Azolla, Azolla spp.

 Uses: Edible, Fodder, Fertilizer. Native to: The Cretaceous Period, worldwide.
     This floating water plant is actually a tiny, prehistoric fern with amazing properties. It fixes atmospheric nitrogen due to its symbiotic relationship with cyanobacteria, like an aquatic version of our more common legumes. Due to its nitrogen-fixation, it is also an extremely protein-rich plant, making it an excellent choice for home-grown fodder. In fact, there are very few land-based livestock animals that will not eat azolla, if any. Chickens, rabbits, goats, ducks, and cows are reported to gorge on this aquatic plant.
     Historically it has been used as a wetland green manure crop in the warmer parts of Asia. In the spring when taro or rice is planted in the boglands, the azolla would be innoculated to the water's surface. There the little ferns would fix nitrogen and proliferate, crowding out any competing algae or weeds via shading. Since the rice and taro have most of their foliage above water-level, it does not hinder their growth. The azolla will thrive, then self-mulch, then die down when the cooler weather comes. The farmers harvest some and shelter it for next year's use. The green manure properties of azolla make it possibly the six most valuable ferns on the planet.

A close-up of the roots which hang into the water.

     It's culture is simple - grow it in clean still or gentle water. Wind and water turbulence can fragment and destroy azolla, so sometimes floating beds are used. Like duckweed, under the right conditions it can become weedy if it escapes into waterways, so care should be taken to prevent escape. Since it provides it's own nitrogen, phosphorous can be a limiting factor to optimal growth. "The symptoms of phosphorous deficiency are red-coloured
fronds (due the presence of the pigment anthocyanin), decreased growth and curled
roots." A quick search of the webs reveals a superior organic source of high quality phosphorous can be made by burning animal bones at high temperatures to ashes. Never has a rocket mass heater in florida sounded more appealing!
     Azolla species can be used as a very nutritious survival food for humans, but it will need to be cooked to destroy any pathogens that might be living in the watery growing media. For animals, this compilated table shows it to have 16.5% crude protein and a bevy of other useful vitamins, like leucine and alanine. This Australian study lists many of the benefits of using azolla as a large-scale fodder source, but points out a simple drawback to commercial production - that contamination of the plantings by tiny freshwater shrimp can occur. This accidental animal protein renders the azolla illegal to sell as a feed source for ruminants in Australia, as they have strict legislation prohibiting animal proteins for ruminant consumption there. They also recommend the azolla be fed fresh or refrigerated, but within a week of harvest.
     A very nice synopsis of azolla research here.

Some free aquatic snail contamination.

I don't speak this language, but the video is very informative even muted.