River silt as topsoil

Hi all,

I am considering purchasing a property for the establishment of a food forest/upick in southwest Michigan. The property I'm looking at was used in a dam removal project several years ago. One foot of topsoil was removed from the property to use as fill for the dam. After that, two feet of river silt, dredged from up river, was spread out across the property. Since then, the field has been abandoned and is just growing "weeds"

I know historically, people used river silt as a way to introduce fertility to fields. Some assuredly still do. My concerns are that the silt may contain a buildup of agricultural byproducts from pesticides and herbicides, and that there is an increased risk of compaction.

Has anybody ever used river silt as topsoil? What are the benefits/drawbacks? Are my concerns valid? Are there other concerns I should consider? Am I looking a gift horse in the mouth?

I really don't have any useful experience, but it does put me in mind of this phenomenon where river-bottom land is super-fertile and everyone wants to farm it but then it floods and the farmers cry about it on the news. (My stance is you only get that fertility by letting it flood every few years, so build your enterprise around knowing that has to happen.) So, on first blush, river silt sounds promising to me, but yeah, I don't really know. And I especially don't know about up-stream pollutants and where they end up.

I guess it might depend on the river and what is upstream. I live near the Ohio River which commonly floods the boat ramps and adjacent parking lots and leaves behind thick layers of silt. Those floods over centuries created the deep super fertile bottom land of the flood plain.  It would be easy to collect that silt and add it to the garden but now the sewage and city run-off of thirty million people, not to mention whatever is left from the Fernald uranium processing facility is in that silt. I won't even touch it.

add - Anything dredged up from the bottom I imagine, would be even worse.

My river is relatively clear, no nuclear plants upstream. So I do dredge mud from sloughs and oxbows and carry it uphill to the garden. It is absolutely wonderful, especially since it avoids the need for expensive compost, as I do not get enough from my regular composting activities. And sometimes, it brings seeds of plants like nettle that I want to have around anyway. After a year or so, it looks like ordinary soil or worm castings, since the worms have been into it. Usually there is a lot of leafy, bokashi-like material in there.

Another benefit of this is preventing wetlands and ponds from reverting to dry land, preserving their imperiled ecosystems, which can hold many benefits for humans and for all life. Nearby, the marsh grows lots of wapato, and iris leaves for weaving (but the wapato is rather scarce.) I am working to help Bolboschoenus and other lovely edible and weaveable bulrushes into these areas too—the former of which is known as the first plant used to make bread that archaeologists have recognized.

I would take a good representative sample of the soil and have it tested. A suitable lab will tell you of the soil composition and the nutrient base. Should also be able to check for contamination, heavy metals and the like. If it is growing weeds it has some fertility, weeds are natures pioneers when it comes to getting soil working.

We're doing it on a tiny scale, but with silt from a very small river (or creek, I guess?) that has no industrial activity upstream of us and very little in the general area. We also haven't been at it for very long, so no long-term data yet. However, it does seem to be working okay so far. I did see a little bit of compaction/crusting on one patch of freshly applied silt during the dry part of last summer, but I expect it'll go away as soil building progresses. Otherwise, I guess adding biochar or something would solve this problem.

As for pollutants, I probably wouldn't worry about it, personally, unless there's something extraordinarily nasty in the area the silt came from. Also, since you're planning for a food forest, you probably wouldn't harvest that many root crops from that land, but more fruits and nuts, yes? That reduces the amount of potential nasty that might end up in the food, if I got it right. Don't know if it's true for all species of plants, or all pollutants, but I've understood that plants tend to keep nasty stuff out of their fruits to some extent. It makes sense, the genetic integrity of the next generation is at stake...