How much is too much natural fertilizer for growing sweet potatoes?

Normally I use not very composted cow manure in my garden. I normally use a LOT. I usually add 6"  of this stuff from the dairy every year, sometimes twice a year. This manure has been washed with tons of water and then strained. I get the solids that are strained out. Cows are so inefficient that it looks like finely chopped straw when it dries. This recipe is generally great for my garden. most everything thrives. A huge and growing number of red wigglers and night crawlers love it!

This is the first year I am growing sweet potatoes. Since folks here have recommended buying slips over the grocery store variety, that is the road I am taking. I have ordered a dozen Georgia Jets and they will be coming soon. Since they are costing me an arm and a leg more than the grocery store variety, I don't want to fail so I can get all future slips from these little babies. I believe I have read that they don't care for too much fertilizer. I do have a sunny spot that I have not yet added the requisite 1/2 foot of cow droppings that I normally place before planting. I am thinking of not adding any fertilizer there and incorporating a whole lot of straw (unlimited supply).  I live in the Willamette Valley of western Oregon. There is not a whole lot of heat here. I was thinking of planting a couple slips in the greenhouse where heat is not in short supply. The base soil here is clay, that is why so much amendments are needed. I also have a bunch of river loam not so far away but it is a lot of work to haul it. Little by little more loam is being added to the whole garden.

What are your suggestions? How should I use the straw? How much straw should I use?  Chop up the straw? Straw between layers of soil? Top dress with a ton of straw?
What about my manure? Don't add any more?

Thank you for your help!

I'd plant as is without straw or adding additional manure. My reasoning for no straw would be to allow it to root. It will root along the stem and produce everywhere it roots.

If you use straw,  maybe periodically shift it from under the plant to over so it can root in.

Don’t add any manure!  You might have too much fertility if you added manure last year.  You could end up with lots of vines and lots of little skinny potatoes.

Are your other vegetables productive with all that manure?

I'm trying to envisage exactly what your "manure" consists of, and it sounds like it may not actually be much more than straw, if you say the rest has been "washed" out?  I wouldn't have thought that added much available fertility at all, and conventional wisdom would say it actually robs the soil of nitrogen whilst decomposing.  Hay is different, because it's a whole plant crop, dried in the prime of life.  Straw is exhausted stems from a plant that's gone to seed.

The "manure" most people use at my allotment site is a similar sort of thing, being horse droppings and associated shavings from their beds.  By the time it's been spread on the surface and the ammonia has gassed off and the rest has been washed out by the rain, they're left with a layer of shavings.  They seem to get good results, but I don't know if they are adding artificial fertiliser as well...

The proof is in the pudding - do you get good results?

My experience with sweet potatoes is that they are tremendously productive with minimal fertilizer.  I don't bother to even plant them anymore -- they come up volunteer all over the place like weeds.  I'll rip them out if they get too far out-of-bounds, but they thrive on neglect and provide us with about a hundred pounds or so that we eat --- and there are a lot more back on my hillside that I don't bother to dig up.

All that to say, I'm sure that your soil already has plenty of nitrogen and fertility already.

As for fancy varieties -- my sweet potatoes all started from a single sweet potato that my wife threw into the compost.  I fished it out, scratched a hole in the ground, planted it, and promptly forgot about it.  We went on summer vacation and by the time we came back a month later, it had taken off and covered a big piece of the garden.  That fall I was digging sweet potatoes up all over the place, so I transplanted a bunch of slips to the hillside where I've got about 25 fruit trees.  It's a ground cover, basically.  Minimal water, zero fertilizer, and zero care --- but we get sweet potatoes like crazy.  I love the commercial variety we've got -- I don't know the name but it tastes great and obviously does well.  

The easiest way to make new slips is to make cuttings of vines and drop them into a tall glass of water.  You'll see roots within days.

Hester Winterbourne wrote:The proof is in the pudding - do you get good results?

Since a couple of people have asked this, the answer is YES! The stuff has not been washed so much that it is useless. Most modern dairies use a huge flush of water to wash the poop off the cement slabs that the cows are standing on as they feed. This water runs like a river down to a large pond where it is stored (to wash down the area again and to use as manure "tea" to water and fertilize their fields. Just before it gets to the pond it goes through an sieve/elevator to take most of the solids away and throw it in a pile. The dairy farmer loads it on my truck -for free- and I take it home. When I get it the manure is HOT from composting which kills most of the weed seeds (all but in the ones on the surface of the pile). Obviously, the best time to add this stuff to my garden is in the fall so the worms can work their magic on it, but because of the high content of "straw" (left-over hay) that is left in the poop which uses up nitrogen, the manure is not too nitrogen hot as to damage young crops. I work it into the soil, use it as a top dressing, and often cover the garden with it in the fall.

I get massively good results. I think that the best results come from worm castings and the aeration that the worms provide for my soil. I also turn the worms into fish in my local rivers and lakes.

It sounds like what you're getting to put on your garden as mulch is great stuff.  While much of the nitrogen has been washed off, there would still be quite a bit left there.  But what makes it so valuable are the microbes that inhabit that biomass/mulch.  There is more than just nitrogen in cow manure, and long after the N has washed through, the medium is still an excellent home for microbial life.  That's why you are seeing so many worms.  Fungi have a hard time dealing with high levels of N, but from the sounds of things, the high carbon mulch that you are getting would be perfect for feeding fungal networks.

I would argue that long term, what you are getting is better for your soil than fresh manure with artificially high levels of N.  You are seeing the results of that thesis daily.

The last load I got was well on its way to being composted. Though it still smells like poop, the smell is faint. When it is fresher the smell is very strong. This stuff also has small mushrooms growing in it.

Sweet potatoes don't seem to need very fertile or loose soil. This pic is of my first sweet potato attempt. Slips were planted directly in hard packed clay & ignored until harvest. I have since grown them in looser soil but not any more fertile. The overall yield was about the same but the potatoes were more uniform in size & less gnarly looking.

I read somewhere that sweet potatoes can be grown directly in straw. Never tried that but this year I planted 200 in a mix of straw, various soils, mulched wood chips with cow manure,& leaves. So far so good but they do seem to be needing a bit of extra nitrogen. I think the thick layer of mulch material is locking up the nitrogen. Planted about 50 more in other parts of the garden with less improved soil as insurance against total sweet potato crop failure. I have started using sweet potatoes & peanuts as a first crop in new areas to help prepare the soil for future use by needier veggies.

As others have suggested it is easy to make your own slips from stem cuttings. Organic potatoes from grocery stores have worked well here. Non-organic usually doesn't sprout.

Variety is the spice of life!!! This was my main source this year. Good luck.

Mike Barkley wrote:Sweet potatoes don't seem to need very fertile or loose soil. This pic is of my first sweet potato attempt. Slips were planted directly in hard packed clay & ignored until harvest. I have since grown them in looser soil but not any more fertile. The overall yield was about the same but the potatoes were more uniform in size & less gnarly looking.

I read somewhere that sweet potatoes can be grown directly in straw. Never tried that but this year I planted 200 in a mix of straw, various soils, mulched wood chips with cow manure,& leaves. So far so good but they do seem to be needing a bit of extra nitrogen. I think the thick layer of mulch material is locking up the nitrogen. Planted about 50 more in other parts of the garden with less improved soil as insurance against total sweet potato crop failure. I have started using sweet potatoes & peanuts as a first crop in new areas to help prepare the soil for future use by needier veggies.Variety is the spice of life!!! This was my main source this year. Good luck.

Mike, You must like sweet potatoes. I think all of us here would be interested in know how your experiments work out. Make sure you report at the end of the season.

You obviously have plenty of room to carry out this experiment. One question: Are you also putting some in different kinds of light? - Full sun, morning sun, afternoon sun, mostly shade, etc.?

Thanks for replying and thanks for your picture.

Jon

Well, we like almost anything edible or useful that will actually grow here. There are a few pix & descriptions from the beginning of the tater hugelhole https://permies.com/t/54477/Comfrey-Hype here.

I've grown them mostly in full sun but a few with late afternoon shade have done just as good.

Here's a pic from yesterday. The stick tripod is directly above the log shown in the original picture. That project has expanded for this year. Second pic is a small glimpse of that. In addition to sweet potatoes there are what some would consider too many peanuts, other legumes, pumpkins, melons, & squashes placed in locations specifically to smother lawn. Part of the logic behind planting much more than we need is the severe flooding in other parts of the country. Expecting to give much away to those who need it most. Trying to establish Seminole pumpkins up the leaning tree in the background but the jungle is blocking too much light now.