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I'm doing soil improvement work in Santa Fe, NM. I've dug down ~24" in an area ca. 2'x 6', creating an empty pit sort of like digging a grave and mixing back in the native dirt in ~4-6" layers with copious amounts of spent coffee grounds, very generous amounts of peat, wetted & diced dried alpaca manure, as well as some fire ash & sawdust.

It's very, very dry here, so I've been wetting down the finished product when I've completed a layer. The mixture can get seriously hot in very little time --when I stick my hand down into the "soil" approx. 4 or 5 inches, it's often too hot too leave there for more than a few seconds, & I'm an Aries who's used to playing with fire & traversing hot coals. (I'm repeating this process in adjacent spaces.)

I'm guessing that the heat is generated by some sort of chemical reaction within the ingredient mix.

So I have two (2) questions:

  1. Is this high heat excessive i.e. can it do any damage to the soil amendments & therefore be counterproductive?
  2. do you see any serious imbalances in my additive program?

The soil here tends to be pretty alkaline, & the soil I've started with is poor-- a clay & sand mixture with many smaller stones, which I've removed.

  • Bacterial life give off heat when they have nitrogen and carbohydrates to work with. It can get hot enough to sterilize seeds and viruses out of the resultant soil. If it goes anaerobic and you can smell sulfur compounds and ammonia, you're losing your nitrogen. – Fiasco Labs Jul 9 '13 at 5:57
  • Got a question - the 'wetted & diced dried alpaca manure'. I interpret this as originally being dried alpaca manure, but that doesn't speak to whether the manure had been composted prior to drying, or more likely, dried out from fresh. If it hadn't been composted prior to drying, then that process is taking place now, and will generate a lot of heat and may result in nitrogen deficiency. – Bamboo Jul 9 '13 at 12:16
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So, let's see here. You are digging down 2' into the ground and removing the local soil which is mostly clay and sand. You're saying the soil is alkaline.

Into this local soil you are mixing:

  • Used coffee grounds
  • Peat moss
  • Aged alpaca manure
  • Wood ash
  • Sawdust

So the used coffee grounds are a bit of a wildcard. The internet is full of claims about it. From my research (which I would not call "exhaustive") I have read that coffee grounds in the 10-20% percentage is good but more (e.g., > 30%) is not better. The carbon:nitrogen ratio will change over time. The pH has varied in experiments from slightly acidic to slightly alkaline. I believe the last time I tested it myself I found the batch I had to be darn near neutral in pH, but that was several years back and I didn't write down the numbers.

Peat is acidic and certainly would help to bring down the pH of your alkaline soil.

The aged manure is lower in nitrogen than the fresh stuff but definitely better for the garden (or whatever you plan to do with the enriched soil).

Wood ash is going to raise the soil pH (as opposed to the lowering of the pH by the peat), so those will in essence contribute to moving the pH of the soil in one way or the other. The ash will also add some trace elements along with potash to the soil. Your local soil may be deficient in those. There are some differences based on the kind of wood used but that's a bit "inside baseball" and probably not particularly important for the purposes of answering your question.

Sawdust is high in carbon. On our farm we've used this or wood chips in the past for stall bedding for horses and chickens. In fact, fresh manure and sawdust or wood chips work really well together in the compost bin and produce some fantastic compost once the microbes have gone to work. The pine sawdust I have used is acidic. Same would go for pine needles.

Now, back to your question which is about compost heat. You've got a bunch of raw materials there long with native soil in a moist environment, so it's going to heat up, particularly if it is mixed well. The microbes will have a field day in there.

The C:N ratio in the range of 25:1 or 30:1 will result in optimal decomposition, or so they say. Personally, I don't worry too much about the ratios being spot on. I keep it wet and turn it and the composting magic just happens. :)

It's heating up quickly because there's a lot of raw material there for the microbes to break down, I believe, and I can also believe that it is too hot to stick your hands in for too long They will definitely generate a lot of heat. Have you put a thermometer in there (or used one of those handheld infrared thermometers) to get a temperature check?

The hotter compost pile will kill weed seeds (and maybe some other things too) and that's what I do with my weed compost bin. Some organisms might be killed by the higher temps. Certainly the worms will move away from anything that is too hot.

It seems like your amendments are more acidic and will balance out the more alkaline soil. I'm not a soil expert by any means and there are likely smarter folks about this subject here than I. I do like the mix you've got there. If it is heating up then that would tell me that it is neither too wet nor too high in carbon (or too low in nitrogen).

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