According to the FAO / IPPC, nearly 80% of agricultural land is being used for the production of animal feed. (download report) That leaves 20% for the production of food to be consumed by humans directly. According to this report, we can feed 4 billion people more if we stop using agricultural land for the production of animal feed and biofuels.
Cultivating Spirulina does not require fertile soil. In fact, Spirulina requires no soil at all, as Spirulina grows in (eg concrete) raceways (eg on barren land or rooftops)
Thus, if we solely use Spirulina for animal feed, we can use all agricultural land for the production of grains etc for direct human consumption. As a result, 5-fold more agricultural land (100% instead of 20%) is available for crops to be consumed by humans directly.
In addition, Spirulina is a very effective source of protein; the same amount of (barren rather than fertile) land for growing maize would produce 125 times more protein from Spirulina.
