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I've got some freshly constructed raised beds and I've filled them with a lovely load of well rotted horse manure.

The manure is so nice to work with, so rich and friable and easy to weed, that next year and the year after I'm going to want to repeat the process, adding in loads more. I've got no shortage of manure. And that's what all the books say - add in "plenty of well rotted horse manure".

So, this may seem like a dumb question, but aren't the beds going to fill up? Does settling alone take care of the problem? Or do some gardeners actually remove some of last year's beds to create the space to add plenty of fresh manure?

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In practice, I don't think it will be a problem.

I wouldn't add substantial manure to every bed every year. Some plants simply don't want it -- your carrots, for example.

I'd add an inch or two for, say, zucchini/courgettes or celery. And, since you're rotating crops, you can plant potatoes or beans there next year to feed off the leftover nutrients. And then the year after that, carrots or onions. Then pile on some more manure and plant tomatoes or corn/maize. Etc.

I don't have enclosed beds, but mine seem to settle somewhat over time after adding tons of horse manure. Unless I'm sifting it, the new additions of even manure from the well-rotted part of the pile always seem to have a few still-recognizable turds. After a year or so in the garden these are completely gone.

I don't think you'll need to specifically shovel soil out of the beds, because you can easily remove some of the soil at the end of the season when you pull out spent crops. If you uproot them, don't shake the soil off too much, and add them to your compost pile, you will end up taking a fair amount of soil out of the beds. This will make plenty of space for adding manure -- and you can mix it in right at that time so it has extra chance to rot down over the winter.

Finally, be careful of overdoing it. You can end up with nutrient imbalances -- too much of one nutrient can interfere with plants' ability to take up some other nutrient, and this can be difficult to correct. (Except possibly the squash family... the best squash I have are always volunteer pumpkins growing directly out of the manure/compost pile.)

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  • Yup... me too. The best plants seem to grown in my compost piles... at least until the chickens find their way in there and tear things up. – itsmatt Jun 6 '13 at 23:56
  • Ahh, chickens, quite possibly the most destructive creature on the farm... – bstpierre Jun 7 '13 at 2:39
  • Yes, I love the heck out of them - fun to watch and they're quite good at breaking down manure and providing us with eggs - but I do have to actively manage where they can go and what they can do because they'll tear up a planting bed in no time. – itsmatt Jun 7 '13 at 11:52
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Good questions. :)

Plants most definitely take up nutrients from the soil. A heavy feeder like corn is hard on the soil - one of the reasons they rotate with soy on factory farms is that corn simply requires a LOT of nitrogen and won't grow well after repeatedly being sowed on the same plot of ground without replacing the lost nitrogen. Other plants aren't quite so hard on the soil.

But this is viewing soil simply as a transfer mechanism for chemicals like NPK and not the right way to look at things in my opinion. Rather, the soil is a living, breathing community of organisms (might sound a bit silly, but this is the truth) which break down compost and manure and aerate the soil and make it quite conducive for growing plants. So adding some compost to the soil is a good thing because it helps feed the soil.

If you fill your beds to the brim they will settle a bit over time but there will come a point where they are definitely full.

You have options. A couple of which are:

Add some compost/manure/worm "tea" to the beds. This is a kind of "liquid fertilizer" that adds nutrients and organisms back into the soil. I've generally just taken, say, worm tea and spread it on the soil prior to planting or later on as a side dressing to the plants. Seems to work quite well.

If you are so inclined, you could shovel off some of the topmost soil into a wheelbarrow and use it for a new bed, or some containers or add a little bit to a compost pile or even spread it on the lawn. Then you could mix in some new composted manure or compost to the soil. That's a bit more labor-intensive but not that bad.

Honestly, I've never got to the point where I was so overloaded with soil because I will tend to just add another bed or a couple of containers or a lettuce table. I do tend to pull a bit of the soil off my beds when creating new seedlings or containers and then will head over to the compost bin and grab a few loads of compost to mix in to the beds. This helps to keep the beds healthy and filled with microbes and nutrients (and worms!). So it's a process and requires some inputs from somewhere, whether that is tilling in a green manure or amending it with manure or compost. You'd end up with the "dust bowl" situation if you never amended the soil and were only ever removing nutrients from it. Legumes will "fix" the soil with nitrogen but there's more to growing than just nitrogen.

A bit long, but I hope that was useful.

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How do I avoid overfilling raised beds if I'm adding substantial manure annually?

Easy. Don't add "substantial manure" annually. Straight composted manure is very nutrient rich and your plants will only use a small portion of it and much of it will not leach out so only add enough to bring your beds back up to the appropriate level. Use whatever you have left over in the rest of your yard. Give some to family, friends, neighbors, etc.

You called it "rotted manure" I hope you mean composted manure. Manure needs to be properly hot composted in a pile with the addition of a little bit of plant material so that the heat kills pathogens and weed seeds.

It seems like this is your first year. My guess based on my experience with straight composted manure is that it won't be as friable after some time. Mixing some peat moss in your beds will help it retain its texture. Square Foot Gardening is based on growing vegetables in raised beds with a mixture of compost (mix of composted manure and plant based compost), peat moss and vermiculite and is practiced by many gardeners. You might want to look into that. Even if you don't use the square foot gardening method there is a lot of useful information for raised bed gardening in general.

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  • In the UK, the stuff you buy in sacks from the garden centre is labelled "Well Rotted Horse Manure". I'm sure it's composted properly. – slim Jul 4 '13 at 14:40
  • @slim thank you for that clarification on regional usage. My main concern was the statement "I've got no shortage of manure". That usually implies they have their own livestock so I wanted to throw in the reminder about properly composting the manure because it's an important safety concern. Better safe than sorry :) – OrganicLawnDIY Jul 4 '13 at 19:23
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The only option is to remove old stuff before adding new manure in order to have similar level.

Personally, when I refill soil mixed with manure to my plants, I try to remove all unwanted old soil that looks unhealthy / rocky, which I think are just empty food plates :)

PS: I just work on upper layer of soil and don't dig down towards roots.

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