Composting 102 - Heat

Alright, now for some application of the principles. First, where does one acquire such copious amounts of mature and immature material? There are three options: don't worry about it and build piles with what you've got, scavenge from your neighborhood or surrounding land, or plan to grow enough of what you need. Each option has its upside and downside.

Using what you have available is easiest, but does not always end in a balanced pile. Usually you will be heavier on the immature material, unless you buy a bale of straw, which leads to the second option.

Scavenging from the woods, fields, lawns and farm supply stores can also lead to an unbalanced pile, but that is less likely. The only three objections I can think of are that it might cost money (gasp!), the material may have qualities you don't want to introduce to your system (like pesticides), and you would technically be stealing fertility from another source. Granted, the local coffee shop may only have one other outlet for all those grounds (the trash) but you are still importing it to your system. Obviously this last objection is more philosophy than practicality. The farmer we learned from in our first internship made a deal with the township that, when they went around in the fall collecting leaves from curbs, they would dump them on his farm. He would then mix these massive piles with some nitrogenous material (horse manure) and let it compost. Often, when spreading this compost, we would find little plastic toys that got raked up with the leaves. It kept us amused... And sometimes made us wonder what else the leaves carried with them. But this farmer had found an incredible amount of mature material, delivered right to his farm, for free. And you can't really argue about the practicality of that. He was farming on a market scale, though, where most of the farm's nutrients are being exported as food. You have to balance things somehow.
Ok, so addressing the first point of the second paragraph: a pile that is unbalanced, favoring the immature. For the record, "unbalanced" is completely relative here, because things will compost no matter what. But when we are trying to get the best possible result, we should keep an eye on what goes in. Allow me to dig briefly into the biology of the compost pile: Once you have built it, the microorganisms in the environment go to work, assuming there is enough moisture. They consume, among other items, nitrogen, carbon, and oxygen. Unless the pile is too wet, in which case the pile turns anaerobic, meaning the microorganisms doing the work operate without oxygen. This will smell bad, your neighbors will resent you, and the finished product will not be so pleasant. I speak from recent experience.
But assuming it is aerobic, meaning oxygen is involved, your compost will smell nice or not at all. As the microorganisms go to work their metabolisms rise and with it their temperature, so much so that the temperature of the whole pile rises. Eventually it will get too hot for the active population of microbes, at which point other populations will become active. This continues until the material available to the microbes is consumed by them.
How hot does it get? That depends on the food you are presenting to the microbes. Two things will affect temperature most: the amount of immature material and the amount of oxygen. If you build a pile only out of grass clippings your pile may get quite hot, upwards of 160° F. If you forget to apply water to a pile of grass clippings and chicken manure (which is HIGH in available nitrogen) it can conceivably get hot enough to combust. Don't try this at home, kids. When a pile gets turned, oxygen is brought into the system in new quantities, which will cause the microbes to become active again, heating up the pile.
To a degree (ha!), heat is beneficial. It means things are working, and at a faster pace than if the temperature were lower. But at a certain level heat becomes unproductive, causing some of the nutrients to which you'd like to hold on turn into gases and exit the pile. Additionally, high heat will kill populations of microbes. Which is actually the point in some composting schools of thought. If you can get your compost pile to reach 160° F or more it will kill most of the disease-causing microbes and many of the seeds that made it into the pile. The downside is that, along with most of the pathogens, all of the beneficial microbes will be killed off. And what holds disease in check? Beneficial microbes. So all you have left in your pile is the most tenacious population. In Grow Biointensive we aim not to eradicate negative populations, but keep them balanced by the beneficial populations.
What, then, is the ideal temperature? We aim for no higher than 140° F. Oops! How did that picture get there? We have compost thermometers with 18" probes to get to the middle - a very handy educational tool. This pile got a little too warm by our standards. The photo below showcases a more reasonable temperature.
These are both examples of classic late-spring piles. Our mature materials from the year before are almost used up, but we still have immature cover crops to pull out. These immature crops are becoming more and more mature in quality, but still have that slightly higher nitrogen content. So the piles, like the days, tend to be warmer. In the fall we have lots and lots of mature material but not much immature. They tend to be cooler. The air temperature plays a role, too. If it is freezing outside you probably won't have a 140° F pile.
As to the turning, it is a trade-off. Some nutrients will be lost in the turning, it's true, but if you don't turn it at least once the materials on the edges of the pile will not break down. We generally compromise by turning each pile once. (For the uninitiated, turning involves taking the pile, piece by piece, and rebuilding it in a neighboring location. Not flipping it over all at once.) There are contraptions called "Compost Tumblers", which are drums that pivot on a horizontal axis. They allow you to put your materials in and turn them as often as you like. Once again, more oxygen, higher heat, and cured compost faster. With greater nutrient loss.
What is turning into a series will probably wrap up with the third post, which will cover manure, undesirable material, compost quantities, and miscellaneous other. Enjoy!