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Composting Manure

Is it safe to use animal manure in your vegetable garden?

Cow ManureHere’s a question we’ve been thinking about: why compost manure? It’s one of those questions we felt we knew the answer to — and we did — but that a reconsideration brought up all the variables and exceptions we’ve either learned from experts or from our own hard experience. So let’s deconstruct. Does all manure need to be composted before being used in the garden? If so, what’s the best ways to compost it? And finally, what about chickens?

We bring that last bit up because more and more people, both in the country and the city, are keeping chickens. And chickens, er, emit some of the richest manure a gardener could hope for, high in nitrogen and phosphorous and full of other nutrients. Best is the fact that a chicken’ digestive system kills weed seeds — 98%! — that might otherwise be spread to the garden. Fresh chicken manure needs to be composted because it contains so much nitrogen that it will discourage germination of many vegetable seeds and burn young seedlings. Which ones? Ironically, it’s those that require a lot of nitrogen later on as they grow.

But there are some manures that are less packed with nitrogen, like horse manure, that usually can be applied to the garden without too much finishing after, er, they’re emitted. But the problem with horse manure and cow manure and goat and sheep manure is that they contain weed seed (PDF). And you don’t want to spread weed seed in your garden.

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So the simple answer to the question — why compost manure? — is to kill off weed seed and/or reduce high nitrogen levels to the point they won’t harm germination or burn seedling (or even full-grown plants). The answer to the next question — what’s the best way to compost fresh manure? — is a bit more complex and with a variety of answers. Most of them have to do with what kind of manure you’re composting and why — to kill seeds or temper its nitrogen? Remember: compost is an amendment, not a fertilizer. If your soil is deficient in nitrogen, you may want to consider adding a safe garden fertilizer as well as compost to bring it up to optimum levels.

The best, maybe only method of killing weed seed, is the hot composting method. Bringing the core of you compost heap up to 135 degrees will kill most seeds as well as any pathogens that might be in it. Some experts claim that raising a compost heap’s internal temperature to 160 degrees is required to kill pathogens, but this is difficult to do and will also kill all the beneficial microbes and other soil organisms you hope your compost will add to your soil. Hot compost is quick compost. You’ll be reducing overly high nitrogen levels at the same time you’re killing weeds. A compost thermometer is a useful tool here. A compost pile that feels hot to the hand may only be at 115 degrees or so. Three days at that temperature should do the trick. Then turn your heap so that the material on the outside is then at the center where it can also heat up to the desired temperature.

If you’re composting chicken manure and not worried about weed seed, then time, rather than temperature is the first factor to consider. Four months or so in a slow, “cool” compost heap will make your manure safe as long as you keep the green-brown ratio in mind. More brown or carbon materials — like straw or leaves — will make your green or nitrogen sources that much safer for your garden. We’ve always found measuring these ratios a tricky prospect; manure tends to be compact and straw and leaves aren’t. So throw in a bunch of brown if you want to be sure. If you’re composting chicken manure a 2:1 or even 1:1 brown-green ratio isn’t too much.

There are other variables in play. Sometimes the composting process is well under way before your manure is cleaned from your coop. The manure has mixed with the bedding and the compost is naturally turned as the chickens scratch. If you clean your coop every week — we’ve known some people to clean their little coops every couple days! — then you’re dealing with fresh manure. But if you allow your bedding to settle and don’t clean for months, then your coop cleanings are probably safe to use as between-row mulch while it breaks down just a bit further.

We’ve found this mulch method very effective when using the bedding from goat stalls. Sure, we still had some problem with weeds but found that pulling them from the mulch was easy and that even walking between rows tended to kill many of the young weed seedlings (love those muck boots). Other things experience has taught us: we once brought a load of chicken-coop bedding to our garden in the fall and, after turning it all into the soil in March and planting a month or so later, had no trouble with burn off. Again your local conditions — this was in the rainy and not-so-cold Pacific Northwet, er, Northwest — and the status of your bedding are determining factors. We found throwing horse manure directly into the garden in spring resulted in no burn off… but plenty of weeds, often right in the spot where the manure landed! Mucking out an old milk cow’s pen and spreading the manure — some of it obviously freshly emitted, some of it had been there for months — early one spring before turning it into the soil caused no problems. Yes, we spread it thinly (another variable).

Commercially available manures? They may claim to be composted or even “fully composted” but use the smell test to decide for yourself. If it smells like manure and not fresh, neutral compost, then throw it in your pile for further breakdown. Commercial compost from factory farms, especially pig farms, may contain copper and other heavy metals as well as high levels of salt. In short, know the source of your manure.

Safest way to use manure around growing plants? Soak a couple cups of it in a five gallon bucket and use it when watering (add the manure tea after a good watering and there’s no fear of it being too nitrogen rich). Water just around the base; try to keep the tea from the plants leaves and stems (remember, pathogens).

There are a lot of variables that go into the effective use of manure as a garden fertilizer. The best rule is to always compost any manure (never use pet manure… pathogens!… unless you have an extremely hot or other specialized method of composting). We’re going to guess you already knew that. Please… share your experiences with us.

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4 Responses to “Composting Manure”

  1. Sandi G on March 9th, 2013 at 6:46 am #

    Personally I use rabbit and worm casting which can both be used directly in the garden. No composting needed. I am working with my city now to enact a new law to allow us to keep chickens. You can bet I will be composting the chicken guano for use in the next years plantings.

  2. Priscilla on June 21st, 2014 at 1:25 pm #

    I’ve been mixing 1/2 4-6 mo old wood shavings with chicken manure from the coops, with 1/2 garden/kitchen scraps with a soupcon of horse poop and a few handfuls of old hay in the tumbler. In hot weather that gets cooked pretty quick then I put it into one half of a 2′ x 6′ compost bin and throw in a bag of Red Wrigglers from Uncle Jim’s Worm Farm. Three loads from the tumbler and those worms will munch it up in no time. We have awesome compost this year with this recipe. And BTW the peppers this year are TERRIFIC! See you next December.

  3. Jon V on July 31st, 2017 at 2:05 pm #

    If You don’t know the source of the manure, there is always the risk of poisoning soil long term with the residual persistent herbicides that make it through the animal and the composting process. Not much will grow for a few years. It is a big gamble.

  4. Farmer Lorran on March 20th, 2018 at 2:17 am #

    I have very sandy loam soil. I have two sheep and 22 chickens. for chicken bedding, I use Douglass fir wood pellets (very absorbant), and pine shavings. For the sheep, I use pellets only. For the past year I’ve been spreading the sheep manure on a future garden patch. I’m actually trying to kill oxalis there. I compost the chicken manure with kitchen scraps. I’m looking forward to tilling the sheep manure area this spring. It seems to be holding much more moisture than the native sandy loam does.