and Composting: Soil Improvement.
