I've created very long raised beds for growing annual and perennial vegetables and herbs on top of cardboard sheet mulch. I'm using mostly decomposed white pine, hay and alfalfa horse bedding compost as the base. Most of it is black like soil and it has a sweet smell like pipe tobacco. There is still plenty of organic material that is still only partially-mostly decomposed, and it appears as if I should add some mineral substance to the compost. What sort of material shall we add to this mostly decomposed horse manure compost to make it fine, fine soil and perhaps decompose faster? I've already made the beds and I'm not willing to rake it all up and compost it further, but I will happily mix in other mediums. What shall it be? We've got plenty of sand, clay, mud, topsoil, chicken manure and hay bedding etc. here at the farm.
(Interesting, the giant compost pile at the equestrian center was so hot that it caught on fire, twice. The owners claimed that a short blue flame was covering the pile and began burning the wood walls containing it!)
A good soil sample will provide the best, most acurate advice on what inputs you should make. Make sure you check your boron, silica, calcium ratios.
If your pile contained any heat I would avoid planting in it at this time. Add nitrogen sources to continue the composting (chicken manures, feathers, moldy hay, etc).
Other that that, I would mix in native topsoil. I would, also, recommend going into a wooded area and finding old, crumbly logs. I would scoop out the pulp and mix that in, as well (lots of good fungi there).
I'm not surprised the big farm's manure pile caught fire. The biological activity within that type of pile can easily reach combustion temperatures.
As an aside, compost should not be allowed to go above 165 degrees F or you are killing off the good microorganisms. It should be turned at or before that temperature. This ensures the pile remains aerobic and healthy. With compost you're trying to add good life to the soil and give these soil microbes great things to eat.
We don't know if you're wanting to plant in these raised beds soon or at a much later date. The suggestion of adding high-N matter to keep the compost going assumes you're going to use the beds later, not sooner.
If you're not willing to rake it up and compost it further, and want to plant in it soon, then you really just want it to decompose slowly, not quickly. You don't say why you think you need need some mineral substance to add. You can seriously throw off the soil's mineral balance by making uninformed decisions and adding material. Boron toxicity comes to mind. Trace elements have to be in trace quantities.
Without a soil test, no one can really answer your question on what more needs to be added. If you just want to make your best guess with no real data, I'd say cover it with 6" of good topsoil, don't till the still-hot compost into the topsoil, and plant in the topsoil. The roots will decide when it is safe to push into the compost. But really, if the compost is not mature, no plant will want to grow in it. Eventually it will be conducive to plants, earthworms, etc. They can do the mixing for you, so you can save your back.
There's really not enough information to answer your question, but maybe this helps.
The horse manure was staged into piles that were far to big and that most likely is why it caught fire. Having trained several stables on proper disposal and composting of horse manure in SC, I highly recommend it be completely composted before you use it in your beds. Horses do not have a digestive system like a cows in that they allow viable weed seeds to pass through. After it is composted, use like any other type of compost, as a soil amendment.
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