I've been researching all morning trying to find out if I can use the excess water lettuce plants (Pistia stratiotes) from my small pond as a soil amendment, either as mulch just dropped upside down on the soil or dehydrated and crumbled/scattered in among the wood chips and leaves.
The more I read, the more confused I get.
Okay, so it's very high in oxalates, which made me think twice about feeding it to my chickens. It seems a lot of peoplecompost it for their gardens, but I'm reading scientific articles (which are hard to understand for me) that note P. stratiotes is allelopathic and suppresses other plant growth, like black walnuts do. But it's also rich in phosphorus and other minerals and nutrients, and I think it's got to be taking up a goodly amount of algae and fish poop from my pond as well. I don't think I have any heavy metal contaminants here, so I'm not so worried about that (although it's being studied as a plant for bioremediation and arsenic removal).
This is such a gangbuster plant and so invasive in so many places, surely somebody knows if it would be bad (or good) to use it to build soil. It certainly does make a lot of organic matter. Has anyone done any testing of its value?
I read a few papers and from what I can tell, the researchers went above and beyond to extract and concentrate the allelopathic chemicals in the water lettuce. It seems like they were testing its use as a herbicide. They soaked seeds in the allelopathic solution to test germination inhibition on weeds and popular crops. So as long as you don't do that, I think you could use your water lettuce right on top of your soil. Maybe don't put it where you just planted seeds. And as long as it's not a huge pile, I think it would be fine.
Alternatively, you could extract the allelopathic chemicals as described in the methods section of any of the papers and use it as an herbicide on your walkways and use the leftover plant material on the garden. That's some work though.
As with most things Permies want to try, there's not clear research or previous examples most of the time. We just have to try it out for ourselves and share what we learned. I'll be curious to see how this turns out for your garden, so please keep us updated!
One caution would be that if there is ANY CHANCE AT ALL, even a thousand year flood or something, that bits of your water lettuce could potentially be washed into a storm drain or nearby water body, please dry it out all the way and make sure it is good and dead before using it in the garden to prevent the spread of invasive species.