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Large Scale Composting in the Desert. Questions about Air Circulation, Moisture, Temperature.


July 3, 2017

I am working in the ____ Zoo as an intern. We have a compost site with several piles. The raw material we use for composting is food waste and vegetable waste, plus some horse bedding. The compost site is in open air. In order to maintain the moisture of the piles, when we weekly use a shovel car to turn the compost. we water the pile a lot, to an extent where the WIP is more wet than wrung out sponges.

I think the water is too much but we don’t have much time to daily take care of the compost site. So I have two questions: 1. How to balance the air circulation and water evaporation? 2. The site is in open air. In summer, the outdoor temperature can be over 120 F. Does this temperature negatively influence the composting process? I think only thermophilic bacteria will be alive and the core temperature might be even higher (we don’t have a thermal probe to measure the core temperature). Do we need to frequently turn the compost to lower the temperature a bit?



Answer by JZ: First - good job! You are doing a fine operation that will provide humus for the soil at the zoo. You have a fine mix of greens and browns. If at all possible, let your pile be in a shaded location, to decrease evaporation.

You could balance air flow and reduce compaction of wet ingredients in the pile by adding coarse bulking material: finger-size, sticks, twigs, pine cones, pine needles.  Bulk as you build the pile.  If you are not currently bulking, you may start adding the bulk, then mix in with what is currently in the pile. This helps with convective air flow in the pile. See our brochure.

If your pile is reaching 5 feet in height, the weight of wet organics will create downward compaction. It would be fine then to subdivide the pile, so that the height is about 3-4 feet. You may have more than one pile. In your environment, I'd suggest that you cover the whole pile with a tarp. This will reduce flying insects AND reduce evaporation from the pile. Hold the tarp in place with rocks around the edges. Of course remove the tarp when you are working with the pile, then replace it.

In thermophilic composting the internally generated heat comes from microbial enzymatic action, breaking down the carbohydrates thus releasing heat.  An ideal target temperature would be 150F maintained for about 2 weeks, then the pile might be turned, churned and watered as necessary. Turning and churning exposes all organics in the pile to heat.  This will destroy plant pathogens and seeds. The moisture level should be similar to brewed coffee grounds - saturated, but not dripping.  You could purchase a temp. probe for about $30.00 at Amazon.

So turning will drop the temperature some, but it will rise again as long as you maintain your current mix of greens and browns.

So if you add bulking, maintain 50% moisture and turn every 2 weeks you will soon have a fine end product.

Response from Questioner: Thank you very much for your quick reply. We are going to try the idea of using tarp. I have one more question for you: I like the idea using bulking for a structure that aids convection. Is it also applicable for large scale composting? Our current practice is we weekly add “new” green waste with some hay to the pile in progress. Meanwhile, pile will be turned and watered. (By the way, after using tarp, we will stop over-watering it.) In winter, the frequency will be higher as we have more green waste. I am afraid adding new stuff and turning the compost will damage the structure. Or maybe it’s not necessary to turn it if the structure can be kept in the first place. 

Answer by JH: John and possibly others may well reply but following are my comments. The purpose of adding bulking materials is to create good airflow so that the pile will not go anaerobic. For this reason, bulking material is always needed unless the operation has some type of mechanical airflow system.

It is not necessary to turn the pile ever. However, if you are wanting to maintain a hot pile or otherwise wanting to decrease the timeline to finished compost, it is necessary to turn the pile. Simply follow JZ's earlier comments on this regardless of time of year or contents of the pile.

Answer by JZ: JH has responded to your question.  Here is my opinion.

What is your description of large scale composting?   What are the dimensions of the set-up?Yes, coarse bulking is appropriate for large scale composting. Adding more greens and browns to an existing hot pile is fine. It would not damage the structure. If your winter additions are very nitrogenous (greens) then adding (mixing in) course bulking wood be an imperative. Bulking only needs to be mixed in the new addition, not the whole pile - its already in there.

In my opinion a pile that is not bulked has poor structure as the bulking will decrease compaction. A non-bulked pile may undergo compaction creating an anaerobic situation then unpleasant odors and acidic conditions may result.

Yes, any pile set-up may be managed statically (no turning) as long as you bulk as you build and maintain 50% moisture and cover the top.

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