As an introduction to this article, let me describe what our compost piles look like on a cold morning (in the last few weeks the temperatures at night have dipped well below freezing on the farm).
Our piles look like small (3-4 feet high) but very long (240 feet) pyramids. On a cold morning, as the sun rises in the East, the eastern edge of the top of the piles looks like it is breathing: a lot of water vapor is rising. This vapor is due to the action of microorganisms which are decomposing the organic material that was added to the piles (fruit residue, leaves, tree prunings, used fruit boxes etc.). The microorganisms produce not only water but also energy (heat) and CO2 and humus. The heat produced is high enough to evaporate water. So, on a cold morning, heat generated inside the pile (usually the top part) heats the water up and the heat from the rising sun also helps. This results in a spectacular steaming on top of each pile. The piles look like they are on fire.
After a very cold night, the sides of the piles might be covered with a beautiful layer of ice crystals while the ridge (top) of the pile is smoking. This is due to the fact that warm air (heated by microorganisms) rises to the top (called convection heating in our kitchen ovens) but not to the sides. On the other hand, the sides also lose heat but this is usually negligible (conduction and radiation); this is why on a very cold morning the sides can frost up while the top is smoking and is visibly wet.
We measure, as frequently as possible, the temperature of the piles because we want to make sure that the temperatures in all parts of the piles are just right to promote rapid decomposition of the organic residues, to kill weed seeds and disease causing organisms (especially plant). But we also want to make sure that the temperatures do not exceed ranges that affect the viability of beneficial microorganisms that are necessary to maintain rich topsoil.
Typically, our piles start (after construction) at around 100 F. This lasts for two or three days then temperatures go up to 140 F- 160 F. This lasts for several days all the way up to two months. We try to keep the piles above 140 F but well below 160 F in order to be sure that most microorganisms stay alive. Above 160 F, most microorganisms die or stop growing.
For all these reasons, several times a week, about 28 temperature readings are taken on each pile. Next week, we will discuss what happens after the temperature readings are recorded.