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I know that you shouldn't use raw manure on plants because it's bad for them, but I don't understand why.

I know you should stack manure before using it (i.e. literally stack it into a big cone), and wait until it no longer smells. How long would this process normally take? Is it different depending on the type of manure, e.g. chicken vs. cow vs. bat etc.

What is the mechanism in play that makes raw (or not well rotted) manure a bad thing for plants?

Dori
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bradley.ayers
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2 Answers2

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There are multiple reasons:

  1. If you use raw manure next to leafy plants or on root crops, you risk bacteria moving from the manure onto your food. E. coli isn't something you want on your food.

  2. Chicken and bat manures are high in nitrogen. It's intense enough to burn your plants.

  3. It can stink. If you dump a bunch of raw cow manure into a flower garden under your front window, your living room or front porch aren't going to be pleasant places to hang out.

  4. The rotting/composting process allows a variety of little organisms to convert the nutrients in the manure into forms that are usable by the plants.

It's not really a blanket prohibition against using raw manures on any plants. I mix regular garden and kitchen wastes into my horse manure piles, and the cucumber and pumpkin volunteers from the kitchen waste thrive in the not-very-well-rotted manure. So I'm guessing that dumping a little semi-raw horse manure on those plants won't hurt.

As for length of time, it depends on several things:

  • the animal (different animal manures have different carbon-to-nitrogen ratios)
  • bedding (which alters the C:N ratio)
  • size of pile
  • whether the pile is aerated regularly
  • whether the pile is wet or dry

It's basically a composting process, so these are the same considerations you'd have for a compost pile.

I turn my horse manure piles regularly (I'm a compulsive composter) and they're pretty well cured within about 6 months. Unturned it can take a year or more. If we used different stall bedding it might take more or less time. If I wasn't mixing in chicken manure it would probably take longer (chicken manure adds more nitrogen and speeds things up). But if it was just a big pile of chicken manure, the chemistry works against you: too much nitrogen. Adding some carbon (leaves, wood chips, etc) would balance it out.

bstpierre
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    +1 esp. for no blanket prohibition. like your cukes and pumpkins, i have found tomatoes go like a train on semi-raw (weeks, not months) horse manure. – Tea Drinker Jun 09 '11 at 09:04
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    The carbon/nitrogen ratio should be about 30/1. Here is a list of ratios: poultry manure-7/1, cow manure-18/1, leaves-50/1, offal (slaughter waste)-2/1, straw-100/1, sawdust-500/1, wood chips-250/1, grass hay-80/1, legume hay-20/1. – J. Musser Sep 16 '11 at 01:30
  • fresh grass clippings has a lot more nitrogen then hay – Ian Ringrose Jul 12 '15 at 16:15
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To add to the above answer, ammonia can be a problem; it's poisonous to some plants, and its production depletes the soil of nitrogen

Jon Spencer
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    This is a good point but can you expand on it further to make a complete answer? How would you know when the ammonia is gone? – kevinskio Apr 14 '13 at 13:50
  • From a [suggested edit](http://gardening.stackexchange.com/review/suggested-edits/3129): A little picky perhaps, but the ammonia production does not deplete nitrogen from the soil. It is a by-product of nitrogen being released from the soil. As nitrogen is consumed, ammonia is released (as ammonium nitrate?). – Niall C. Feb 28 '14 at 14:43