Spirulina has a very high protein content per gram, so I would imagine it would break down into a lot of nitrogen. It also contains other key plant nutrient elements such as potassium, calcium, iron, phosphorous, and a few other micros. However, using spirulina as fertilizer seems very costly and therefore not financially sustainable. There are much more economical ways of feeding your plants nutrients from organic sources. I think you would be better off just blending it into a smoothie and having your body assimilate all those awesome nutrients 🙂