Danger of Horse Manure

dirtydan(8-9 Lancaster, CA)April 2, 2010

Here in north Los Angeles County we have practically unlimited amounts of composted horse manure available for free on Craig'slist. It seemed like a good deal to me so I filled up my truck and headed home with a load.

Most discussions regarding the negative aspects of horse manure revolve around the problems associated with weed seeds and pathogens that may be present in the material. After top dressing my vegetable garden with a couple inches of the stuff I began to wonder about the chemical composition.

I mixed a large sample of manure with distilled water, I inserted my pH meter into the water and got a reading of 10.1. Thinking that the meter must be out of whack I checked my tap water and then some vinegar for calibration reference. The meter was working properly.

After a little research I learned that stables frequently add lime to horse manure for odor control. This will result in manure that is so alkaline that it could be disastrous to add to a vegetable garden, especially here in the west where soil is frequently quite alkaline to begin with. Adding limed manure could raise the soil pH significantly for many years, causing severe nutrient deficiencies in the plants.

I have removed all of the manure top dressing from my gardens. Now I need to decide whether I should try to amend this pile with acid or just throw it all out in the garbage.

Here is a link that might be useful: Long Term Effect of a Single Application of Horse Manure

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I did not know this. Thx for posting it.


    Bookmark   April 2, 2010 at 1:10PM
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I've seen horse manure composts that contain as much as 43 lbs. of lime per cu. yd. I am not a fan of that stuff at all for around here. Although for acid soils.....two birds with one stone. (and a free stone to boot)

    Bookmark   April 2, 2010 at 1:18PM
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Gee..........this old dog just learned something of value. Thanks. I am here in the Northeast in moderately acid soil. We have no reason to fear reasonable use of horse manure because we usually need more lime too.

I would compost that pile of horse manure. When it is finished compost it will be at PH 7 or very close to it.
Then you will have a usable product. You of course would need to add lots of browns which could be at a ratio of 1 green to 6 - 10 browns. Time will solve your problem if you have the time and space to work with it.

    Bookmark   April 3, 2010 at 2:10PM
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When it is finished compost it will be at PH 7 or very close to it.
That's a pretty good rule of thumb and works well in regards to most composts when you're just dealing with decomposing organic matter.
However, the lime makes for a special case. The organic material can break down completely but the lime is still reacting. The compost I was talking about in my last post was well decomposed and determined "mature" via Solvita testing but still had a pH of 9.5.

    Bookmark   April 3, 2010 at 3:07PM
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We've seen other members post some things to consider before using HM. De-wormers, pest control chemicals used in the stable, other meds given to the horses being among them.

What I am taking from all these is that one should ask the supplier what else is in the manure. I suppose that would apply to any manures one gets where one does not know the animals or owner.

Again, thx for bringing this up.


    Bookmark   April 3, 2010 at 3:38PM
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How would composting the manure (even if it was fresh) neutralize the lime? I don't think that is possible; lime is a mineral, it's PH should not be altered at all. I figure if pure limestone is alkaline and lime (kiln-dried limestone) is as well and portland cement (kiln-dried to the point of molecular breakdown) is as well, then nothing can alter the PH of limestone other than an acid.

You can add sulphur to it (gypsum is limestone that has been chemically changed by sulfuric acid), or maybe even vinegar. When invading Ancient Rome Hannibal (I think it was Hannibal's army) climbed up a mountain and down an inaccessible pass by pouring sour wine (aka: vinegar) on the limestone and heating it until it broke apart in order to carve a walkable pass through the mountain.

Normally I'd just add a lot of pine needles, but I have naturally acidic soil, so that'd be enough. In your case, with a PH of 10, and pine needles slowly neutralizing, it probably would not be enough. I'd chemically alter the PH with an acid. If you have a truckload of free manure I'd think a few bottles of cheap vinegar would be worth a try; they sell white vinegar in gallon bottles for a couple dollars.

    Bookmark   April 4, 2010 at 3:08AM
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Wow! That was a scary read! Obviously too much lime is bad for pine trees, but now I'm freaked out because I just amended the clay I added to my raised veggie bed with a lot of peat moss and lime just to make it workable. I figured the combo would neutralize each other, but now I'm afraid I added too much lime. It was added to clay, not sandy soil, and it is for veggies, so I guess I'll find out later this season. Hopefully the organic material I added earlier will help as well; I just hope my wormies don't kill me.

    Bookmark   April 4, 2010 at 3:32AM
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The high PH is probably not an issue for many people, if their soil is acidic, but for those of us with alkaline soils, it's worth thinking about.

Another thing to consider (if you're in an arid climate) is that manures are often high in salt. That's not a problem in areas that get enough rainfall, but it's a consideration for those of us in arid climates.

    Bookmark   April 4, 2010 at 3:47AM
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Lime is added to manures to aid in odor and fly control where there is too little organic matter in the manure to do that. I can remember a discussion with my farmer uncle many years ago, after this was posted, about why that was done and his comment that only those that do not properly manage their manures would do that.

    Bookmark   April 4, 2010 at 7:25AM
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toxcrusadr Clay Soil(Zone 6a - MO)

Wow, that's not ground limestone either, which would be pH 8.3. That's ag lime, the powerful stuff.

Lime also reduces the offgassing of ammonia from the urine, by keeping it in the soluble ammonium hydroxide form, if I'm not mistaken.

Re: neutralizing during composting, I would think that the weak organic acids produced during composting would react with the lime and neutralize at least some of it. I wouldn't depend on it getting to neutral from that alone, but I would be surprised if it was still where it started after composting was complete.

    Bookmark   April 5, 2010 at 12:08PM
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toxcrusadr Clay Soil(Zone 6a - MO)

After thinking about this I may have mis-spoken about the effects of lime on the urine nitrogen. If ammonia is present, it is normally in equilibrium with ammonium hydroxide:

NH3 + H2O NH4+ + OH-

The OH- being hydroxide, i.e. alkalinity. If you were to add hydroxide to this system, it pushes the equilibrium to the left, toward ammonia, which can evaporate away. This would not have the desired effect of reducing odors.

Of course, it doesn't start out as nitrogen, it starts as urea or urinary porphyrins or who knows what, so I may be off center here.

Somebody tell me exactly why lime is used on horse manure for odor control, and how it works, because I don't get it.

    Bookmark   April 5, 2010 at 3:51PM
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No major concern for horse manure. Yes there are some stables that use lime for odour and horses do get de-wormers etc but this is minimal. If you get a small farm, maybe someone with a few horses the manure will be stable.

    Bookmark   May 1, 2011 at 7:39PM
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"Somebody tell me exactly why lime is used on horse manure for odor control, and how it works, because I don't get it."

Lime really isn't for the manure or even odor control, it is used to neutralize the ammonia created by the urine in the confined space of a stall. Breathing ammonia--regardless how it smells is hazardous to a horse's respiratory system.

The lime is usually spread on the stall floor during the mucking out process then fresh bedding placed over it. I've heard of it used most often in stalls with dirt floors as there is often a preferred spot for the horses to urinate that becomes saturated and difficult to keep dry. Ammonia is only a problem when the urine soaked bedding and dirt floor become saturated and don't have good air flow.

I agree with the farmer uncle, lime is a stop gap measure to compensate for poor horse management. When stalls have mats on the floor (or other impervious material) with bedding on top of it, and/or the stalls are cleaned on a daily basis there is no need for lime.

From a composter's point of view I would assume composted manure (which would almost always include some sort of bedding material) from a barn that uses stall mats would be preferable anyway. In those cases the bedding material would hold and contain most of the urine as opposed to it being absorbed in the stall floor.

    Bookmark   May 4, 2011 at 10:25AM
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curtludwig(New England)

So the reality here is the horse manure is fine...

Its lime in the west which is a problem...

I hate posts that say "The Danger of Horse Manure" when in reality they mean "sometimes people put lime in horse manure" and its really "Know your Horse Manure!"

    Bookmark   May 4, 2011 at 4:16PM
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Well, another thing to be concerned about. I've used lots of horse manure over the years and made sure I avoided manure after deworming meds had been administered. So, after this scare, I emailed my horse manure source this afternoon. She told me she has never used lime in the stables and doesn't know any horse owners who do. What's next?

    Bookmark   May 4, 2011 at 4:52PM
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tsugajunkie z5 SE WI

And then, of course, there is this.


    Bookmark   May 4, 2011 at 6:12PM
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curtludwig(New England)

@vermontkingdom might be a western thing, I've been around horse stables and never seen it done before either. Still around central MA I wouldn't mind at all, it'd cut down on the lime I have to spread...

    Bookmark   May 4, 2011 at 6:18PM
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We've got a major garden emergency on our hands. We've been getting horse, cow and chicken manure from neighbors for 35 years and have never had any problem other than weeds which are easy enough to control. But I suspect this lot of manure has had a lot of lime put in it. The pH when we finally tested it was 8! I realize that is a lot better than 10 but still does not make for happy plants. I'm wondering if any of you recognize these symptoms: necrotic edging on brassica leaves, general failure to grow, lower leaves of tomatoes yellowing, squash plants yellow and dying and miniaturization of broccoli plants and heads.

Oddly, the celery looks fine. We've been amending the beds that have the least manure in them with home made compost and scraping off the manure where it is possible. I should mention that this is aged and rotted manure which in addition, sat over the winter with a tarp over it.

Unfortunately we had added quite a bit to our beds and incorporated it into about 1/3 of the garden and luckily have mostly not planted in there. Do any of these symptoms correspond to just a high pH or do you suspect there might be some other chemical that was used to treat the horses that might have affected the composted manure this way? I haven't been able to talk to the manure providers. They seem to be away.

The area of the garden where I used compost rather than the manure are thriving and looking uncommonly healthy and well.

Been gardening a long time and never bumped into this before!

    Bookmark   May 31, 2012 at 10:10AM
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Miriam, did you find out if your neighbor added Lime to the manure?

    Bookmark   May 31, 2012 at 12:30PM
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toxcrusadr Clay Soil(Zone 6a - MO)

Also, have they changed their source for hay for the horses or did someone start using a herbicide they haven't before? Is it possibly a herbicide carryover?

pH 8 is not bad, remember it's being incorporated into several inches of soil. Even a 50-50 mix of pH 8 manure with pH 6 soil still gives you a pH of 7.

    Bookmark   May 31, 2012 at 1:13PM
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My neighbor seems to be away. But my husband just told me that when he tested the pH it was bright purple, not murky like the 8 that is registered on the color code. It's just a cheap little kit. I suspect that the manure is closer to 10 and the beds that have a little in it are closer to 8.

ai, yai, yai. No, I don't know if herbicides are at play but as soon as my neighbor is back I guess I'll find out.

    Bookmark   May 31, 2012 at 4:31PM
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I don't have access to horse manure, but I am curious by the statement that when you compost manure that's limed, the PH will go to neutral. How does that work? Does the microbes actually break down the minerals?


    Bookmark   May 31, 2012 at 4:58PM
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toxcrusadr Clay Soil(Zone 6a - MO)

I will take a stab at that. There are weak organic acids floating around in an active biological system that can react with lime to produce salts. It all depends on the relative amounts, and you have to keep in mind that every 1 unit of pH = 10 times more alkali as you go up the scale. So I'd say the composting process will have *some* effect in bringing the pH *towards* neutral, but if it's very high the resulting compost can still be at high pH.

    Bookmark   May 31, 2012 at 6:16PM
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I am pleased to announce that with the help of our soil and plant extension agents we have figured out the problem with horse manure in the garden. The problem was not Ph as I originally thought. It's true the horse owner did use PDZ in moderate amounts and that might have contributed to bringing the Ph up. But the real problem was that the manure he brought was not nearly as aged as I'd thought. Apparently even fairly fresh horse manure doesn't have much odor so I couldn't tell. The pelletized sawdust they use for bedding is the problem. It sequestered the nitrogen in the soil so plants were suffering from nitrogen deficiency. I'm very relived it wasn't a problem with herbicide or some other chemical. As time goes by, plants are recovering. I assume the sawdust is mostly broken down and the nitrogen in the soil is freed up. We also side dressed with Pro Boost that has 1/3 chilean nitrate in it so the plants got a major pick-me-up. Thanks all of you for your help!

    Bookmark   June 18, 2012 at 3:02PM
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Thanks all for this great, educational discussion and article links!

    Bookmark   June 18, 2012 at 10:09PM
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Animal manures are somewhat alkaline and may raise the soils pH some. But if those animal manures are composted with adequate amounts of vegetative waste the bacteria digesting that material will change it and the result will be a soil amendment with a near neutral pH that does remarkable things in soils.

Here is a link that might be useful: About animal manures

    Bookmark   June 19, 2012 at 7:21AM
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I have checked & have not found a stable that use lime in the stable.
Some have said line in stables is a bad sign & you should not keep your horses there.

    Bookmark   June 19, 2012 at 4:31PM
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I learn SO much when reading threads like this. Thanks, everyone!!

    Bookmark   June 20, 2012 at 11:35AM
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