The question is when the nitrogen is added to the soil, what form, and how long will it take to convert it into usable food for the plant. Most organic materials need to be converted and since it is a multiple step process, the nitrogen may not be available to the plant for up to two months. This is why organic fertilizers as manure and alfalfa are applied soon after pruning, so they have time to break down and be available for the plants by springtime. There are some fast and slow acting forms of organic nitrogen. Blood meal and fish emulsion are fast acting nitrogen fertilizers, where manures and compost are slow acting.
Organic fertilizers tend to have low amounts of nitrogen.
Humus is an organic material that cannot be broken down to available nitrogen and is a poor source of nutrients. Be careful of uncomposted materials such as sawdust and straw, which will actually use up available nitrogen as they decompose. Once the nitrate is absorbed by the plant, it is converted to ammonium using the energy from photosynthesis. It is interesting that the microorganisms will convert the ammonium ion to a nitrate, then once in the plant it is converted back. Manmade or chemical fertilizers come in several forms.