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Sweet potatoes as a soil amendment :>)

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Location: Utah
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I had two (unknown commercial variety) sweet potatoes in pots that are about 18 inches deep. I found nothing in the top foot of the soil when I dug them up. The sweet potatoes were on the bottom of the pots. I think in lower water areas the roots reach for the water, so they'll penetrate much deeper. Yay! I can use them to fix my soil at the 12-18 inch level!

The seedlings as well; I think I left much of the usable roots in the ground unintentionally, because I wasn't looking for roots that deep.
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Location: northern California
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Ordinarily when I grow sweet potatoes in the ground, under reliable rain or regular irrigation, most of the roots form right around the base of the plant.  Most varieties are bred thus for ease of harvest.  There are always a few outliers, and some varieties do this more than others.  But yeah, I'd say if the majority of the roots were at the bottom they were after moisture.  
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Location: Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Sweet potatoes are a root veg that tends to stop depth wise at the first horizon differentiation it encounters.
That, and moisture following gravity, is why the tubers are at the bottom of container grown plants. (if you replant the tops after harvest you have the possibility of them re-rooting and producing more crop)
Sweets are fairly cold die back prone but the tuber can over winter in the soil, sort of a store it where you grow it vegetable.

This horizon differentiation issue is why Sweet potatoes are not usually used for getting organic matter deeper into soils.
The daikon radish does a far better job at going deep, these roots will go into a layer of heavy clay where the sweet potato would stop above that clay layer and form tubers there, in your already top soil instead of deepening that top soil layer.
Another really good tuber for extending the top soil layer and thus increasing the microbiome depth is Rape, rape is more like a carrot or daikon and like both of those it cycles on a two year seeding schedule, and it makes a good animal feed.

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